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The Peripheral Nervous System
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Nervous structures outside the brain and spinal cord
Nerves allow the CNS to receive information and
take action
Functional components of the PNS
• Sensory inputs and motor outputs
• Categorized as somatic or visceral
• Sensory inputs also classified as general or special
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Functional Organization of the PNS
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Figure 14.1
Basic Structural Components of the PNS
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Sensory Receptors: pick up stimuli from inside or
outside the body
Motor Endings: axon terminals of motor neurons
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Nerves and Ganglia:
• Innervate effectors (muscle fibers and glands)
• Nerves – bundles of peripheral axons
• Ganglia – clusters of peripheral neuronal cell bodies
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Structural Organization of PNS in Region of a Spinal Nerve
Peripheral Nerve Endings:
• Afferent: Sensory Receptors
• Efferent: Somatic Motor
• Efferent: Autonomic Nervous System
Peripheral Sensory Receptors
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Structures that pick up sensory stimuli
• Initiate signals in sensory axons
Two main categories of sensory receptors
• Special nerve endings of sensory neurons
• Monitor general sensory information
• Independent receptor cells – specialized epithelial cells or
small neurons
Sensory receptors can be classified in three ways:
• Type of stimulus detected
• Location
• Structural Complexity
• Monitor most types of special sensory information
Classification by Type of Stimuli
• Mechanoreceptors – respond to mechanical forces
• Thermoreceptors – respond to temperature changes
• Chemoreceptors – respond to chemicals in solution
• Photoreceptors – respond to light – located in the eye
• Nociceptors – respond to harmful stimuli that result in pain
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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, and temperature
• Interoceptors: – (visceroceptors) receive stimuli from
within the body
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internal viscera & blood vessels
Monitor a variety of stimuli such as chemical changes,
stretching of tissue & temperature
• Proprioceptors: – monitor degree of stretch
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Located in musculoskeletal organs
Classification by Structure
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Simple:
• Vast majority
• Modified dendritic extensions of sensory neurons
• Found in skin, mucous membranes, muscles & connective
tissue
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Complex:
• Associated with the special senses (vision, hearing, smell
& taste)
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General Sensory Receptors (simple receptors)
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Widely distributed
Nerve endings of sensory neurons monitor:
• Touch, pressure, vibration, stretch
• Pain, temperature, proprioception
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Divided into two groups
• Free dendritic (nerve) endings
• Encapsulated dendritic (nerve) endings
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Free Nerve Endings
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Abundant in epithelia and underlying connective tissue
Most are unmyelinated with a small diameter
Respond to pain and temperature
Monitor affective senses
Two specialized types of free nerve endings
• Merkel discs – lie in the epidermis
• Slowly adapting receptors for light touch
• Hair follicle receptors – wrap around hair follicles
• Rapidly adapting receptors
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Encapsulated Nerve Endings
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Consist of one or more end fibers of sensory neurons
Enclosed in connective tissue
Mechanoreceptors
Include four main types:
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Meissner’s corpuscles
Pacinian corpuscles
Ruffini’s corpuscles
Proprioceptors – monitor stretch in locomotory organs
• Muscle spindles
• Golgi tendon organs
• Joint kinesthetic receptors
Three Types of Proprioceptors
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Muscle spindles – measure the changing length of
a muscle
• Imbedded in the perimysium between muscle
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fascicles
Golgi tendon organs – located near the muscletendon junction
• Monitor tension within tendons
Joint kinesthetic receptors
• Sensory nerve endings within the joint capsules
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Structure of Receptors in Skin
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Innervation of Skeletal Muscle
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Motor axons innervate skeletal
muscles
• Neuromuscular junctions
(motor end plates)
Similar to synapses between
neurons
Acetylcholine diffuses across
the synaptic cleft
• Binds with molecules on
the sarcolemma
Motor axons branch to
innervate muscle fibers
Motor unit – a motor neuron
and all the muscle fibers it
innervates
The Neuromuscular Junction
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Figure 14.5a, b
Innervation of Visceral Muscle and Glands
• Simpler than neuromuscular junctions of skeletal muscle
• Near the smooth muscle or gland it innervates
• Visceral motor axon swells into a row of varicosities
• Visceral motor responses
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Slower than somatic motor reflexes
Cranial Nerves
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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
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Primarily serve head and neck structures
• The vagus nerve (X) extends into the abdomen
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The 12 Pairs of Cranial Nerves
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Figure 14.8
CN I: Olfactory Nerves
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Sensory nerves of smell
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Table 14.2
CN II: Optic Nerve
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Sensory nerve of vision
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Table 14.2
CN III: Oculomotor Nerve
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Innervates four of the extrinsic eye muscles
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Table 14.2
CN IV: Trochlear Nerve
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Innervates an extrinsic eye muscle
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Table 14.2
CN V: Trigeminal Nerve
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Provides sensory innervation to the face
• Motor innervation to chewing muscles
CN VI: Abducens Nerve
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Abducts the eyeball
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Table 14.2
CN VII: Facial Nerve
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Innervates muscles of facial expression
Sensory innervation of face
Taste
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Table 14.2
CN VIII: Vestibulocochlear Nerve
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Sensory nerve of hearing and balance
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Table 14.2
CN IX: Glossopharyngeal Nerve
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Sensory and motor innervation of structures of the
tongue and pharynx
Taste
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Table 14.2
CN X: Vagus Nerve (pneumogastric nerve)
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A mixed sensory and
motor nerve
Main parasympathetic
nerve
• “Wanders” into thorax
and abdomen
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Table 14.2
CN XI: Accessory Nerve
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An accessory part of the vagus nerve
Somatic motor function of pharynx, larynx, neck
muscles
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Table 14.2
CN XII: Hypoglossal Nerve
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Runs inferior to the tongue
• Innervates the tongue muscles
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Table 14.2
Spinal Nerves
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31 pairs – contain thousands of nerve fibers
Connect to the spinal cord
Named for point of issue from the spinal cord
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8 pairs of cervical nerves (C1-C8)
12 pairs of thoracic nerves (T1-T12)
5 pairs of lumbar nerves (L1-L5)
5 pairs of sacral nerves (S1-S5)
1 pair of coccygeal nerves (Co1)
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Spinal Nerves Posterior View
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Figure 14.9
Spinal Nerves
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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 (cell bodies in gray matter of spinal cord – no
ganglia)
Spinal Nerves
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Branch into dorsal ramus and ventral ramus
Rami communicantes connect to the base of the ventral ramus
• Lead to the sympathetic chain ganglia (gray and white
ramus)
Dorsal and ventral rami contain sensory and motor fibers
Innervation of the Skin: Dermatomes
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Dermatome – an area of skin
• Innervated by cutaneous branches of a single spinal
nerve
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Upper limb – skin is supplied by nerves of the
brachial plexus
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Lower limb
• Lumbar nerves – anterior surface
• Sacral nerves – posterior surface
Map of Dermatomes – Anterior & Posterior View
Reflex Activity
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A reflex is a rapid, predictable motor response to a
stimulus
The basic reflex is unlearned, unpremeditated and
involuntary,
Some reflexes we are aware of (pulling away from
hot pot) and others, usually visceral, we are not
In addition to basic reflexes there are also learned or
acquired reflexes (automatic actions of an
experienced driver)
Reflexes occur in highly specific neural paths called
reflex arcs
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The Reflex Arc
1. Receptor: site of stimulus action
2. Sensory Neuron: transmits afferent impulses to the CNS
3. Integration Center: synapse between sensory & motor neuron or
multiple synapses within a chain of interneurons
4. Motor Neuron: conducts the afferent impulses from integration center
to effector organ
5. Effector: the muscle fiber or gland cell that response to the efferent
impulses in a characteristic way
Spinal Reflexes
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Spinal reflexes are somatic reflexes mediated by the
spinal cord
• In the stretch reflex the muscle spindle is stretched and
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excited by either an external stretch or an internal stretch
The Golgi tendon reflex produces muscle relaxation and
lengthening in response to contraction
The flexor, or withdrawal, reflex is initiated by a painful
stimulus and causes automatic withdrawal of the
threatened body part from the stimulus
They crossed extensor reflex is a complex spinal reflex
consisting of an ipsilateral withdrawal reflex and a
contralateral extensor reflex
Superficial reflexes are elicited by gentle cutaneous
stimulation
The Stretch Reflex
Deep Golgi Tendon Reflex
Disorders: Anencephaly
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Various degrees of absence of the
various of the skull bones, cerebrum
and the brain stem
One of the most common neural tube
defects. Neural tube.
Occurs early in the development
when the upper part of the neural
tube fails to close.
Possible causes include
environmental toxins and low intake
of folic acid by the mother during
pregnancy.
Often results in miscarriage, if child
survives to birth he/she will be
totally vegetative and unable to see,
hear or process sensory input.
Death will occur soon after birth.
Disorders: Shingles
• Viral infection
• Childhood chicken pox dormant in spinal ganglia
• Often brought on by stress
• Mostly experienced by those over 50
• Unilateral patch of skin blisters and discoloration along
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the path of one or more spinal nerves
Most commonly around the waist. The symptoms may
persist for 3 to 6 months.
Disorders: Cerebal Palsy
• Group of neuromuscular
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disorders in which the
voluntary muscles are poorly
controlled or paralyzed due to
brain damage
Can result in spasticity,
speech difficulties, seizures,
deafness or reduced cognitive
ability
Can be due to lack of oxygen
during delivery, smoking or
drug use, or maternal
infection during pregnancy
Disorders: Spina Bifida & Lumbar Myelomeningocele
• Incomplete formation of vertebral
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arches, usually in lumbosacral
region
Laminae & spinal processes
missing in at least one vertebrae
In the severe form, spina bifida
cyctica, it can result in a sac-like
cyst protruding dorsally
The cyst may contain meninges
and CSF and possibly portiosns of
the spinal cord and spinal root
nerves
primary functional deficits are
lower limb paralysis and sensory
loss, bladder and bowel
dysfunction, and cognitive
dysfunction
Disorders :Migraine Headache
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Migraine headache
• Relates to sensory innervation of cerebral arteries
• Arteries dilate
• Compresses and irritates sensory nerve endings
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Disorders: Poliomyelitis
• “polio” means gray matter
• caused by Poliovirus hominis.
• CNS infection, but its major effect
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is on the peripheral nerves and
the muscles they supply.
virus infects the motor neurons
in the anterior horn of the central
gray matter of the spinal cord.
The infection causes
degeneration of the motor
neurons, which results in
paralysis and atrophy of the
muscles innervated by those
nerves.
Disorders: Anesthetic leprosy
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Bacterial infection of the
peripheral nerves caused
by Mycobacterium leprae.
Infection results in
anesthesia, paralysis,
ulceration, and gangrene
Disorders :Myasthenia Gravis
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Myasthenia gravis
• Progressive weakening of the skeletal muscles
• An autoimmune disorder
• Antibodies destroy acetylcholine receptors
Ptosis due to weakness of
eyelid muscles
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