The
Peripheral
Nervous
System
The Peripheral Nervous System
• 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
Functional Organization of the PNS
Basic Structural Components of the
PNS
• Sensory receptors – pick up stimuli from inside
or outside the body
• Motor endings – axon terminals of motor
neurons
– Innervate effectors (muscle fibers and glands)
• Nerves and ganglia
– Nerves – bundles of peripheral axons
– Ganglia – clusters of peripheral neuronal cell
bodies
Structural Organization of PNS in
Region of a Spinal Nerve
Peripheral Endings
• Afferent: Sensory Receptors
• Efferent: Somatic Motor
• Efferent: Autonomic Nervous System
Afferent: Sensory
Peripheral Sensory Receptors
• Structures that pick up sensory stimuli
– Initiate signals in sensory axons
Peripheral Sensory Receptors
• 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
• Monitor most types of special sensory information
Sensory Receptors
Peripheral Sensory Receptors
• Sensory receptors also classified according to:
– Location
– Type of stimulus detected
– Structure
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 internal viscera
– Monitor a variety of stimuli
• Proprioceptors – monitor degree of stretch
– Located in musculoskeletal organs
Classification by Modality
• 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
Classification by Structure
• General sensory receptors
– Widely distributed
– Nerve endings of sensory neurons monitor:
• Touch, pressure, vibration, stretch
• Pain, temperature, proprioception
• Divided into two groups
• Free nerve endings
• Encapsulated nerve endings
Free Nerve Endings
• Abundant in epithelia and underlying
connective tissue
• 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
Unencapsulated Nerve Endings
Encapsulated Nerve Endings
• Consist of one or more end fibers of sensory
neurons
• Enclosed in connective tissue
• Mechanoreceptors
• Include four main types
Encapsulated Nerve Endings
• Encapsulated nerve endings: dendrites with
special supporting structures
(mechanoreceptors and proprioceptors)
Encapsulated Nerve Endings
•
•
•
•
Meissner’s corpuscles
Pacinian corpuscles
Ruffini’s corpuscles
Proprioceptors
Encapsulated Receptors
Proprioceptors
– Monitor stretch in locomotory organs
– Three types of proprioceptors
Three Types of Proprioceptors
• Muscle spindles – measure the changing length of a
muscle
– Imbedded in the perimysium between muscle fascicles
• Golgi tendon organs – located near the muscletendon junction
– Monitor tension within tendons
• Joint kinesthetic receptors
– Sensory nerve endings within the joint capsules
Proprioceptors
Structure of Receptors in Skin
Somatic Efferent: Innervation of
Skeletal Muscles
Innervation of Skeletal Muscle
• 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
The Neuromuscular Junction
Innervation of Skeletal Muscle
• Motor unit – a motor neuron and all the
muscle fibers it innervates
Efferent: Autonomic Nervous System
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
– Slower than somatic motor reflexes
Innervation of Smooth Muscle
Cranial Nerves
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
– The vagus nerve (X) extends into the abdomen
The 12 Pairs of Cranial Nerves
CN I: Olfactory Nerves
• Sensory nerves of smell
• Sense of smell
• Damage causes impaired sense of smell
CN II: Optic Nerve
• Sensory nerve of vision
• Provides vision
• Damage causes blindness in visual field
CN III: Oculomotor Nerve
• Innervates four of the extrinsic eye muscles
• Somatic and Autonomic motor function
• Eye movement (Superior, inferior, medial rectus muscles and inferior oblique
muscle), opening of eyelid (levator palpebrae superioris), constriction of
pupil (circular muscle), focusing (ciliary muscle and accomodation)
• Damage causes drooping eyelid, dilated pupil, double vision, difficulty
focusing and inability to move eye in certain directions
CN IV: Trochlear Nerve
• Innervates an extrinsic eye muscle
• Eye movement (superior oblique muscle)
• Damage causes double vision and inability to
rotate eye inferolaterally
CN V: Trigeminal Nerve
• Provides sensory innervation to the face
– Motor innervation to chewing muscles
• Ophthalmic branch – sensations from nasal cavity, skin of
forehead, upper eyelid, eyebrow, nose
• Maxillary branch – sensations from lower eyelid, upper lips and
gums, teeth of the maxilla, cheek, nose, palate, pharynx
• Mandibular branch – sensations from teeth of the mandible,
lower gums and lips, palate, tongue. Motor function of
temporalis and masseter muscles.
• Damage produces loss of sensation and impaired chewing
Trigeminal Nerve
CN VI: Abducens Nerve
• Abducts the eyeball
• Provides eye movement (lateral rectus m.)
• Damage results in inability to rotate eye
laterally and at rest eye rotates medially
CN VII: Facial Nerve
• Innervates muscles of facial expression
• Sensory innervation of face
• Taste
• Somatic Motor - facial expressions
• Autonomic Motor - salivary and lacrimal glands, mucous
membranes of nasal and palatine mucosa
• Special Sensory - taste on anterior 2/3’s of tongue
• Damage produces sagging facial muscles and disturbed sense
of taste (no sweet and salty)
Branches of Facial Nerve
Clinical test: Test anterior 2/3’s of tongue with substances such as
sugar, salt, vinegar, and quinine; test response of tear glands to
ammonia fumes; test motor functions by asking subject to close
eyes, smile, whistle, frown, raise eyebrows, etc.
CN VIII: Vestibulocochlear Nerve
• Sensory nerve of hearing and balance
• Special Sensory
• Provides hearing (cochlear branch) and sense of balance
(vestibular branch)
• Damage produces deafness, dizziness, nausea, loss of balance
and nystagmus
CN IX: Glossopharyngeal Nerve
• Sensory and motor innervation of structures of the tongue
and pharynx
• Taste
•
•
•
•
•
Somatic motor – Swallowing and voice production via pharyngeal muscles
Autonomic motor - salivation, gagging, control of BP and respiration
Sensations from posterior 1/3 of tongue including taste
Sensations from baroreceptors and chemoreceptors
Damage results in loss of bitter and sour taste and impaired swallowing, blood
pressure anomalies (with CN X).
CN X: Vagus Nerve
• A mixed sensory and
motor nerve
• Main
parasympathetic
nerve
– “Wanders” into
thorax and abdomen
Vagus Nerve X
• Sensations from skin at back of ear,
external acoustic meatus, part of
tympanic membrane, larynx, trachea,
espophagus, thoracic and abdominal
viscera
• Sensations from bararoceptors and
chemoreceptors
• Special sensory – taste from epiglottis
and pharynx
• Somatic motor – Swallowing and voice
production via pharyngeal muscles
• Autonomic motor – smooth muscle of
abdominal viscera, visceral glands
secretions, relaxation of airways, and
normal or decreased heart rate.
• Damage causes hoarseness or loss of
voice, impaired swallowing, GI
dysfunction, blood pressure anomalies
(with CN IX), fatal if both are cut
CN XI: Accessory Nerve
• An accessory part of the vagus nerve
• Somatic motor function of pharynx, larynx,
neck muscles
• Swallowing, head, neck and shoulder movement via trapezius and
sternocleidomastoid and pharyngeal muscles
• Damage causes impaired head, neck, shoulder movement
CN XII: Hypoglossal Nerve
• Runs inferior to the tongue
– Innervates the
tongue muscles
• Tongue movements for speech, food manipulation
and swallowing
• If both are damaged – can’t protrude tongue
• If one side is damaged – tongue deviates towards
injured side
Cranial Nerve Disorders
• Trigeminal neuralgia (tic douloureux)
– recurring episodes of intense stabbing pain in
trigeminal nerve area (near mouth or nose)
– pain triggered by touch, drinking, washing face
– treatment may require cutting nerve
• Bell’s palsy
– disorder of facial nerve causes paralysis of facial
muscles on one side
– may appear abruptly with full recovery within 3-5
weeks
Spinal Nerves
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)
– 5 pairs of sacral nerves (S1-S5)
– 1 pair of coccygeal nerves (Co1)
Spinal Nerves Posterior View
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 (cell bodies in gray matter of spinal cord – no ganglia)
Spinal Nerves
• 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
• 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
Map of Dermatomes – Anterior View
Map of Dermatomes – Posterior View
Disorders of the PNS: Shingles
• Shingles (herpes
zoster)
– Viral infection
– Stems from
childhood chicken
pox
– Often brought on
by stress
– Mostly experienced
by those over 50
Disorders of the PNS: Migraine
Headache
• Migraine headache
– Relates to sensory innervation of cerebral arteries
– Arteries dilate
– Compresses and irritates sensory nerve endings
Disorders of the PNS: Myasthenia Gravis
• Myasthenia gravis
– Progressive weakening of the skeletal muscles
– An autoimmune disorder
– Antibodies destroy acetylcholine receptors
Ptosis due to weakness of
eyelid muscles