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prepared by
Janice Meeking,
Mount Royal College
CHAPTER
13
The Peripheral
Nervous
System and
Reflex Activity:
Part A
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Peripheral Nervous System (PNS)
• All neural structures outside the brain
• Sensory receptors
• Peripheral nerves and associated ganglia
• Motor endings
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Central nervous system (CNS)
Peripheral nervous system (PNS)
Sensory (afferent)
division
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Motor (efferent) division
Somatic nervous
system
Autonomic nervous
system (ANS)
Sympathetic
division
Parasympathetic
division
Figure 13.1
Sensory Receptors
• Specialized to respond to changes in their
environment (stimuli)
• Activation results in graded potentials that
trigger nerve impulses
• Sensation (awareness of stimulus) and
perception (interpretation of the meaning of
the stimulus) occur in the brain
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Classification of Receptors
• Based on:
• Stimulus type
• Location
• Structural complexity
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Classification by Stimulus Type
• Mechanoreceptors—respond to touch, pressure,
vibration, stretch, and itch
• Thermoreceptors—sensitive to changes in
temperature
• Photoreceptors—respond to light energy (e.g.,
retina)
• Chemoreceptors—respond to chemicals (e.g., smell,
taste, changes in blood chemistry)
• Nociceptors—sensitive to pain-causing stimuli (e.g.
extreme heat or cold, excessive pressure,
inflammatory chemicals)
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Classification by Location
1. Exteroceptors
•
Respond to stimuli arising outside the body
•
Receptors in the skin for touch, pressure,
pain, and temperature
•
Most special sense organs
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Classification by Location
2. Interoceptors (visceroceptors)
•
Respond to stimuli arising in internal viscera
and blood vessels
•
Sensitive to chemical changes, tissue
stretch, and temperature changes
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Classification by Location
3. Proprioceptors
•
Respond to stretch in skeletal muscles,
tendons, joints, ligaments, and connective
tissue coverings of bones and muscles
•
Inform the brain of one’s movements
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Classification by Structural Complexity
1. Complex receptors (special sense organs)
•
Vision, hearing, equilibrium, smell, and taste
(Chapter 15)
2. Simple receptors for general senses:
•
Tactile sensations (touch, pressure, stretch,
vibration), temperature, pain, and muscle
sense
•
Unencapsulated (free) or encapsulated
dendritic endings
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Unencapsulated Dendritic Endings
• Thermoreceptors
• Cold receptors (10–40ºC); in superficial dermis
• Heat receptors (32–48ºC); in deeper dermis
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Unencapsulated Dendritic Endings
• Nociceptors
• Respond to:
• Pinching
• Chemicals from damaged tissue
• Temperatures outside the range of
thermoreceptors
• Capsaicin
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Unencapsulated Dendritic Endings
• Light touch receptors
• Tactile (Merkel) discs
• Hair follicle receptors
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Table 13.1
Encapsulated Dendritic Endings
• All are mechanoreceptors
• Meissner’s (tactile) corpuscles—discriminative touch
• Pacinian (lamellated) corpuscles—deep pressure and
vibration
• Ruffini endings—deep continuous pressure
• Muscle spindles—muscle stretch
• Golgi tendon organs—stretch in tendons
• Joint kinesthetic receptors—stretch in articular
capsules
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Table 13.1
From Sensation to Perception
• Survival depends upon sensation and
perception
• Sensation: the awareness of changes in the
internal and external environment
• Perception: the conscious interpretation of
those stimuli
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Sensory Integration
• Input comes from exteroceptors,
proprioceptors, and interoceptors
• Input is relayed toward the head, but is
processed along the way
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Processing at the Circuit Level
• Pathways of three neurons conduct sensory
impulses upward to the appropriate brain regions
• First-order neurons
• Conduct impulses from the receptor level to the
second-order neurons in the CNS
• Second-order neurons
• Transmit impulses to the thalamus or cerebellum
• Third-order neurons
• Conduct impulses from the thalamus to the
somatosensory cortex (perceptual level)
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Processing at the Perceptual Level
• Identification of the sensation depends on the
specific location of the target neurons in the sensory
cortex
• Aspects of sensory perception:
• Perceptual detection—ability to detect a stimulus
(requires summation of impulses)
• Magnitude estimation—intensity is coded in the
frequency of impulses
• Spatial discrimination—identifying the site or pattern of
the stimulus (studied by the two-point discrimination
test)
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Main Aspects of Sensory Perception
• Feature abstraction—identification of more
complex aspects and several stimulus
properties
• Quality discrimination—the ability to identify
submodalities of a sensation (e.g., sweet or
sour tastes)
• Pattern recognition—recognition of familiar or
significant patterns in stimuli (e.g., the melody
in a piece of music)
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Perceptual level (processing in
cortical sensory centers)
3
Motor
cortex
Somatosensory
cortex
Thalamus
Reticular
formation
Pons
2 Circuit level
(processing in
Spinal
ascending pathways) cord
Cerebellum
Medulla
Free nerve
endings (pain,
cold, warmth)
Muscle
spindle
Receptor level
(sensory reception Joint
and transmission
kinesthetic
to CNS)
receptor
1
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Figure 13.2
Perception of Pain
• Warns of actual or impending tissue damage
• Stimuli include extreme pressure and
temperature, histamine, K+, ATP, acids, and
bradykinin
• Impulses travel on fibers that release
neurotransmitters glutamate and substance P
• Some pain impulses are blocked by inhibitory
endogenous opioids
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Structure of a Nerve
• Cordlike organ of the PNS
• Bundle of myelinated and unmyelinated
peripheral axons enclosed by connective
tissue
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Structure of a Nerve
• Connective tissue coverings include:
• Endoneurium—loose connective tissue that
encloses axons and their myelin sheaths
• Perineurium—coarse connective tissue that
bundles fibers into fascicles
• Epineurium—tough fibrous sheath around a
nerve
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Endoneurium
Axon
Myelin sheath
Perineurium
Epineurium
Fascicle
Blood
vessels
(b)
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Figure 13.3b
Classification of Nerves
• Most nerves are mixtures of afferent and efferent
fibers and somatic and autonomic (visceral) fibers
• Pure sensory (afferent) or motor (efferent) nerves are
rare
• Types of fibers in mixed nerves:
• Somatic afferent and somatic efferent
• Visceral afferent and visceral efferent
• Peripheral nerves classified as cranial or spinal
nerves
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Ganglia
• Contain neuron cell bodies associated with
nerves
• Dorsal root ganglia (sensory, somatic)
(Chapter 12)
• Autonomic ganglia (motor, visceral)
(Chapter 14)
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Regeneration of Nerve Fibers
• Mature neurons are amitotic
• If the soma of a damaged nerve is intact, axon will
regenerate
• Involves coordinated activity among:
• Macrophages—remove debris
• Schwann cells—form regeneration tube and secrete
growth factors
• Axons—regenerate damaged part
• CNS oligodendrocytes bear growth-inhibiting proteins
that prevent CNS fiber regeneration
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Cranial Nerves
• Twelve pairs of nerves associated with the
brain
• Most are mixed in function; two pairs are
purely sensory
• Each nerve is identified by a number
(I through XII) and a name
“On occasion, our trusty truck acts funny—very
good vehicle anyhow”
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Frontal lobe
Temporal lobe
Infundibulum
Facial
nerve (VII)
Vestibulocochlear
nerve (VIII)
Glossopharyngeal
nerve (IX)
Vagus nerve (X)
Accessory nerve (XI)
Hypoglossal nerve (XII)
Filaments of
olfactory
nerve (I)
Olfactory bulb
Olfactory tract
Optic nerve
(II)
Optic chiasma
Optic tract
Oculomotor
nerve (III)
Trochlear
nerve (IV)
Trigeminal
nerve (V)
Abducens
nerve (VI)
Cerebellum
Medulla
oblongata
(a)
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Figure 13.5 (a)
Cranial nerves
I – VI
I
II
III
IV
V
Olfactory
Optic
Oculomotor
Trochlear
Trigeminal
VI Abducens
Cranial nerves
VII – XII
VII Facial
VIII Vestibulocochlear
IX
X
XI
XII
(b)
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Glossopharyngeal
Vagus
Accessory
Hypoglossal
Sensory
function
Motor
function
PS*
fibers
Yes (smell)
Yes (vision)
No
No
Yes (general
sensation)
No
No
Yes
Yes
Yes
No
No
Yes
No
No
No
Yes
No
Sensory
function
Motor
function
PS*
fibers
Yes (taste)
Yes (hearing
and balance)
Yes
Some
Yes
No
Yes (taste)
Yes (taste)
No
No
Yes
Yes
Yes
Yes
Yes
Yes
No
No
*PS = parasympathetic
Figure 13.5 (b)
I: The Olfactory Nerves
• Arise from the olfactory receptor cells of nasal
cavity
• Pass through the cribriform plate of the
ethmoid bone
• Fibers synapse in the olfactory bulbs
• Pathway terminates in the primary olfactory
cortex
• Purely sensory (olfactory) function
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Table 13.2
II: The Optic Nerves
• Arise from the retinas
• Pass through the optic canals, converge and
partially cross over at the optic chiasma
• Optic tracts continue to the thalamus, where
they synapse
• Optic radiation fibers run to the occipital
(visual) cortex
• Purely sensory (visual) function
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Table 13.2
III: The Oculomotor Nerves
• Fibers extend from the ventral midbrain
through the superior orbital fissures to the
extrinsic eye muscles
• Functions in raising the eyelid, directing the
eyeball, constricting the iris
(parasympathetic), and controlling lens shape
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Table 13.2
IV: The Trochlear Nerves
• Fibers from the dorsal midbrain enter the
orbits via the superior orbital fissures to
innervate the superior oblique muscle
• Primarily a motor nerve that directs the
eyeball
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Table 13.2
V: The Trigeminal Nerves
• Largest cranial nerves; fibers extend from pons to
face
• Three divisions
• Ophthalmic (V1) passes through the superior orbital
fissure
• Maxillary (V2) passes through the foramen rotundum
• Mandibular (V3) passes through the foramen ovale
• Convey sensory impulses from various areas of the
face (V1) and (V2), and supplies motor fibers (V3) for
mastication
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Table 13.2
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Table 13.2
VI: The Abducens Nerves
• Fibers from the inferior pons enter the orbits
via the superior orbital fissures
• Primarily a motor, innervating the lateral
rectus muscle
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Table 13.2
VII: The Facial Nerves
• Fibers from the pons travel through the internal
acoustic meatuses, and emerge through the
stylomastoid foramina to the lateral aspect of the
face
• Chief motor nerves of the face with 5 major branches
• Motor functions include facial expression,
parasympathetic impulses to lacrimal and salivary
glands
• Sensory function (taste) from the anterior two-thirds
of the tongue
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Table 13.2
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Table 13.2
VIII: The Vestibulocochlear Nerves
• Afferent fibers from the hearing receptors
(cochlear division) and equilibrium receptors
(vestibular division) pass from the inner ear
through the internal acoustic meatuses, and
enter the brain stem at the pons-medulla
border
• Mostly sensory function; small motor
component for adjustment of sensitivity of
receptors
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Table 13.2
IX: The Glossopharyngeal Nerves
• Fibers from the medulla leave the skull via the
jugular foramen and run to the throat
• Motor functions: innervate part of the tongue
and pharynx for swallowing, and provide
parasympathetic fibers to the parotid salivary
glands
• Sensory functions: fibers conduct taste and
general sensory impulses from the pharynx
and posterior tongue, and impulses from
carotid chemoreceptors and baroreceptors
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Table 13.2
X: The Vagus Nerves
• The only cranial nerves that extend beyond the head
and neck region
• Fibers from the medulla exit the skull via the jugular
foramen
• Most motor fibers are parasympathetic fibers that
help regulate the activities of the heart, lungs, and
abdominal viscera
• Sensory fibers carry impulses from thoracic and
abdominal viscera, baroreceptors, chemoreceptors,
and taste buds of posterior tongue and pharynx
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Table 13.2
XI: The Accessory Nerves
• Formed from ventral rootlets from the C1–C5
region of the spinal cord (not the brain)
• Rootlets pass into the cranium via each
foramen magnum
• Accessory nerves exit the skull via the jugular
foramina to innervate the trapezius and
sternocleidomastoid muscles
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Table 13.2
XII: The Hypoglossal Nerves
• Fibers from the medulla exit the skull via the
hypoglossal canal
• Innervate extrinsic and intrinsic muscles of the
tongue that contribute to swallowing and
speech
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Table 13.2
Spinal Nerves
• 31 pairs of mixed nerves named according to
their point of issue from the spinal cord
• 8 cervical (C1–C8)
• 12 thoracic (T1–T12)
• 5 Lumbar (L1–L5)
• 5 Sacral (S1–S5)
• 1 Coccygeal (C0)
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Cervical plexus
Brachial plexus
Cervical
enlargement
Intercostal
nerves
Cervical
nerves
C1 – C8
Thoracic
nerves
T1 – T12
Lumbar
enlargement
Lumbar plexus
Sacral plexus
Cauda equina
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Lumbar
nerves
L1 – L5
Sacral nerves
S1 – S5
Coccygeal nerve Co1
Figure 13.6
Spinal Nerves: Roots
• Each spinal nerve connects to the spinal cord
via two roots
• Ventral roots
• Contain motor (efferent) fibers from the ventral
horn motor neurons
• Fibers innervate skeletal muscles)
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Spinal Nerves: Roots
• Dorsal roots
• Contain sensory (afferent) fibers from sensory
neurons in the dorsal root ganglia
• Conduct impulses from peripheral receptors
• Dorsal and ventral roots unite to form spinal
nerves, which then emerge from the vertebral
column via the intervertebral foramina
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Gray matter
White matter
Ventral root
Dorsal root
Dorsal root
ganglion
Dorsal ramus
of spinal nerve
Ventral ramus
of spinal nerve
Spinal nerve
Dorsal and
ventral rootlets
of spinal nerve
Rami communicantes
Sympathetic trunk
ganglion
Anterior view showing spinal cord, associated nerves, and vertebrae.
The dorsal and ventral roots arise medially as rootlets and join
laterally to form the spinal nerve.
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Figure 13.7 (a)
Spinal Nerves: Rami
• Each spinal nerve branches into mixed rami
• Dorsal ramus
• Larger ventral ramus
• Meningeal branch
• Rami communicantes (autonomic pathways)
join to the ventral rami in the thoracic region
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Spinal Nerves: Rami
• All ventral rami except T2–T12 form interlacing
nerve networks called plexuses (cervical,
brachial, lumbar, and sacral)
• The back is innervated by dorsal rami via
several branches
• Ventral rami of T2–T12 as intercostal nerves
supply muscles of the ribs, anterolateral
thorax, and abdominal wall
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Dorsal ramus
Ventral ramus
Spinal nerve
Rami communicantes
Sympathetic trunk
ganglion
Intercostal nerve
Dorsal root
ganglion
Dorsal root
Ventral root
Branches of intercostal
nerve
• Lateral cutaneous
• Anterior cutaneous
Sternum
(b) Cross section of thorax showing the main roots and
branches of a spinal nerve.
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Figure 13.7 (b)
Cervical Plexus
• Formed by ventral rami of C1–C4
• Innervates skin and muscles of the neck, ear,
back of head, and shoulders
• Phrenic nerve
• Major motor and sensory nerve of the
diaphragm (receives fibers from C3–C5)
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Ventral rami
Segmental
branches
Hypoglossal
nerve (XII)
Lesser occipital
nerve
Greater auricular
nerve
Transverse
cervical nerve
Ansa cervicalis
Ventral
rami:
C1
C2
C3
C4
Accessory nerve (XI)
Phrenic nerve
C5
Supraclavicular
nerves
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Figure 13.8
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Table 13.3
Brachial Plexus
• Formed by ventral rami of C5–C8 and T1 (and often
C4 and T2)
• It gives rise to the nerves that innervate the upper
limb
• Major branches of this plexus:
• Roots—five ventral rami (C5–T1)
• Trunks—upper, middle, and lower
• Divisions—anterior and posterior
• Cords—lateral, medial, and posterior
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Roots (ventral rami):
C4
C5
Dorsal scapular
Nerve to
subclavius
Suprascapular
Cords
C6
Posterior
divisions
C7
Lateral
C8
Posterior
T1
Upper
Middle
Trunks
Lower
Long thoracic
Medial pectoral
Lateral pectoral
Medial
Axillary
Musculocutaneous
Radial
Upper subscapular
Median
Ulnar
Medial cutaneous
nerves of the arm
and forearm
Lower subscapular
Thoracodorsal
(a) Roots (rami C5 – T1), trunks, divisions, and cords
Anterior
divisions
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Posterior
divisions
Trunks
Roots
Figure 13.9 (a)
Brachial Plexus: Nerves
• Axillary—innervates the deltoid, teres minor, and skin
and joint capsule of the shoulder
• Musculocutaneous—innervates the biceps brachii
and brachialis and skin of lateral forearm
• Median—innervates the skin, most flexors and
pronators in the forearm, and some intrinsic muscles
of the hand
• Ulnar—supplies the flexor carpi ulnaris, part of the
flexor digitorum profundus, most intrinsic muscles of
the hand, and skin of medial aspect of hand
• Radial—innervates essentially all extensor muscles,
supinators, and posterior skin of limb
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Axillary
nerve
Anterior
divisions
Posterior
divisions
Trunks
Roots
Humerus
Radial nerve
Musculocutaneous nerve
Ulna
Radius
Ulnar nerve
Median nerve
Radial nerve (superficial branch)
Dorsal branch of ulnar nerve
Superficial branch of ulnar nerve
Digital branch of ulnar nerve
Muscular branch
Median nerve
Digital branch
(c) The major nerves of the upper limb
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Figure 13.9 (c)
Lumbar Plexus
• Arises from L1–L4
• Innervates the thigh, abdominal wall, and
psoas muscle
• Femoral nerve—innervates quadriceps and
skin of anterior thigh and medial surface of leg
• Obturator nerve—passes through obturator
foramen to innervate adductor muscles
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Ventral rami
Iliohypogastric
Ilioinguinal
Genitofemoral
Lateral femoral
cutaneous
Obturator
Femoral
Lumbosacral
trunk
Ventral
rami:
Iliohypogastric
L1
Ilioinguinal
Femoral
Lateral femoral
L2
cutaneous
Obturator
L3
Anterior femoral
cutaneous
Saphenous
L4
L5
(a) Ventral rami and major branches
of the lumbar plexus
(b) Distribution of the major nerves from
the lumbar plexus to the lower limb
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Figure 13.10
Sacral Plexus
• Arises from L4–S4
• Serves the buttock, lower limb, pelvic structures, and
perineum
• Sciatic nerve
• Longest and thickest nerve of the body
• Innervates the hamstring muscles, adductor magnus,
and most muscles in the leg and foot
• Composed of two nerves: tibial and common fibular
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Ventral rami
Ventral rami:
L4
Superior
gluteal
Lumbosacral
trunk
Inferior
gluteal
Common
fibular
Tibial
Posterior
femoral
cutaneous
Pudendal
Sciatic
L5
S1
S2
S3
S4
S5
Co1
Ventral rami and major branches
of the sacral plexus
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Figure 13.11 (a)
Superior gluteal
Inferior gluteal
Pudendal
Sciatic
Posterior femoral
cutaneous
Common fibular
Tibial
Sural (cut)
Deep fibular
Superficial fibular
Plantar branches
(b) Distribution of the major nerves from
the sacral plexus to the lower limb
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Figure 13.11 (b)
Innervation of Skin
• Dermatome: the area of skin innervated by
the cutaneous branches of a single spinal
nerve
• All spinal nerves except C1 participate in
dermatomes
• Most dermatomes overlap, so destruction of a
single spinal nerve will not cause complete
numbness
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Innervation of Joints
• Hilton’s law: Any nerve serving a muscle that
produces movement at a joint also innervates
the joint and the skin over the joint
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Motor Endings
• PNS elements that activate effectors by
releasing neurotransmitters
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Levels of Motor Control
• Segmental level
• Projection level
• Precommand level
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Precommand Level
(highest)
• Cerebellum and basal
nuclei
• Programs and instructions
(modified by feedback)
Internal
feedback
Feedback
Projection Level (middle)
• Motor cortex (pyramidal
system) and brain stem
nuclei (vestibular, red,
reticular formation, etc.)
• Convey instructions to
spinal cord motor neurons
and send a copy of that
information to higher levels
Segmental Level (lowest)
• Spinal cord
• Contains central pattern
generators (CPGs)
Sensory
input
Reflex activity
Motor
output
(a) Levels of motor control and their interactions
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Figure 13.13a
Precommand level
• Cerebellum
• Basal nuclei
Projection level
• Primary motor cortex
• Brain stem nuclei
Segmental level
• Spinal cord
(b) Structures involved
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Figure 13.13b
Reflexes
• Inborn (intrinsic) reflex: a rapid, involuntary,
predictable motor response to a stimulus
• Learned (acquired) reflexes result from
practice or repetition,
• Example: driving skills
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Reflex Arc
•
Components of a reflex arc (neural path)
1. Receptor—site of stimulus action
2. Sensory neuron—transmits afferent impulses to the
CNS
3. Integration center—either monosynaptic or
polysynaptic region within the CNS
4. Motor neuron—conducts efferent impulses from the
integration center to an effector organ
5. Effector—muscle fiber or gland cell that responds to
the efferent impulses by contracting or secreting
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Spinal Reflexes
• Spinal somatic reflexes
• Integration center is in the spinal cord
• Effectors are skeletal muscle
• Testing of somatic reflexes is important
clinically to assess the condition of the
nervous system
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Stretch and Golgi Tendon Reflexes
• For skeletal muscle activity to be smoothly
coordinated, proprioceptor input is necessary
• Muscle spindles inform the nervous system of
the length of the muscle
• Golgi tendon organs inform the brain as to the
amount of tension in the muscle and tendons
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Stretch Reflexes
• Maintain muscle tone in large postural
muscles
• Cause muscle contraction in response to
increased muscle length (stretch)
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Flexor and Crossed-Extensor Reflexes
• Flexor (withdrawal) reflex
• Initiated by a painful stimulus
• Causes automatic withdrawal of the
threatened body part
• Ipsilateral and polysynaptic
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Flexor and Crossed-Extensor Reflexes
• Crossed extensor reflex
• Occurs with flexor reflexes in weight-bearing
limbs to maintain balance
• Consists of an ipsilateral flexor reflex and a
contralateral extensor reflex
• The stimulated side is withdrawn (flexed)
• The contralateral side is extended
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Superficial Reflexes
• Elicited by gentle cutaneous stimulation
• Depend on upper motor pathways and cordlevel reflex arcs
• Plantar reflex
• Stimulus: stroking lateral aspect of the sole of
the foot
• Response: downward flexion of the toes
• Tests for function of corticospinal tracts
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Superficial Reflexes
• Babinski’s sign
• Stimulus: as above
• Response: dorsiflexion of hallux and fanning of
toes
• Present in infants due to incomplete
myelination
• In adults, indicates corticospinal or motor
cortex damage
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Superficial Reflexes
• Abdominal reflexes
• Cause contraction of abdominal muscles and
movement of the umbilicus in response to
stroking of the skin
• Vary in intensity from one person to another
• Absent when corticospinal tract lesions are
present
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Developmental Aspects of the PNS
• Spinal nerves branch from the developing
spinal cord and neural crest cells
• Supply both motor and sensory fibers to
developing muscles to help direct their
maturation
• Cranial nerves innervate muscles of the head
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Developmental Aspects of the PNS
• Distribution and growth of spinal nerves
correlate with the segmented body plan
• Sensory receptors atrophy with age and
muscle tone lessens due to loss of neurons,
decreased numbers of synapses per neuron,
and slower central processing
• Peripheral nerves remain viable throughout
life unless subjected to trauma
Copyright © 2010 Pearson Education, Inc.