Digital histology an interactive cd atlas with review text phần 4 potx

Region of the cell body where axon originates 䉬 Devoid of rough endoplasmic reticulum 䉬 Continuous with initial segment of the axon that is a highly elec-trically excitable zone for ini

Extrafusal muscle fibers

Intrafusal muscle fibers

M bandMitochondriaMyofibrilsMyofilamentsNucleusSarcomereZ-linesCapillariesConnective tissueSmooth muscleAutonomic ganglionSmall intestineInner circular layer of smoothmuscle

Outer longitudinal layer ofsmooth muscle

MucosaSmooth muscle fascicleSmooth muscle fibersNucleus

SarcolemmaSpindle shapeAutonomic neuron cell body

䊉 Multiunit smooth muscle

䉬 Richly innervated

䉬 Specialized for precise, graded contraction (e.g., iris of the eye)

Structures Identified in this Section

General Considerations

➢ Nervous tissue is highly specialized to employ modifications inmembrane electrical potentials to relay signals throughout the body.Neurons form intricate circuits that (1) relay sensory informationfrom the internal and external environments; (2) integrate informa-tion among millions of neurons; and (3) transmit effector signals tomuscles and glands

➢ Anatomical subdivisions of nervous tissue

䊉 Central nervous system (CNS)

䉬 Brain

䉬 Spinal cord

䊉 Peripheral nervous system (PNS)

䉬 Nerves

䉬 Ganglia (singular, ganglion)

Cells of Nervous Tissue

➢ Neurons

䊉 Functional units of the nervous system; receive, process, store, andtransmit information to and from other neurons, muscle cells, orglands

䊉 Composed of a cell body, dendrites, axon and its terminal tion, and synapses

arboriza-䊉 Form complex and highly integrated circuits

Structure of a “Typical” Neuron

➢ Cell body (soma, perikaryon)

䊉 Nucleus

䉬 Large, spherical, usually centrally located in the soma

䉬 Highly euchromatic with a large, prominent nucleolus

䊉 Usually multiple and highly branched at acute angles

䊉 May possess spines to increase surface area for synaptic contact

䊉 Collectively, form the majority of the receptive field of a neuron;conduct impulses toward the cell body

䊉 Organelles

䉬 Microtubules and neurofilaments

䉬 Rough endoplasmic reticulum and polysomes

䉬 Smooth endoplasmic reticulum

䉬 Mitochondria

➢ Axon

䊉 Usually only one per neuron

䊉 Generally of smaller caliber and longer than dendrites

䊉 Branches at right angles, fewer branches than dendrites

䊉 Organelles

䉬 Microtubules and neurofilaments

䉬 Lacks rough endoplasmic reticulum and polysomes

䉬 Smooth endoplasmic reticulum

䉬 Mitochondria

䊉 Axon hillock Region of the cell body where axon originates

䉬 Devoid of rough endoplasmic reticulum

䉬 Continuous with initial segment of the axon that is a highly

elec-trically excitable zone for initiation of nervous impulse

䊉 Usually ensheathed by supporting cells

䊉 Transmits impulses away from the cell body to

䉬 Neurons

䉬 Effector structures Muscle and glands

䊉 Terminates in a swelling, the terminal bouton, which is the naptic element of a synapse

presy-FIGURE 8.1 Types of neurons based on shape.

Type of Neurons by Shape and Function

➢ Multipolar neuron Most numerous and structurally diverse type

䊉 Efferent Motor or integrative function

䊉 Found throughout the CNS and in autonomic ganglia in the PNS

➢ Pseudounipolar neuron

䊉 Afferent Sensory function

䊉 Found in selected areas of the CNS and in sensory ganglia ofcranial nerves and spinal nerves (dorsal root ganglia)

➢ Bipolar neuron

䊉 Afferent Sensory function

䊉 Found associated with organs of special sense (retina of the eye,olfactory epithelium, vestibular and cochlear ganglia of the innerear)

䊉 Developmental stage for all neurons

Arrangement of Neuronal Cell Bodies and

Their Processes

➢ In both CNS and PNS, cell bodies are found in clusters or layers andaxons travel in bundles These groupings are based on common func-tions and/or common connections

Group of cell bodies Bundle of processes

Central nervous system Nucleus or cortex Tract

(gray matter) (white matter) Peripheral nervous system Ganglion Nerve

Synapse

➢ The function of the synapse is to alter the membrane potential of thepostsynaptic target cell to either facilitate or inhibit the likelihood ofthe stimulus to be propagated by the postsynaptic cell Most neuronsreceive thousands of synaptic contacts, both stimulatory andinhibitory, and the algebraic sum of these inputs determines whetherthe postsynaptic cell will depolarize

➢ Classified according to postsynaptic target

䊉 Axodendritic Most common

䊉 Axosomatic

䊉 Axoaxonic Mostly occur at presynaptic terminals

䊉 Neuromuscular junction

➢ Structure of the synapse

䊉 Presynaptic component

䉬 Distal end of the axon branches, each branch terminating in a

swelling or button called the terminal bouton.

䉬 Bouton contains synaptic vesicles/granules, which contain

neuro-transmitters and numerous mitochondria

䊉 Synaptic gap/cleft Separation (20–30 nm) between pre- and

䊉 Bouton en passant “Bouton-like” swellings along the length of an

axon, allows a single axon to contact many distant cells Common

in smooth muscle innervation

The Reflex Arc

➢ The reflex arc is the simplest neuronal circuit and includes each ofthe elements discussed above These circuits provide rapid, stereo-typed reactions to help maintain homeostasis To begin the reflex, apseudounipolar, sensory neuron is activated by a receptor The axoncarries an afferent signal from the skin into the spinal cord where itsynapses on a multipolar association neuron or interneuron Theinterneuron signals a multipolar, motor neuron whose axon thencarries an efferent signal to skeletal muscle to initiate contraction

䊏 Fibrous astrocytes in white matter

䊏 Protoplasmic astrocytes in gray matter

䉬 Present in white and gray matter

䉬 Interfascicular oligodendrocytes are located in the white matter of

the CNS, where they produce the myelin sheath

䊉 Ependymal cells Line ventricles

䊉 Microglia

䉬 Not a true neuroglial cell; derived from mesoderm whereas roglial cells, as well as neurons, are derived from ectoderm

neu-䉬 Highly phagocytic cells

➢ Supporting cells of the PNS Schwann cells

䊉 Satellite Schwann cells surround cell bodies in ganglia

FIGURE 8.2 The reflex arc.

䊉 Ensheathing Schwann cells

䉬 Surround unmyelinated axons Numerous axons indent theSchwann cell cytoplasm and are ensheathed only by a singlewrapping of plasma membrane

䉬 Produce the myelin sheath around axons

Myelin Sheath

➢ The myelin sheath is formed by the plasma membrane of supporting

cells wrapping around the axon The sheath consists of lar, lipid-rich segments produced by Schwann cells in the PNS andoligodendrocytes in the CNS

multilamel-➢ Functions

䊉 Increases speed of conduction (saltatory conduction)

䊉 Insulates the axon

➢ Similar structure in CNS and PNS with some differences in proteincomposition

➢ Organization

䊉 Internode Single myelin segment

䊉 Paranode Ends of each internode where they attach to the axon

䊉 Node of Ranvier Specialized region of the axon between myelin

internodes where depolarization occurs

➢ In the PNS, each Schwann cell associates with only one axon andforms a single internode of myelin

➢ In the CNS, each oligodendrocyte associates with many (40–50) axons(i.e each oligodendrocyte forms multiple internodes on differentaxons)

Connective Tissue Investments of Nervous Tissue

➢ Peripheral nervous system

䊉 Endoneurium Delicate connective tissue surrounding Schwann

cells; includes the basal lamina secreted by Schwann cells as well

as reticular fibers

䊉 Perineurium Dense tissue surrounding groups of axons and their

surrounding Schwann cells, forming fascicles; forms the nerve barrier

blood-䊉 Epineurium Dense connective tissue surrounding fascicles and the

entire nerve

Glial cells

Astrocyte, protoplasmic

Astrocyte, fibrous

Astrocyte nuclei

Astrocytic end feet

Microglial cell nuclei

Organ of CortiBone

Multipolar neuronsAxon

Axon hillockCell bodyDendriteNissl substanceNucleolusNucleusTypes

➢ Central nervous system

䊏 Separated from pia mater by connective tissue trabeculae

䊏 Encloses the subarachnoid space, which contains blood vessels and the cerebrospinal fluid (CSF) produced by the cells of the

choroid plexus

䊏 Together with pia mater, constitute the leptomeninges;

inflam-mation of these membranes produces meningitis

䉬 Dura mater

䊏 Outermost of the meninges

䊏 Dense connective tissue that includes the periosteum of theskull

Structures Identified in This Section

Autonomic ganglion

Purkinje cell (neuron)

Purkinje cell body

Purkinje cell dendrites

AxonMeissner’s corpuscleMuscle spindleSkeletal muscle fibersModified skeletal muscle fibersCapsule

Sensory axonPacinian corpusclePerineurial cellsSpinal cord

Spinal nerve rootsSynapses

Motor end plateSkeletal muscleAxons

CNS synapseTerminal boutonSynaptic vesicles (Neurotrans-mitter vesicles)

MitochondriaSynaptic cleftPostsynaptic cellPostsynaptic densityDendrite

Dendritic spineWhite matter

➢ Types of membranes

䊉 Cutaneous membrane or skin

䉬 Covers the exterior surface of the body

䉬 Composition

䊏 Stratified squamous keratinized epithelium called epidermis

䊏 Two layers of connective tissue, loose connective tissue anddense irregular connective tissue, called the dermis

䉬 May possess hair follicles, sweat glands, and sebaceous glands

䊉 Mucous membrane or mucosa

䉬 Lines all interior lumens of organs that open to the exterior, such

as stomach, uterus, and trachea

䉬 Composition

䊏 Epithelium varies depending on the location

䊏 The lamina propria, composed of loose connective tissue, liesbeneath the epithelium

䊏 Muscularis mucosae, a layer of smooth muscle, is frequently,but not always, present in a mucous membrane

䉬 Many mucous membranes are associated with mucus-secretingcells or glands that lubricate the surface of the membrane

䊉 Serous membrane or serosa

䉬 Location

䊏 Lines the perimeter of all internal body cavities that do notopen to the exterior, such as peritoneal cavity (called peri-toneum), pleural cavity (called pleura), and pericardial cavity(called pericardium) This lining constitutes the parietal layer

of these cavities

䊏 Covers and forms the outer layer of any organs that protrudeinto those body cavities, such as stomach and jejunum (peri-toneum), lungs (pleura), and heart (pericardium) This covering constitutes the visceral layer of these cavities

䉬 Composition

䊏 Simple squamous epithelium called mesothelium

䊏 Loose connective tissue

FIGURE 9.1 Cross-section through the abdomen, illustrating epithelial membranes.

Serous membraneSimple squamous epitheliumSubmucosa

Cortex and medullaCortex

MedullaStroma and parenchymaParenchyma

Stroma

Cortex vs Medulla

➢ Components

䊉 Cortex The outer region or portion of some organs, such as kidney,

lymph nodes, and adrenal glands; surrounds the internal medulla

䊉 Medulla The center portion of some organs; surrounded by a

➢ Parenchyma The cells and tissues of an organ that perform the

func-tion of the organ; composed of epithelium, muscle, nerve and, times, connective tissue

some-Structures Identified in This Section

General Considerations

➢ Continuous tubular system for transporting blood, carrying oxygen,carbon dioxide, hormones, nutrients, and wastes

➢ Components of the circulatory system

䊉 Heart Highly modified, muscular blood vessel specialized for

pumping the blood Composed of two atria and two ventricles

䊉 Closed circuit of vessels The vessels are listed below in the order that

blood would follow as it leaves the heart

䉬 Elastic arteries (e.g., aorta and pulmonary arteries)

䉬 Muscular arteries (remaining named arteries)

䉬 Small arteries and arterioles

䉬 Capillaries

䉬 Venules and small veins

䉬 Medium veins (most named veins)

䉬 Large veins (e.g., venae cavae return blood to the heart)

➢ Circuitry of the circulatory system

䉬 Blood leaves the right ventricle of the heart through pulmonary

arteries and proceeds through a series of smaller arteries to

supply pulmonary capillaries in the lungs Blood returns through

a series of increasingly larger veins to the pulmonary veins to the

capillaries throughout the body Blood returns through a series

of increasingly larger veins via the superior and inferior venae

cavae to the right atrium.

䉬 Functions for exchange of carbon dioxide and oxygen, andnutrients and metabolic wastes between the blood and tissues;distribution of hormones

䊉 Lymphatic circulation Consists of a system of blind-ended lymph

vessels positioned throughout the body, which return tissue fluid

to the venous circulation

Basic Structural Organization

➢ The walls of the entire cardiovascular system, consists of three centric layers or tunics that are continuous between both the heartand vessels The constituents and thickness of these layers varydepending on the mechanical and metabolic functions of the vessel

con-➢ Inner tunic

䊉 In the heart, this layer is called the endocardium; in vessels it is termed the tunica intima.

䊉 Composition

䉬 Simple squamous epithelium (endothelium)

䉬 Varying amounts and types of connective tissue

䉬 In the largest vessels, longitudinally oriented smooth musclemay be present in the connective tissue layer

➢ Middle tunic

䊉 In the heart this layer is composed of cardiac muscle and is called

the myocardium.

䊉 In vessels this layer is composed of circularly oriented smoothmuscle or smooth muscle plus connective tissue and is called the

tunica media.

➢ Outer tunic

䊉 In the heart, this layer consists of a serous membrane, called the

epicardium (visceral pericardium) composed of connective tissue

covered with a simple squamous epithelium (mesothelium)

䊉 In vessels, this layer is called the tunica adventitia and is composed

of connective tissue; variable amount of longitudinally arrangedsmooth muscle is present in this layer in the largest veins

䊉 Possesses blood vessels that supply the wall of the heart or largerblood vessels

䉬 Coronary blood vessels Supply the heart wall

䉬 Vasa vasorum Consists of a system of small blood vessels that

supply the outer wall of larger vessels

Arteries

➢ General considerations

䊉 Carry blood away from the heart and toward capillary beds

䊉 Have thicker walls and smaller lumens than veins of similar size

䊉 Tunica media is the predominate tunic

䊉 Cross-sectional outlines are more circular in arteries than in veins

FIGURE 10.1 Structure of a muscular artery.

䊉 Types

䉬 Elastic (large) arteries (aorta, pulmonary arteries)

䊏 Internal elastic lamina is present but difficult to distinguish

䊏 Tunica media is composed of fenestrated sheets of elastic

tissue (elastic lamellae) and smooth muscle.

䊏 Passively maintain blood pressure by distension and recoil ofthe elastic sheets

䉬 Muscular (medium, distributing)

䊏 Tunica media is composed of smooth muscle

䊏 Internal elastic lamina Single, fenestrated, elastic sheet; lies

internal to the smooth muscle of the tunica media

䊏 External elastic laminae Multiple elastic sheets; lie external to

the smooth muscle of the tunica media

䊏 Regulate blood pressure and blood distribution by tion and relaxation of smooth muscle in the tunica media

contrac-䉬 Small arteries and arterioles

䊏 Less than 200 microns in diameter

䊏 Small arteries have an internal elastic lamina and up to eightlayers of smooth muscle in the tunica media

䊏 Arterioles usually lack an internal elastic lamina and have one

to two layers of smooth muscle in the tunica media

䊏 Arterioles are the vessels that regulate blood pressure anddeliver blood under low pressure to capillaries