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PNS
Chapter 13
Peripheral Nervous System (PNS)
• Provides links from and to world outside body
• All neural structures outside brain
– Sensory receptors
– Peripheral nerves and associated ganglia
– Efferent motor endings
Sensory Receptors
• Specialized to respond to changes in
environment (stimuli)
• Activation results in graded potentials that
trigger nerve impulses
• Sensation (awareness of stimulus) and
perception (interpretation of meaning of
stimulus) occur in brain
Classification of Receptors
• Based on
– Type of stimulus they detect
– Location in body
– Structural complexity
Classification by Stimulus Type
• Mechanoreceptors—respond to touch, pressure,
vibration, and stretch
• 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)
Classification by Location
• Exteroceptors
– Respond to stimuli arising outside body
– Most special sense organs
• Interoceptors (visceroceptors)
– Respond to stimuli arising in internal viscera and
blood vessels
• Proprioceptors
– Inform brain of body movements
Classification by Receptor Structure
• General senses (simple receptors)
– Tactile sensations (touch, pressure, stretch,
vibration), temperature, pain, and muscle sense
– Modified dendritic endings of sensory neurons
• Non-encapsulated (free nerve endings)
• Encapsulated
• Special senses
– Vision, hearing, equilibrium, smell, and taste
(Chapter 15)
Sensory Integration
• Somatosensory system – part of
sensory system serving body wall and
limbs
– Receives inputs from Exteroceptors,
proprioceptors, and interoceptors
– Input relayed toward head, but
processed along way
– Levels of neural integration in sensory
systems:
1. Receptor level—sensory
receptors
2. Circuit level—processing in
ascending pathways
3. Perceptual level—processing in
cortical sensory areas
Adaptation of Sensory Receptors
• Adaptation is change in sensitivity in presence
of constant stimulus
– Phasic (fast-adapting) receptors signal beginning
or end of stimulus
• Examples - receptors for pressure, touch, and smell
– Tonic receptors adapt slowly or not at all
• Examples - nociceptors and most proprioceptors
Processing at the Perceptual Level
• Interpretation of sensory input depends on
specific location of target neurons in sensory
cortex
• Aspects of sensory perception:
– Perceptual detection—ability to detect a stimulus
(requires summation of impulses)
– Magnitude estimation—intensity coded in
frequency of impulses
– Spatial discrimination—identifying site or pattern
of stimulus (studied by two-point discrimination
test)
Perception of Pain
• Warns of actual or impending tissue damage 
protective action
• Impulses travel on fibers that release
neurotransmitters glutamate and substance P
• Some pain impulses are blocked by inhibitory
endogenous opioids (e.g., endorphins)
• All perceive pain at same stimulus intensity, but pain
tolerance varies
– "Sensitive to pain" means low pain tolerance, not low pain
threshold
– Pain tolerance, and response to pain medication, is rooted
in genetics.
Homeostatic Imbalance
• Hyperalgesia (pain amplification),
– NMDA receptors-allow spinal cord to "learn"
hyperalgesia
• Phantom limb pain – felt in limb no longer
present
– epidural use during anesthesia to prevent
Visceral and Referred Pain
• Stimulation of visceral organ
receptors
– Felt as vague aching, gnawing, burning
– Activated by tissue stretching,
ischemia, chemicals, muscle spasms
• Referred pain
– Pain from one body region perceived
from different region
– Visceral and somatic pain fibers travel
in same nerves; brain assumes
stimulus from common (somatic)
region
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
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?
• Peripheral nerves classified as cranial or spinal
nerves
• Ganglia
– Contain neuron cell bodies associated with nerves
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
Endoneurium
Schwann cells
Droplets
of myelin
1 The axon
becomes
fragmented at
the injury site.
Fragmented
axon
Site of nerve damage
Figure 13.4 (1 of 4)
Schwann cell
Macrophage
2 Macrophages
clean out the
dead axon distal
to the injury.
Figure 13.4 (2 of 4)
Aligning Schwann cells
form regeneration tube
3 Axon sprouts,
or filaments,
grow through a
regeneration tube
formed by
Schwann cells.
Fine axon sprouts
or filaments
Figure 13.4 (3 of 4)
Schwann cell
Site of new
myelin sheath
formation
4 The axon
regenerates and
a new myelin
sheath forms.
Single enlarging
axon filament
Figure 13.4 (4 of 4)
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”
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)
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)
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
• Purely sensory (olfactory) function
• Afferent impulses for sense of smell
• Injury diagnosis: partial or total loss of smell
(anosmia)
Table 13.2
II: The Optic Nerves
• Pass through the optic canals, converge and
partially cross over at the optic chiasma
• Purely sensory (visual) function
• Injury diagnosis: blindness
or partial loss of vision
in affected eye
Table 13.2
III: The Oculomotor Nerves
• Functions in raising the eyelid, directing the
eyeball, constricting the iris
(parasympathetic), and controlling lens shape
• Injury: eye cannot be
moved (up, down, inward)
or rotates laterally when at
rest, upper eyelid droops,
double vision, and difficulty
focusing on close objects
Table 13.2
IV: The Trochlear Nerves
• Primarily a motor nerve that directs the
eyeball
• Injury: double vision, reduced ability to rotate
eye inferolaterally
• Tested w/ III
Table 13.2
V: The Trigeminal Nerves
• Three divisions
–
–
–
–
Ophthalmic (V1)
Maxillary (V2)
Mandibular (V3)
Convey sensory impulses from various
areas of the face (V1) and (V2), and
supplies motor fibers (V3) for
mastication
• Injury: produces excruciating pain for a
few seconds to a minute, recurring
many times a day and by various
causes (brushing teeth, breeze hitting
the face)
VI: The Abducens Nerves
• Primarily a motor nerve, innervating the
lateral rectus muscle
• Injury: eye cannot be moved laterally, at rest
eyeball rotates medially
VII: The Facial Nerves
• Chief motor nerves of the face
with 5 major branches
– Temporal, zygomatic, buccal,
mandibular, cervical
• Motor functions include facial
expression, parasympathetic
impulses to lacrimal and salivary
glands
• Sensory function (taste) from the
anterior two-thirds of the tongue
• Injury: Bell’s palsy
VIII: The Vestibulocochlear Nerves
• Mostly sensory function; small motor
component for adjustment of sensitivity of
receptors
• Injury:
– Cochlear: deafness
– Vestibular:
dizziness, rapid
involuntary eye
movements, loss of
balance, nausea,
vomiting
IX: The Glossopharyngeal Nerves
• 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
• Injury: Impaired
swallowing or taste
X: The Vagus Nerves
• 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
X: The Vagus Nerves
• Injury: Hoarseness or loss of voice, difficulty
swallowing, impaired digestive system
motility.
• Total destruction is incompatible with life
XI: The Accessory Nerves
• 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
• Injury: head turns toward
side of injury, shrugging of
that shoulder difficult
XII: The Hypoglossal Nerves
• Innervate extrinsic and intrinsic muscles of the
tongue that contribute to swallowing and
speech
• Injury: difficulties in
speech/swallowing,
problems with the tongue.
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)
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
– Dorsal roots
• Contain sensory (afferent) fibers from sensory neurons
in the dorsal root ganglia
• Dorsal + Ventral = spinal nerves
Spinal Nerves: Rami
• Each spinal nerve branches into mixed rami
– Dorsal ramus
– Ventral ramus - larger
– Meningeal branch – tiny, reenters vertebral canal,
innervates meninges and blood vessels
– Rami communicantes (autonomic pathways) join
ventral rami in thoracic region
Spinal Nerves: Rami
• All ventral rami (except T2–T12) form nerve plexuses
(cervical, brachial, lumbar, and sacral)
• Back innervated by dorsal rami via several branches
• Ventral rami of T2–T12 as intercostal nerves supply
muscles of ribs, anterolateral thorax, and abdominal
wall
• Spinal roots get longer as move inferiorly in cord
– Lumbar and sacral roots extend as cauda equina
Spinal Nerves: Plexuses
• Within plexus fibers criss-cross
– Each branch contains fibers from several spinal
nerves
– Fibers from as single ventral ramus go to body
periphery via several routes
• Each limb muscle innervated by more than one spinal
nerve
Cervical Plexus and the Neck
• Formed by ventral rami of C1–C4
• Most branches form cutaneous nerves
– Innervate skin of neck, ear, back of head, and
shoulders
– Other branches innervate neck muscles
Phrenic nerve
• Major motor and sensory
nerve of diaphragm (receives
fibers from C3–C5)
• Irritation  hiccups
Brachial Plexus and Upper Limb
• Formed by ventral rami of C5–C8 and T1 (and
often C4 and/or T2)
• Gives rise to nerves that innervate upper limb
Figure 13.10a The brachial plexus.
Anterior
divisions
Posterior
divisions
Trunks
Roots
Dorsal scapular
Nerve to
subclavius
Suprascapular
Cords
Roots (ventral rami):
C4
C5
C6
Posterior
divisions
C7
Lateral
C8
Posterior
T1
Upper
Middle
Trunks
Lower
Long thoracic
Medial
Medial pectoral
Lateral pectoral
Axillary
Upper subscapular
Musculocutaneous
Lower subscapular
Radial
Thoracodorsal
Median
Ulnar
Roots (rami C5–T1), trunks, divisions, and cords
© 2013 Pearson Education, Inc.
Medial cutaneous
nerves of the arm
and forearm
Brachial Plexus: Five Important Nerves
•
•
•
•
•
Axillary
Musculocutaneous
Median
Ulnar
Radial
Lumbar Plexus
• Arises from L1–L4
• Innervates 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
Figure 13.11 The lumbar plexus.
Ventral rami
Ventral
rami:
L1
L2
Iliohypogastric
Ilioinguinal
Iliohypogastric
Femoral
Ilioinguinal
Lateral
femoral
cutaneous
Genitofemoral
Lateral femoral
cutaneous
L3
Obturator
L4
Anterior
femoral
cutaneous
Saphenous
Obturator
Femoral
L5
Lumbosacral
trunk
Ventral rami and major branches of the lumbar plexus
Distribution of the major nerves from the
lumbar plexus to the lower limb
Sacral Plexus
• Arises from L4–S4
• Serves the buttock, lower limb, pelvic
structures, and perineum
• Sciatic nerve
– Longest and thickest nerve of body
– Innervates hamstring muscles, adductor magnus,
and most muscles in leg and foot
– Composed of two nerves: tibial and common
fibular
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
Stimulus
Reflex Arc
Skin
1 Receptor
Interneuron
2 Sensory neuron
3 Integration center
4 Motor neuron
5 Effector
Spinal cord
(in cross section)
Figure 13.14
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
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
Stretch Reflexes
• Maintain muscle tone in large postural
muscles
• Cause muscle contraction in response to
increased muscle length (stretch)
• How a stretch reflex works:
– Stretch activates the muscle spindle
– sensory neurons synapse directly with motor
neurons in the spinal cord
– motor neurons cause the stretched muscle to
contract
Stretch Reflexes
• All stretch reflexes are monosynaptic and
ipsilateral
• Reciprocal inhibition also occurs
– fibers synapse with interneurons that inhibit the
motor neurons of antagonistic muscles
• Example: In the patellar reflex, the stretched
muscle (quadriceps) contracts and the
antagonists (hamstrings) relax
Stretched muscle spindles initiate a stretch reflex,
causing contraction of the stretched muscle and
inhibition of its antagonist.
The events by which muscle stretch is damped
1 When muscle spindles are activated
2 The sensory neurons synapse directly with alpha
motor neurons (red), which excite extrafusal fibers
by stretch, the associated sensory
of the stretched muscle. Afferent fibers also
neurons (blue) transmit afferent impulses synapse with interneurons (green) that inhibit motor
at higher frequency to the spinal cord.
neurons (purple) controlling antagonistic muscles.
Sensory
neuron
Cell body of
sensory neuron
Initial stimulus
(muscle stretch)
Spinal cord
Muscle spindle
Antagonist muscle
3a Efferent impulses of alpha motor neurons
3b Efferent impulses of alpha motor
cause the stretched muscle to contract,
which resists or reverses the stretch.
neurons to antagonist muscles are
reduced (reciprocal inhibition).
Figure 13.17 (1 of 2)
The patellar (knee-jerk) reflex—a specific example of a stretch reflex
2
Quadriceps
(extensors)
1
3a
3b
3b
Patella
Muscle
spindle
Spinal cord
(L2–L4)
Hamstrings
(flexors)
Patellar
ligament
1 Tapping the patellar ligament excites
muscle spindles in the quadriceps.
2 Afferent impulses (blue) travel to the
spinal cord, where synapses occur with
motor neurons and interneurons.
3a The motor neurons (red) send
+
–
Excitatory synapse
Inhibitory synapse
activating impulses to the quadriceps
causing it to contract, extending the
knee.
3b The interneurons (green) make
inhibitory synapses with ventral horn
neurons (purple) that prevent the
antagonist muscles (hamstrings) from
resisting the contraction of the
quadriceps.
Figure 13.17 (2 of 2)
Golgi Tendon Reflexes
• Polysynaptic reflexes
• Help to prevent damage due to excessive
stretch
• Important for smooth onset and termination
of muscle contraction
Golgi Tendon Reflexes
• Produce muscle relaxation (lengthening) in response
to tension
– Contraction or passive stretch activates Golgi
tendon organs
– Afferent impulses are transmitted to spinal cord
– Contracting muscle relaxes and the antagonist
contracts (reciprocal activation)
– Information transmitted simultaneously to the
cerebellum is used to adjust muscle tension
1 Quadriceps strongly
2 Afferent fibers synapse
contracts. Golgi tendon
organs are activated.
with interneurons in the
spinal cord.
Interneurons
Quadriceps
(extensors)
Spinal cord
Golgi
tendon
organ
Hamstrings
(flexors)
+ Excitatory synapse
– Inhibitory synapse
3a Efferent impulses
3b Efferent
to muscle with
stretched tendon are
damped. Muscle
relaxes, reducing
tension.
impulses to
antagonist
muscle cause
it to contract.
Figure 13.18
Flexor and Crossed-Extensor Reflexes
• Flexor (withdrawal) reflex
– Initiated by a painful stimulus
– Causes automatic withdrawal of the threatened
body part
– Ipsilateral and polysynaptic
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
+ Excitatory synapse
– Inhibitory synapse
Interneurons
Efferent
fibers
Afferent
fiber
Efferent
fibers
Extensor
inhibited
Flexor
stimulated
Site of stimulus: a noxious
stimulus causes a flexor
reflex on the same side,
withdrawing that limb.
Arm
movements
Flexor
inhibited
Extensor
stimulated
Site of reciprocal
activation: At the
same time, the
extensor muscles
on the opposite
side are activated.
Figure 13.19