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The Peripheral Nervous System

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Title: The Peripheral Nervous System


1
The Peripheral Nervous System
  • Chapter 14

2
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

3
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

4
(No Transcript)
5
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

6
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

7
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 fibers
    associated myelin sheath

8
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

9
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

10
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

11
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

12
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

13
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

14
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

15
Peripheral Sensory Receptors
  • Dendritic endings of sensory neurons monitor most
    types of general sensory information (touch,
    pain, pressure, temperature, and proprioception)

16
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)

17
Sensory Receptors
  • Sensory receptors are classified by
  • The type of stimulus they detect
  • Their location in the body
  • Their structure

18
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

19
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

20
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

21
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

22
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

23
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

24
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

25
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

26
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

27
Free Dendritic Endings
  • Free nerve endings have small knoblike swellings
  • Chiefly respond to pain, temperature, and
    possible mechanical pressure caused by tissue
    movement

28
Free Dendritic Endings
  • The receptors are simple and widely dispersed
    everywhere in the body
  • Particularly abundant in epithelia and connective
    tissue underlying epithelial tissue

29
Merkel Discs
  • Certain free dendritic endings contribute to
    Merkel discs
  • These discs lie in the epidermis of the skin

30
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

31
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

32
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

33
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
34
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

35
Encapsulated Dendritic Endings
  • Encapsulated receptors vary widely in shape,
    size, and distribution in the body
  • The main types are
  • Meissners corpuscles
  • Krauses end bulbs
  • Pacinian corpuscles
  • Ruffinis corpuscles
  • Proprioceptors

36
Meissners Corpuscles
  • In a Meissners 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

37
Meissners Corpuscles
  • These corpuscles are found in the dermal papillae
    beneath the epidermis
  • These corpuscles are rapidly adapting receptors
    for fine, light touch

38
Meissners Corpuscles
  • Meissners corpuscles occur in sensitive and
    hairless areas of the skin, such as the soles of
    the feet, palms, fingertips, nipples, and lips
  • Apparently, Meissners corpuscles perform the
    same light touch function in hairless skill
    that root hair plexuses perform in hairy skin

39
Krauses End Bulbs
  • Krauses End Bulbs are a type of Meissners
    corpuscle for fine touch
  • Krauses end bulbs occur in mucous membranes in
    the lining of the mouth and the conjunctiva of
    the eye

40
Pacinian Corpuscle
  • Pacinian corpuscle are scattered throughout the
    deep connective tissues of the body
  • Occur in the hypodermis of the skin

41
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

42
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

43
Ruffinis Corpuscle
  • Ruffinis 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

44
Ruffinis Corpuscle
  • Ruffinis corpuscle respond to pressure and touch
  • They adapt slowly and thus can monitor continuous
    pressure placed on the skin

45
Proprioceptors
46
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

47
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

48
Proprioceptors
  • An average muscle contains some 50 to 100 muscle
    spindles, which are embedded in the perimysium
    between muscle fascicles

49
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

50
Proprioceptors
  • The intrafusal fibers are innervated by the
    dendrites of several sensory neurons

51
Proprioceptors
  • Some of these sensory dendrites twirl around the
    middle of the middle of the intrafusal fibers as
    annulospiral sensory endings

52
Proprioceptors
  • Flower spray sensory endings supply the ends of
    the intrafusal fibers

53
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

54
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

55
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

56
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

57
Proprioceptors
  • Also innervating the intrafusal fibers of the
    muscle spindle are the axons of spinal motor
    neurons call gamma efferent fibers

58
Proprioceptors
  • Gamma efferent fibers let the brain preset the
    sensitivity of the spindle to stretch

59
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

60
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

61
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

62
Golgi Tendon Organs
  • The receptors induce a spinal reflex that both
    relaxes the contracting muscle and activates its
    antagonist

63
Golgi Tendon Organs
  • Relaxation reflex is important in motor
    activities that involve the rapid alternation
    between flexion and extension such as in sprinting

64
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?)

65
Joint Kinesthetic Receptors
  • Pacinian corpuscles are rapidly adapting stretch
    receptors that are ideal for measuring
    acceleration and rapid movement of the joints
  • Ruffini corpuscles are slowly adapting stretch
    receptors that are ideal for measuring the
    positions of non-moving joints and the stretch of
    joints that undergo slow, sustained movements

66
Joint Kinesthetic Receptors
  • Free dendritic endings in joint may serve as pain
    receptors
  • Receptors resembling Golgi tendon organs have
    been identified in joints but their function is
    not yet known

67
Joint Kinesthetic Receptors
  • Joint receptors, like the other two classes of
    proprioceptors, send information on body
    movements to the cerebellum and cerebrum, as well
    as to spinal reflex arcs

68
Innervation of Skeletal Muscle
  • Motor axons innervate skeletal muscle fibers at
    junctions called neuromuscular junctions, or
    motor end plates

69
Innervation of Skeletal Muscle
  • A single neuromuscular is associated with each
    muscle fiber
  • These junctions are similar to the synapses
    between neurons

70
Innervation of Skeletal Muscle
  • The neural part of the junction is a cluster of
    typical axon terminals separated from the plasma
    membrane (sarcolemma) of the underlying muscle
    cell by a synaptic cleft

71
Innervation of Skeletal Muscle
  • As in typical synapses, the axon terminals
    contain synaptic vesicles that release a
    neurotransmitter when a nerve impulse reaches the
    terminals
  • The neurotransmitter (acetylcholine) diffuses
    across the synaptic cleft and binds to receptor
    molecules on the sarcolemma, where it induces an
    impulse that signals the muscle cell to contract

72
Innervation of Skeletal Muscle
  • Although neuromuscular junctions resemble
    synapses they have several unique features

73
Innervation of Skeletal Muscle
  • Each axon terminal lies in a trough-like
    depression of the sarcolemma, which in turn shows
    groove-like invaginations

74
Innervation of Skeletal Muscle
  • The invaginations and the synaptic cleft contain
    a basal lamina that does not appear in synapses
    between neurons

75
Innervation of Skeletal Muscle
  • This basal lamina contains the enzyme
    acetylcholinesterase which breaks down
    acetylcholine immediately after the
    neurotransmitter signals a single contraction
  • This assures that each nerve impulse in the motor
    axon produces just one twitch of the muscle cell,
    preventing any undersireable additional twitches
    that would occur acetylcholine lingered in the
    synaptic cleft

76
Innervation of Skeletal Muscle
  • Each motor axon branches to innervate a number of
    muscle fibers within a skeletal muscle
  • A motor neuron and all the muscle fibers it
    innervates is called a motor unit
  • When a motor unit fires, all the skeletal muscle
    cells in the motor unit contract together

77
Innervation of Skeletal Muscle
  • Although the average number of muscle fibers in a
    motor unit is 150, a motor unit may contain as
    many as several hundred fibers or as few as four
    muscle fibers
  • Muscles that require very fine control, such as
    the muscles moving the fingers and eyes have few
    muscle fibers per motor unit, whereas
    weight-bearing muscles whose movements are less
    precise have many muscle fibers per unit

78
Innervation of Skeletal Muscle
  • The muscle fibers of a single motor unit are not
    clustered together but spread throughout the
    muscle
  • As a result, stimulation of a single motor unit
    causes a weak contraction of the entire muscle

79
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

80
Innervation of Visceral Muscle
  • Varicosities are the presynaptic terminals which
    contain synaptic vesicles filled with
    neruotransmitter
  • Some of the axon terminals form shallow
    indentations on the membrane of the effector
    cell, but many axon terminals remain a
    considerable distance from any cell

81
Innervation of Visceral Muscle
  • Because it takes time for neurotransmitters to
    diffuse across these wide synaptic clefts,
    visceral motor responses tend to be slower that
    somatic motor reflexes

82
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

83
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

84
Location of Cranial Nerves
  • The cranial nerves as they emerge from the brain
    and spinal cord

85
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

86
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

87
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

88
Oculomotor Nerve III
  • Fibers extend from midbrain to eye
  • Mixed nerve that contains a few proprio- ceptors,
    but is chiefly motor
  • Supplies four of six extrinsic muscles that move
    the eye in its orbit

89
Trochlear Nerves IV
  • Fibers emerge from midbrain to enter orbits
  • Mixed nerve primarily motor
  • Innervates extrinsic muscles in the orbit

90
Trigeninal Nerves V
  • Extends from pons to face
  • Forms three divisions
  • Ophthalmic
  • Maxillary
  • Mandibular
  • Mixed nerve innervating the face, forehead and
    muscle of mastication

91
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)

92
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

93
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

94
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

95
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

96
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

97
Hypoglossal Nerves XII
  • Fibers arise from the medulla to travel to tongue
  • Mixed nerve but primarily motor
  • Innervates muscles that move the tongue

98
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

99
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

100
Innervation of the Back
  • Each spinal nerve connects to the spinal cord by
    two roots
  • Each root forms from a series of rootlets

101
Innervation of the Back
  • Ventral roots contain motor (efferent) fibers
  • Dorsal roots contain sensory (afferent) fibers

102
Innervation of the Back
  • The spinal root pass laterally from the cord, and
    unite just distal to the dorsal root ganglion, to
    form a spinal nerve before emerging from the
    vertebral column

103
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

104
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

105
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

106
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

107
Innervation of the Back
  • The innervation of the posterior body trunk is by
    the dorsal rami
  • Each dorsal ramus innervates a narrow strip of
    muscle and skin
  • Pattern follows a neat, segmented pattern in line
    with emergence from spinal cord

108
Innervation of Thorax Abdomem
  • Only in the thorax are the ventral rami arranged
    in a simple segmental pattern corresponding to
    that of the dorsal rami
  • Ventral rami of T1-T12 course anteriorly deep to
    each rib as intercostal nerves supplying the
    inter- costal muscles most of abdominal wall

109
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

110
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

111
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 students nightmare

112
Brachial Plexus and Upper Limb
  • The plexus is formed by the intermixing of the
    ventral rami of the four inferior cervical nerves
    C5-C8 and most of T1
  • It often receives fibers from C4 or T2

113
Brachial Plexus and Upper Limb
  • The terms used to describe the plexus from medial
    to lateral are
  • Roots / Trunks / Divisions / Cords

114
Brachial Plexus and Upper Limb
  • The five roots (rami C5-T1) of the brachial
    plexus lie deep to the sternocleidomastoid muscle
  • At the lateral border of that muscle, these
    nerves unite to form the upper, middle, and lower
    trunks

115
Brachial Plexus and Upper Limb
  • Each of the three trunks divides almost
    immediately to form anterior and posterior
    divisions
  • The divisions generally reflect which fibers will
    serve the front or back of the limb

116
Brachial Plexus and Upper Limb
  • The divisions give rise to three large fiber
    bundles called the lateral, medial, and posterior
    cords
  • All along the divisions and cords small nerve
    branch off to supply muscles of the shoulder and
    arm

117
Brachial Plexus and Upper Limb
  • A summary of the differentiation of the brachial
    plexus reveals how it gives rise to common nerves
  • The five peripheral nerves that emerge are the
    main nerves of the upper limb

118
Brachial Plexus and Upper Limb
  • The main nerves that emerge from the brachial
    plexus are
  • Axillary
  • Musculotaneous
  • Median
  • Ulnar
  • Radial

Roots
119
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

120
Axillary Nerve
  • Muscular branches
  • Deltoid
  • Teres minor
  • Cutaneous branches
  • Some of the skin of shoulder region

121
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

122
Musculocutaneous Nerve
  • Muscular branches
  • Biceps brachii
  • Brachialis
  • Coracobrachialis
  • Cutaneous branches
  • Skin on anterolateral aspect of forearm

123
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

124
Median Nerve
  • Muscular branches
  • Palmaris longus
  • Flexor carpi radialis
  • Flexor digitorium superficialis
  • Flexor pollicus longus
  • Flexor digitorium profundus
  • Pronator
  • Intrinsic muscles of fingers 2 and 3
  • Cutaneous branches
  • Skin of lateral two-thirds of hand, palm side and
    dorsum of fingers 2 and 3

125
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

126
Ulnar Nerve
  • Muscular branches
  • Flexor carpi ulnaris
  • Flexor digitorium profundus (medial half)
  • Intrinsic muscles of the hand
  • Cutaneous branches
  • Skin of medial third of hand, both anterior and
    posterior aspects

127
Radial Nerve
  • The radial nerve is a continuation of the
    posterior cord
  • The nerve wraps around humerous, runs anteriorly
    by the lateral epicondyle at the elbow
  • Divides into a super- ficial branch that follows
    the radius and a deep branch that runs posteriorly

128
Radial Nerve
  • Muscular branches
  • Triceps brachii
  • Anconeus
  • Supinator
  • Brachioradialis
  • Extensor capri radialis
  • Extensor carpi brevis
  • Extensor carpi ulnaris
  • Muscles that extend fingers
  • Cutaneous branches
  • Skin of posterior surface of entire limb

129
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

130
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

131
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

132
Femoral Nerve
  • Muscular branch
  • Quadiceps group
  • Rectus femoris, vastus laterialis, vastus
    medialis, vastus intermedius
  • Sartorius
  • Pertineus
  • Iliacus
  • Cutaneous branches
  • Anterior femoral cutaneous
  • Skin of anterior and medial thigh
  • Saphenous
  • Skin of medial leg and foot, hip and knee joints

133
Obturator Nerve
  • The obturator nerve enters the medial thigh via
    the obturator foramen and innervates the adductor
    muscles

134
Obturator Nerve
  • Muscular branch
  • Adductor magnus (part)
  • Adductor longus
  • Adductor brevis
  • Gracilis
  • Obturator externus
  • Cutaneous branches
  • Sensory for skin of medial thigh and hip and knee
    joints

135
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

136
Sacral Plexus and Lower Limb
  • Half the nerves serve muscles of the buttocks and
    lower limb while others innervate pelvic
    structures and the perineum

137
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

138
Sciatic Nerve
  • Muscular branch
  • Bicep femoris
  • Semitendinous
  • Semimembranous
  • Adductor magnus
  • Cutaneous branches
  • Posterior thigh

139
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

140
Tibial Nerve
  • Muscular branch
  • Triceps surae
  • Tibialis posterior
  • Popliteus
  • Flexor digitorum longus
  • Flexor hallicus longus
  • Intrinsic muscle of the foot
  • Cutaneous branches
  • Skin of the posterior surface of the leg and the
    sole of the foot

141
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

142
Common Peroneal Nerve
  • Muscular branch
  • Biceps foemoris (short head)
  • Peroneal muscles (longus, brevis, tertius)
  • Tibialis anterior
  • Extensor hallicus longus
  • Extensor digitorum longus
  • Extensor digitorum brevis
  • Cutaneous branches
  • Skin of the anterior surface of leg and dorsum of
    foot

143
Sarcal Plexus Nerves
  • Superior and inferior gluteal
  • Innervate the gluteal muscles and tensor fasciae
    latae
  • Pudendal
  • Innervates the muscles of the skin of the
    perineum
  • Mediates the act of erection
  • Voluntary control of urination
  • External anal sphinter

144
Innervation of the Joints
  • Hiltons law . . . any nerve serving a muscle
    producing movement at a joint also innervates the
    joint itself and the skin over the joint

145
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

146
Innervation of Skin Desmatomes
  • The skin of the upper limbs is supplied by C5-T1
  • The ventral rami of the lumbar nerves supply most
    of the anterior muscles of the thighs and legs

147
Innervation of Skin Desmatomes
  • The ventral rami of sacral nerves serve most of
    the posterior surfaces of the lower limbs

148
End of Chapter
  • Chapter 14

149
Reflex Activity
  • Many of the bodys 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

150
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

151
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

152
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

153
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

154
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

155
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)

156
Anatomy of Muscle Spindle
  • The central region of the intrafusal fibers which
    lack myofilaments and are noncontractile, serving
    as the receptive surface of the spindle

157
Anatomy of Muscle Spindle
  • Intrafusal fibers are wrapped by two types of
    afferent endings that send sensory inputs to the
    CNS
  • Primary sensory endings
  • Type Ia fibers
  • Secondary sensory endings
  • Type II fibers

158
Anatomy of Muscle Spindle
  • Primary sensory endings
  • Type Ia fibers
  • Stimulated by both the rate and amount of stretch
  • Innervate the center of the spindle

159
Anatomy of Muscle Spindle
  • Secondary sensory endings
  • Type II fibers
  • Associated with the ends of the spindle and are
    stimulated only by degree of stretch

160
Anatomy of Muscle Spindle
  • The contractile region of the intrafusal muscle
    fibers are limited to their ends as only these
    areas contain actin and myosin filaments
  • These regions are innervated by gamma (?)
    efferent fibers

161
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)

162
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

163
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

164
The Stretch Reflex
  • The reflexive muscle contraction that follows (an
    example of serial processing) resists further
    stretching of the muscle

165
The Stretch Reflex
  • Branches of the afferent fibers also synapse with
    inter- neurons that inhibit motor neurons
    controlling the antagonistic muscles inhibiting
    their contraction

166
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

167
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

168
The Deep Tendon Reflex
  • Deep tendon reflexes cause muscle relaxation and
    lengthening in response to the muscles
    contraction
  • This effect is opposite of those elicited by
    stretch reflexes

169
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

170
The Deep Tendon Reflex
  • Upon reaching the spinal cord, informa- tion is
    sent to the cerebellum, where it is used to
    adjust muscle tension
  • Simultaneously, motor neurons in the spinal cord
    supplying the contracting muscle are imhibited
    and antagonistic muscle are activated (activation)

171
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

172
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

173
The Crossed Extensor Reflex
  • The crossed extensor reflex is a complex spinal
    reflex consisting of an ipsilateral withdrawal
    reflex and a contralateral extensor reflex

174
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

175
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

176
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)

177
End of Chapter
178
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

179
Regeneration of Nerve Fibers
  • Almost immediately after a peripheral axon has
    been cut, the separated ends seal themselves off
    and swell as substances being transported along
    the axon begin to accumulate

180
Regeneration of Nerve Fibers
  • Wallerian degeneration spreads distally from the
    injury site completely fragmenting the axon

181
Regeneration of Nerve Fibers
  • Macrophages that migrate into the trauma zone
    from adjacent tissues, phagocytize the
    disintegrating myelin and axonal debris
  • Generally, the entire axon distal to the injury
    degrades within a week
  • However, the nucleus and neurilemma remain intact
    with the endoneurium

182
Regeneration of Nerve Fibers
  • Schwann cells then proliferate and migrate to the
    injury site
  • They release growth factors that encourage axon
    growth
  • Additionally, they form cellular cords that guide
    the regenerating axon to their original contacts

183
Regeneration of Nerve Fibers
  • The same Schwann cells then protect, support, and
    remyelinate the regenerating axons

184
Regeneration of Nerve Fibers
  • Axons regenerate at a rate of 1 to 5 mm a day
  • The greater the distance between the severed
    nerve endings, the greater the time for
    regeneration
  • Greater distances also lessen the chance of
    successful regeneration because adjacent tissues
    often block growth by protruding into larger gaps

185
Regeneration of Nerve Fibers
  • CNS nerve fibers never regenerate under normal
    circumstances
  • Brain and spinal cord damage is considered as
    irreversible
  • The difference in regenerative capacity is
    largely due to the support cells of the CNS
  • Macrophage invasion in the CNS is much slower
    than in the PNS
  • Oligodendrocytes surrounding the damaged axon die
    and thus cannot guide axon regeneration and growth

186
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
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