The Peripheral Nervous System

Chapter 14

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

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 Neurons 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 are 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

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

Peripheral Sensory Receptors Peripheral sensory receptors are

structures that pick up sensory stimuli and then initiate signals in the sensory axons

Most receptors fit into two main categories;– Dendritic endings of sensory neurons– Complete receptor cells

Peripheral Sensory Receptors

Dendritic endings of sensory neurons monitor most types of general sensory information (touch, pain, pressure, temperature, and proprioception)

Peripheral Sensory Receptors Complete receptor cells are specialized

epithelial cells or small neurons that transfer sensory information to sensory neurons

Specialized receptor cells monitor most types of special sensory information (taste, vision, hearing, and equilibrium)

Sensory Receptors Sensory receptors are classified by

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

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 internal viscera and body organs,

– Internoceptors monitor a variety of internal stimuli

• Changes in chemical concentration• Taste stimuli• The stretching of tissues• Temperature

– Their activation causes us to feel visceral pain, nausea, hunger, or fullness

Classification by Location Proprioceptors

– Located in the musculoskeletal organs such as skeletal muscles, tendons, joints and ligaments

– Proprioceptors monitor the degree of stretch of these locomotor organs and send input to the CNS

Classification by Stimulus Detected Mechanoreceptors

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

• Touch

• Pressure

• Vibration

• Stretch

• Itch

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 over-stimulation of any of

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

Classification by Structure General sensory receptors are divided

into two broad groups– Free (naked) endings– Encapsulated dendritic endings

It should be pointed out that there is no one receptor - one function relationship

Rather, one receptor type can respond to several different kinds of stimuli, and different receptor types can respond to similar stimuli

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

Free Dendritic Endings Free nerve endings

have small knoblike swellings

Chiefly respond to pain, temperature, and possible mechanical pressure caused by tissue movement

Free Dendritic Endings The receptors are

simple and widely dispersed everywhere in the body

Particularly abundant in epithelia and connective tissue underlying epithelial tissue

Merkel Discs Certain free

dendritic endings contribute to Merkel discs

These discs lie in the epidermis of the skin

Merkel Cells Merkel cells attach

to the basal layer of the skin epidermis

Each Merkel disc consists of a disc- shaped epithelial cell innervated by a dendrite

Functions as light touch receptors

Merkel Discs Merkel cells seem to be slowly adapting

receptors for light touch Slowly adapting means that they

continue to respond to stimuli present and send out action potentials even long after a period of continual stimulation

Root Hair Plexuses Root hair plexuses

are free dendritic endings that wrap around hair follicles

These are receptors for light touch that monitor the bending of hairs

Root Hair Plexuses Root hair plexuses

are rapidly adapting This means that the

sensation disappears quickly even if the stimulus is maintained

The landing of a mosquito is mediated by root hair plexuses Root Hair

Plexus

Encapsulated Dendritic Endings All encapsulated dendritic endings

consist of one or more end fibers of sensory neurons enclosed in a capsule of connective tissue

All seem to be mechanoreceptors, and their capsules serve to either amplify the stimulus or to filter out the wrong types of stimuli

Encapsulated Dendritic Endings Encapsulated receptors vary widely in

shape, size, and distribution in the body The main types are

– Meissner’s corpuscles– Krause’s end bulbs– Pacinian corpuscles– Ruffini’s corpuscles – Proprioceptors

Meissner’s Corpuscles In a Meissner’s

corpuscle (tactile corpuscle) a few spiraling dendrites are surrounded by Schwann cells, which in turn are surrounded by an egg-shaped capsule of connective tissue

Meissner’s Corpuscles These corpuscles

are found in the dermal papillae beneath the epidermis

These corpuscles are rapidly adapting receptors for fine, light touch

Meissner’s Corpuscles Meissner’s corpuscles occur in sensitive

and hairless areas of the skin, such as the soles of the feet, palms, fingertips, nipples, and lips

Apparently, Meissner’s corpuscles perform the same “light touch” function in hairless skill that root hair plexuses perform in hairy skin

Krause’s End Bulbs Krause’s End Bulbs

are a type of Meissner’s corpuscle for fine touch

Krause’s end bulbs occur in mucous membranes in the lining of the mouth and the conjunctiva of the eye

Pacinian Corpuscle Pacinian corpuscle

are scattered throughout the deep connective tissues of the body

Occur in the hypodermis of the skin

Pacinian Corpuscles Pacinian corpuscles

contains a single dendrite surrounded by up to 60 layers of Schwann cells and is in turn enclosed by connective tissue

Respond to deep pressure

Rapidly adapting as they respond to only the initial pressure

Pacinian Corpuscles Pacinian corpuscles are rapidly adapting

receptors and are best suited to monitor vibrations which is an on-off stimulus

These corpuscles are large enough to be visible to the naked eye

Ruffini’s Corpuscle Ruffini’s corpuscle

are located in the dermis of the skin and joint capsules of the body

The corpuscle contains an array of dendritic endings enclosed in a thin flattened capsule

Ruffini’s Corpuscle Ruffini’s corpuscle

respond to pressure and touch

They adapt slowly and thus can monitor continuous pressure placed on the skin

Proprioceptors

Proprioceptors Virtually all proprioceptors are

encapsulated dendritic endings that monitor stretch in the locomotor organs

Proprioceptors include…– Muscle spindles– Golgi tendon organs– Joint kinesthetic receptors

Proprioceptors Muscle spindles

measure the changing length of a muscle as that muscle contracts and as it is stretched back to its original length

Muscle spindles are found throughout skeletal muscle

Proprioceptors An average muscle

contains some 50 to 100 muscle spindles, which are embedded in the perimysium between muscle fascicles

Muscle Spindles Structurally each muscle

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

Infrafusal fibers have fewer striations than do the ordinary muscle cells

Proprioceptors

The intrafusal fibers are innervated by the dendrites of several sensory neurons

Proprioceptors

Some of these sensory dendrites twirl around the middle of the middle of the intrafusal fibers as annulospiral sensory endings

Proprioceptors

Flower spray sensory endings supply the ends of the intrafusal fibers

Proprioceptors Muscles are stretched by the contraction

of antagonist muscles and also by the movements that occur when we lose our balance

The muscle spindles sense these changes and compensate for the stretch

Proprioceptors Muscle spindles sense changes in muscle

length by the simple fact that as the muscle is stretched the muscle spindle is also stretched

The stretching activates the sensory neurons that innervate the spindle, causing them to signal the spinal cord and brain

Proprioceptors

The CNS then activates spinal motor neurons called alpha efferent neurons that cause the entire muscle to generate contractile force and resist further stretching

Proprioceptors This response to stretching can take the

form of a monosynapatic spinal reflex that makes a rapid adjustment to prevent a fall

Alternatively, the stretch response can be controlled by the cerebellum, in which case it is involved in the regulation of muscle tone – The steady force generated by non-

contracting muscle to resist stretching

Proprioceptors

Also innervating the intrafusal fibers of the muscle spindle are the axons of spinal motor neurons call gamma efferent fibers

Proprioceptors

Gamma efferent fibers let the brain preset the sensitivity of the spindle to stretch

Proprioceptors When the brain signals gamma motor

neurons to fire, the intrafusal muscle fibers contract and become tense so that very little stretch is needed to stimulate the sensory dendrites

Making the spindles highly sensitive to stretch is advantageous when balance reflexes have little margin for error

Golgi Tendon Organs GTO are proprioceptors

located in tendons, close to the skeletal muscle - tendon junction

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

Golgi Tendon Organs

When a contracting muscle pulls on its tendon, Golgi tendon organs are stimulated, and their sensory neurons send this information to the cerebellum

Golgi Tendon Organs

The receptors induce a spinal reflex that both relaxes the contracting muscle and activates its antagonist

Golgi Tendon Organs Relaxation reflex is important in motor

activities that involve the rapid alternation between flexion and extension such as in sprinting

Joint Kinesthetic Receptors These proprioceptors monitor stretch in

the synovial joints Specifically, they are sensory dendritic

endings within the joint capsules Four types of receptors are present

within each joint capsule– Pacinian corpuscles– Ruffini corpuscles– Free dendritic endings– Golgi tendon organs (kinda?)

Joint Kinesthetic Receptors Pacinian corpuscles are rapidly adapting

stretch receptors that are ideal for measuring acceleration and rapid movement of the joints

Ruffini corpuscles are slowly adapting stretch receptors that are ideal for measuring the positions of non-moving joints and the stretch of joints that undergo slow, sustained movements

Joint Kinesthetic Receptors Free dendritic endings in joint may serve

as pain receptors Receptors resembling Golgi tendon

organs have been identified in joints but their function is not yet known

Joint Kinesthetic Receptors Joint receptors, like the other two classes

of proprioceptors, send information on body movements to the cerebellum and cerebrum, as well as to spinal reflex arcs

Innervation of Skeletal Muscle

Motor axons innervate skeletal muscle fibers at junctions called neuromuscular junctions, or motor end plates

Innervation of Skeletal Muscle

A single neuromuscular is associated with each muscle fiber

These junctions are similar to the synapses between neurons

Innervation of Skeletal Muscle

The neural part of the junction is a cluster of typical axon terminals separated from the plasma membrane (sarcolemma) of the underlying muscle cell by a synaptic cleft

Innervation of Skeletal Muscle As in typical synapses, the axon terminals

contain synaptic vesicles that release a neurotransmitter when a nerve impulse reaches the terminals

The neurotransmitter (acetylcholine) diffuses across the synaptic cleft and binds to receptor molecules on the sarcolemma, where it induces an impulse that signals the muscle cell to contract

Innervation of Skeletal Muscle

Although neuromuscular junctions resemble synapses they have several unique features

Innervation of Skeletal Muscle

Each axon terminal lies in a trough-like depression of the sarcolemma, which in turn shows groove-like invaginations

Innervation of Skeletal Muscle

The invaginations and the synaptic cleft contain a basal lamina that does not appear in synapses between neurons

Innervation of Skeletal Muscle This basal lamina contains the enzyme

acetylcholinesterase which breaks down acetylcholine immediately after the neurotransmitter signals a single contraction

This assures that each nerve impulse in the motor axon produces just one twitch of the muscle cell, preventing any undersireable additional twitches that would occur acetylcholine lingered in the synaptic cleft

Innervation of Skeletal Muscle Each motor axon

branches to innervate a number of muscle fibers within a skeletal muscle

A motor neuron and all the muscle fibers it innervates is called a motor unit

When a motor unit fires, all the skeletal muscle cells in the motor unit contract together

Innervation of Skeletal Muscle Although the average number of muscle

fibers in a motor unit is 150, a motor unit may contain as many as several hundred fibers or as few as four muscle fibers

Muscles that require very fine control, such as the muscles moving the fingers and eyes have few muscle fibers per motor unit, whereas weight-bearing muscles whose movements are less precise have many muscle fibers per unit

Innervation of Skeletal Muscle The muscle fibers of a single motor unit

are not clustered together but spread throughout the muscle

As a result, stimulation of a single motor unit causes a weak contraction of the entire muscle

Innervation of Visceral Muscle The contacts between visceral motor

endings and the visceral effectors are much simpler than the elaborate neuromuscular junctions present on skeletal muscle

Near the smooth muscle of gland cells it innervates, a visceral motor axon swells into a row of knobs (varicosities) resembling the beads on a necklace

Innervation of Visceral Muscle Varicosities are the presynaptic terminals

which contain synaptic vesicles filled with neruotransmitter

Some of the axon terminals form shallow indentations on the membrane of the effector cell, but many axon terminals remain a considerable distance from any cell

Innervation of Visceral Muscle Because it takes time for neurotransmitters

to diffuse across these wide synaptic clefts, visceral motor responses tend to be slower that somatic motor reflexes

Innervation of Cardiac Muscle The motor innervation of cardiac muscle

cells resembles that of smooth muscle fibers and glands

However, the axon terminals are of a uniform diameter and do not include varicosities at the sites where they release their neurotransmitters

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) Note that C1 has nerves

that exit superior and inferior to the vertebrae to add to the total of 8 cervical nerves

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

Cervical 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 diaphragm

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 or 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

End of Chapter

Chapter 14

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

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)

End of Chapter

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

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 potentials are generated as long as the stimulus is applied

Stimulus strength is determined by the frequency of impulse transmission