BIOL 2304
Peripheral Nervous System
PNS Functions
Provides vital link between brain, body, and outside world
Allows CNS to receive information and initiate action
Sensory inputs and motor outputs categorized as
Somatic or visceral
General or special
Basic Structure of PNS
Sensory receptors—pick up stimuli from inside or outside the body
Nerves and ganglia
Nerves—bundles of peripheral axons
Ganglia—clusters of peripheral neuronal cell bodies
Motor endings—axon terminals of motor neurons
Innervate effectors (muscle fibers and glands)
Cranial Nerves
Numbered from I–XII
Attach to the brain and pass through foramina of the skull
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) is the only cranial nerve that extends into the abdomen
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Cranial Nerves
Cranial Nerve I – Olfactory
A sensory nerve involved in the sense of smell.
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Cranial Nerve II – Optic
Transmits visual information from the retina to the brain
Cranial Nerve III – Oculomotor
Controls most of the eye's movements, including constriction of the pupil and maintaining an open
eyelid by innervating the Levator palpebrae superiors muscle.
Cranial Nerve IV – Trochlear
A motor nerve (a “somatic efferent” nerve) that innervates a single
muscle: the superior oblique muscle of the eye.
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Cranial Nerve V – Trigeminal
Contains both sensory and motor fibers. It is responsible for sensation in the face and certain motor
functions such as biting, chewing, and swallowing.
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Cranial Nerve VI – Abducens
A somatic efferent nerve that controls the movement of a single muscle, the lateral rectus muscle of the
eye, in humans
Cranial Nerve VII – Facial
Controls the muscles of facial expression, and functions in the conveyance of taste sensations from the
anterior two-thirds of the tongue and oral cavity. It also supplies preganglionic parasympathetic fibers to
several head and neck ganglia.
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Cranial Nerve VIII – Vestibulocochlear
Is responsible for transmitting sound and equilibrium (balance) information from the inner ear to the
brain.
Cranial Nerve IX – Glossopharyngeal
Receives general sensory fibers (ventral trigeminothalamic tract) from the tonsils, the pharynx, the
middle ear and the posterior 1/3 of the tongue.
Receives special sensory fibers (taste) from the posterior one-third of the tongue.
Receives visceral sensory fibers from the carotid bodies, carotid sinus.
Supplies parasympathetic fibers to the parotid gland via the otic ganglion.
Supplies motor fibers to stylopharyngeus muscle, the only motor component of this cranial nerve.
Contributes to the pharyngeal plexus.
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Cranial Nerve X – Vagus
Parasympathetic innervation of organs
The vagus nerve contributes to the innervation of the viscera.
Besides output to the various organs in the body, the vagus nerve conveys sensory information about
the state of the body's organs to the central nervous system.
80-90% of the nerve fibers in the vagus nerve are afferent (sensory) nerves communicating the state of
the viscera to the brain.
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Cranial Nerve XI – Accessory
The Spinal Accessory nerve controls specific muscles of the shoulder and neck.
It is considered a cranial nerve because part of it was formerly believed to originate in the brain.
It provides motor innervation from the central nervous system to two muscles of the neck: the
sternocleidomastoid muscle and the trapezius muscle.
Cranial Nerve XII – Hypoglossal
It supplies motor fibers to all of the muscles of the tongue, except the palatoglossus muscle, which is
innervated by the vagus nerve
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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)
5 pairs of sacral nerves (S1–S5)
1 pair of coccygeal nerves (Co1)
Spinal Nerves
Although there are seven cervical vertebrae (C1-C7), there are eight cervical nerves (C1-C8)
All nerves emerge above their corresponding vertebrae except for C8
The C8 nerve emerges below the C7 vertebra
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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
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
Singular: Ramus communicans
Nerve Plexuses
Ventral rami form nerve plexuses
Cervical
Located under sternocleidomastoid
Formed by rami of C1-C4
Supplies nerves of neck (phrenic nerve)
Brachial
Located in neck and axilla
Formed by rami of C5-C8
Supplies nerves of upper limbs (radial, ulnar nerves)
Lumbar
Formed by rami of L1-L4
Supplies nerves of anterior lower limb (femoral nerve)
Sacral
Formed by rami of L4-S4
Supplies nerves of gluteal muscles, perineum, and posterior lower limb (sciatic nerve)
Autonomic Nervous System
The ANS responds to stimuli by activating smooth muscle, cardiac muscle, and glands.
Sympathetic division – fight/flight
Parasympathetic division – rest/digest
ANS Regulates visceral functions
Heart rate
Blood pressure
Digestion
Urination
Contrast with the somatic nervous system which responds to stimuli by activating skeletal muscle
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Comparing SNS and ANS
Somatic Nervous System
1 motor neuron from spinal cord to skeletal muscle
Axon is myelinated; impulse is rapid
Skeletal muscle movement
Autonomic Nervous System
2 motor neuron chain from brain stem or spinal cord
Axon is unmyelinated
Smooth or cardiac muscle movement or gland secretion
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Comparison of Somatic and Autonomic Systems
Autonomic Nervous System (ANS)
Sympathetic NS
Pre-ganglionic neuron arises in thoracic or lumbar spinal cord segment and is short
Post-ganglionic neuron long and synapses onto target organ/gland.
Parasympathetic NS
Pre-ganglionic neuron arises in brainstem or sacral spinal cord segment and is long
Post-ganglionic neuron short and close to and synapses onto target organ/gland
Most organs are dually innvervated by both branches of ANS
Allows for fine tuning of body for homeostasis
Autonomic Pathways
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