Copyright © 2010 Pearson Education, Inc. Chapter 13 The Peripheral Nervous System Part D Shilla Chakrabarty, Ph.D

Copyright © 2010 Pearson Education, Inc.

Chapter 13

The Peripheral Nervous System

Part D

Shilla Chakrabarty, Ph.D.


Motor Endings

• Are PNS elements that activate effectors by releasing neurotransmitters

• Found at neuromuscular junctions

Terminals of somatic fibers innervating voluntary muscles forms elaborate neuromuscular junctions with their effector cells

As each axon branch reaches its target, a single muscle cell, the ending splits into a cluster of axon terminals

Axon terminals branch like a tree over the junctional folds of sarcolemma of muscle fiber

Axon terminals contain mitochondria and synaptic vesicles filled with the neurotransmitter acetylcholine (ACh)

When a nerve impulse reaches an axon terminal, Ach is released by exocytosis and a series of events is initiated.


Nucleus

Actionpotential (AP)

Myelinated axonof motor neuron

Axon terminal of neuromuscular junction

Sarcolemma ofthe muscle fiber

Ca2+Ca2+

Axon terminalof motor neuron

Synaptic vesiclecontaining ACh

Mitochondrion

Synaptic cleft

Junctionalfolds of sarcolemma

Fusing synaptic vesicles

ACh

Sarcoplasm ofmuscle fiber

Postsynaptic membraneion channel opens;ions pass.

Na+ K+

AChNa+

K+

Degraded ACh

Acetylcholinesterase

Postsynaptic membraneion channel closed;ions cannot pass.

Action potential arrives at axon terminal of motor neuron.

Voltage-gated Ca2+

channels open and Ca2+

enters the axon terminal.

Ca2+ 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.

1

2

3

4

5

6

Events At A Neuromuscular Junction When A Nerve Impulse Arrives


Review of Innervation of Visceral Muscle and Glands

• Autonomic motor endings and visceral effectors (such as smooth and cardiac muscles and glands) are simpler than somatic junctions

• Branches form synapses en passant via varicosities

Varicosities are knoblike swellings containing mitochondria and synaptic vesicles, and appear like a string of beads

• Autonomic synaptic vesicles typically contain acetylcholine or norepinephrine, both of which act indirectly via second messengers

• Visceral motor responses are slower than somatic responses which directly open ion channels


Smoothmusclecell

Varicosities releasetheir neurotransmittersinto a wide synaptic cleft (a diffuse junction).

Synapticvesicles

Mitochondrion

Autonomicnerve fibersinnervatemost smoothmuscle fibers.

Varicosities

Innervations Of Smooth Muscles


Feedback

Reflex activity Motoroutput

Sensoryinput

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)

Internalfeedback

Levels of Motor Control And Their Interactions


Levels of Motor Control: 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


Levels of Motor Control: Projection Level

• Consists of:

Upper motor neurons that direct the pyramidal system to directly produce voluntary skeletal muscle movements

Brain stem motor areas that oversee the indirect (extrapyramidal) system to control reflex and CPG-controlled motor actions

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


Levels of Motor Control: Precommand Level

• Neurons in the cerebellum and basal nuclei

Regulate motor activity

Precisely start or stop movements

Coordinate movements with posture

Block unwanted movements

Monitor muscle tone

Perform unconscious planning and discharge in advance of willed movements


Precommand Level

• Cerebellum

• Lacks direct connections to the spinal cord

• Acts on motor pathways through projection areas of the brain stem

• Acts on the motor cortex via the thalamus

• Basal nuclei

• Receive inputs from all cortical areas and send output to premotor and prefrontal cortical areas via thalamus

• Inhibit various motor centers under resting conditions


(b) Structures involved

Precommand level • Cerebellum• Basal nuclei

Projection level • Primary motor cortex• Brain stem nuclei

Segmental level • Spinal cord

Levels of Motor Control: Segmental, Projection and Precommand


Reflexes

• Inborn (intrinsic) reflex: a rapid, involuntary, predictable motor response to a stimulus

• Learned (acquired) reflexes result from practice or repetition,

Example: driving skills


Reflex Arc• Reflexes occur over highly specific neural paths

called reflex arcs

• Components of a reflex arc (neural path)

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

Copyright © 2010 Pearson Education, Inc. Figure 13.14

Receptor

Sensory neuron

Integration center

Motor neuron

Effector

Spinal cord(in cross section)

Interneuron

Stimulus

Skin

1

2

3

4

5


Spinal Reflexes

• Spinal somatic reflexes

• Integration center is in the spinal cord

• Effectors are skeletal muscle

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


Stretch and Golgi Tendon Reflexes

• For skeletal muscle activity to be smoothly coordinated, proprioceptor input is necessary

Muscle spindles inform the nervous system of the length of the muscle

Golgi tendon organs inform the brain as to the amount of tension in the muscle and tendons


Secondary sensoryendings (type II fiber)

Efferent (motor)fiber to muscle spindle

Primary sensoryendings (type Iafiber)

Connectivetissue capsule

Muscle spindle

Tendon

Sensory fiber

Golgi tendonorgan

Efferent (motor)fiber to extrafusalmuscle fibers

Extrafusal musclefiber

Intrafusal musclefibers

Muscle Spindles

• Composed of 3–10 short intrafusal muscle fibers in a connective tissue capsule

• Intrafusal fibers

Noncontractile in their central regions (lack myofilaments)

Wrapped with two types of afferent endings: primary sensory endings of type Ia fibers and secondary sensory endings of type II fibers

• Contractile end regions are innervated by gamma () efferent fibers that maintain spindle sensitivity

Note: Extrafusal fibers (contractile muscle fibers) are innervated by alpha () efferent fibers


Operation Of The Muscle Spindle

(a) Unstretched muscle. Action potentials (APs) are generated at a constant rate in the associated sensory (la) fiber.

Musclespindle

Intrafusalmuscle fiber

Primarysensory (la)nerve fiberExtrafusalmuscle fiber

Time

(b) Stretched muscle. Stretching activates the muscle spindle, increasing the rate of APs.

Time

(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

Action potentials generated in sensory fibers are shown as black lines in yellow bars


Stretch Reflexes• Maintain muscle tone in large postural muscles

• Cause muscle contraction in response to increased muscle length (stretch)

How a stretch reflex works:

• Stretch activates the muscle spindle

• IIa sensory neurons synapse directly with motor neurons in the spinal cord

• motor neurons cause the stretched muscle to contract

• All stretch reflexes are monosynaptic and ipsilateral

• 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

Copyright © 2010 Pearson Education, Inc. Figure 13.17 (1 of 2)

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

When muscle spindles are activatedby stretch, the associated sensoryneurons (blue) transmit afferent impulsesat higher frequency to the spinal cord.

The sensory neurons synapse directly with alphamotor neurons (red), which excite extrafusal fibersof the stretched muscle. Afferent fibers alsosynapse with interneurons (green) that inhibit motorneurons (purple) controlling antagonistic muscles.

The events by which muscle stretch is damped

Efferent impulses of alpha motor neuronscause the stretched muscle to contract,which resists or reverses the stretch.

Efferent impulses of alpha motorneurons to antagonist muscles arereduced (reciprocal inhibition).

Initial stimulus(muscle stretch)

Cell body ofsensory neuron

Sensoryneuron

Muscle spindleAntagonist muscle

Spinal cord

12

3a 3b

Copyright © 2010 Pearson Education, Inc. Figure 13.17 (2 of 2)

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

Musclespindle

Quadriceps(extensors)

Hamstrings(flexors)

Patella

Patellarligament

Spinal cord(L2–L4)

Tapping the patellar ligament excitesmuscle spindles in the quadriceps.

The motor neurons (red) sendactivating impulses to the quadricepscausing it to contract, extending theknee.

Afferent impulses (blue) travel to thespinal cord, where synapses occur withmotor neurons and interneurons.

The interneurons (green) makeinhibitory synapses with ventral horn neurons (purple) that prevent theantagonist muscles (hamstrings) fromresisting the contraction of thequadriceps.

Excitatory synapseInhibitory synapse

+

–

1

2

3a

3b

1

2

3a3b 3b

Copyright © 2010 Pearson Education, Inc. Figure 13.17 (2 of 2), step 3b

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

Musclespindle


Hamstrings(flexors)

Patella

Patellarligament

Spinal cord(L2–L4)

Tapping the patellar ligament excitesmuscle spindles in the quadriceps.

The motor neurons (red) sendactivating impulses to the quadricepscausing it to contract, extending theknee.

Afferent impulses (blue) travel to thespinal cord, where synapses occur withmotor neurons and interneurons.

The interneurons (green) makeinhibitory synapses with ventral horn neurons (purple) that prevent theantagonist muscles (hamstrings) fromresisting the contraction of thequadriceps.

Excitatory synapseInhibitory synapse

+

–

1

2

3a

3b

1

2

3a3b 3b


Golgi Tendon Reflexes

• Polysynaptic reflexes

• Help to prevent damage due to excessive stretch

• Important for smooth onset and termination of muscle contraction


Golgi Tendon Reflexes

• Produce muscle relaxation (lengthening) in response 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


+ Excitatory synapse– Inhibitory synapse

Quadriceps strongly contracts. Golgi tendon organs are activated.

Afferent fibers synapse with interneurons in the spinal cord.

Efferent impulses to muscle with stretched tendon are damped. Muscle relaxes, reducing tension.

Efferent impulses to antagonist muscle cause it to contract.

Interneurons

Spinal cord


Golgitendon

organHamstrings

(flexors)

1 2

3a 3b


Flexor and Crossed-Extensor Reflexes

• Flexor (withdrawal) reflex

• Initiated by a painful stimulus

• Causes automatic withdrawal of the threatened body part

• Ipsilateral and polysynaptic


Flexor and Crossed-Extensor Reflexes

• Crossed extensor reflex

• Occurs with flexor reflexes in weight-bearing limbs to maintain balance

• Consists of an ipsilateral flexor reflex and a contralateral extensor reflex

• The stimulated side is withdrawn (flexed)

• The contralateral side is extended


Afferentfiber

Efferentfibers

Extensorinhibited

Flexorstimulated

Site of stimulus: a noxiousstimulus causes a flexorreflex on the same side,withdrawing that limb.

Site of reciprocalactivation: At thesame time, theextensor muscleson the oppositeside are activated.

Armmovements

Interneurons

Efferentfibers

FlexorinhibitedExtensorstimulated

+ Excitatory synapse– Inhibitory synapse


Superficial Reflexes

• Elicited by gentle cutaneous stimulation

• Depend on upper motor pathways and cord-level reflex arcs


Superficial Reflexes: Plantar Reflex

• Plantar reflex

Stimulus: stroking lateral aspect of the sole of the foot

Response: downward flexion of the toes

Tests for function of corticospinal tracts



• Babinski’s sign

• Stimulus: as above

• Response: dorsiflexion of hallux and fanning of toes

• Present in infants due to incomplete myelination

• In adults, indicates corticospinal or motor cortex damage



• Abdominal reflexes

• Cause contraction of abdominal muscles and movement of the umbilicus in response to stroking of the skin

• Vary in intensity from one person to another

• Absent when corticospinal tract lesions are present


Developmental Aspects of the PNS

• Spinal nerves branch from the developing spinal cord and neural crest cells

• Supply both motor and sensory fibers to developing muscles to help direct their maturation

• Cranial nerves innervate muscles of the head


Developmental Aspects of the PNS

• Distribution and growth of spinal nerves correlate with the segmented body plan

• Sensory receptors atrophy with age and muscle tone lessens due to loss of neurons, decreased numbers of synapses per neuron, and slower central processing

• Peripheral nerves remain viable throughout life unless subjected to trauma

Documents

Copyright © 2010 Pearson Education, Inc. Chapter 13 The Peripheral Nervous System Part D Shilla Chakrabarty, Ph.D