Duus topical diagnosis in neurology anatomy, physiology, signs, symptoms, 5th edition PDF (jan 19, 2012) (3136128052) (springer)

cor-Peripheral Nerve, Dorsal Root Ganglion, Posterior Root The further “w ay stations” through w hich an ent im pulse m ust travel as it m akes its w ay to theCNS are the peripheral nerv

Un iversity of Göttin gen

Göttingen, Germ any

Michael Frotscher, MD

Hertie Sen ior Research Professor

for Neuroscience an d Ch airm an

Departm en t of Structural Neurobiology

Cen ter for Molecular Neurobiology

Ham burg (ZMNH)

Un iversity of Ham burg

Ham burg, Germ any

With contribution s by

Wilh elm Kueker

Foun ding auth or Peter Duus

40 0 illustration s, m ost in color,

by Professor Gerhard Spitzer an d Barbara Gay

Th iem e

Stuttgart · New York

Library of Congress Cataloging-in-Publication Data

Baeh r, Math ias.

[Duus’ n eurologisch -topisch e Diagn ostik English ]

Duus’ topical diagnosis in neurology : anatom y,

Physiology, sign s, sym ptom s/Math ias Baehr,

Michael Frotsch er ; w ith contributions by

Wilh elm Kueker ; translated by Ethan Taub ;

Illustrated by Gerhard Spitzer 5th , rev ed.

p ; cm

Rev translation of the 8th Germ an ed c20 03.

Includes in dex.

ISBN 978-3-13-612805-3 (GTV : alk paper)

1 Nervous system Diseases Diagnosis 2 Neuroan atom y.

3 Anatom y, Pathological 4 Nervous system

Pathophysi-ology I Frotscher,

M (Mich ael), 1947- II Duus, Peter, 1908- Topical

diagnosis in neurology III Title IV Title: Topical

diagnosis in neurology [DNLM: 1 Nervous System

Diseases–diagn osis 2 Nervous System –an atom y &

histology 3 Nervous System –physiopath ology.

WL 141 B139d 2011]

RC347.D8813 2011

Im portant note: Medicin e is an everchan ging science un

-dergoing continual developm ent Research and clinical perien ce are continually expanding our know ledge, in par- ticular our know ledge of proper treatm ent and drug ther- apy Insofar as this book m entions any dosage or application , readers m ay rest assured that the auth ors, editors, and pub- lish ers h ave m ade every effort to ensure that such refer-

ex-ences are in accordance w ith the state of know ledge at the

time of production of the book.

Nevertheless, this does not involve, im ply, or express any guaran tee or respon sibility on the part of the publish ers in respect to any dosage instruction s and form s of applications

stated in the book Every user is requested to examine

care-fully the m an ufacturers’ leaflets accom panyin g each drug

and to check, if n ecessary in consultation w ith a physician or specialist, w heth er the dosage schedules m en tion ed th erein

or the contraindications stated by the m an ufacturers differ from th e statem ents m ade in the present book Such exam i-

n ation is particularly im portant w ith drugs th at are either rarely used or have been new ly released on th e m arket Every dosage sch edule or every form of application used is

en tirely at the user’s ow n risk an d responsibility Th e authors an d publish ers request every user to report to the publishers any discrepancies or inaccuracies noticed If errors in th is w ork are found after publication, errata w ill be posted at w w w.thiem e.com on the product description page.

Th is book is an auth orized and revised tran slation of th e 9th Germ an edition publish ed an d copyrighted 20 09 by Georg

Th iem e Verlag, Stuttgart, Germ any Title of the Germ an edition: Neurologisch -topische Diagnostik Anatom ie— Fun ktion—Klinik.

Con tributor: Wilh em Kueker, MD, Radiological Clinic, partm en t of Neuroradiology, University Hospital Tü bingen , Germ any

De-4th edition translated by Eth an Taub, MD, Klin ik im Park, Zurich, Sw itzerlan d Updates tran slated by Geraldin e O’Sullivan, Dublin , Rep of Irelan d

Illustrators: Gerhard Spitzer, Fran kfurt/M, Germ any;

Barbara Gay, Stuttgart, Germ any

Som e of th e product n am es, patents, an d registered designs referred to in this book are in fact registered tradem arks or proprietary n am es even th ough specific referen ce to this fact is not alw ays m ade in the text Th erefore, th e appear- ance of a nam e w ithout designation as proprietary is n ot to

be construed as a representation by th e publisher that it is

in the public dom ain.

This book, in cludin g all parts th ereof, is legally protected

by copyright Any use, exploitation , or com m ercialization outside th e narrow lim its set by copyrigh t legislation,

w ithout th e publisher’s con sen t, is illegal and liable to ecution This applies in particular to photostat reproduction , copying, m im eograph ing, preparation of m icrofilm s, and electronic data processin g an d storage.

pros-© 2012 Georg Thiem e Verlag,

Rü digerstrasse 14, 70469 Stuttgart,

Germ any

http://w w w.th iem e.de

Th iem e New York, 333 Seven th Avenue,

New York, NY 10 0 01 USA

http://w w w.th iem e.com

Cover design : Thiem e Publishin g Group

Typesettin g by prim ustype Hurler, Notzin gen, Germ any

Printed in China by Everbest Printing Co., Ltd

edition 1996 1st Italian edition 1987 1st Japanese edition 1982 2nd Japanese edition 1984 3rd Japanese edition 1988 4th Japanese edition 1999 1st Korean edition 1990 1st Polish edition 1990 1st Portuguese edition 2008 1st Russian edition 1996 2nd Russian edition 2009 1st Spanish edition 1985 1st Turkish edition 2001

In 2005 w e published a com plete revision of Duus’

textbook of topical diagnosis in neurology, the first

new edition since the death of its original author,

Professor Peter Duus, in 1994 Feedback from

read-ers w as extrem ely positive and the book w as

trans-lated into num erous languages, proving that the

concept of this book w as a successful one: com

bin-ing an integrated presentation of basic

neu-roanatom y w ith the subject of neurological

syn-drom es, including m odern im aging techniques In

this regard w e thank our neuroradiology

col-leagues, and especially Dr Kueker, for providing us

w ith im ages of very high quality

In this fifth edition of “Duus,” w e have preserved

the rem arkably effective didactic concept of the

book, w hich particularly m eets the needs of m

edi-cal students Modern m ediedi-cal curricula require

in-tegrative know ledge, and m edical students should

be taught how to apply theoretical know ledge in a

clinical setting and, on the other hand, to recognize

clinical sym ptom s by delving into their basic

know ledge of neuroanatom y and neurophysiology

Our book fulfils these requirem ents and illustrates

the im portance of basic neuroanatom ical know

l-edge for subsequent practical w ork, as it includes

actual case studies We have color-coded the

sec-tion headings to enable readers to distinguish at a

glance betw een neuroanatom ical (blue) and

clini-cal (green) m aterial, w ithout disrupting the

the-m atic continuity of the text

Although the book w ill be useful to advancedstudents, also physicians or neurobiologists inter-ested in enriching their know ledge of neu-roanatom y w ith basic inform ation in neurology, orfor revision of the basics of neuroanatom y w illbenefit even m ore from it

This book does not pretend to be a textbook ofclinical neurology That w ould go beyond the scope

of the book and also contradict the basic conceptdescribed above First and forem ost w e w ant to de-

m onstrate how, on the basis of theoretical tom ical know ledge and a good neurological exam i-nation, it is possible to localize a lesion in thenervous system and com e to a decision on furtherdiagnostic steps The cause of a lesion is initiallyirrelevant for the prim ary topical diagnosis, andelucidation of the etiology takes place in a secondstage Our book contains a cursory overview of the

ana-m ajor neurological disorders, and it is not intended

to replace the system atic and com prehensivecoverage offered by standard neurological text-books

We hope that this new “Duus,” like the earliereditions, w ill m erit the appreciation of itsaudience, and w e look forw ard to receiving read-ers’ com m ents in any form

Professor M Baehr Professor M Frotscher

1 Elements of the Nervous System 2

Inform ation Flow in the Nervous System 2 Neurons and Synapses 2

Neurons 2

Synapses 4

Neurotransm itters and Receptors 7

Functional Groups of Neurons 7

Glial Cells 7

Developm ent of the Nervous System 8

2 Somatosensory System 12

Peripheral Com ponents of the Som ato-sensory System and Peripheral Regulatory Circuits 12

Receptor Organs 12

Periph eral Nerve, Dorsal Root Ganglion, Posterior Root 14

Peripheral Regulatory Circuits 18

Central Com ponents of the Som ato-sensory System 24

Posterior and Anterior Spinocerebellar Tracts 25

Posterior Colum ns 28

Anterior Spinothalam ic Tract 30

Lateral Spinothalam ic Tract 30

Other Afferent Tracts of the Spinal Cord 31

Central Processing of Som atosensory Inform ation 32

Som atosensory Deficits due to Lesions at Specific Sites along the Som atosensory Pathw ays 32

3 Motor System 36

Central Com ponents of the Motor System and Clinical Syndrom es of Lesions Affect-ing Them 36

Motor Cortical Areas 36

Corticospinal Tract (Pyram idal Tract) 38

Corticonuclear (Corticobulbar) Tract 39

Other Central Com ponents of the Motor System 39

Lesions of Central Motor Pathw ays 41

Peripheral Com ponents of the Motor System and Clinical Syndrom es of Lesions Affecting Them 43

Clinical Syndrom es of Motor Unit Lesions 44

Com plex Clinical Syndrom es due to Lesions of Specific Com ponents of the Nervous System 45

Spinal Cord Syndrom es 45

Vascular Spinal Cord Syndrom es 56

Nerve Root Syndrom es (Radicular Syndrom es) 57

Plexus Syndrom es 62

Peripheral Nerve Syndrom es 67

Syndrom es of the Neurom uscular Junction and Muscle 72

4 Brainstem 74

Surface Anatom y of the Brainstem 74

Medulla 74

Pons 75

Midbrain 75

Cranial Nerves 77

Origin, Com ponents, an d Fun ctions 77

Olfactory System (CN I) 81

Visual System (CN II) 84

Eye Movem ents (CN III, IV, and VI) 89

Trigem inal Nerve (CN V) 103

Facial Nerve (CN VII) an d Nervus In term edius 109

Vestibulocochlear Nerve (CN VIII)—Cochlear Com ponent and the Organ of Hearing 113

Vestibulocochlear Nerve (CN VIII)— Vestibular Com ponent and Vestibular System 120

Vagal System (CN IX, X, and the Cranial Portion of XI) 126

Hypoglossal Nerve (CN XII) 132

Topographical Anatom y of the Brainstem 134 Internal Structure of the Brainstem 134

Brainstem Disorders 145

Ischem ic Brainstem Syndrom es 145

5 Cerebellum 158

Surface Anatom y 158

Internal Structure 159

Cerebellar Cortex 159

Cerebellar Nuclei 160

Afferent and Efferent Projections of the Cerebellar Cortex and Nuclei 162

Connections of the Cerebellum w ith Other Parts of the Nervous System 162

Cerebellar Function and Cerebellar Syndrom es 164

Vestibulocerebellum 164

Spinocerebellum 165

Cerebrocerebellum 166

Cerebellar Disorders 167

Cerebellar Ischem ia and Hem orrh age 167

Cerebellar Tum ors 167

6 Diencephalon and Autonomic Nervous System 170

Location and Com ponents of the Diencephalon 170

Thalam us 172

Nuclei 172

Position of the Thalam ic Nuclei in Ascending and Descending Pathw ays 172

Functions of the Thalam us 176

Syn drom es of Th alam ic Lesions 176

Thalam ic Vascular Syndrom es 177

Epithalam us 177

Subthalam us 178

Hypothalam us 178

Location and Com ponents 178

Hypothalam ic Nuclei 179

Afferen t and Efferent Projections of the Hypothalam us 180

Functions of the Hypoth alam us 184

Peripheral Autonom ic Nervous System 188

Fundam entals 188

Sym pathetic Nervous System 190

Parasym pathetic Nervous System 192

Autonom ic Innervation and Functional Disturbances of Individual Organs 193

Visceral and Referred Pain 199

7 Limbic System 202

Anatom ical Overview 202

Internal and Extern al Connections 203

Major Com ponents of the Lim bic System 203 Hippocam pus 203

Microanatom y of the Hippocam pal Form ation 203

Am ygdala 205

Functions of the Lim bic System 206

Types of Mem ory 206

Mem ory Dysfunction—the Am nestic Syndrom e and Its Causes 208

8 Basal Ganglia 214

Prelim inary Rem arks on Term inology 214

The Role of the Basal Ganglia in the Motor System : Phylogenetic Aspects 214

Com ponents of the Basal Ganglia and Their Connections 215

Nuclei 215

Connections of the Basal Ganglia 217

Function and Dysfunction of the Basal Ganglia 219

Clinical Syndrom es of Basal Ganglia Lesions 219

9 Cerebrum 226

Developm ent 226

Gross Anatom y and Subdivision of the Cerebrum 228

Gyri and Sulci 228

Histological Organization of the Cerebral Cortex 231

Lam inar Arch itecture 231

Cerebral White Matter 235

Projection Fibers 235

Association Fibers 236

Com m issural Fibers 238

Functional Localization in the Cerebral Cortex 238

Prim ary Cortical Fields 239

Association Areas 247

Frontal Lobe 248

Higher Cortical Functions and Their Im pairm ent by Cortical Lesions 248

10 Coverings of the Brain and Spinal Cord; Cerebrospinal Fluid and Ventricular System 260

Coverings of the Brain and Spinal Cord 260

Dura Mater 260

Arachnoid 262

Pia Mater 262

Cerebrospinal Fluid and Ventricular System 263

Structure of th e Ven tricular System 263

Cerebrospinal Fluid Circulation and Resorption 263

Disturbances of Cerebrospinal Fluid Circulation—Hydroceph alus 266

11 Blood Supply and Vascular Disorders

of the Central Nervous System 270

Arteries of the Brain 270

Extradural Course of th e Arteries of the Brain 270

Arteries of the Anterior and Middle Cran ial Fossae 273

Arteries of the Posterior Fossa 275

Collateral Circulation in the Brain 278

Veins of the Brain 279

Superficial and Deep Veins of the Brain 279

Dural Sinuses 280

Blood Supply of the Spinal Cord 281

Arterial Anastom otic Netw ork 281

Venous Drainage 283

Cerebral Ischem ia 283

Arterial Hypoperfusion 283

Particular Cerebrovascular Syndrom es 295

Im paired Venous Drainage from the Brain 302

Intracranial Hem orrhage 305

In tracerebral Hem orrhage (Nontraum atic) 305

Subarachnoid Hem orrhage 307

Subdural and Epidural Hem atom a 311

Vascular Syndrom es of the Spinal Cord 312

Arterial Hypoperfusion 312

Im paired Venous Drainage 312

Spin al Cord Hem orrhage and Hem atom a 314

Further Reading 315

Index 319

ACA anterior cerebral artery

ACTH adrenocorticotropic horm one

(corti-cotropin)

AIDS acquired im m unodeficiency syndrom e

AMPA α-am ino-3-hydroxy-5-m

ethyl-4-isox-azolepropionate acidARAS ascending reticular activating system

BAEP brainstem auditory evoked potentials

BPPV benign paroxysm al positioning

vertigo

CCA com m on carotid artery

CRF corticotropin-releasing factor

DREZ dorsal root entry zone (also called the

EPSP excitatory postsynaptic potential

FLAIR fluid-attenuated inversion recovery

im agingFSH follicle-stim ulating horm one

GH (STH) grow th horm one (som atotropic

horm one)

GPi globus pallidus, internal segm ent

HMSN hereditary m otor and sensory

poly-neuropathy

ICA internal carotid artery

IPSP inhibitory postsynaptic potential

MCA m iddle cerebral artery

MD m edial dorsal nucleus of the thalam us

one-inhibiting factor

MLF m edial longitudinal fasciculus

one-releasing factor

NMDA N-m ethyl-D-aspartatePCA posterior cerebral arteryPET positron em ission tom ographyPICA posterior inferior cerebellar artery

(= dopam ine)PPRF param edian pontine reticular form a-

tion

rCBF regional cerebral blood flow

VOR vestibulo-ocular reflexVPL ventral posterolateral nucleus of the

thalam usVPM ventral posterom edial nucleus of the

thalam us

1 Elements of the

Nervous System

Information Flow in the

Nervous System . 2

Neurons and Synapses 2

Neurotransmitters and Receptors 7

Functional Groups of Neurons 7

Glial Cells 7

Development of the Nervous

System 8

1 Elements of the Nervous System

The nervous system is com posed of cells, called

neurons, that are specialized for inform ation

pro-cessing and transm ission Neurons m ake contact

w ith each other at junctions called synapses, at

w hich inform ation is transferred from one neuron

to the next by m eans of chem ical m essenger

substances called neurotransmitters In general, neurons can be divided into tw o classes: exci-

tatory and inhibitory The organization of the

nervous system is easier to understand after abrief consideration of its (ontogenetic) develop-

m ent

Information Flow in the Nervous

System

Inform ation flow in the nervous system can be

broken dow n schem atically into three steps

(Fig 1.1): an external or internal stim ulus im

ping-ing on the sense organs induces the generation of

nerve im pulses that travel tow ard the central

nervous system (CNS) (afferent impulses); com

-plex processing occurs w ithin the CNS

(informa-tion processing); and, as the product of this

pro-cessing, the CNS generates im pulses that travel

tow ard the periphery (efferent impulses) and

ef-fect the (m otor) response of the organism to the

stim ulus Thus, w hen a pedestrian sees a green

traffic light, afferent im pulses are generated in the

optic nerves and visual system that convey

infor-m ation about the specific color present Then, at

higher levels in the CNS, the stim ulus is interpreted

and assigned a m eaning (green light = go) Efferent

im pulses to the legs then effect the m otor response

(crossing the street)

In the sim plest case, inform ation can be

trans-ferred directly from the afferent to the efferent

Fig 1.1 Basic organization of information processing in

the nervous system

at the body surface

or in the internal organs

CNS

processing

arm , w ithout any intervening com plex processing

in the CNS; this is w hat happens, for exam ple, in anintrinsic m uscle reflex such as the knee-jerk(patellar) reflex

Neurons and Synapses

Dendrites and axons. Neurons transfer inform ation

in one direction only because they are bipolar: they

receive inform ation from other neurons at one end,and transm it inform ation to other neurons at theother end

The receptive structures of a nerve cell, called

dendrites, are branched processes attached to the

cell body Neurons vary considerably w ith regard

to the num ber and branching pattern of their

den-drites The forw ard conducting structure is the

axon, w hich in hum ans can be up to a m eter in

length In contrast to the variable num ber of

den-drites, each neuron possesses only a single axon At

its distal end, the axon splits into a num ber of

ter-m inal branches, each of w hich ends in a so-calledterm inal bouton that m akes contact w ith the next

neuron (Fig 1.2).

The long peripheral processes of the unipolar neurons of the spinal ganglia are an im -portant special case These are the fibers that relayinform ation regarding touch, pain, and tem pera-ture from the body surface to the CNS Although

pseudo-Axon ending (term inal) with term inal bouton

Perikaryon

body

Nucleolus Nucleus

Axon hillock Axon (neurite) Myelin sheath

Collateral axon

Collateral axon

Fig 1.2 Structure of a neuron (schematic drawing).

From: Kahle W and Frotscher M: Color Atlas of Human omy, Vol 3, 6th ed., Thieme, Stuttgart, 2010.

Anat-they are receptive structures, Anat-they nonetheless

possess the structural characteristics of axons and

are designated as such

The trophic (nutritive) center of the neuron is its

cell body (soma or perikaryon), w hich contains the

cell nucleus and various different types of

sub-cellular organelles

Axonal transport. The neurotransm itters, or the

enzym es catalyzing their biosynthesis, are

syn-thesized in the perikaryon and then carried dow n

axonal m icrotubules to the end of the axon in a

process know n as axoplasm ic transport The

neu-rotransm itter m olecules are stored in synaptic

vesicles inside the term inal boutons (each bouton

contains m any synaptic vesicles) Axoplasm ic

transport, generally speaking, can be in either

direction—from the cell body tow ard the end of

the axon (anterograde transport), or in the

reverse direction (retrograde transport) Rapid

axoplasm ic transport proceeds at a speed of 20 0–

40 0 m m /day This is distinct from axoplasm ic

flow, w hose speed is 1–5 m m /day Axoplasm ic

transport is exploited in the research laboratory

by anterograde and retrograde tracer techniques

for the anatom ical dem onstration of neural

pro-jections (Fig 1.3).

Axon myelination. Axons are surrounded by a

sheath of m yelin (Fig 1.4) The m yelin sheath,

w hich is form ed by oligodendrocytes (a special

class of glial cells) in the central nervous system

and by Schw ann cells in the peripheral nervous

system , is a sheetlike continuation of the

oligoden-drocyte or Schw ann cell m em brane that w raps

it-self around the axon m ultiple tim es, providing

electrical insulation Many oligodendrocytes or

Schw ann cells form the m yelin surrounding a

single axon The segm ents of m yelin sheath form ed

by tw o adjacent cells are separated by an area of

uncovered axonal m em brane called a node of

Ran-vier Because of the insulating property of m yelin,

an action potential causes depolarization only at

the nodes of Ranvier; thus, neural excitation jum ps

from one node of Ranvier to the next, a process

know n as saltatory conduction It follow s that

neural conduction is fastest in neurons that have

thick insulating m yelin w ith nodes of Ranvier

spaced w idely apart On the other hand, in axons

that lack a m yelin covering, excitation m ust travel

relatively slow ly dow n the entire axonal m em

-brane Betw een these tw o extrem es there areaxons w ith m yelin of interm ediate thickness Thus,

axons are divided into thickly myelinated, thinly

myelinated, and unmyelinated axons (nerve

fibers); these classes are also designated by the ters A, B, and C The thickly m yelinated A fibers are

let-of 3–20 µm diam eter and conduct at speeds up to

120 m /s The thinly m yelinated B fibers are up to

3 µm thick and conduct at speeds up to 15 m /s The

Fig 1.3 Tracing of neuronal projections w ith

retro-grade and anteroretro-grade tracer substances Tracer

sub-stances, such as fluorescent dyes, are injected either at the

site of origin or at the destination of the neuronal pathway

in question The tracer substances are then transported

along the neurons, either from the cell bodies to the axon

terminals (anterograde transport) or in the reverse

direc-tion (retrograde transport) It is thus possible to trace the

entire projection from one end to the other.

a Retrograde transport.

b Retrograde transport from multiple projection areas of a

single neuron.

c Anterograde transport from a single cell body into

mul-tiple projection areas.

From: Kahle W and Frotscher M: Color Atlas of Human

Anat-omy, vol 3, 6th ed., Thieme, Stuttgart, 2010.

6 5

3

2

4 7

1

Fig 1.4 Nerve fiber in the central nervous system, w ith

oligodendrocyte and myelin sheath (schematic drawing).

1, Oligodendrocyte 2, Axon 3, Myelin sheath 4, Node of

Ranvier 5, Inner mesaxon 6, Outer mesaxon 7, Pockets of

cytoplasm From: Kahle W and Frotscher M: Color Atlas of Human Anatomy, Vol 3, 6th ed., Thieme, Stuttgart, 2010.

unm yelinated C fibers conduct no faster than

2 m /s

Synapses

General structure. As late as the 1950s, it w as stillunclear w hether neurons w ere connected to eachother in a continuous netw ork (syncytium ), w hich

w ould theoretically allow rapid electrical com

-m unication betw een neurons, or w hether eachneuron w as entirely enclosed in its ow n m em -brane Subsequent visualization of synapses underthe electron m icroscope settled the question: there

is no direct spatial continuity betw een neurons.The axon ends on one side of the synapse, andneural im pulses are conveyed across it by special

transm itter substances (Fig 1.5) The axon term inal (bouton) is the presynaptic part of the synapse, and

the m em brane of the cell receiving the transm itted

inform ation is the postsynaptic part The

presyn-aptic and postsynpresyn-aptic m em branes are separated

7

5 2

4

1 3

Fig 1.5 Synaptic structure (schematic drawing) 1,

Pre-synaptic membrane with gridlike thickening, leaving

hexa-gonal spaces in between 2, Synaptic cleft 3, Postsynaptic membrane 4, Synaptic vesicle 5, Fusion of a synaptic ves-

icle with the presynaptic membrane (so-called Ω [omega] figure), with release of the neurotransmitter (green) into

the synaptic cleft 6, Vesicle with neurotransmitter molecules taken back up into the terminal bouton 7, Axon

filaments From: Kahle W and Frotscher M: Color Atlas of Human Anatomy, Vol 3, 6th ed., Thieme, Stuttgart, 2010.

Fig 1.6 Synaptic transmission at a glutamatergic

(exci-tatory) synapse (schematic drawing) The arriving action

potential induces an influx of Ca 2+ (1), which, in turn, causes the synaptic vesicles (2) to fuse with the presynaptic mem-

brane, resulting in the release of neurotransmitter (in this

case, glutamate) into the synaptic cleft (3) The

neu-rotransmitter molecules then diffuse across the cleft to the

specific receptors in the postsynaptic membrane (4) and bind to them, causing ion channels (5) to open, in this case

Na + channels The resulting Na + influx, accompanied by a

Ca 2+ influx, causes an excitatory depolarization of the synaptic neuron (excitatory postsynaptic potential, EPSP) This depolarization also removes a blockade of the so-called NMDA receptor by Mg 2+ ions From: Kahle W and Frotscher M: Taschenatlas der Anatomie, vol 3, 8th ed., Thieme, Stuttgart, 2002.

post-by the synaptic cleft The bouton contains vesicles

filled w ith the neurotransm itter substance

Exam ination of synapses under the electron m

i-croscope reveals specialized, osm iophilic

thickenings of the presynaptic and postsynaptic m em

-branes, w hich are m ore pronounced on the

postsy-naptic side in so-called asymmetrical synapses,

and are approxim ately equally thick on both sides

in so-called symmetrical synapses These tw o

types of synapse are also know n, after their

origi-nal describer, as Gray type I and Gray type II

syn-apses, respectively Asym m etrical synapses w ere

found to be excitatory and sym m etrical synapses

to be inhibitory (see below for the concepts of

exci-tation and inhibition) This hypothesis w as later

confirm ed by im m unocytochem ical studies using

antibodies directed against neurotransm itter

sub-stances and the enzym es involved in their

bio-synthesis

Synaptic transmission (Fig 1.6) is essentially a

sequence of three different processes:

¼ The excitatory im pulse (action potential)

arriv-ing at the axon term inal depolarizes the

presy-naptic m em brane, causing voltage-dependent

calcium channels to open As a result, calcium

ions flow into the term inal bouton and then

in-teract w ith various proteins to cause fusion of

synaptic vesicles w ith the presynaptic m em

-brane The neurotransm itter m olecules w ithin

the vesicles are thereby released into the

synap-tic cleft

¼ The neurotransm itter m olecules diffuse across

the synaptic cleft and bind to specific receptors

on the postsynaptic m em brane

¼ The binding of neurotransm itter m olecules to

receptors causes ion channels to open, inducing

ionic currents that cause either a depolarization

or a hyperpolarization of the postsynaptic

m em brane—i.e., either an excitatory

postsynap-tic potential (EPSP) or an inhibitory postsynappostsynap-tic

potential (IPSP) Thus, synaptic transm ission

re-sults in either an excitation or an inhibition of

the postsynaptic neuron

In addition to these fast-acting transm itter-gated

or ligand-gated ion channels, there are also

G-pro-tein-coupled receptors that generate a m uch slow er

response by m eans of an intracellular signal

cas-cade

Chemical and electrical synapses. The type of

syn-aptic transm ission described above, involving the

release and receptor binding of a neurotransm itter,

is the type m ost com m only found There are also

so-called electrical synapses in w hich the

excita-tion is transm itted directly to the next neuron

across a gap junction.

Types of synapses. Synapses m ediate the transfer of

inform ation from one neuron to the next; the

syn-apses that bring inform ation to a particular cell are

know n as its input synapses Most input synapses

are to be found on a cell’s dendrites (axodendritic

synapses) The dendrites of m any neurons (e.g.,

cor-tical pyram idal cells) possess thornlike processes,

the dendritic spines, that enable the com partm

en-talization of synaptic input Many spines contain a

spine apparatus for the internal storage of calcium

ions The synapses on dendritic spines are m ainly

asym m etrical, excitatory synapses

Input synapses are found not only on the

den-drites but also on the cell body itself (perikaryon;

axosomatic synapses) and even on the axon and its

initial segm ent, the axon hillock (axo-axonal

syn-apses).

Convergence and divergence of synaptic

connec-tions. In general, each individual neuron receives

inform ation through synapses from m any different

neurons and neuron types (convergence of

infor-m ation transfer) The neuron can, in turn, infor-m ake

synaptic contact w ith a large num ber of other

neu-Fig 1.7 Three types of

rons through num erous collateral axonal branches

(divergence of inform ation transfer).

Excitation and inhibition. The nervous system isconstructed in such a w ay that each neuron can be

in one of tw o basic states at any m om ent: eitherthe neuron is electrically discharging and trans-

m itting inform ation via synapses to other neurons,

or else it is silent Excitatory input to the neuroncauses it to discharge, w hile inhibitory inputcauses it to be silent

It follow s that neurons can be classified as tatory and inhibitory in term s of their effect on

exci-the neurons to w hich exci-they provide input

Exci-tatory neurons are usually principal neurons (e.g.,

the pyram idal cells of the cerebral cortex), w hichoften project over long distances and thus have

long axons Inhibitory neurons, on the other hand,

are often interneurons and have short axons

Principles of neuronal inhibition (Fig 1.7)

Collater-als of excitatory cells can activate inhibitory neurons, w hich then inhibit the principal neuron

inter-itself (recurrent inhibition, a form of negative back) In forw ard inhibition, collaterals of principal

feed-neurons activate inhibitory interfeed-neurons that theninhibit other principal neurons When an inhibi-tory neuron inhibits another inhibitory neuron, theresulting decrease in inhibition of the postsynapticprincipal cell causes a net increase in its activity

(disinhibition).

Neurotransmitters and

Receptors

Excitatory and inhibitory neurotransmitters. In

classic neuroanatom ical studies, neurons w ere

divided into tw o m ajor types on the basis of their

shape and the length of their projections: principal

neurons w ith distant projections w ere called Golgi

type I neurons, w hile interneurons w ith short

axons w ere called Golgi type II neurons Currently,

neurons are usually classified according to their

neurotransm itter phenotype, w hich generally

de-term ines w hether they are excitatory or inhibitory

The com m onest excitatory neurotransm itter in the

CNS is glutamate, w hile the com m onest inhibitory

neurotransm itter is γ-aminobutyric acid (GABA).

The inhibitory neurotransm itter in the spinal cord

is glycine Acetylcholine and norepinephrine are

the m ost im portant neurotransm itters in the

au-tonom ic nervous system but are also found in the

CNS Other im portant neurotransm itters include

dopamine, serotonin, and various neuropeptides,

m any of w hich have been (and continue to be)

identified; these are found m ainly in interneurons

Ligand-gated receptors. Ligand-gated ion channels

are constructed of m ultiple subunits that span the

cell m em brane The binding of neurotransm itter to

the receptor opens the ion channel (i.e., m akes it

perm eable) for one or m ore particular species of

ion

recep-tors are subdivided into three types called AMPA,

NMDA, and kainate receptors Glutam ate binding to

an AMPA receptor results in an influx of Na+ ions,

w hich depolarizes the cell The activation of an

NMDA receptor also causes an Na+ influx, accom

-panied by a Ca2+ influx The NMDA receptor,

how ever, can be activated only after the blockade

of its ion channel by a m agnesium ion is rem oved;

this, in turn, is accom plished through an

AMPA-receptor-induced m em brane depolarization

(Fig 1.6) The excitatory neurotransm itter

gluta-m ate thus has a graded effect: it activates AMPA

re-ceptors first and NMDA rere-ceptors later, after the

m em brane has been depolarized

activa-tion of either of these tw o types of receptor causes

an influx of negatively charged chloride ions, andthus a hyperpolarization of the postsynaptic cell.Other types of ligand-gated ion channel include

the nicotinic acetylcholine receptor and the

sero-tonin (5-HT3) receptor

G-protein-coupled receptors. The response to astim ulus acting through a G-protein-coupled re-ceptor lasts considerably longer, as it results fromthe activation of an intracellular signal cascade Theresponse m ay consist of changes in ion channels or

in gene expression Exam ples of G-protein-coupledreceptors include m uscarinic acetylcholine recep-tors and m etabotropic glutam ate receptors

Functional Groups of Neurons

As discussed earlier, neurons are currentlyclassified according to the neurotransm itters that

they release Thus, one speaks of the glutam atergic,

GABAergic, cholinergic, and dopam inergic system s,

am ong others These system s have distinct ties Glutam atergic neurons m ake point-to-pointconnections w ith their target cells, w hile thedopam inergic system , for exam ple, has rather m orediffuse connections: a single dopam inergic neurongenerally projects to a large num ber of target neu-rons The connections of the GABAergic system areparticularly highly specialized Som e GABAergicneurons (basket cells) m ake num erous synapticconnections onto the cell body of the postsynapticneuron, form ing a basketlike structure around it;others form m ainly axodendritic or axo-axonalsynapses The latter are found at the axon hillock

proper-Neurotransm itter analogues or receptor blockers can

be applied pharm acologically for the specific hancem ent or w eakening of the effects of a partic-ular neurotransm itter on neurons

en-Glial Cells

The num erically m ost com m on cells in the nervoussystem are, in fact, not the neurons, but the glialcells (also called glia or neuroglia) These cells play

an indispensable supportive role for the function ofneurons The three types of glial cells in the CNS

are the astroglial cells (astrocytes), oligodendroglia

(oligodendrocytes), and m icroglial cells

Astrocytes are divided into tw o types:

proto-plasm ic and fibrillary In the intact nervous system ,

astrocytes are responsible for the m aintenance of

the internal environm ent (hom eostasis),

particu-larly w ith respect to ion concentrations Fine

astro-cyte processes surround each synapse, sealing it off

from its surroundings so that the neurotransm itter

cannot escape from the synaptic cleft When the

central nervous system is injured, astrocytes are

responsible for the form ation of scar tissue

(glio-sis)

The oligodendrocytes form the m yelin sheaths

of the CNS (see p 7) The microglial cells are

phago-cytes that are activated in inflam m atory and

degenerative processes affecting the nervous

sys-tem

Development of the Nervous

System

A detailed discussion of the developm ent of the

nervous system w ould be beyond the scope of this

book and not directly relevant to its purpose The

physician should understand som e of the basic

principles of neural developm ent, how ever, as

developm ental disturbances account for a large

num ber of diseases affecting the nervous system

The nervous system develops from the (initially)

longitudinally oriented neural tube, w hich consists

of a solid w all and a central fluid-filled cavity The

cranial portion of the neural tube grow s m ore

ex-tensively than the rest to form three distinct brain

vesicles, the rhom bencephalon (hindbrain), the

m esencephalon (m idbrain), and the prosencephalon

(forebrain) The prosencephalon, in turn, becom es

further differentiated into a caudal part, the

dien-cephalon, and the m ost cranial portion of the entire

neural tube, the paired telencephalon (endbrain).

The central cavity of the tw o telencephalic

ven-tricles com m unicates w ith that of the

dien-cephalon through the interventricular foram en

(destined to becom e the foram en of Monro) The

central cavity undergoes its greatest enlargem ent

in the areas w here the neural tube has its m ost

pronounced grow th; thus, the lateral ventricles

form in the tw o halves of the telencephalon, the

third ventricle w ithin the diencephalon, and the

fourth ventricle in the brainstem In those

seg-m ents of the neural tube that grow to a relativelylesser extent, such as the m esencephalon, no ven-tricle is form ed (in the fully developed organism ,the cerebral aqueduct runs through the m esen-cephalon)

Over the course of vertebrate phylogeny, gressive enlargem ent of the telencephalon hascaused it to overlie the brainstem and to rotateback on itself in sem icircular fashion This rotation

pro-is reflected in the structure of various com ponents

of the telencephalic gray m atter, including the date nucleus and hippocam pus; in the course ofcertain w hite m atter tracts, such as the fornix; and

cau-in the shape of the lateral ventricles, each of w hich

is com posed of a frontal horn, a central portion(atrium ), and a tem poral horn, as show n in

Fig 10.3, p 262.

Cellular proliferation. Im m ature neurons blasts) proliferate in the ventricular zone of theneural tube, i.e., the zone neighboring its centralcavity It is a m ajor aim of current research in neu-roem bryology to unveil the m olecular m echanism scontrolling neuronal proliferation

(neuro-Neuronal migration. New ly form ed nerve cellsleave the ventricular zone in w hich they arise, m i-grating along radially oriented glial fibers tow ardtheir definitive location in the cortical plate Migra-tory processes are described in greater detail on

pp 227 ff

Grow th of cellular processes. Once they have rived at their destinations, the postm igratory neu-roblasts begin to form dendrites and axons One ofthe m ajor questions in neurobiology today is howthe new ly sprouted axons find their w ay to theircorrect targets over w hat are, in som e cases, verylong distances Im portant roles are played in thisprocess by m em brane-bound and soluble factorsthat are present in a concentration gradient, as

ar-w ell as by extracellular m atrix proteins There areligand–receptor system s that exert both attractiveand repulsive influences to steer the axon into theappropriate target area These system s cannot bedescribed in greater detail here

Synaptogenesis. The axon term inals, having foundtheir w ay to their targets, proceed to form synapticcontacts Recent studies have show n that the for-

m ation of synapses, and of dendritic spines, is

ac-tivity-dependent Much evidence suggests that

new synapses can be laid dow n throughout the

lifespan of the individual, providing the basis of

adaptive processes such as learning and m em ory

Physiological neuronal death (program m ed celldeath, apoptosis) Many neurons die as the CNSdevelops, presum ably as part of the m echanismenabling the precise and specific form ation of in-terneuronal connections The regulation of neu-ronal survival and neuronal death is a m ajor topic

of current research

2 Somatosensory

System

Peripheral Components of the

Somatosensory System and

Peripheral Regulatory Circuits 12

Central Components of the

Somatosensory System . 24

Central Processing of Somatosensory

Information 32

Somatosensory Deficits due

to Lesions at Specific Sites along

the Somatosensory Pathw ays 32

2 Somatosensory System

After a prelim inary chapter on the structural

ele-m ents of the nervous systeele-m , the discussion of its

m ajor functional com ponents and m echanism s

now begins w ith the perceptual processes m

edi-ated by receptor organs: as depicted earlier in

Figure 1.1, these organs are the site of origin of

in-form ation flow in the nervous system , in

accor-dance w ith the basic organizing principle,

perception processing response Som atosensory im

-pulses from the periphery are conducted along an

afferent nerve fiber to its neuronal cell body, w hich

lies in a dorsal root ganglion (spinal ganglion) The

im pulses are then conducted onw ard into the

central nervous system, w ithout any intervening

synapses, along the central process (axon) of thesam e neuron This axon m akes synaptic contact

w ith a second neuron in the spinal cord or

brain-stem , w hose axon, in turn, proceeds further

cen-trally, and crosses the midline to the opposite side

at som e level along its path The third neuron lies

in the thalamus, the so-called “gatew ay to

con-sciousness”; it projects to various cortical areas,

m ost im portantly the prim ary som atosensory

cortex, w hich is located in the postcentral gyrus of

the parietal lobe

Peripheral Components of the

Somatosensory System and

Peripheral Regulatory Circuits

Receptor Organs

Receptors are specialized sensory organs that

reg-ister physical and chem ical changes in the external

and internal environm ent of the organism and

convert (transduce) them into the electrical im

-pulses that are processed by the nervous system

They are found at the peripheral end of afferent

nerve fibers Som e receptors inform the body

about changes in the nearby external environm ent

(exteroceptors) or in the distant external

environ-m ent (teleceptors, such as the eye and ear)

Propri-oceptors, such as the labyrinth of the inner ear,

convey inform ation about the position and m

ove-m ent of the head in space, tension in ove-m uscles and

tendons, the position of the joints, the force

needed to carry out a particular m ovem ent, and so

on Finally, processes w ithin the body are reported

on by enteroceptors, also called visceroceptors

(in-cluding osmoceptors, chemoceptors, and

barocep-tors, am ong others) Each type of receptor

re-sponds to a stim ulus of the appropriate, specific

kind, provided that the intensity of the stim ulus is

above threshold

Sensory receptor organs are abundantly present

in the skin but are also found in deeper regions of

the body and in the viscera

Receptors in the Skin

Most receptors in the skin are exteroceptors Theseare divided into tw o classes: (1) free nerve endingsand (2) encapsulated end organs

The encapsulated, differentiated end organs areprobably m ainly responsible for the m ediation ofepicritic sensory m odalities such as fine touch, dis-crim ination, vibration, pressure, and so forth,

w hile the free nerve endings m ediate protopathic

m odalities such as pain and tem perature The dence for this functional distinction is incom plete,how ever (see below )

evi-Various receptor organs of the skin and its

ap-pendages are depicted in Figure 2.1, including

mechanoreceptors (for touch and pressure), moreceptors (for w arm and cold), and nociceptors

ther-(for pain) These receptors are located m ainly inthe zone betw een the epiderm is and the connec-tive tissue The skin can thus be regarded as asensory organ that covers the entire body

Special receptor organs The peritrichial nerve

endings around the hair follicles are found in all

areas of hair-bearing skin and are activated by the

m ovem ent of hairs In contrast, the tactile

cor-puscles of Meissner are found only on glabrous

skin, particularly on the palm s and soles but also

on the lips, the tip of the tongue, and the genitals,and respond best to touch and light pressure The

laminated Vater–Pacini corpuscles (pacinian

cor-puscles) are found in deeper layers of the skin,especially in the area betw een the cutis and the

subcutis, and m ediate pressure sensations The end

bulbs of Krause w ere once thought to be cold

re-ceptors, w hile the corpuscles of Ruffini w ere

thought to be w arm receptors, but there is som e

doubt about this at present Free nerve endings

have been found to be able to transm it inform ation

about w arm th and cold as w ell as about position

In the cornea, for exam ple, only free nerve endings

are present to transm it inform ation about all of

these sensory m odalities Aside from the receptor

types specifically m entioned here, there are also

m any others in the skin and elsew here w hose

function m ostly rem ains unclear

Free nerve endings (Fig 2.1) are found in the clefts

betw een epiderm al cells, and som etim es also on

m ore specialized cells of neural origin, such as the

tactile disks of Merkel Free nerve endings are

pre-sent, how ever, not just in the skin but in practically

all organs of the body, from w hich they convey

nociceptive and therm al inform ation relating to

cellular injury Merkel’s disks are m ainly located in

the pads of the fingers and respond to touch and

light pressure

Receptors in Deeper Regions of the Body

A second group of receptor organs lies deep to the

skin, in the m uscles, tendons, fasciae, and joints

(Fig 2.2) In the m uscles, for exam ple, one finds

m uscle spindles, w hich respond to stretching of

the m usculature Other types of receptors are

found at the transition betw een m uscles and

ten-dons, in the fasciae, or in joint capsules

Muscle spindles are very thin, spindle-shaped

bo-dies that are enclosed in a connective-tissue

cap-sule and lie betw een the striated fibers of the

skeletal m usculature Each m uscle spindle itself

usually contains 3–10 fine striated m uscle fibers,

w hich are called intrafusal muscle fibers in

con-trast to the extrafusal fibers of the m uscular tissue

proper The tw o ends of each spindle, com posed of

connective tissue, are fixed w ithin the connective

tissue betw een m uscle fascicles, so that they m ove

in conjunction w ith the m uscle An afferent nerve

fiber called an annulospiral ending or prim ary

ending w inds around the m iddle of the m uscle

spindle This afferent fiber has a very thick m yelin

sheath and belongs to the m ost rapidly conducting

group of nerve fibers in the body, the so-called Ia

fibers For further details, see p 18 ff (m

onosynap-tic intrinsic m uscle reflex; polysynaponosynap-tic reflexes)

Fig 2.1 Somatosensory receptors in the skin a Free nerve ending (pain, temperature) b Tactile disk of Merkel.

c Peritrichial nerve endings around a hair follicle (touch).

d Tactile corpuscle of Meissner e Vater−Pacini corpuscle

(pressure, vibration) f End bulb of Krause (cold?) g Ruffini

corpuscle (warmth?).

Golgi tendon organs contain fine nerve endings,derived from branches of thickly m yelinated nervefibers, that surround a group of collagenous tendonfibers They are enclosed in a connective-tissuecapsule, are located at the junction betw een ten-don and m uscle, and are connected in series to theadjacent m uscle fibers Like m uscle spindles, theyrespond to stretch (i.e., tension), but at a higher

threshold (see Fig 2.12, p 22).

Other receptor types. In addition to the m usclespindles and Golgi tendon organs, receptor types inthe deep tissues include the lam inated Vater–

Fig 2.2 Receptors in muscle, tendons, and fascia a

An-nulospiral ending of a muscle spindle (stretch) b Golgi

ten-don organ (tension) c Golgi−Mazzoni corpuscle (pressure).

Pacini corpuscles and the Golgi–Mazzoni puscles as w ell as other term inal nerve endingsthat m ediate pressure, pain, etc

cor-Peripheral Nerve, Dorsal Root Ganglion, Posterior Root

The further “w ay stations” through w hich an ent im pulse m ust travel as it m akes its w ay to theCNS are the peripheral nerve, the dorsal root gan-glion, and the posterior nerve root, through w hich

affer-it enters the spinal cord

Peripheral nerve. Action potentials arising in a ceptor organ of one of the types described aboveare conducted centrally along an afferent fiber,

re-w hich is the peripheral process of the first som tosensory neuron, w hose cell body is located in adorsal root ganglion (see p 16) The afferent fibersfrom a circum scribed area of the body run together

a-in a peripheral nerve; such nerves contaa-in not only

fibers for superficial and deep sensation (som atic

afferent fibers) but also efferent fibers to striated

m uscle (som atic efferent fibers) and fibers

inner-vating the internal organs, the sw eat glands, and

vascular sm ooth m uscle (visceral afferent and

visceral efferent fibers) Fibers (axons) of all of these

types are bundled together inside a series of nective-tissue coverings (endoneurium , peri-neurium , and epineurium ) to form a “nerve cable”

con-(Fig 2.3) The perineurium also contains the blood

vessels that supply the nerve (vasa nervorum ).

Nerve plexus and posterior root. Once the eral nerve enters the spinal canal through the in-tervertebral foram en, the afferent and efferentfibers go their separate w ays: the peripheral nervedivides into its tw o “sources,” the anterior and

periph-posterior spinal roots (Fig 2.4) The anterior root

contains the efferent nerve fibers exiting the spinalcord, w hile the posterior root contains the afferentfibers entering it A direct transition from the pe-ripheral nerve to the spinal nerve roots is found,how ever, only in the thoracic region At cervicaland lum bosacral levels, nerve plexuses are inter-posed betw een the peripheral nerves and the spi-nal nerve roots (the cervical, brachial, lum bar, andsacral plexuses) In these plexuses, w hich are lo-cated outside the spinal canal, the afferent fibers ofthe peripheral nerves are redistributed so thatfibers from each individual nerve ultim ately join

Fat

Blood vessel

Unmyelinated fibers, mostly automatic

Myelinated, segmented fibres, motor

T12 T11 T10 T9 T8

T7 T6 T5 T4

T3 T2 T1

Fig 2.4 Nerve root

seg-ments and their ship to the vertebral bodies a Anatomy of the

relation-anterior and posterior nal roots.

spi-b Enumeration of the nerve

root segments and the levels of exit of the spinal nerves from the spinal canal The spinal cord grows to a shorter final length than the vertebral column, so that the nerve roots (proceeding caudally) must travel increasingly long distances to reach their exit foramina See also p 45, Chapter 3 (Motor System).

spinal nerves at m ultiple segm ental levels

(Fig 2.5) (In analogous fashion, the m otor fibers of

a single segm ental nerve root travel to m ultiple

pe-ripheral nerves; cf Fig 2.5 and p 62 ff in

Chap-ter 3.) The redistributed afferent fibers then enChap-ter

the spinal cord at m ultiple levels and ascend a

vari-able distance in the spinal cord before m aking

syn-aptic contact w ith the second sensory neuron,

w hich m ay be at or near the level of the entering

af-ferent fibers or, in som e cases, as high as the stem Thus, in general, a peripheral nerve is com -posed of fibers from m ultiple radicular segm ents;this is true of both afferent and efferent fibers

brain-Digression: Anatomy of the spinal roots and nerves.

In total, there are 31 pairs of spinal nerves; eachspinal nerve is form ed by the junction of an ante-rior and a posterior nerve root w ithin the spinal

canal The num bering of the spinal nerves is based

on that of the vertebral bodies (Fig 2.4) Even

though there are only seven cervical vertebrae,

there are eight pairs of cervical nerves, because the

highest spinal nerve exits (or enters) the spinal

canal just above the first cervical vertebra Thus,

this nerve, the first cervical nerve (C1), exits the

spinal canal betw een the occipital bone and the

first cervical vertebra (atlas); the rem aining

cervi-cal nerves, dow n to C7, exit above the

correspond-ingly num bered vertebra; and C8 exits betw een

the seventh (low est) cervical vertebra and the first

thoracic vertebra At thoracic, lum bar, and sacral

levels, each spinal nerve exits (or enters) the spinal

canal below the correspondingly num bered

verte-bra There are, therefore, just as m any pairs of

Fig 2.5 Redistribution of afferent and efferent nerve

fibers in a nerve plexus The sensory fibers contained in a

single peripheral nerve are distributed to multiple dorsal

spinal nerve roots, and, analogously, the motor fibers of a

single nerve root are distributed to multiple peripheral

nerves a In the periphery, the sensory fibers of a single

radicular segment are grouped together once again to

supply a characteristic segmental region of the skin

(der-matome) b Radicular and peripheral nerve innervation of

muscle: each muscle is supplied by a single peripheral

nerve, which, however, generally contains fibers from

mul-tiple nerve roots (so-called polyradicular or plurisegmental

innervation).

nerves in each of these regions as there are

verte-brae (12 thoracic, 5 lum bar, and 5 sacral) (Fig 2.4).

Lastly, there is a single pair of coccygeal nerves (or,occasionally, m ore than one pair)

Spatial organization of som atosensory fibers in the

som atosensory m odalities originate in differenttypes of peripheral receptor and are conductedcentrally in separate groups of afferent fibers,

w hich are spatially arranged in the posterior root in

a characteristic pattern As show n in Figure 2.15

(p 25), the m ost thickly m yelinated nerve fibers,

w hich originate in m uscle spindles, run in the m dial portion of the root; these fibers are responsiblefor proprioception Fibers originating in receptororgans, w hich m ediate the senses of touch, vibra-tion, pressure, and discrim ination, run in the cen-tral portion of the root, and the sm all and thinly

e-m yelinated fibers e-m ediating pain and tee-m peraturesensation run in its lateral portion

Dorsal root ganglion. The dorsal root ganglion is

m acroscopically visible as a sw elling of the dorsalroot, im m ediately proxim al to its junction w ith the

ventral root (Fig 2.4) The neurons of the dorsal root

ganglion are pseudounipolar, i.e., they possess asingle process that divides into tw o processes ashort distance from the cell, in a T-shaped configu-ration One of these tw o processes travels to the re-ceptor organs of the periphery, giving off num erouscollateral branches along the w ay, so that a singleganglion cell receives input from m ultiple receptororgans The other process (the central process)travels by w ay of the posterior root into the spinalcord, w here it either m akes synaptic contact w iththe second sensory neuron im m ediately, or else as-

cends tow ard the brainstem (see Fig 2.17, p 27).

There are no synapses w ithin the dorsal root glion itself

gan-Somatosensory Innervation by Nerve Roots and Peripheral Nerves

The fibers of individual nerve roots are tributed into m ultiple peripheral nerves by w ay ofthe plexuses (cf p 14 ff.), and each nerve containsfibers from m ultiple adjacent radicular segm ents

redis-(see also Figs 3.31, 3.32, and 3.33, pp 63, 64) The

fibers of each radicular segm ent regroup in the

pe-riphery, how ever (Fig 2.5), to innervate a

particu-Spinal

cord

Nerve root (posterior root)

Plexus Peripheral n Dermatome

Myotome

Radicular segments

Nerve root (anterior root) Plexus Peripheral n.

a

b

C2 C3 C4

C5

C6

C7 C8

L1

L2

L3

L4 L5

S1

S1

S2

T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12

C6

C7 C8

T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12

L1 L2 L3

L4

L5

L5 L4 S1

S1

S2 S3

S5 S4

T1 T1

Fig 2.6 Segmental

inner-vation of the skin (after

Hansen−Schliack) a rior view b Posterior view.

Ante-lar segm ental area of the skin (dermatome) Each

derm atom e corresponds to a single radicular

seg-m ent, w hich, in turn, corresponds to a single

“spi-nal cord segm ent.” The latter term is used even

though the m ature spinal cord no longer displays

its original m etam eric segm entation

The derm atom es on the anterior and posterior

body surfaces are show n in Figure 2.6 The m

et-am eric organization of the derm atom es is easiest

to see in the thoracic region

As show n in Figure 2.5, the derm atom es of

neighboring roots overlap considerably, so that a

lesion confined to a single root often causes a

barely discernible sensory deficit, or none at all

Sensory deficits due to radicular lesions. A

de-m onstrable sensory deficit in a segde-m ental

distribu-tion is usually found only w hen m ultiple adjacent

nerve roots are involved by a lesion As each

der-m atoder-m e corresponds to a particular spinal cord orradicular level, the derm atom e(s) in w hich asensory deficit is located is a highly valuable in-dicator of the level of a lesion involving the spinalcord or one or m ore nerve roots The schem atic

representation of Figure 2.7 is intended for didactic

purposes, to help the student rem em ber w here theboundaries betw een the cervical, thoracic, lum bar,and sacral derm atom al areas are located

The derm atom es for the sense of touch overlap

to a greater extent than those for pain andtem perature It follow s that, in a lesion of one or

tw o adjacent roots, a derm atom al deficit of touch

is generally hard to dem onstrate, w hile one of painand tem perature sensation is m ore readily ap-parent Thus, nerve root lesions can be m ore sensi-tively detected by testing for hypalgesia or analge-sia, rather than hypesthesia or anesthesia

Sensory deficits due to peripheral nerve lesions. It

is easy to see w hy a lesion affecting a nerve plexus

or a peripheral nerve produces a sensory deficit of

an entirely different type than a radicular lesion As

plexus lesions usually cause a prom inent m otor

deficit in addition, w e w ill defer further discussion

of plexus lesions to the next chapter on the m otor

system (pp 62–66)

When a peripheral nerve is injured, the fibers

w ithin it, derived from m ultiple nerve roots, can no

longer rejoin in the periphery w ith fibers derived

from the sam e nerve roots but belonging to other

peripheral nerves—in other w ords, the fibers in the

injured nerve can no longer reach their assigned

derm atom es The sensory deficit thus has a

differ-ent distribution from that of the derm atom al

defi-cit seen after a radicular injury (Fig 2.8)

Further-m ore, the cutaneous areas innervated by

in-dividual peripheral nerves overlap m uch less that

those innervated by adjacent nerve roots Sensory

deficits due to peripheral nerve lesions are,

there-S1 T1

C2

L1

Fig 2.7 Segmental innervation of the skin: simplified

diagram of dermatomal topography

fore, m ore readily apparent than those due toradicular lesions

Peripheral Regulatory Circuits

In the next section after this one, w e w ill trace theascending fiber pathw ays responsible for pain andtem perature sensation, and for sensory m odalitiessuch as touch and pressure, as they travel up thespinal cord and into the brain Before doing so,how ever, w e w ill explain the function of a num ber

of im portant peripheral regulatory circuits Eventhough the current chapter is devoted to thesensory system , it w ill be useful, in this lim itedcontext, to describe not only the afferent (sensory)arm of these regulatory circuits, but their efferent(m otor) arm as w ell

Monosynaptic and Polysynaptic Reflexes

Monosynaptic intrinsic reflex. As illustrated in

Figure 2.11 (p 22), the large-diam eter afferent

fiber arising in a m uscle spindle gives off m any

ter-m inal branches shortly after entering the spinalcord; som e of these branches m ake direct synapticcontact onto neurons in the gray m atter of theanterior horn These neurons, in turn, are the origin

of efferent m otor fibers, and are therefore called

motor anterior horn cells The efferent neurites

exit the spinal cord by w ay of the anterior root andthen travel, along peripheral nerves, to the skeletal

m uscles

A neural loop is thus created from a skeletal

m uscle to the spinal cord and back again, com posed of tw o neurons—an afferent sensory neuronand an efferent m otor neuron This loop consti-tutes a sim ple, m onosynaptic reflex arc Because

-the arc begins and ends in -the sam e m uscle, -the

as-sociated reflex is called an intrinsic (or

propriocep-tive) muscle reflex.

Such m onosynaptic reflex arcs provide the roanatom ical basis for the regulation of m usclelength (see below )

sense, the m onosynaptic reflex is not truly m synaptic, because it also has a polysynaptic com -ponent The reflex is m anifested not only in con-traction of the m uscle in question, but also in re-laxation of its antagonist m uscle(s) The inhibition

ono-of m uscle cells that leads these m uscles to relax is apolysynaptic process occurring by w ay of inter-

Medial brachial cutaneous n.

Posterior antebrachial cutaneous n.

Lateral antebrachial cutaneous n.

Medial antebrachial cutaneous n.

Ilio-Cluneal nn.

Dorsal rami

of lumbar nn.

Sural n.

Lateral femoral cutaneous n.

Posterior femoral cutaneous n.

Femoral n.

Common peroneal (fibular) n.

Superficial peroneal (fibular) n.

Saphenous n.

Transverse cervical n C2 – C3

Deep peroneal (fibular) n.

neurons in the spinal gray m atter Were this not

the case, tension in the antagonist m uscles w ould

counteract agonist contraction (see Fig 2.14, p 24).

Polysynaptic flexor reflex. Another im portant

re-flex arc is that of the polysynaptic re-flexor rere-flex, a

protective and flight reflex that is m ediated by

m any interneurons and is thus polysynaptic.

When a finger touches a hot stove, the hand ispulled back w ith lightning speed, before any pain isfelt The action potentials that arise in the cu-taneous receptor (nociceptor) for this reflex travel

by w ay of afferent fibers to the substantia nosa of the spinal cord, w here they are then relayed,across synapses, into cells of various types belong-ing to the cord’s intrinsic neuronal apparatus

gelati-Fig 2.8 Innervation of

the skin by peripheral nerves a Anterior view.

b Posterior view c The

areas innervated by the three divisions of the trigeminal nerve and by the cervical cutaneous nerves.

(interneurons, association neurons, and com m

is-sural neurons) Som e of these cells—particularly

the association neurons—project their processes

m ultiple spinal levels upw ard and dow nw ard, in

the so-called fasciculus proprius (Fig 2.9) After

crossing m ultiple synapses, excitatory im pulses

fi-nally reach the m otor neurons and travel along

their efferent axons into the spinal nerve roots,

pe-ripheral nerves, and m uscle, producing the m

uscu-lar contraction that pulls the hand back from the

stove

A reflex of this type requires the coordinated

contraction of m ultiple m uscles, w hich m ust

con-tract in the right sequence and w ith the right

in-tensity, w hile others (the antagonist m uscles)

m ust relax at the appropriate tim es The intrinsic

neuronal apparatus of the spinal cord is the com

-puterlike, interconnected netw ork of cells that

m akes this process possible

In another paradigm atic situation, stepping on a

sharp rock generates nociceptive im pulses that

in-itiate a com plex but unvarying sequence of events

(Fig 2.10): the painful foot is raised by flexion of

the hip, knee, and ankle, w hile the opposite leg is

Interneuron

Fasciculus

Funicular neuron Lissauer zone

Commissural

neuron

Association neuron

Fig 2.9 Intrinsic neurons and polysynaptic connections

in the spinal cord Note: interneurons are also called

“in-tercalated” or “internuncial” neurons (from Latin nuntius,

messenger).

extended so that the individual can stand on it

alone (crossed extensor reflex) The sudden

redis-tribution of w eight does not cause the individual tofall over, because it is im m ediately com pensatedfor by reflex contraction of m uscles of the trunk,shoulders, arm s, and neck, m aintaining the body’supright posture This process requires synapticcom m unication am ong m any different neurons inthe spinal cord, w ith sim ultaneous participation ofthe brainstem and cerebellum All of this happens

in a fraction of a second; only afterw ard does theindividual feel pain, look to see w hat caused it, andcheck w hether the foot has been injured

These m onosynaptic and polysynaptic reflexesare unconscious processes occurring m ainly in thespinal cord, yet the last exam ple show s that highercom ponents of the CNS m ust often be activated atthe sam e tim e, e.g., to preserve balance (as in theexam ple)

Regulation of Muscle Length and Tension

As discussed above, m onosynaptic and tic reflex arcs serve different purposes: polysynap-tic reflex arcs m ediate protective and flight re-sponses, w hile m onosynaptic reflex arcs are incor-porated in functional circuits that regulate thelength and tension of skeletal m uscle Each m uscle,

polysynap-in fact, contapolysynap-ins tw o servo-control (feedback) tem s:

sys-¼ A control system for length, in w hich the

nu-clear bag fibers of the m uscle spindles serve aslength receptors

¼ A control system for tension, in w hich the Golgi

tendon organs and the nuclear chain fibers ofthe m uscle spindles serve as tension receptors

Stretch and tension receptors Muscle spindles are

receptors for both stretch (length) and tension.These tw o distinct m odalities are subserved by tw odifferent kinds of intrafusal fibers, the so-called

nuclear bag and nuclear chain fibers (Figs 2.11 and 2.12) Fibers of both of these types are typically

shorter and thinner than extrafusal m uscle fibers.The tw o types of intrafusal fiber are depicted sepa-

rately for didactic reasons in Figures 2.11 and 2.12,

but, in reality, the shorter and thinner nuclearchain fibers are directly attached to the som ew hatlonger nuclear bag fibers Muscle spindles gener-ally consist of tw o nuclear bag fibers and four orfive nuclear chain fibers In the m iddle of a nuclear

Cerebrum Brainstem Cerebellum

Painful stimulus

Fig 2.10 Flexor reflex w ith polysynaptic connections

bag fiber, the intrafusal m uscle fibers w iden to a

form a bag containing about 50 nuclei, w hich is

covered by a netw ork of sensory nerve fibers

know n as a prim ary or annulospiral ending (from

Latin annulus, ring) This spiral ending reacts very

sensitively to m uscle stretch, m ainly registering

changes in m uscle length; the nuclear bag fibers

are thus stretch receptors The nuclear chain fibers,

on the other hand, m ainly register a persistently

stretched state of the m uscle, and are thus tension

receptors

Maintenance of constant muscle length. The

extra-fusal m uscle fibers have a certain length at rest,

w hich the organism alw ays tries to m aintain

con-stant Whenever the m uscle is stretched beyond

this length, the m uscle spindle is stretched along

w ith it This generates action potentials in the

an-nulospiral ending, w hich travel very rapidly in Ia

afferent fibers and are then relayed across a

syn-apse to m otor neurons in the anterior horn of the

spinal cord (Fig 2.11) The excited m otor neurons

fire im pulses that travel in equally rapidly

con-ducting, large-diam eter α1 efferent fibers back to

the w orking extrafusal m uscle fibers, causing them

to contract to their form er length Any stretch of

the m uscle induces this response

The physician tests the intactness of this

regu-latory circuit w ith a quick tap on a m uscle tendon,

e.g., the patellar tendon for elicitation of the

quad-riceps (knee-jerk) reflex The resulting m uscular

stretch activates the m onosynaptic reflex arc

In-trinsic m uscle reflexes are of m ajor value for

local-ization in clinical neurology because the reflex arc

for a particular m uscle occupies only one or tw o

radicular or spinal cord segm ents; thus, a finding

of an abnorm al reflex enables the physician to

infer the level of the underlying radicular or spinal

lesion The m ore im portant intrinsic m uscle

re-flexes in clinical practice, the m anner in w hich

they are elicited, and the segm ents that

partici-pate in their reflex arcs are show n in Figure 2.13.

It should be realized that the clinical elicitation of

intrinsic m uscle reflexes is an artificial event: a

brief m uscular stretch such as that produced w ith

a reflex ham m er is a rarity in everyday life

contraction of a stretched m uscle to m aintain

con-stant length is accom panied by reflex relaxation of

its antagonist m uscle(s) The regulatory circuit for

Central input

Pyramidal tract

Nuclear bag muscle spindle with annulospiral ending:

receptor for changes in muscle length (stretch)

α1 motor neuron

Pyramidal tract

Reticulospinal tract

Tendon organ (Golgi organ):

receptor for muscle tension

Nuclear chain muscle spindle with a primary ending and

a flower-spray ending Tonic stretch reflex

Fig 2.11 Regulatory

cir-cuit for muscle length

Fig 2.12 Regulatory

cir-cuit for muscle tension

this likew ise begins in the m uscle spindles The

nu-clear chain fibers of m any m uscle spindles contain

secondary endings called flow er-spray endings in

addition to the prim ary (annulospiral) endings

dis-cussed above These secondary endings react to

stretch as the prim ary endings do, but the afferent

im pulses generated in them travel centrally in II

fibers, w hich are thinner than the Ia fibers

as-sociated w ith the prim ary endings The im pulses

are relayed via spinal interneurons to produce a

net inhibition—and thus relaxation—of the

antago-nist m uscle(s) (reciprocal antagoantago-nist inhibition,

Fig 2.14).

Setting of target values for muscle length. There is

a special m otor system w hose function is to set justable target values in the regulatory circuit for

ad-m uscle length

As show n in Figure 2.11, the anterior horn of

the spinal cord contains not only the large

α m otor neurons but also the sm aller γ m otorneurons These cells project their axons (γ fibers)

Fig 2.13 The most

im-portant intrinsic muscle reflexes

Biceps

C5 C6

C6 C7

cutaneous n.

Muscolo-Radius

Ulna

Radial n.

Triceps reflex Biceps reflex

L5 S1 S2 Tibial n.

cnem ius

Gastro-Femorale n.

Quadriceps fem oris

L2 L3 L4

Quadriceps reflex

(patellar reflex, knee-jerk reflex)

Triceps surae reflex

(Achilles reflex, ankle-jerk reflex)

Triceps

to the sm all, striated intrafusal fibers of the

m uscle spindles Excitation by γ fibers induces

contraction of the intrafusal m uscle fibers at

either end of a m uscle spindle This stretches the

m idportion of the spindle, leading the

annulospi-ral ending to fire action potentials, w hich, in turn,

elevate tension in the w orking m uscle

The γ m otor neurons are under the influence ofseveral descending m otor pathways, including thepyram idal, reticulospinal, and vestibulospinaltracts They thus serve as interm ediaries for thecontrol of m uscle tone by higher m otor centers,

w hich is clearly an im portant aspect of voluntary

m ovem ent The γ efferents enable precise control

of voluntary m ovem ents and also regulate the

+ –

–

– –

Motor neuron Associationneuron

Interneuron Fasciculus

proprius

Fig 2.14 Monosynaptic reflex w ith polysynaptic

inhibi-tion of antagonist muscles

sensitivity of the stretch receptors When the

in-trafusal m uscle fibers contract and stretch the

m idportion of a m uscle spindle, the threshold of

the stretch receptors is low ered, i.e., they require

m uch less m uscular stretch to be activated In the

norm al situation, the target m uscle length that is

to be m aintained is autom atically set by the

fusim otor (γ) innervation of the m uscle

If both the prim ary receptors (nuclear bag fibers

w ith annulospiral endings) and the secondary

re-ceptors (nuclear chain fibers w ith flow er-spray

endings) are slow ly stretched, the response of the

spindle receptors is static, i.e., unchanging in tim e

On the other hand, if the prim ary receptors are very

rapidly stretched, a dynam ic (rapidly changing)

sponse ensues Both the static and the dynam ic

re-sponses are controlled by efferent γ neurons

pre-sum ed to be tw o types of γ m otor neurons,

dy-nam ic and static The form er innervate m ainly the

intrafusal nuclear bag fibers, the latter m ainly the

intrafusal nuclear chain fibers Excitation of

nu-clear bag fibers by dynam ic γ neurons induces a

strong, dynam ic response m ediated by the

an-nulospiral ending, w hile excitation of nuclear

chain fibers by static γ neurons induces a static,

tonic response

Muscle tone. Every m uscle possesses a certaindegree of tone, even in its m axim ally relaxed (rest-ing) state In the clinical neurological exam ination,the physician assesses m uscle tone by noting theresistance to passive m ovem ent of the lim bs (e.g.,flexion and extension)

Total loss of m uscle tone can be produced perim entally either by transection of all of the ante-rior roots or, perhaps m ore surprisingly, by transec-tion of all of the posterior roots Resting tone, there-fore, is not a property of the m uscle itself, but rather

ex-is m aintained by the reflex arcs described in thex-issection

Adaptation of m uscle tone to gravity and m ovem ent.

The hum an body is continually subject to theearth’s gravitational field When an individualstands or walks, anti-gravity m uscles m ust be acti-vated (am ong them the quadriceps fem oris, thelong extensors of the trunk, and the cervical

m uscles) to keep the body erect

When a heavy object is lifted, the tone norm allypresent in the quadriceps m uscle no longer suffices

to keep the body erect Buckling at the knees can beprevented only by an im m ediate increase in quad-riceps tone, w hich occurs as a result of tonic intrin-sic reflexes induced by the stretching of the m uscleand of the m uscle spindles w ithin it This feedback

m echanism or servom echanism enables autom aticadaptation of the tension in a m uscle to the loadthat is placed upon it Thus, w henever an individualstands, w alks, or lifts, action potentials are con-stantly being relayed back and forth to ensure the

m aintenance of the correct am ount of m uscle sion

ten-Central Components of the Somatosensory System

Having traced the path of afferent im pulses fromthe periphery to the spinal cord in the precedingsections, w e w ill now proceed to discuss theirfurther course w ithin the central nervous system

Root entry zone and posterior horn. Individual

so-m atosensory fibers enter the spinal cord at thedorsal root entry zone (DREZ; also called the Red-lich–Obersteiner zone) and then give off num erouscollaterals that m ake synaptic contact w ith other

neurons w ithin the cord Fibers subserving

differ-ent sensory m odalities occupy differdiffer-ent positions

in the spinal cord (Fig 2.15) It is im portant to note

that the m yelin sheaths of all afferent fibers

be-com e considerably thinner as the fibers traverse

the root entry zone and enter the posterior horn

The type of m yelin changes from peripheral to

cen-tral, and the m yelinating cells are no longer

Schw ann cells, but rather oligodendrocytes

The afferent fiber pathw ays of the spinal cord

subserving individual som atosensory m odalities

(Fig 2.16) w ill now be described individually.

Posterior and Anterior

Spinocerebellar Tracts

Som e of the afferent im pulses arising in organs of

the m usculoskeletal system (the m uscles, tendons,

and joints) travel by w ay of the spinocerebellar

tracts to the organ of balance and coordination, the

cerebellum There are tw o such tracts on each side,

one anterior and one posterior (Fig 2.16a).

Posterior spinocerebellar tract. Rapidly conducting

Ia fibers from the m uscle spindles and tendon

or-gans divide into num erous collaterals after

enter-Fig 2.15 Position of fibers of different somatosensory modalities in the posterior root and root entry zone, and their

further course in the spinal cord

ing the spinal cord Som e of these collateral fibers

m ake synaptic contact directly onto the large

α m otor neurons of the anterior horn (m

onosynap-tic reflex arc, Figs 2.15 and 2.11) Other collateral

fibers arising at thoracic, lum bar, and sacral levelsterm inate in a colum n-shaped nucleus occupyingthe base of the posterior horn at levels C8–L2,

w hich is variously nam ed the interm ediolateralcell colum n, thoracic nucleus, Clarke’s colum n, andStilling’s nucleus The postsynaptic second neu-rons w ith cell bodies lying in this nucleus are theorigin of the posterior spinocerebellar tract, w hosefibers are am ong the m ost rapidly conducting ofany in the body The posterior spinocerebellar tract

ascends the spinal cord ipsilaterally in the posterior

portion of the lateral funiculus and then travels by

w ay of the inferior cerebellar peduncle to the

cere-bellar verm is (p 165; Figs 2.16a and 2.17) Afferent

fibers arising at cervical levels (i.e., above the level

of the interm ediolateral cell colum n) travel in thecuneate fasciculus to m ake a synapse onto theircorresponding second neurons in the accessory

cuneate nucleus of the m edulla (Fig 2.17), w hose

output fibers ascend to the cerebellum

Posterior columns Anterior spinocerebellar tract

Posterior spinocerebellar tract

Motor fiber Anterior spinothalamic tract

Lateral spinothalamic tract (pain, temperature)

Posterior columns

Cuneate fasciculus (of Burdach)

Verm is

Via superior

m edullary velum Anterior spinocere- bellar tract,

2nd neuron Posterior spinocere- bellar tract

2nd neuron Thoracic nucleus (Clarke’s column, Stilling’s nucleus) 1st neuron

3rd neuron

Thalamus

2nd neuron

Medial lemniscus Cuneate nucleus and gracile nucleus

Gracile fasciculus Cuneate fasciculus

Fig 2.16 Major fiber

tracts of the spinal cord and the sensory modali- ties that they subserve.

a The anterior and

poste-rior spinocerebellar tracts.

b The posterior funiculus

(posterior columns) c The

anterior spinothalamic

tract d The lateral

spinothalamic tract.

Anterior spinocerebellar tract. Other afferent Ia

fibers entering the spinal cord form synapses w ith

funicular neurons in the posterior horns and in the

central portion of the spinal gray m atter (Figs 2.15,

2.16a, and 2.17) These second neurons, w hich are

found as low as the low er lum bar segm ents, are the

cells of origin of the anterior spinocerebellar tract,

w hich ascends the spinal cord both ipsilaterally and

contralaterally to term inate in the cerebellum In

contrast to the posterior spinocerebellar tract, theanterior spinocerebellar tract traverses the floor ofthe fourth ventricle to the m idbrain and then turns

in a posterior direction to reach the cerebellar

ver-m is by w ay of the superior cerebellar peduncle and

the superior m edullary velum The cerebellum ceives afferent proprioceptive input from all re-gions of the body; its polysynaptic efferent output,

re-in turn, re-influences m uscle tone and the

cerebellum

Paleo-Posterior cerebellar tract Anterior spino- cerebellar tract

Position sense, vibration, pressure, discrimination, touch

(cutaneous receptors, muscle and tendon receptors, Vater–Pacini corpuscles)

Pressure, touch

(peritrichial nerve endings and various cutaneous receptors) 1st neuron

Cuneate nucleus and gracile nucleus

Fig 2.17 Spinal cord

w ith major ascending pathw ays and their further course to target structures in the cere- brum and cerebellum

(schematic drawing)

nated action of the agonist and antagonist m uscles

(synergistic m uscles) that participate in standing,

w alking, and all other m ovem ents Thus, in

addi-tion to the low er regulatory circuits in the spinal

cord itself, w hich w ere described in earlier

sec-tions, this higher functional circuit for the

regula-tion of m ovem ent involves other, nonpyram idalpathw ays and both α and γ m otor neurons All ofthese processes occur unconsciously

Posterior Columns

We can feel the position of our lim bs and sense thedegree of m uscle tension in them We can feel the

w eight of the body resting on our soles (i.e., w e

“feel the ground under our feet”) We can also ceive m otion in the joints Thus, at least som e pro-prioceptive im pulses m ust reach consciousness.Such im pulses are derived from receptors in

per-m uscles, tendons, fasciae, joint capsules, and nective tissue (Vater–Pacini and Golgi–Mazzonicorpuscles), as w ell as cutaneous receptors The af-ferent fibers conveying them are the distalprocesses of pseudounipolar neurons in the spinalganglia The central processes of these cells, inturn, ascend the spinal cord and term inate in theposterior colum n nuclei of the low er m edulla (Figs

con-2.16b and 2.17).

Central continuation of posterior column ways. In the posterior funiculus of the spinal cord,the afferent fibers derived from the low er lim bs oc-cupy the m ost m edial position The afferent fibersfrom the upper lim bs join the cord at cervical levelsand lie m ore laterally, so that the posterior funiculushere consists of tw o colum ns (on either side): the

path-m edial gracile fasciculus (colupath-m n of Goll), and the lateral cuneate fasciculus (colum n of Burdach) The

fibers in these colum ns term inate in the spondingly nam ed nuclei in the low er m edulla, i.e.,the gracile nucleus and cuneate nucleus, respec-tively These posterior colum n nuclei contain thesecond neurons, w hich project their axons to the

corre-thalam us (bulbocorre-thalam ic tract) All of the

bul-bothalam ic fibers cross the m idline to the other side

as they ascend, form ing the m edial lem niscus (Figs.

2.16b and 2.17) These fibers traverse the m edulla,

pons, and m idbrain and term inate in the ventral

posterolateral nucleus of the thalam us (VPL, p 173;

Fig 6.4, p 174) Here they m ake synaptic contact

w ith the third neurons, w hich, in turn, give off the

thalam ocortical tract; this tract ascends by w ay of

the internal capsule (posterior to the pyram idal tract) and through the corona radiata to the prim ary som atosensory cortex in the postcentral gyrus The

som atotopic organization of the posterior colum npathw ay is preserved all the w ay up from the spinal

cord to the cerebral cortex (Fig 2.19 a) The som

ato-topic projection on the postcentral gyrus resem bles

a person standing on his head—an inverted

“hom unculus” (Fig 9.19, p 240).

Gracile fasciculus, from lower limb

To the posterior column nuclei

Cuneate fasciculus, from upper limb

Fig 2.18 Posterior funiculus, containing the posterior

columns: gracile fasciculus (medial, afferent fibers from

lower limb) and cuneate fasciculus (lateral, afferent fibers

from upper limb)

Postcentral gyrus

Abdomen, viscera

Pharynx Tongue Jaw Lower lip Upper lip

Face Eye Thumb

Finger

II III

IV V Hand

Forearm

Arm Shoulder Head

Neck Trunk Hip Thigh Leg

Toes, genitalia

Tail of caudate nucleus

Internal capsule Head of caudate nucleus

Corticospinal tract Medial lemniscus Lateral spinothalamic tract

Claustrum Insula

Th alamus Pu

tamen

Pa llid um

Fig 2.19 Course of the

sensory pathw ays by w ay

of the thalamus and ternal capsule to the cerebral cortex

in-Posterior column lesions. The posterior colum ns

m ainly transm it im pulses arising in the

proprio-ceptors and cutaneous reproprio-ceptors If they are

dys-functional, the individual can no longer feel the

position of his or her lim bs; nor can he or she

rec-ognize an object laid in the hand by the sense of

touch alone or identify a num ber or letter draw n by

the exam iner’s finger in the palm of the hand

Spa-tial discrim ination betw een tw o stim uli delivered

sim ultaneously at different sites on the body is no

longer possible As the sense of pressure is also

dis-turbed, the floor is no longer securely felt under

the feet; as a result, both stance and gait are im

-paired (gait ataxia), particularly in the dark or w ith

the eyes closed These signs of posterior colum n

disease are m ost pronounced w hen the posterior

colum ns them selves are affected, but they can also

be seen in lesions of the posterior colum n nuclei,

the m edial lem niscus, the thalam us, and the central gyrus

post-The clinical signs of a posterior colum n lesion are,therefore, the follow ing:

¼ Loss of the sense of position and m ovem ent

(kinesthetic sense): the patient cannot state theposition of his or her lim bs w ithout looking

¼ Astereognosis: the patient cannot recognize and

nam e objects by their shape and w eight usingthe sense of touch alone

¼ Agraphesthesia: the patient cannot recognize by

touch a num ber or letter draw n in the palm ofthe hand by the exam iner’s finger

¼ Loss of tw o-point discrim ination.

¼ Loss of vibration sense: the patient cannot

per-ceive the vibration of a tuning fork placed on abone

¼ Positive Rom berg sign: The patient cannot stand

for any length of tim e w ith feet together and

eyes closed w ithout w obbling and perhaps

fal-ling over The loss of proprioceptive sense can

be com pensated for, to a considerable extent, by

opening the eyes (w hich is not the case, for

ex-am ple, in a patient w ith a cerebellar lesion)

The fibers in the posterior colum ns originate in the

pseudounipolar neurons of the spinal ganglia, but

the fibers in the anterior and posterior

spinothalam ic tracts do not; they are derived from

the second neurons of their respective pathw ays,

w hich are located w ithin the spinal cord

(Fig 2.16c, d, p 26).

Anterior Spinothalamic Tract

The im pulses arise in cutaneous receptors

(per-itrichial nerve endings, tactile corpuscles) and are

conducted along a m oderately thickly m yelinated

peripheral fiber to the pseudounipolar dorsal root

ganglion cells, and thence by w ay of the posterior

root into the spinal cord Inside the cord, the

cen-tral processes of the dorsal root ganglion cells

travel in the posterior colum ns som e 2–15

m ents upw ard, w hile collaterals travel 1 or 2

seg-m ents dow nward, seg-m aking synaptic contact onto

cells at various segm ental levels in the gray m atter

of the posterior horn (Fig 2.16c, p 26) These cells

(the second neurons) then give rise to the anterior

spinothalam ic tract, w hose fibers cross in the

ante-rior spinal com m issure, ascend in the contralateral

anterolateral funiculus, and term inate in the

ven-tral posterolateral nucleus of the thalam us, together

w ith the fibers of the lateral spinothalam ic tract

and the m edial lem niscus (Fig 2.17, p 27) The

third neurons in this thalam ic nucleus then project

their axons to the postcentral gyrus in the

thalam ocortical tract.

Lesions of the anterior spinothalamic tract. As

ex-plained above, the central fibers of the first

neu-rons of this tract ascend a variable distance in the

ipsilateral posterior colum ns, giving off collaterals

along the w ay to the second neurons, w hose fibers

then cross the m idline and ascend further in the

contralateral anterior spinothalam ic tract It

fol-low s that a lesion of this tract at a lum bar or

thoracic level generally causes m inim al or no im

pairm ent of touch, because m any ascending im

-pulses can circum vent the lesion by w ay of theipsilateral portion of the pathw ay A lesion of the

anterior spinothalam ic tract at a cervical level,

how ever, w ill produce m ild hypesthesia of the tralateral low er lim b

con-Lateral Spinothalamic Tract

The free nerve endings of the skin are the eral receptors for noxious and therm al stim uli.These endings constitute the end organs of thingroup A fibers and of nearly unm yelinated group Cfibers that are, in turn, the peripheral processes ofpseudounipolar neurons in the spinal ganglia Thecentral processes pass in the lateral portion of theposterior roots into the spinal cord and then dividelongitudinally into short collaterals that term inate

periph-w ithin one or tperiph-w o segm ents in the substantia

gelatinosa, m aking synaptic contact w ith funicular

neurons (second neurons) w hose processes form

the lateral spinothalam ic tract (Fig 2.16d, p 26).

These processes cross the m idline in the anterior

spinal com m issure before ascending in the tralateral lateral funiculus to the thalam us Like theposterior colum ns, the lateral spinothalam ic tract

con-is som atotopically organized; here, how ever, thefibers from the low er lim b lie laterally, w hile thosefrom the trunk and upper lim b lie m ore m edially

(Fig 2.20).

The fibers m ediating pain and tem perature sation lie so close to each other that they cannot beanatom ically separated Lesions of the lateralspinothalam ic tract thus im pair both sensory m od-alities, though not alw ays to the sam e degree

sen-Central continuation of the lateral spinothalamic tract. The fibers of the lateral spinothalam ic tracttravel up through the brainstem together w ith

those of the m edial lem niscus in the spinal lem

nis-cus, w hich term inates in the ventral posterolateral nucleus of the thalam us (VPL, pp 172, 173; see

Fig 6.4, p 174, and Fig 2.19) The third neurons in

the VPL project via the thalam ocortical tract to the

postcentral gyrus in the parietal lobe (Fig 2.19).

Pain and tem perature are perceived in a rough

m anner in the thalam us, but finer distinctions arenot m ade until the im pulses reach the cerebral cor-tex

Lesions of the lateral spinothalamic tract. Thelateral spinothalam ic tract is the m ain pathw ay for

Fig 2.20 Somatotopic

organization of spinal cord tracts in cross sec- tion The laminae of Rexed

are also designated with Roman numerals (cytoar- chitectural organization of the spinal gray matter).

Anterior corticospinal tract

Tectospinal tract Reticulospinal tract

cuneatus (of Burdach)

Fasciculus gracilis (of Goll)

Semilunar tract (comma of Schultz)

T L

S C

IX VIII

VII X

Lower lim b

Trunk

Up p er lim b

pain and tem perature sensation It can be

neuro-surgically transected to relieve pain (cordotomy);

this operation is m uch less com m only perform ed

today than in the past, because it has been

sup-planted by less invasive m ethods and also because

the relief it provides is often only tem porary The

latter phenom enon, long recognized in clinical

ex-perience, suggests that pain-related im pulses

m ight also ascend the spinal cord along other

routes, e.g., in spinospinal neurons belonging to

the fasciculus proprius

If the lateral spinothalam ic tract is transected in

the ventral portion of the spinal cord, pain and

tem perature sensation are deficient on the

op-posite side one or tw o segm ents below the level of

the lesion, w hile the sense of touch is preserved

(dissociated sensory deficit).

Other Afferent Tracts of the Spinal

Cord

In addition to the spinocerebellar and

spino-thalam ic tracts discussed above, the spinal cord

contains yet other fiber pathw ays ascending to

various target structures in the brainstem and deepsubcortical nuclei These pathw ays, w hich originate

in the dorsal horn of the spinal cord (second afferentneuron) and ascend in its anterolateral funiculus,

include the spinoreticular, spinotectal, spino

-olivary, and spinovestibular tracts The

spinovesti-bular tract is found in the cervical spinal cord, fromC4 upw ard, in the area of the (descending) vesti-bulospinal tract and is probably a collateral pathw ay

of the posterior spinocerebellar tract

Figure 2.20 is a schem atic draw ing of the various

sensory (ascending) tracts, as seen in a cross tion of the spinal cord The m otor (descending)tracts are also indicated, so that the spatial rela-tionships betw een the various tracts can be appre-ciated Finally, in addition to the ascending and de-scending tracts, the spinal cord also contains a so-called intrinsic apparatus, consisting of neuronsthat project upward and dow nw ard over several

sec-spinal segm ents in the fasciculus proprius (Fig 2.9,

p 20)