The Peripheral Nervous System

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The Peripheral Nervous System

• Chapter 14

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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Peripheral Sensory Receptors

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

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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)

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Sensory Receptors

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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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Free Dendritic Endings

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

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Free Dendritic Endings

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

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Merkel Discs

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

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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

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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

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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

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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
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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

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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

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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

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Meissners Corpuscles

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

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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

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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

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Pacinian Corpuscle

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

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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

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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

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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

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Ruffinis Corpuscle

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

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Proprioceptors
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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

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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

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Proprioceptors

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

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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

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Proprioceptors

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

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Proprioceptors

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

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Proprioceptors

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

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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

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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

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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

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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

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Proprioceptors

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

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Proprioceptors

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

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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

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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

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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

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Golgi Tendon Organs

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

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Golgi Tendon Organs

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

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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)

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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

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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

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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

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Innervation of Skeletal Muscle

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

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Innervation of Skeletal Muscle

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

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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

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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

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Innervation of Skeletal Muscle

• Although neuromuscular junctions resemble
  synapses they have several unique features

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Innervation of Skeletal Muscle

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

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Innervation of Skeletal Muscle

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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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Location of Cranial Nerves

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

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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

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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

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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

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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

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Trochlear Nerves IV

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

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Trigeninal Nerves V

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

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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)

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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

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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

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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

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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

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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

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Hypoglossal Nerves XII

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

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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

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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

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Innervation of the Back

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

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Innervation of the Back

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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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Brachial Plexus and Upper Limb

• The main nerves that emerge from the brachial
  plexus are
• Axillary
• Musculotaneous
• Median
• Ulnar
• Radial

Roots
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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

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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

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Musculocutaneous Nerve

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

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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

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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