The peripheral nervous system
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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

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Introduction
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


Introduction1
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


Introduction2
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
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 ganglia1
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 ganglia2
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 ganglia3
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 ganglia4
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 ganglia5
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 ganglia6
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

  • 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

  • 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 receptors1
Peripheral Sensory Receptors

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


Peripheral sensory receptors2
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 receptors1
Sensory Receptors

  • Sensory receptors are classified by

    • The type of stimulus they detect

    • Their location in the body

    • Their structure


Classification by location
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 location1
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 location2
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 location3
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
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 detected1
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 detected2
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
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 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 Dendritic Endings

  • Free nerve endings have small knoblike swellings

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


Free dendritic endings1
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
Merkel Discs

  • Certain free dendritic endings contribute to Merkel discs

  • These discs lie in the epidermis of the skin


Merkel cells
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 discs1
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

  • 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 plexuses1
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
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 endings1
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
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 corpuscles1
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 corpuscles2
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

  • 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

  • Pacinian corpuscle are scattered throughout the deep connective tissues of the body

  • Occur in the hypodermis of the skin


Pacinian corpuscles
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 corpuscles1
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

  • 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 corpuscle1
Ruffini’s Corpuscle

  • Ruffini’s corpuscle respond to pressure and touch

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



Proprioceptors1

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


Proprioceptors2
Proprioceptors endings that monitor stretch in the locomotor organs

  • 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


Proprioceptors3
Proprioceptors endings that monitor stretch in the locomotor organs

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


Muscle spindles
Muscle Spindles endings that monitor stretch in the locomotor organs

  • 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


Proprioceptors4
Proprioceptors endings that monitor stretch in the locomotor organs

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


Proprioceptors5
Proprioceptors endings that monitor stretch in the locomotor organs

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


Proprioceptors6
Proprioceptors endings that monitor stretch in the locomotor organs

  • Flower spray sensory endings supply the ends of the intrafusal fibers


Proprioceptors7
Proprioceptors endings that monitor stretch in the locomotor organs

  • 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


Proprioceptors8
Proprioceptors endings that monitor stretch in the locomotor organs

  • 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


Proprioceptors9
Proprioceptors endings that monitor stretch in the locomotor organs

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


Proprioceptors10
Proprioceptors endings that monitor stretch in the locomotor organs

  • 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


Proprioceptors11
Proprioceptors endings that monitor stretch in the locomotor organs

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


Proprioceptors12
Proprioceptors endings that monitor stretch in the locomotor organs

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


Proprioceptors13
Proprioceptors endings that monitor stretch in the locomotor organs

  • 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
Golgi Tendon Organs endings that monitor stretch in the locomotor 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 organs1
Golgi Tendon Organs endings that monitor stretch in the locomotor 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 organs2
Golgi Tendon Organs endings that monitor stretch in the locomotor organs

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


Golgi tendon organs3
Golgi Tendon Organs endings that monitor stretch in the locomotor organs

  • Relaxation reflex is important in motor activities that involve the rapid alternation between flexion and extension such as in sprinting


Joint kinesthetic receptors
Joint Kinesthetic Receptors endings that monitor stretch in the locomotor organs

  • 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 receptors1
Joint Kinesthetic Receptors endings that monitor stretch in the locomotor organs

  • 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 receptors2
Joint Kinesthetic Receptors endings that monitor stretch in the locomotor organs

  • 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 receptors3
Joint Kinesthetic Receptors endings that monitor stretch in the locomotor organs

  • 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
Innervation of Skeletal Muscle endings that monitor stretch in the locomotor organs

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


Innervation of skeletal muscle1
Innervation of Skeletal Muscle endings that monitor stretch in the locomotor organs

  • A single neuromuscular is associated with each muscle fiber

  • These junctions are similar to the synapses between neurons


Innervation of skeletal muscle2
Innervation of Skeletal Muscle endings that monitor stretch in the locomotor organs

  • 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 muscle3
Innervation of Skeletal Muscle endings that monitor stretch in the locomotor organs

  • 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 muscle4
Innervation of Skeletal Muscle endings that monitor stretch in the locomotor organs

  • Although neuromuscular junctions resemble synapses they have several unique features


Innervation of skeletal muscle5
Innervation of Skeletal Muscle endings that monitor stretch in the locomotor organs

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


Innervation of skeletal muscle6
Innervation of Skeletal Muscle endings that monitor stretch in the locomotor organs

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


Innervation of skeletal muscle7
Innervation of Skeletal Muscle endings that monitor stretch in the locomotor organs

  • 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 muscle8
Innervation of Skeletal Muscle endings that monitor stretch in the locomotor organs

  • 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 muscle9
Innervation of Skeletal Muscle endings that monitor stretch in the locomotor organs

  • 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 muscle10
Innervation of Skeletal Muscle endings that monitor stretch in the locomotor organs

  • 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
Innervation of Visceral Muscle endings that monitor stretch in the locomotor organs

  • 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 muscle1
Innervation of Visceral Muscle endings that monitor stretch in the locomotor organs

  • 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 muscle2
Innervation of Visceral Muscle endings that monitor stretch in the locomotor organs

  • 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
Innervation of Cardiac Muscle endings that monitor stretch in the locomotor organs

  • 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
Cranial Nerves endings that monitor stretch in the locomotor organs

  • 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
Location of Cranial Nerves endings that monitor stretch in the locomotor organs

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


Cranial nerves1
Cranial Nerves endings that monitor stretch in the locomotor organs

  • 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
Olfactory Nerve: I endings that monitor stretch in the locomotor organs

  • 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
Optic Nerves: II endings that monitor stretch in the locomotor organs

  • 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
Oculomotor Nerve: III endings that monitor stretch in the locomotor organs

  • 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
Trochlear Nerves: IV endings that monitor stretch in the locomotor organs

  • Fibers emerge from midbrain to enter orbits

  • Mixed nerve; primarily motor

  • Innervates extrinsic muscles in the orbit


Trigeninal nerves v
Trigeninal Nerves: V endings that monitor stretch in the locomotor organs

  • Extends from pons to face

  • Forms three divisions

    • Ophthalmic

    • Maxillary

    • Mandibular

  • Mixed nerve innervating the face, forehead and muscle of mastication


Abducens nerves vi
Abducens Nerves: VI endings that monitor stretch in the locomotor organs

  • 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
Facial Nerves: VII endings that monitor stretch in the locomotor organs

  • 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
Vestibulocochlear Nerves: VIII endings that monitor stretch in the locomotor organs

  • 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
Glossopharyngeal endings that monitor stretch in the locomotor organs

  • 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
Vagus Nerves: X endings that monitor stretch in the locomotor organs

  • 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
Accessary Nerves: XI endings that monitor stretch in the locomotor organs

  • 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
Hypoglossal Nerves: XII endings that monitor stretch in the locomotor organs

  • Fibers arise from the medulla to travel to tongue

  • Mixed nerve but primarily motor

  • Innervates muscles that move the tongue


Distribution of spinal nerves
Distribution of Spinal Nerves endings that monitor stretch in the locomotor organs

  • 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 nerves1
Distribution of Spinal Nerves endings that monitor stretch in the locomotor organs

  • 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
Innervation of the Back endings that monitor stretch in the locomotor organs

  • Each spinal nerve connects to the spinal cord by two roots

  • Each root forms from a series of rootlets


Innervation of the back1
Innervation of the Back endings that monitor stretch in the locomotor organs

  • Ventral roots contain motor (efferent) fibers

  • Dorsal roots contain sensory (afferent) fibers


Innervation of the back2
Innervation of the Back endings that monitor stretch in the locomotor organs

  • 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
Dorsal & ventral rami endings that monitor stretch in the locomotor organs

  • 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 rami1
Dorsal & ventral rami endings that monitor stretch in the locomotor organs

  • 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
Innervation of Body Regions endings that monitor stretch in the locomotor organs

  • 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 regions1
Innervation of Body Regions endings that monitor stretch in the locomotor organs

  • 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 back3
Innervation of the Back endings that monitor stretch in the locomotor organs

  • 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
Innervation of Thorax & Abdomem endings that monitor stretch in the locomotor organs

  • 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
Cervical Plexus and the Neck endings that monitor stretch in the locomotor organs

  • 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 neck1
Cervical Plexus and the Neck endings that monitor stretch in the locomotor organs

  • 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
Brachial Plexus and Upper Limb endings that monitor stretch in the locomotor organs

  • 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 limb1
Brachial Plexus and Upper Limb endings that monitor stretch in the locomotor organs

  • 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 limb2
Brachial Plexus and Upper Limb endings that monitor stretch in the locomotor organs

  • The terms used to describe the plexus from medial to lateral are:

    • Roots / Trunks / Divisions / Cords


Brachial plexus and upper limb3
Brachial Plexus and Upper Limb endings that monitor stretch in the locomotor organs

  • 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 limb4
Brachial Plexus and Upper Limb endings that monitor stretch in the locomotor organs

  • 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 limb5
Brachial Plexus and Upper Limb endings that monitor stretch in the locomotor organs

  • 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 limb6
Brachial Plexus and Upper Limb endings that monitor stretch in the locomotor organs

  • 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 limb7
Brachial Plexus and Upper Limb endings that monitor stretch in the locomotor organs

  • The main nerves that emerge from the brachial plexus are

    • Axillary

    • Musculotaneous

    • Median

    • Ulnar

    • Radial

Roots


Axillary nerve
Axillary Nerve endings that monitor stretch in the locomotor organs

  • 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 nerve1
Axillary Nerve endings that monitor stretch in the locomotor organs

  • Muscular branches

    • Deltoid

    • Teres minor

  • Cutaneous branches

    • Some of the skin of shoulder region


Musculocutaneous nerve
Musculocutaneous Nerve endings that monitor stretch in the locomotor organs

  • 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 nerve1
Musculocutaneous Nerve endings that monitor stretch in the locomotor organs

  • Muscular branches

    • Biceps brachii

    • Brachialis

    • Coracobrachialis

  • Cutaneous branches

    • Skin on anterolateral aspect of forearm


Median nerve
Median Nerve endings that monitor stretch in the locomotor organs

  • 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 nerve1
Median Nerve endings that monitor stretch in the locomotor organs

  • 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
Ulnar Nerve endings that monitor stretch in the locomotor organs

  • 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 nerve1
Ulnar Nerve endings that monitor stretch in the locomotor organs

  • 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
Radial Nerve endings that monitor stretch in the locomotor organs

  • 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 nerve1
Radial Nerve endings that monitor stretch in the locomotor organs

  • 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
Lumbosacral Plexus endings that monitor stretch in the locomotor organs

  • 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
Lumbar Plexus and Lower Limb endings that monitor stretch in the locomotor organs

  • 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
Femoral Nerve endings that monitor stretch in the locomotor organs

  • 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 nerve1
Femoral Nerve endings that monitor stretch in the locomotor organs

  • 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
Obturator Nerve endings that monitor stretch in the locomotor organs

  • The obturator nerve enters the medial thigh via the obturator foramen and innervates the adductor muscles


Obturator nerve1
Obturator Nerve endings that monitor stretch in the locomotor organs

  • 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
Sacral Plexus and Lower Limb endings that monitor stretch in the locomotor organs

  • 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 limb1
Sacral Plexus and Lower Limb endings that monitor stretch in the locomotor organs

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


Sciatic nerve
Sciatic Nerve endings that monitor stretch in the locomotor organs

  • 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 nerve1
Sciatic Nerve endings that monitor stretch in the locomotor organs

  • Muscular branch

    • Bicep femoris

    • Semitendinous

    • Semimembranous

    • Adductor magnus

  • Cutaneous branches

    • Posterior thigh


Tibial nerve
Tibial Nerve endings that monitor stretch in the locomotor organs

  • 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 nerve1
Tibial Nerve endings that monitor stretch in the locomotor organs

  • 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
Common Peroneal Nerve endings that monitor stretch in the locomotor organs

  • 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 nerve1
Common Peroneal Nerve endings that monitor stretch in the locomotor organs

  • 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
Sarcal Plexus Nerves endings that monitor stretch in the locomotor organs

  • 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
Innervation of endings that monitor stretch in the locomotor organsthe 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
Innervation of Skin: Desmatomes endings that monitor stretch in the locomotor organs

  • 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 desmatomes1
Innervation of Skin: Desmatomes endings that monitor stretch in the locomotor organs

  • 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 desmatomes2
Innervation of Skin: Desmatomes endings that monitor stretch in the locomotor organs

  • The ventral rami of sacral nerves serve most of the posterior surfaces of the lower limbs


End of chapter

End of Chapter endings that monitor stretch in the locomotor organs

Chapter 14


Reflex activity
Reflex endings that monitor stretch in the locomotor organsActivity

  • 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 activity1
Reflex Activity endings that monitor stretch in the locomotor organs

  • 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
Components of a Reflex Arc endings that monitor stretch in the locomotor organs

  • 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 arc1
Components of a Reflex Arc endings that monitor stretch in the locomotor organs

  • Reflexes are classified functionally as

    • Somatic reflexes

      • (activate skeletal muscle)

    • Visceral reflexes (autonomic reflexes)

      • (activate smooth, cardiac muscle or visceral organs


Spinal reflexes
Spinal Reflexes endings that monitor stretch in the locomotor organs

  • 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
Stretch and Deep Tendon Reflexes endings that monitor stretch in the locomotor organs

  • 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
Anatomy of Muscle Spindle endings that monitor stretch in the locomotor organs

  • 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 spindle1
Anatomy of Muscle Spindle endings that monitor stretch in the locomotor organs

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


Anatomy of muscle spindle2
Anatomy of Muscle Spindle endings that monitor stretch in the locomotor organs

  • 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 spindle3
Anatomy of Muscle Spindle endings that monitor stretch in the locomotor organs

  • Primary sensory endings

    • Type Ia fibers

  • Stimulated by both the rate and amount of stretch

  • Innervate the center of the spindle


Anatomy of muscle spindle4
Anatomy of Muscle Spindle endings that monitor stretch in the locomotor organs

  • Secondary sensory endings

    • Type II fibers

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


Anatomy of muscle spindle5
Anatomy of Muscle Spindle endings that monitor stretch in the locomotor organs

  • 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
The Stretch Reflex endings that monitor stretch in the locomotor organs

  • 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 reflex1
The Stretch Reflex endings that monitor stretch in the locomotor organs

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


The stretch reflex2
The Stretch Reflex endings that monitor stretch in the locomotor organs

  • 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 reflex3
The Stretch Reflex endings that monitor stretch in the locomotor organs

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


The stretch reflex4
The Stretch Reflex endings that monitor stretch in the locomotor organs

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


The stretch reflex5
The Stretch Reflex endings that monitor stretch in the locomotor organs

  • 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 reflex6
The Stretch Reflex endings that monitor stretch in the locomotor organs

  • 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


The deep tendon reflex1
The Deep Tendon Reflex in response to the muscle’s contraction

  • 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 reflex2
The Deep Tendon Reflex in response to the muscle’s contraction

  • 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 reflex3
The Deep Tendon Reflex in response to the muscle’s contraction

  • 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 Withdrawal Reflex in response to the muscle’s contraction

  • 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 in response to the muscle’s contraction

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


The crossed extensor reflex1
The Crossed Extensor Reflex in response to the muscle’s contraction

  • 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 in response to the muscle’s contraction

  • 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 structure1
Classification by Structure in response to the muscle’s contraction

  • 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 chapter1

End of Chapter in response to the muscle’s contraction


Regeneration of nerve fibers
Regeneration of Nerve Fibers in response to the muscle’s contraction

  • 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 fibers1
Regeneration of Nerve Fibers in response to the muscle’s contraction

  • 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 fibers2
Regeneration of Nerve Fibers in response to the muscle’s contraction

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


Regeneration of nerve fibers3
Regeneration of Nerve Fibers in response to the muscle’s contraction

  • 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 fibers4
Regeneration of Nerve Fibers in response to the muscle’s contraction

  • 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 fibers5
Regeneration of Nerve Fibers in response to the muscle’s contraction

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


Regeneration of nerve fibers6
Regeneration of Nerve Fibers in response to the muscle’s contraction

  • 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 fibers7
Regeneration of Nerve Fibers in response to the muscle’s contraction

  • 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 Receptor Potentials in response to the muscle’s contraction

  • 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


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