The Peripheral Nervous System

Chapter 13

Introduction The CNS would be useless without a

means of sensing our own internal as well as the external environments

In addition, we need a means by which we can effect our external environment

The peripheral nervous system provides these links to the CNS

Introduction The peripheral nervous system includes

all the neural structures outside the brain and spinal cord – Sensory receptors– Peripheral nerves and their ganglia– Efferent motor endings

Introduction Basic components of

the PNS Sensory components

provide the information interpreted by the CNS

Motor components stimulate the effectors of the CNS

The CNS commands; the PNS acts

Sensory Receptors Sensory receptors are structures that are

specialized to respond to changes in their environment

Such environmental changes are called stimuli

Typically activation of a sensory receptor by an adequate stimulus results in depolarization or graded potentials that trigger nerve impulses along the afferent fibers coursing to the CNS

Sensory Receptors Sensory receptors are classified by

– The type of stimulus they detect– Their location in the body– The relative complexity of their structure

Classification by Stimulus Detected Mechanoreceptors

– general nerve impulses when they, or adjacent tissues, are deformed by mechanical forces

• Touch

• Pressure

• Vibration

• Stretch

Thermoreceptors – Sensitive to temperature changes

Classification by Stimulus Detected Photoreceptors

– Respond to light energy Chemoreceptors

– Respond to chemicals in solution• Smell

• Taste

• Blood chemistry

Nociceptors– Respond to potentially damaging stimuli that

result in pain

Classification by Stimulus Detected Note that the overstimulation of any of the

aforementioned receptors is painful and thus virtually all receptors function as nociceptors at one time or another

Classification by Location Receptors are recognized according to

their location or the location of the stimuli to which they respond– Externoceptors– Internoceptors or visceroceptors– Proprioceptors

Classification by Location Externoceptors

– Sensitive to stimuli arising from outside of the body

– Typically located near the surface of the body– Include receptors for

• Touch

• Pressure

• Pain

• Temperature

• Special sense receptors

Classification by Location Internoceptors or visceroceptors

– Respond to stimuli arising from within the body organs, internal viscera and blood vessels

– Monitor a variety of internal stimuli• Feel pain

• Discomfort

• Hunger

• Thirst

– However, we are usually unaware of the working of our internal visceroceptors

Classification by Location Proprioceptors

– Respond to internal stimuli in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles

• Monitor the degree of stretch of the tissue in which they are located

Classification by Structure Based on structural complexity there

simple and complex receptors– Simple are equivalent to modified dendritic

endings of sensory neurons• Found in skin, mucous membranes, muscles and

connective tissue– Monitor general sensory information

– Complex receptors are associated with the special senses

• Located in the special sensory organs– Specific sensory information (sight, hearing, etc)

General Sense Organs The widely distributed general sensory

receptors are involved in – Tactile sensation

• Touch / pressure / stretch / vibration

– Temperature monitoring• Heat / cold

– Pain– Muscle stretch

Free Dendritic Endings Simple, widely dispersed receptors

present everywhere in the body Particularly abundant in epithelia and

connective tissue Most are unmyelinated

Free Nerve Endings Free nerve endings

have small knoblike swellings

Respond to pain, temperature, and possible mechanical pressure

Merkel Cells Merkel cells attach

to the deeper layers of the skin epidermis

Functions as light touch receptors

Root Hair Plexuses Root hair plexuses

are free dendritic endings that entwine the hair follicles

They function as light touch receptors that detect slight bending of hairs

Root HairPlexus

Encapsulated Dendritic Endings Consist of one or more terminal fibers of

sensory neurons enclosed in a connective tissue capsule

Virtually all are mechanoreceptors Vary greatly in shape, size, and

distribution in the body

Messner’s Corpuscles Small receptors in

which a few spiraling dendrites are surrounded by Schwann’s cells and then by an egg shaped sheath of connective tissue

Found in the dermal papillae of sensitive areas of the body

Light touch, tactile receptor

Krause End Bulbs A variation of

Meissner’s corpuscles

Receptors for discriminative touch

Most abundant in mucous membranes

Pacinian Corpuscles Pacinian corpuscles

are scattered deep in the dermis, and in the subcutaneous tissue under the skin

It contains a single dendrite surrounded by up to 60 layers of Schwann cells and is enclosed by connect- ive tissue

Respond to pressure Rapidly adapting

Ruffini’s Corpuscles Found in the dermis

subcutaneous tissue, and joint capsules

Contains a spray of dendritic endings enclosed by a flattened capsule

Monitor deep and continuous pressure

Muscle Spindles Proprioceptors found

throughout skeletal muscle

Each muscle spindle consists of a bundle of modified skeletal muscle fibers called intrafusal fibers enclosed in a connective tissue capsule

The infrafusal fibers detect when a muscle is stretched and initiate a reflex to resist the stretch

Golgi Tendon Organs Proprioceptors located in

tendons, close to the point of skeletal muscle insertion

They consist of small bundles of tendon fibers enclosed in a layered capsule with dendrites coiling around the fibers

When GTO activate, the contracting muscle is inhibited, which causes it to relax

Joint Kinesthetic Receptors Proprioceptors that monitor stretch in

the capsules of synovial joints Includes Pacinian and Ruffini corpuscles,

free dendritic endings and receptors resembling Golgi tendon organs

Joint position and motion

Sensory Receptor Potentials Sensory stimuli reaches us as many

different forms of energy Sensory receptors associated with sensory

neurons convert the energy of the stimulus into electrical energy

The energy changes the action potential of the receptor

Action potential are generated as long as the stimulus is applied

Stimulus strength is determined by the frequency of impulse transmission

Adaptation of Sensory Receptors Adaptation occurs in certain sensory

receptors when they are subjected to an unchanging stimulus

As a result, the receptor potentials decline in frequency or stop

Some receptors adapt quickly (pressure, touch and smell)

Nocioceptors and proprioceptors adapt slowly or not at all as they serve a protective function

Nerves and Associated Ganglia A nerve is a cordlike organ

that is part of the peripheral nervous system

Every nerve consists of parallel bundles of peripheral axons enclosed by successive wrappings of connective tissue

Nerves and Associated Ganglia Within a nerve, each axon is

surrounded by a delicate layer of loose connective tissue called endoneurium

The endoneurium layer also encloses the fiber’s associated myelin sheath

Nerves and Associated Ganglia Groups of fibers are bound

into bundles or fascicles by a courser connective tissue wrapping called the perineurium

All the fascicles are enclosed by a tough fibrous sheath called the epineurium to form a nerve

Nerves and Associated Ganglia Neuron are actually only a

small fraction of the nerve The balance is myelin, the

protective connective tissue wrappings, blood vessels, and lymphatic vessels

Nerves and Associated Ganglia Nerves are classified according to the

direction in which they transmit impulses– Nerves containing both sensory and motor

fibers are called mixed nerves– Nerves that carry impulses toward the

CNS only as sensory (afferent) nerves– Nerves that carry impulses only away from

the CNS are motor (efferent) nerves Most nerves are mixed as purely sensory

or motor nerves are extremely rare

Nerves and Associated Ganglia Since mixed nerves often carry both

somatic and autonomic (visceral) nervous system fibers, the fibers within them may be classified further according to the region they innervate as– Somatic afferent– Somatic efferent– Visceral afferent– Visceral efferent

Nerves and Associated Ganglia Peripheral nerves are generally classified

on whether they arise from the brain or spinal cord as– Cranial nerves / brain and brain stem– Spinal nerves / spinal cord

Ganglia are collections of neuron cell bodies associated with nerves in the PNS– Ganglia associated with afferent nerve fibers

contain cell bodies of sensory neurons– Ganglia associated with efferent nerve fibers

contain cell bodies of autonomic neurons, as well as a variety of integrative neurons

Regeneration of Nerve Fibers Damage to nervous tissue is serious

because mature neurons do not divide If the damage is severe or close to the cell

body, the entire neuron may die, and other neurons that are normally stimulated by its axon may die as well

However, in certain cases, cut or compressed axons on peripheral nerves can regenerate successfully

Regeneration of Nerve Fibers Almost immediately

after a peripheral axon has been cut, the separated ends seal themselves off and swell as substances being transported along the axon begin to accumulate

Regeneration of Nerve Fibers Wallerian degeneration

spreads distally from the injury site completely fragmenting the axon

Regeneration of Nerve Fibers Macrophages that

migrate into the trauma zone from adjacent tissues, phagocytize the disintegrating myelin and axonal debris

Generally, the entire axon distal to the injury degrades within a week

However, the nucleus and neurilemma remain intact with the endoneurium

Regeneration of Nerve Fibers Schwann cells then

proliferate and migrate to the injury site

They release growth factors that encourage axon growth

Additionally, they form cellular cords that guide the regenerating axon to their original contacts

Regeneration of Nerve Fibers The same Schwann cells

then protect, support, and remyelinate the regenerating axons

Regeneration of Nerve Fibers Axons regenerate at a rate of 1 to 5 mm a

day The greater the distance between the

severed nerve endings, the greater the time for regeneration

Greater distances also lessen the chance of successful regeneration because adjacent tissues often block growth by protruding into larger gaps

Regeneration of Nerve Fibers CNS nerve fibers never regenerate under

normal circumstances Brain and spinal cord damage is

considered as irreversible The difference in regenerative capacity is

largely due to the support cells of the CNS Macrophage invasion in the CNS is much

slower than in the PNS Oligodendrocytes surrounding the

damaged axon die and thus cannot guide axon regeneration and growth

Cranial Nerves Twelve pair of cranial nerves are

associated with the brain and pass through various foramina of the skull

The first two attach to the forebrain, while the rest originate from the brain stem

Cranial nerves serve only the head and neck structures with the exception of the vagus nerves

In most cases, the nerve are named for the structures they serve or their primary functions

Location of Cranial Nerves

The cranial nerves as they emerge from the brain and spinal cord

Cranial Nerves The cranial nerves

are numbered from the most rostal to the most caudal

Some cranial nerves are exclusively sensory and others are exclusively motor and still others are mixed

The differences are due to the functions the nerves serve

Olfactory Nerve: I Fibers arise from

olfactory epithelium of nasal cavity

Synapse with olfactory bulb which extends as olfactory tract

Purely sensory; carries afferent impulses for sense of smell

Optic Nerves: II Fibers arise from

retina to form sensory nerve

Converge to form optic chiasma with partial crossover

Enter thalamus and synapse there

Thalamic fibers runs as optic radiation to visual cortex for interpretation

Oculomotor Nerve: III Fibers extend

from midbrain to eye

Mixed nerve that contains a few proprio- ceptors, but is chiefly motor

Supplies four of six extrinsic muscles that move the eye in its orbit

Trochlear Nerves: IV Fibers emerge

from midbrain to enter orbits

Mixed nerve; primarily motor

Innervates extrinsic muscles in the orbit

Trigeninal Nerves: V Extends from

pons to face Forms three

divisions– Ophthalmic

– Maxillary

– Mandibular Mixed nerve

innervating the face, forehead and muscle of mastication

Abducens Nerves: VI Fibers leave

inferior pons and enter orbit to run to eye

Mixed nerve; but primarily motor

This nerve controls the extrinsic eye muscles that abduct the eye (turn it laterally)

Facial Nerves: VII Fibers issue from the

pons, enters temporal bone, emerges from inner ear cavity to run to the lateral aspect of the face

Mixed nerve with five major branches– Temporal, zygomatic,

buccal, mandibular, and cervical

Innervates muscles of facial expression

Vestibulocochlear Nerves: VIII Fibers arise

from hearing and equilibrum apparatus to enter brain stem at pons medulla border

Purely sensory This nerve

provides for hearing and balance

Glossopharyngeal Fibers emerge

from medulla and run to throat

Mixed nerve provide motor control of tongue and pharynx

Sensory fibers conduct taste and general sensory info

Vagus Nerves: X Fibers emerge from

medulla and descend into neck, thorax and abdomen

Mixed nerve; fibers are parasympathetic except those serving muscles of pharynx and larynx

Parasympathetic fibers supply heart, lungs, abdominal viscera

Accessary Nerves: XI Unique in that it

is formed by branches of cranial and spinal nerves

Mixed nerve, but primarily motor in function supplying fibers to innervate the trapezius and sternocledio- mastoid

Hypoglossal Nerves: XII Fibers arise

from the medulla to travel to tongue

Mixed nerve but primarily motor

Innervates muscles that move the tongue

Distribution of Spinal Nerves There are 31 pairs of

spinal nerves each containing thousands of nerve fibers

All arise from the spinal cord and supply all parts of the body except the head and neck

All are mixed nerves Spinal nerves are named

according to where they exit the spinal cord

Distribution of Spinal Nerves The distribution of

spinal nerves – Cervical (8)

– Thoracic (12)

– Lumbar (5)

– Sacral (5)

– Coccyx (1) C1-C7 nerves exits

above the vertebrae C8 and below nerves exit

below the vertebrae

Innervation of the Back Each

spinal nerve connects to the spinal cord by two roots

Each root forms from a series of rootlets

Innervation of the Back

Ventral roots contain motor (efferent) fibers Dorsal roots contain sensory (afferent) fibers

Innervation of the Back

The spinal root pass laterally from the cord, and unite just distal to the dorsal root ganglion, to form a spinal nerve before emerging from the vertebral column

Dorsal & ventral rami A spinal nerve is

short (1-2 cm) because it divides almost immediately after emerging to form a small dorsal ramus, a larger ventral ramus, and a tiny meningeal branch

Dorsal & ventral rami In the thoracic

region there is also a rami communicantes joined to the base of the ventral rami

These rami contain auto-nomic (visceral) nerve fibers

Rami are both motor & sensory

Innervation of Body Regions Except for T2-T12, all

ventral rami branch and join one another lateral to the vertebral column forming nerve plexuses– Cervical

– Brachial

– Lumbar

– Sacral Note that only ventral

roots form plexuses

Innervation of Body Regions Within plexuses the different ventral rami

crisscross each other and become redistributed so that– Each branch of the plexus contains fibers from

several different spinal nerves– Fibers from each ventral ramus travel to the body

periphery via several different routes or branches Thus, each muscle in a limb receives its nerve

supply from more than one spinal nerve Damage to a single root cannot completely

paralyze any limb muscle

Innervation of the Back The innervation

of the posterior body trunk is by the dorsal rami

Each dorsal ramus innervates a narrow strip of muscle and skin

Pattern follows a neat, segmented pattern in line with emergence from spinal cord

Innervation of Thorax & Abdomem Only in the thorax

are the ventral rami arranged in a simple segmental pattern corresponding to that of the dorsal rami

Ventral rami of T1-T12 course anteriorly deep to each rib as intercostal nerves supplying the inter- costal muscles & most of abdominal wall

Cervical Plexus and the Neck The cervical plexus

lies deep under the sternocleidomastoid muscle

Plexus is formed by the ventral rami of the first 4 cervical nerves

Most branches are cutaneous nerve that transmit sensory impulses from the skin

Cevical Plexus and the Neck The single most

important nerve of the plexus is the phrenic nerve

It receives its fibers from C3 - C4

The phrenic nerve runs inferiorly through the thorax and supplies motor and sensory fibers to diaphram

Breathing

Brachial Plexus and Upper Limb The large important brachial plexus is

situated partly in the neck and partly in the axilla

It gives rise to virtually all the nerves that innervate the upper limb

The brachial plexus is very complex and is often referred to as the anatomy student’s nightmare

Brachial Plexus and Upper Limb

The plexus is formed by the intermixing of the ventral rami of the four inferior cervical nerves C5-C8 and most of T1

It often receives fibers from C4 as well as T2

Brachial Plexus and Upper Limb

The terms used to describe the plexus from medial to lateral are:– Roots / Trunks / Divisions / Cords

Brachial Plexus and Upper Limb

The five roots (rami C5-T1) of the brachial plexus lie deep to the sternocleidomastoid muscle

At the lateral border of that muscle, these nerves unite to form the upper, middle, and lower trunks

Brachial Plexus and Upper Limb

Each of the three trunks divides almost immediately to form anterior and posterior divisions

The divisions generally reflect which fibers will serve the front or back of the limb

Brachial Plexus and Upper Limb

The divisions give rise to three large fiber bundles called the lateral, medial, and posterior cords

All along the divisions and cords small nerve branch off to supply muscles of the shoulder and arm

Brachial Plexus and Upper Limb

A summary of the differentiation of the brachial plexus reveals how it gives rise to common nerves

The five peripheral nerves that emerge are the main nerves of the upper limb

Brachial Plexus and Upper Limb The main nerves

that emerge from the brachial plexus are– Axillary

– Musculotaneous

– Median

– Ulnar

– Radial

Roots

Axillary Nerve The axillary nerve

branches off the posterior cord and runs posterior to the surgical neck of the humerous

It innervates the deltoid and teres minor muscles and the skin and joint capsule of the shoulder

Axillary Nerve Muscular branches

– Deltoid – Teres minor

Cutaneous branches– Some of the skin of shoulder region

Musculocutaneous Nerve Musculocutaneous

nerve is the major end of the lateral cord, courses inferiorly within the anterior arm, supplying motor fibers to the elbow flexors

Beyond the elbow it provides for cutaneous sensation of lateral forearm

Musculocutaneous Nerve Muscular branches

– Biceps brachii– Brachialis– Coracobrachialis

Cutaneous branches– Skin on anterolateral aspect of forearm

Median Nerve The median nerve

descends through the arm without branching

In the anterior forearm, it gives off branches to the skin and most of the flexor muscles

It innervates the five intrinsic muscles of the lateral palm

Median Nerve Muscular branches

– Palmaris longus– Flexor carpi radialis– Flexor digitorium superficialis– Flexor pollicus longus– Flexor digitorium profundus– Pronator– Intrinsic muscles of fingers 2 and 3

Cutaneous branches– Skin of lateral two-thirds of hand, palm side

and dorsum of fingers 2 and 3

Ulnar Nerve The ulnar nerve

branches off the medial cord of the plexus

It descends along the medial aspect of the arm toward the elbow, swings behind the medial epicondyle, then follows the ulna along the forearm

Innervates most intrinsic hand muscles

Ulnar Nerve Muscular branches

– Flexor carpi ulnaris– Flexor digitorium profundus (medial half)– Intrinsic muscles of the hand

Cutaneous branches– Skin of medial third of hand, both anterior

and posterior aspects

Radial Nerve The radial nerve is a

continuation of the posterior cord

The nerve wraps around humerous, runs anteriorly by the lateral epicondyle at the elbow

Divides into a super- ficial branch that follows the radius and a deep branch that runs posteriorly

Radial Nerve Muscular branches

– Triceps brachii– Anconeus– Supinator– Brachioradialis– Extensor capri radialis– Extensor carpi brevis– Extensor carpi ulnaris– Muscles that extend fingers

Cutaneous branches– Skin of posterior surface of entire limb

Lumbosacral Plexus The sacral and lumbar plexuses overlap

substantially Since many of the fibers of the lumbar

plexus contribute to the sacral plexus via the lumbosacral trunk, the two plexuses are often referred to as the lumbosacral plexus

Although the lumbosacral plexus mainly serves the lower limb, it also sends some branches to the abdomen, pelvis and buttocks

Lumbar Plexus and Lower Limb The lumbar plexus

arises from the first four spinal nerves and lies within the psoas major muscle

Its proximal branches innervate parts of the abdominal wall and iliopsoas

Major branches of the plexus descend to innervate the medial and anterior thigh

Femoral Nerve The femoral nerve, the

largest of the lumbar plexus, runs deep to the inguinal ligament to enter the thigh and then divides into a number of large branches

The motor branches innervate the anterior thigh muscles while the cutaneous branch serves anterior thigh

Femoral Nerve Muscular branch

– Quadiceps group• Rectus femoris, vastus laterialis, vastus medialis,

vastus intermedius

– Sartorius– Pertineus– Iliacus

Cutaneous branches– Anterior femoral cutaneous

• Skin of anterior and medial thigh

– Saphenous• Skin of medial leg and foot, hip and knee joints

Obturator Nerve The obturator nerve

enters the medial thigh via the obturator foramen and innervates the adductor muscles

Obturator Nerve Muscular branch

– Adductor magnus (part)– Adductor longus– Adductor brevis– Gracilis– Obturator externus

Cutaneous branches– Sensory for skin of medial thigh and hip and

knee joints

Sacral Plexus and Lower Limb

The sacral plexus arises from spinal nerves L4-S4 and lies immediately caudal to the lumbar plexus

The sacral plexus has about a dozen named nerves

Sacral Plexus and Lower Limb

Half the nerves serve muscles of the buttocks and lower limb while others innervate pelvic structures and the perineum

Sciatic Nerve The sciatic nerve is the

thickest and longest nerve in the body

The sciatic nerve leaves the pelvis via the greater sciatic notch

Actually the tibial and common peroneal nerves

It courses deep to the gluteus maximus muscle

It gives off branches to the hamstrings and adductor magnus

Sciatic Nerve Muscular branch

– Bicep femoris– Semitendinous– Semimembranous– Adductor magnus

Cutaneous branches– Posterior thigh

Tibial Nerve The tibial nerve through

the popliteal fossa and supplies the posterior compartment muscles of the leg and the skin of the posterior calf and sole of foot

Important branches of the tibial nerve are the sural, which serves the skin of the posterior leg and the plantar nerves which serve the foot

Tibial Nerve Muscular branch

– Triceps surae– Tibialis posterior– Popliteus– Flexor digitorum longus– Flexor hallicus longus – Intrinsic muscle of the foot

Cutaneous branches– Skin of the posterior surface of the leg and

the sole of the foot

Common Peroneal Nerve The common peroneal

nerve descends the leg, wraps around the head of the fibula, and then divides into superficial and deep branches

These branches innervate the knee joint, the skin of the lateral calf and dorsum of the foot and the muscles of the anterolateral leg

Common Peroneal Nerve Muscular branch

– Biceps foemoris (short head)– Peroneal muscles (longus, brevis, tertius)– Tibialis anterior– Extensor hallicus longus– Extensor digitorum longus– Extensor digitorum brevis

Cutaneous branches– Skin of the anterior surface of leg and

dorsum of foot

Sarcal Plexus Nerves Superior and inferior gluteal

– Innervate the gluteal muscles and tensor fasciae latae

Pudendal– Innervates the muscles of the skin of the

perineum– Mediates the act of erection– Voluntary control of urination– External anal sphinter

Innervation of the Joints Hilton’s law “. . . any nerve serving a

muscle producing movement at a joint also innervates the joint itself and the skin over the joint”

Innervation of Skin: Desmatomes The are of skin that is innervated by the

cutaneous branch of a spinal nerve is called a dermatome

All spinal nerves except C1 participate in dermatomes

Adjacent dermatomes on the body trunk are fairly uniform in width, almost horizontal, and in direct line with their spinal nerves

Innervation of Skin: Desmatomes The skin of the

upper limbs is supplied by C5-T1

The ventral rami of the lumbar nerves supply most of the anterior muscles of the thighs and legs

Innervation of Skin: Desmatomes The ventral rami of

sacral nerves serve most of the posterior surfaces of the lower limbs

Reflex Activity Many of the body’s control systems

belong to the general category of stimulus response consequences known as reflexes

A reflex is a rapid, predictable motor response to a stimulus

It is unlearned, unpremeditated, and involuntary

Basic reflexes may be considered to be built into our neural anatomy

Reflex Activity In addition to these basic, inborn types of

reflexes, there are many learned, or acquired reflexes that result from practice of repetition

There is no clear cut distinction between basic and learned reflexes

Components of a Reflex Arc

All reflex arcs have five essential components– The receptor

– The sensory neuron, afferent impulses to CNS

– Integration center• Monosynaptic (one neuron)

• Polysynaptic (more than one chain of neurons)

– The motor neuron, efferent impulses to effector organ

– The effector, the muscle spindle or gland

Components of a Reflex Arc Reflexes are classified functionally as

– Somatic reflexes • (activate skeletal muscle)

– Visceral reflexes (autonomic reflexes) • (activate smooth, cardiac muscle or visceral organs

Spinal Reflexes Somatic reflexes mediated by the spinal

cord are called spinal reflexes These reflexes may occur without the

involvement of higher brain centers Other reflexes may require the activity of

the brain for their successful completion Additionally, the brain is “advised” of

most types of spinal cord reflex activity and can facilitate or inhibit them

Stretch and Deep Tendon Reflexes If skeletal muscles are to perform

normally – The brain must be continually informed of

the current state of the muscles• Depends on information from muscle spindles

and Golgi tendon organs

– The muscles must exhibit healthy tone• Depends on stretch reflexes initiated by the

muscle spindles

These processes are important to normal skeletal muscle function, posture and locomotion

Anatomy of Muscle Spindle Each spindle

consists of 3-10 infrafusal muscle fibers enclosed in a connective tissue capsule

These fibers are less than one quarter of the size of extrafusal muscle fibers (effector fibers)

Anatomy of Muscle Spindle The central

region of the intrafusal fibers which lack myofilaments and are noncontractile, serving as the receptive surface of the spindle

Anatomy of Muscle Spindle Intrafusal fibers

are wrapped by two types of afferent endings that send sensory inputs to the CNS

Primary sensory endings – Type Ia fibers

Secondary sensory endings– Type II fibers

Anatomy of Muscle Spindle Primary sensory

endings – Type Ia fibers

Stimulated by both the rate and amount of stretch

Innervate the center of the spindle

Anatomy of Muscle Spindle Secondary

sensory endings – Type II fibers

Associated with the ends of the spindle and are stimulated only by degree of stretch

Anatomy of Muscle Spindle The contractile

region of the intrafusal muscle fibers are limited to their ends as only these areas contain actin and myosin filaments

These regions are innervated by gamma () efferent fibers

The Stretch Reflex Exciting a muscle spindle occurs in two

ways– Applying a force that lengthens the entire

muscle (external stretch - either by weight or by the action of an antagonist)

– Activing the motor neurons that stimulate the distal ends of the intrafusal fibers to contact, thus stretching the mid-portion of the spindle (internal stretch)

The Stretch Reflex Whatever the

stimulus, when the spindles are activated their associated sensory neurons transmit impulses at a higher frequency to the spinal cord

The Stretch Reflex

At spinal cord sensory neurons synapse directly (mono- synaptically) with the motor neurons which rapidly excite the extrafusal muscle fibers of stretched muscle

The Stretch Reflex

The reflexive muscle contraction that follows (an example of serial processing) resists further stretching of the muscle

The Stretch Reflex

Branches of the afferent fibers also synapse with inter- neurons that inhibit motor neurons controlling the antagonistic muscles inhibiting their contraction

The Stretch Reflex Inhibition of the antagonistic muscles is

called reciprocal inhibition In essence, the stretch stimulus causes the

antagonists to relax so that they cannot resist the shortening of the “stretched” muscle caused by the main reflex arc

While this spinal reflex is occurring, impulses providing information on muscle length and the velocity of shortening are also being relayed to the brain

The Stretch Reflex The stretch reflex is most important in

large extensor muscles which sustain upright posture

Contractions of the postural muscles of the spine are almost continuously regulated by stretch reflexes initiated first on one side of the spine and then the other

The Deep Tendon Reflex Deep tendon reflexes cause muscle

relaxation and lengthening in response to the muscle’s contraction

This effect is opposite of those elicited by stretch reflexes

The Deep Tendon Reflex When muscle tension

increases moderately during muscle contraction or passive stretching, GTO receptors are activated and afferent impulses are transmitted to the spinal cord

The Deep Tendon Reflex Upon reaching the

spinal cord, informa- tion is sent to the cerebellum, where it is used to adjust muscle tension

Simultaneously, motor neurons in the spinal cord supplying the contracting muscle are imhibited and antagonistic muscle are activated (activation)

The Deep Tendon Reflex Golgi tendon organs help ensure smooth

onset and termination of muscle contraction and are particularly important in activities involving rapid switching between flexion and extension such as in running

The Flexor Withdrawal Reflex

The flexor, or withdrawal reflex is initiated by a painful stimulus (actual or perceived) and causes automatic withdrawal of the threatened body part from the stimulus

The Crossed Extensor Reflex

The crossed extensor reflex is a complex spinal reflex consisting of an ipsilateral withdrawal reflex and a contralateral extensor reflex

The Crossed Extensor Reflex

The reflex is can occur when you step on a sharp object There is a rapid lifting of the affected foot, while the

contralateral response activates the extensor muscles of the opposite leg to support the weight shifted to it

Superficial Reflexes Superficial reflexes are elicited by

cutaneous stimulation These reflexes are dependent upon

functional upper motor pathways and spinal cord reflex arcs

Babinski reflex

End of Chapter