Lecture Human anatomy and physiology - Chapter 13: The peripheral nervous system and reflex activity (part d)

Chapter 13 (part d) provides knowledge of motor endings and motor activity. In this chapter, students will be able to compare and contrast the motor endings of somatic and autonomic nerve fibers, outline the three levels of the motor hierarchy, compare the roles of the cerebellum and basal nuclei in controlling motor activity.

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PowerPoint® Lecture Slides

prepared by Janice Meeking, Mount Royal College

C H A P T E R

13

The Peripheral Nervous

System and Reflex Activity: Part D

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Motor Endings

• PNS elements that activate effectors by releasing neurotransmitters

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Review of Innervation of Skeletal Muscle

• Takes place at a neuromusclular junction

• Acetylcholine (ACh) is the neurotransmitter

• ACh binds to receptors, resulting in:

potential

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Nucleus

Action potential (AP)

Myelinated axon

of motor neuron

Axon terminal of

neuromuscular junction

Sarcolemma of the muscle fiber

Ca 2+ Ca 2+

Axon terminal

of motor neuron

Synaptic vesicle containing ACh Mitochondrion

Synaptic cleft

Junctional folds of sarcolemma

Fusing synaptic vesicles ACh

Sarcoplasm of muscle fiber

Postsynaptic membrane ion channel opens;

Acetylcholinesterase

Postsynaptic membrane ion channel closed;

ions cannot pass.

Action potential arrives

at axon terminal of motor neuron.

Voltage-gated Ca 2+

channels open and Ca 2+

enters the axon terminal.

Ca 2+ entry causes some synaptic vesicles to release their contents (acetylcholine)

by exocytosis.

Acetylcholine, a neurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma.

ACh binding opens ion channels that allow simultaneous passage of

Na + into the muscle fiber and K + out of the muscle fiber.

ACh effects are terminated by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase.

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Review of Innervation of Visceral Muscle and Glands

• Autonomic motor endings and visceral

effectors are simpler than somatic junctions

• Branches form synapses en passant via

varicosities

• Acetylcholine and norepinephrine act

indirectly via second messengers

• Visceral motor responses are slower than somatic responses

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Smooth muscle cell

Varicosities release

their neurotransmitters into a wide synaptic cleft (a diffuse junction).

Synaptic vesicles

Mitochondrion

Autonomic nerve fibers

innervate most smooth muscle fibers.

Varicosities

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Levels of Motor Control

• Segmental level

• Projection level

• Precommand level

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Feedback

Reflex activity Motor

output

Sensory input

(a) Levels of motor control and their interactions

Precommand Level(highest)

• Cerebellum and basal

nuclei

• Programs and instructions

(modified by feedback)

Projection Level (middle)

• Motor cortex (pyramidal

system) and brain stem

nuclei (vestibular, red,

reticular formation, etc.)

• Convey instructions to

spinal cord motor neurons

and send a copy of that

information to higher levels

Segmental Level (lowest)

• Spinal cord

• Contains central pattern

generators (CPGs)

Internal feedback

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Segmental Level

• The lowest level of the motor hierarchy

• Central pattern generators (CPGs): segmental circuits that activate networks of ventral horn neurons to stimulate specific groups of

muscles

• Controls locomotion and specific, oft-repeated motor activity

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Projection Level

• Consists of:

(pyramidal) system to produce voluntary skeletal muscle movements

indirect (extrapyramidal) system to control reflex and CPG-controlled motor actions

• Projection motor pathways keep higher command levels informed of what is

happening

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Precommand Level

• Neurons in the cerebellum and basal nuclei

in advance of willed movements

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Precommand Level

• Cerebellum

areas of the brain stem

• Basal nuclei

conditions

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Feedback

Reflex activity Motor

output

Sensory input

(a) Levels of motor control and their interactions

Precommand Level(highest)

• Cerebellum and basal

nuclei

• Programs and instructions

(modified by feedback)

Projection Level (middle)

• Motor cortex (pyramidal

system) and brain stem

nuclei (vestibular, red,

reticular formation, etc.)

• Convey instructions to

spinal cord motor neurons

and send a copy of that

information to higher levels

Segmental Level (lowest)

• Spinal cord

• Contains central pattern

generators (CPGs)

Internal feedback

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• Primary motor cortex

• Brain stem nuclei

Segmental level

• Spinal cord

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Reflex Arc

1 Receptor—site of stimulus action

2 Sensory neuron—transmits afferent impulses to the

CNS

3 Integration center—either monosynaptic or

polysynaptic region within the CNS

4 Motor neuron—conducts efferent impulses from the

integration center to an effector organ

5 Effector—muscle fiber or gland cell that responds to

the efferent impulses by contracting or secreting

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Receptor Sensory neuron Integration center Motor neuron

Effector

Spinal cord (in cross section)

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Spinal Reflexes

• Spinal somatic reflexes

• Testing of somatic reflexes is important clinically to assess the condition of the nervous system

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Stretch and Golgi Tendon Reflexes

• For skeletal muscle activity to be smoothly

coordinated, proprioceptor input is necessary

the length of the muscle

amount of tension in the muscle and tendons

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primary sensory endings of type Ia fibers and secondary sensory endings of type II fibers

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Secondary sensory endings (type II fiber)

Efferent (motor) fiber to muscle spindle

Primary sensory endings (type Ia fiber)

Connective tissue capsule

Muscle spindle

Tendon

Sensory fiber

Golgi tendon organ

Efferent (motor) fiber to extrafusal muscle fibers

Extrafusal muscle fiber

Intrafusal muscle fibers

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Muscle Spindles

• Excited in two ways:

spindle

the ends to contract, thereby stretching the spindle

• Stretch causes an increased rate of

impulses in Ia fibers

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• – coactivation maintains the tension and sensitivity of the spindle during muscle

contraction

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(d) - Coactivation.

Both extrafusal and intrafusal muscle fibers contract

Muscle spindle tension is main- tained and it can still signal changes

in length.

Time

(c) Only motor

neurons activated.

Only the extrafusal

muscle fibers contract

The muscle spindle

becomes slack and no

APs are fired It is

unable to signal further

length changes.

Time

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Stretch Reflexes

• How a stretch reflex works:

motor neurons in the spinal cord

to contract

• All stretch reflexes are monosynaptic and

ipsilateral

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Stretch Reflexes

• Reciprocal inhibition also occurs—IIa fibers synapse with interneurons that inhibit the motor neurons of antagonistic muscles

• Example: In the patellar reflex, the stretched muscle (quadriceps) contracts and the

antagonists (hamstrings) relax

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Stretched muscle spindles initiate a stretch reflex,causing contraction of the stretched muscle andinhibition of its antagonist

When muscle spindles are activated

by stretch, the associated sensory

neurons (blue) transmit afferent impulses

at higher frequency to the spinal cord.

The sensory neurons synapse directly with alpha motor neurons (red), which excite extrafusal fibers

of the stretched muscle Afferent fibers also synapse with interneurons (green) that inhibit motor neurons (purple) controlling antagonistic muscles.

The events by which muscle stretch is damped

Efferent impulses of alpha motor neurons

cause the stretched muscle to contract,

which resists or reverses the stretch.

Efferent impulses of alpha motor neurons to antagonist muscles are reduced (reciprocal inhibition).

Initial stimulus

(muscle stretch)

Cell body of sensory neuron

Sensory neuron

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Initial stimulus

Sensory neuron

Muscle spindle

Antagonist muscle

Spinal cord

1

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Initial stimulus

Sensory neuron

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Initial stimulus

Sensory neuron

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Efferent impulses of alpha motor neurons to antagonist muscles are reduced (reciprocal inhibition).

Initial stimulus

Sensory neuron

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The patellar (knee-jerk) reflex—a specific example of a stretch reflex

Muscle spindle

Quadriceps (extensors)

Hamstrings (flexors)

Patella

Patellar ligament

Spinal cord (L 2 –L 4 )

Tapping the patellar ligament excites muscle spindles in the quadriceps.

The motor neurons (red) send activating impulses to the quadriceps causing it to contract, extending the knee.

Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons.

The interneurons (green) make inhibitory synapses with ventral horn neurons (purple) that prevent the antagonist muscles (hamstrings) from resisting the contraction of the

quadriceps.

Excitatory synapse Inhibitory synapse

+ –

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Muscle spindle

Patella

+ –

1

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Muscle spindle

Patella

+ –

1

2

1

2

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Muscle spindle

Patella

+ –

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Muscle spindle

Patella

The interneurons (green) make inhibitory synapses with ventral horn neurons (purple) that prevent the antagonist muscles (hamstrings) from resisting the contraction of the

quadriceps.

+ –

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Golgi Tendon Reflexes

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Golgi Tendon Reflexes

to tension

• Contraction or passive stretch activates Golgi tendon organs

• Afferent impulses are transmitted to spinal cord

• Contracting muscle relaxes and the antagonist

contracts (reciprocal activation)

• Information transmitted simultaneously to the

cerebellum is used to adjust muscle tension

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