The Peripheral Nervous System

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

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

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

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 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

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

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

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

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

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

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

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

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

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

These corpuscles are found in the dermal papillae beneath the epidermis These corpuscles are rapidly adapting receptors for fine, light touch

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?)

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

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 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

A single neuromuscular is associated with each muscle fiber These junctions are similar to the synapses between neurons

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

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

Although neuromuscular junctions resemble synapses they have several unique features

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

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

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

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

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

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

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

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 proprioceptors, 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

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

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

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 intercostal 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

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

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

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

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

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

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

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 Cutaneous 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 Cutaneous 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 Cutaneous 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 superficial branch that follows the radius and a deep branch that runs posteriorly

Radial Nerve Muscular branches Cutaneous 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 Cutaneous branches 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 Cutaneous branches
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 Cutaneous branches 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 Cutaneous branches 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 Cutaneous branches
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 Pudendal
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

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

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)

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

Primary sensory endings Type Ia fibers Stimulated by both the rate and amount of stretch Innervate the center of the spindle

Secondary sensory endings Type II fibers Associated with the ends of the spindle and are stimulated only by degree of stretch

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, information 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

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

Wallerian degeneration spreads distally from the injury site completely fragmenting the axon

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

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

The same Schwann cells then protect, support, and remyelinate the regenerating axons

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

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

The Peripheral Nervous System

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The Peripheral Nervous System

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