The Peripheral Nervous System

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The Peripheral Nervous System

• Chapter 14

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Introduction to the PNS

• Nervous structures outside the brain and spinal
  cord
• - sensory and motor connections to the outside
  world
• - nerves thread throughout the body to allow the
  CNS to receive information and take action
• Functional components of the PNS
• - sensory inputs and motor outputs
• - categorized as somatic or visceral
• - also classified as general or special

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Functional Components of the PNS

• Basic structural components
• 1. Sensory receptors pick up stimuli from
  inside and outside the body, then initiate
  impulses in sensory axons
• 2. Motor endings the axon terminals of motor
  neurons that innervate the effectors
• 3. Nerves bundles of peripheral axons and
• Ganglia - clusters of peripheral neuronal cell
  bodies
• - most are mixed nerves, contain both sensory
  and motor axons
• - some cranial nerves are purely sensory or
  purely motor in function

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Autonomic Nervous System

• General visceral motor part of the PNS
• Has 2 divisions (with opposite effects)
• - Parasympathetic housekeeping activities
  (rest and digest)
• - Sympathetic extreme situations (fight or
  flight)

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Functional Organization of the PNS
Figure 14.1
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Peripheral Sensory Receptors

• Most fit into 2 main categories
• 1. free nerve endings of sensory neurons
• - monitor general sensory information such as
  touch, pain, pressure, temperature, and
  proprioception
• 2. complete receptor cells specialized
  epithelial cells or small neurons that transfer
  sensory information to sensory neurons
• - monitor most special sensory information such
  as taste, vision, hearing, and equilibrium

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Basic Anatomical Scheme of the PNS in the Region
of a Spinal Nerve
Figure 14.2

• A nerve is composed of numerous nerve fibers

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Sensory Receptors of the PNS

• Also classified according to
• a) Location based on body location or location
  of stimuli to which they respond
• b) Type of stimulus detected kinds of stimuli
  that most readily activate them
• c) Structure divided into 2 broad categories
  free or encapsulated nerve endings

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Classification by Location

• Exteroceptors sensitive to stimuli arising from
  outside the body
• - located at or near body surfaces
• - include receptors for touch, pressure, pain,
  temperature, and most receptors of the special
  sense organs
• Interoceptors (visceroceptors) receive stimuli
  from internal viscera (digestive tube, bladder,
  lungs)
• - monitor a variety of stimuli such as changes
  in chemical concentration, taste stimuli,
  stretching of tissues, and temperature
• - activation causes visceral pain, nausea,
  hunger, or satiety

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Classification by Location

• Proprioceptors monitors degree of stretch and
  sends input on body movements to the CNS
• - located in musculoskeletal organs such as
  skeletal muscles, tendons, joints, and ligaments

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Classification by Stimulus Detected

• Mechanoreceptors respond to mechanical forces
• - such as touch, pressure, stretch, vibrations,
  and itch
• Thermoreceptors respond to temperature changes
• Chemoreceptors respond to chemicals in solution
  (molecules tasted or smelled) and to change in
  blood chemistry
• Photoreceptors in the eye respond to light
• Nociceptors respond to harmful stimuli that
  result in pain (noci harm)

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Classification by Structure

• General sensory receptors widely distributed
• Nerve endings of sensory neurons moniter
• - Touch
• - Pressure
• - Vibration
• - Stretch
• - Pain
• - Temperture
• - Proprioception

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Classification by Structure

• General sensory receptors are divided into 2
  groups
• - Free nerve endings
• - Encapsulated nerve endings
• Note there is no perfect one receptor one
  function
• - one receptor can respond to several kinds of
  stimuli and different receptor types can respond
  to similar stimuli

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Free Nerve Endings

• Abundant in epithelia and underlying CT
• Respond to pain and temperature
• Monitor affective senses those to which people
  have an emotional response (pain)
• 2 specialized types of free nerve endings
• - Merkel discs lie in the epidermis
• - Hair follicle receptors wrap around hair
  follicles

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Free Nerve Endings

• Merkel discs a disc-shaped epithelial cell
  innervated by a sensory nerve ending
• - slowly adapting receptors for light touch
  (respond and send out action potentials even
  after continual stimulation)
• Hair follicle receptors receptors for light
  touch
• - monitor the bending of hairs
• - rapidly adapting (sensation disappears quickly
  even if the stimulus is maintained)
• Itch receptor in the dermis (newly discovered)

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Table 14.1 (1 of 4)
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Encapsulated Nerve Endings

• Consist of one or more end fibers of sensory
  neurons enclosed in connective tissue
• All seem to be mechanoreceptors capsules either
  amplify the stimulus or filter out the wrong
  types of stimuli
• 4 main types
• - Meissners corpuscles
• - Pacinian corpuscles
• - Ruffini endings
• - Proprioceptors

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Meissners Corpuscles

• Spiraling nerve ending surrounded by Schwann
  cells
• - occur in the dermal papillae
• - rapidly adapting receptors for discriminative
  touch
• - occur in sensitive, hairless areas of the skin

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Meissners Corpuscles
Table 14.1 (2 of 4)
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Pacinian Corpuscles and Ruffini Endings

• Pacinian corpuscle - single nerve ending
• - surrounded by layers of flattened Schwann
  cells
• - occur in the hypodermis
• - sensitive to deep pressure
• - rapidly adapting receptors
• Ruffini endings located in the dermis
• - monitor continuous pressure on the skin (adapt
  slowly)

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Table 14.1 (3 of 4)
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Proprioceptors

• 3 types - monitor stretch in locomotory organs
• Muscle spindles - measure the changing length of
  a muscle
• - imbedded in the perimysium between muscle
  fascicles
• Golgi tendon organs located near the
  muscle-tendon junction
• - monitor tension within tendons
• Joint kinesthetic receptors sensory nerve
  endings within the joint capsules

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Table 14.1 (4 of 4)
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Structure of Proprioceptors
Figure 14.4
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Cranial Nerves

• Attach to the brain and pass through foramina of
  the skull
• Numbered from I XII
• Cranial nerves I and II attach to the forebrain
• - all others attach to the brain stem
• Primarily serve head and neck structures
• - except the vagus nerve (X) that extends into
  the abdomen

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• The 12 pairs of cranial nerves

Figure 14.5
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Olfactory Nerves

• Sensory nerves of smell

Table 14.3 (1 of 12)
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Optic Nerve

• Sensory nerve of vision

Table 14.3 (2 of 12)
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Oculomotor Nerve

• Innervates four of the extrinsic eye muscles

Table 14.3 (3 of 12)
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Trochlear Nerve

• Innervates the superior oblique muscle (extrinsic
  eye muscle)

Table 14.3 (4 of 12)
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Trigeminal Nerve
Table 14.3 (5 of 12)

• Provides sensory innervation to the face and
  motor innervation to chewing muscles

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Abducens Nerve

• Abducts the eyeball innervates lateral rectus
  muscle

Table 14.3 (6 of 12)
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Facial Nerve

• Innervates muscles of facial expression

Table 14.3 (7 of 12)
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Vestibulocochlear Nerve

• Sensory nerve of hearing and balance

Table 14.3 (8 of 12)
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Glossopharyngeal Nerve

• Innervates structures of the tongue and pharynx

Table 14.3 (9 of 12)
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Vagus Nerve

• A mixed sensory and motor nerve
• Wanders into thorax and abdomen
• Parasympathetic innervation of organs

Table 14.3 (10 of 12)
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Accessory Nerve

• An accessory part of the vagus nerve
• Innervates trapezius muscle

Table 14.3 (11 of 12)
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Hypoglossal Nerve

• Runs inferior to the tongue - innervates the
  tongue muscles

Table 14.3 (12 of 12)
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Spinal Nerves

• 31 pairs contain thousands of nerve fibers
• Connect to the spinal cord
• Named for point of issue from the spinal cord
• - 8 pairs of cervical nerves (C1 C8)
• - 12 pairs of thoracic nerves (T1 T12)
• - 5 pairs of lumbar nerves (L1 L5)
• - 1 pair of coccygeal nerves (Co1)

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Spinal Nerves Posterior View
Figure 14.6
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Spinal Nerves

• Connect to the spinal cord by the dorsal root and
  ventral root
• Dorsal root contains sensory fibers
• - cell bodies located in the dorsal root
  ganglion
• Ventral root contains motor fibers arising from
  anterior gray column

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Spinal Nerves

• Branch into dorsal ramus and ventral ramus
• - dorsal and ventral rami contain sensory and
  motor fibers
• Rami communicantes connect to the base of the
  ventral ramus
• - lead to the sympathetic chain ganglia

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Spinal Nerves
Figure 14.7a
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Innervation of the Back

• Dorsal rami innervate back muscles
• - follow a neat, segmented pattern
• - innervate a horizontal strip of muscle and
  skin (in line with emergence point from the
  vertebral column)

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Innervation of the Back
Figure 14.7b
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Innervation of the Anterior Thoracic and
Abdominal Wall

• Thoracic region ventral rami arranged in
  simple, segmented pattern
• Intercostal nerves supply intercostal muscles,
  skin, and abdominal wall
• - each gives off lateral and anterior cutaneous
  branches

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Introduction to Nerve Plexuses

• A network of nerves
• Ventral rami (except T2 T12)
• - branch and join with one another
• - form nerve plexuses in the cervical, brachial,
  lumbar, and sacral regions
• - primarily serve the limbs
• - fibers from ventral rami crisscross

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The Cervical Plexus

• Buried deep in the neck under the
  sternocleidomastoid muscle
• Formed by ventral rami of first 4 cervical nerves
  (C1 C4)
• Most are cutaneous nerves
• Some innervate muscles of the anterior neck
• Phrenic nerve major nerve

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The Brachial Plexus and Innervation of the Upper
Limb

• Brachial plexus lies in the neck and axilla
• Formed by ventral rami of C5 C8
• Cords give rise to main nerves of the upper limb

Fig 14.9d
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Nerves from Lateral and Medial Cords

• Musculocutaneous main branch of the lateral
  cord
• - innervates the biceps brachii and brachialis
• Median originates from both lateral and medial
  cords
• - innervates anterior forearm muscles and
  lateral palm
• Ulnar branches from the medial cord
• - innervates intrinsic hand muscles and skin of
  the medial hand
• Radial continuation of the posterior cord
• - largest branch innervates muscles of posterior
  upper limb
• Axillary innervates the deltoid and teres minor

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The Brachial Plexus
Fig 14.9a
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Axillary and Radial Nerves
Figure 14.11
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Lumbar Plexus and Innervation of the Lower Limb

• Arises from L1 L4
• Smaller branches innervate the posterior
  abdominal wall and psoas muscle
• Main branches innervate the anterior thigh
• - femoral nerve innervates anterior thigh
  muscles
• - obturator nerve innervates adductor muscles

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The Lumbar Plexus
Fig 14.12a, b
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Sacral Plexus

• Arises from spinal nerves L4 S4
• Caudal to the lumbar plexus
• Often considered with the lumbar lumbosacral
  plexus
• Sciatic nerve largest nerve
• - 2 nerves in one sheath
• Tibial nerve innervates most of the posterior
  lower limb
• Common fibular (peroneal) nerve innervates
  muscles of the anterolateral leb

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Innervation of the Lower Limb

• Superior and inferior gluteal nerves
• - innervate the gluteal muscles
• Pudendal nerve
• - innervates muscles of the perineum

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The Sacral Plexus
Figure 14.13
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Innervation of the Skin Dermatomes

• Dermatome an area of skin
• Innervated by cutaneous branches of a single
  spinal nerve
• Upper limb skin is supplied by nerves of the
  brachial plexus
• Lower limb
• Lumbar nerves anterior surface
• Sacral nerves posterior surface

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Map of Dermatomes
Posterior
Anterior
Figure 14.14
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Disorders of the PNS

• Shingles (herpes zoster)
• Viral infection stems from childhood chicken
  pox
• Often brought on by stress
• Mostly experienced by those over 50

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Disorders of the PNS

• Migraine headache relates to sensory
  innervation of cerebral arteries
• - arteries dilate compresses and irritates
  sensory nerve endings
• Myasthenia gravis progressive weakening of the
  skeletal muscles
• - an autoimmune disorder
• - antibodies destroy acetylcholine receptors

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PNS Throughout Life

• Spinal nerves form late in week 4
• Each of the 31 pairs of spinal nerves
• - sends motor fibers to an individual myotome
• - sends sensory fibers to the overlying band of
  skin
• During week 5 nerves reach the organs they
  innervate

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PNS Throughout Life

• Embryonic muscles migrate to new locations
• Some skin dermatomes become displaced
• Muscles and skin always retain their original
  nerve supply