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Chapter 13

Peripheral Nervous System (PNS)


Peripheral Nervous System (PNS)

•      PNS – all neural structures outside the brain and spinal cord

•      Includes: sensory receptors, peripheral nerves, associated ganglia, and motor endings

•      Provides links to and from the external environment

Sensory Receptors

•      Structures specialized to respond to stimuli

•      Activation of sensory receptors results in depolarizations that trigger impulses to the CNS

•      The realization of these stimuli, sensation and perception, occur in the brain

Receptor Classification by Stimulus

•      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

Receptor Class by Location: Exteroceptors

•      Respond to stimuli arising outside the body

•      Found near the body surface

•      Sensitive to touch, pressure, pain, and temperature

•      Includes the special sense organs

Receptor Class by Location: Interoceptors

•      Respond to stimuli arising within the body

•      Found in internal viscera and blood vessels

•      Sensitive to chemical changes, stretch, and temperature changes

Receptor Class by Location: Proprioceptors

•      Respond to degree of stretch of the organs they occupy

•      Found in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles

•      Constantly “advise” the brain of one’s movements

Receptor Classification by Structure

•      Receptors are structurally classified as either simple or complex

•      Most receptors are simple and include encapsulated and unencapsulated varieties

•      Complex receptors are special sense organs

Simple Receptors: Unencapsulated

•      Free dendritic nerve endings

•      Merkel discs

•      Root hair plexuses

Simple Receptors: Encapsulated

•      Meissner’s corpuscles and Krause’s end bulbs

•      Pacinian corpuscles

•      Muscle spindles, Golgi tendon organs, and Ruffini’s corpuscles

•      Joint kinesthetic receptors

Structure of a Nerve

•      Nerve – cordlike organ of the PNS consisting of peripheral axons enclosed by connective tissue

•      Connective tissue coverings include:

•    Endoneurium – loose connective tissue  that surrounds axons

•    Perineurium – coarse connective tissue  that bundles fibers into fascicles

•    Epineurium – tough fibrous sheath around a nerve

Classification of Nerves

•      Sensory and motor divisions

•      Sensory (afferent) – carry impulse to the CNS

•      Motor (efferent) – carry impulses from CNS

•      Mixed – sensory and motor fibers carry impulses to and from CNS; most common type of nerve

Peripheral Nerves

•      Mixed nerves – carry somatic and autonomic (visceral) impulses

•      The four types of mixed nerves are:

•    Somatic afferent and somatic efferent

•    Visceral afferent and visceral efferent

•      Peripheral nerves originate from the brain or spinal column

Regeneration of Nerve Fibers

•      Damage to nerve tissue is serious because mature neurons are amitotic

•      If the soma of a damaged nerve remains intact, damage can be repaired

•      Regeneration involves coordinated activity among:

•    Macrophages – remove debris

•    Schwann cells – form regeneration tube and secrete growth factors

•    Axons – regenerate damaged part

Motor Endings

•      PNS elements that activate effectors by releasing neurotransmitters at:

•    Neuromuscular junctions

•    Varicosities at smooth muscle and glands

Cranial Nerves

•      Twelve pairs of cranial nerves arise from the brain

•      They have sensory, motor, or both sensory and motor functions.

•      Each nerve is identified by a number (I through XII) and a name

•      Four cranial nerves carry parasympathetic fibers that serve muscles and glands

Summary of Function of Cranial Nerves

Cranial Nerve I: Olfactory

•      Arises from the olfactory epithelium

•      Passes through the cribriform plate of the ethmoid bone

•      Fibers run through the olfactory bulb and terminate in the primary olfactory cortex

•      Functions solely by carrying afferent impulses for the sense of smell

Cranial Nerve I: Olfactory

Cranial Nerve II: Optic

•      Arises from the retina of the eye

•      Optic nerves pass through the optic canals and converge at the optic chiasm

•      They continue to the thalamus where they synapse

•      From there, the optic radiation fibers run to the visual cortex

•      Functions solely by carrying afferent impulses for vision

Cranial Nerve III: Oculomotor

•      Fibers extend from the ventral midbrain, pass through the superior orbital fissure, and go to the extrinsic eye muscles

•      Functions in raising the eyelid, directing the eyeball, constricting the iris, and controlling lens shape

Cranial Nerve IV: Trochlear

•      Fibers emerge from the dorsal midbrain and enter the orbits via the superior orbital fissures; innervate the superior oblique muscle

•      Primarily a motor nerve that directs the eyeball

Cranial Nerve V: Trigeminal

•      Composed of three divisions: ophthalmic (V1), maxillary (V2), and mandibular (V3)

•      Fibers run from the face to the pons via the superior orbital fissure (V1), the foramen rotundum (V2), and the foramen ovale (V3)

•      Conveys sensory impulses from various areas of the face (V1) and (V2), and supplies motor fibers (V3) for mastication

Cranial Nerve VI: Abducens

•      Fibers leave the inferior pons and enter the orbit via the superior orbital fissure

•      Primarily a motor nerve innervating the lateral rectus muscle

Cranial Nerve VII: Facial

•      Fibers leave the pons, travel through the internal acoustic meatus, and emerge through the stylomastoid foramen to the lateral aspect of the face

•      Mixed nerve with five major branches

•      Motor functions include facial expression, and the transmittal of autonomic impulses to lacrimal and salivary glands

•      Sensory function is taste from the anterior two-thirds of the tongue

Cranial Nerve VIII: Vestibulocochlear

•      Fibers arise from the hearing and equilibrium apparatus of the inner ear, pass through the internal acoustic meatus, and enter the brainstem at the pons-medulla border

•      Two divisions – cochlear (hearing) and vestibular (balance)

•      Functions are solely sensory for the sense of equilibrium and of hearing

Cranial Nerve IX: Glossopharyngeal

•      Fibers emerge from the medulla, leave the skull via the jugular foramen, and run to the throat

•      Nerve IX is a mixed nerve with motor and sensory functions

•      Motor – innervates part of the tongue and pharynx, and provides motor fibers to the parotid salivary gland

•      Sensory – fibers conduct taste and general sensory impulses from the tongue and pharynx

Cranial Nerve X: Vagus

•      The only cranial nerve that extends beyond the head and neck

•      Fibers emerge from the medulla and emerge via the jugular foramen

•      The vagus is a mixed nerve

•      Most motor fibers are parasympathetic fibers to the heart, lungs, and visceral organs

•      Its sensory function is in taste

Cranial Nerve XI: Accessory

•      Formed from a cranial root emerging from the medulla and a spinal root arising from the superior region of the spinal cord

•      The spinal root passes upward into the cranium via the foramen magnum

•      The accessory nerve leaves the cranium via the jugular foramen

•      Primarily a motor nerve supplying:

•    Fibers to the larynx, pharynx, and soft palate

•    Innervates the trapezius and sternocleidomastoid, which move the head and neck

Cranial Nerve XII: Hypoglossal

•      Fibers arise from the medulla and exit the skull via the hypoglossal canal

•      Innervates both extrinsic and intrinsic muscles of the tongue, which contribute to swallowing and speech

Spinal Nerves

•      Thirty-one pairs of mixed nerves arise from the spinal cord and supply all parts of the body except the head

•      They are named according to their point of issue

•    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 medial roots

•      Each root forms a series of rootlets that attach to the spinal cord

•      Ventral roots arise from the anterior horn and contain motor (efferent) fibers

•      Dorsal roots arise from sensory neurons in the dorsal root ganglion and contain sensory (afferent) fibers

Spinal Nerves: Rami

•      The short spinal nerves branch into three or four mixed, distal rami:

•    Small dorsal ramus

•    Larger ventral ramus

•    Tiny meningeal branch

•    Rami communicantes at the base of the ventral rami in the thoracic region

Nerve Plexuses

•      All ventral rami except T2-T12 form interlacing nerve networks called plexuses

•      Plexuses are found in the cervical, brachial, lumbar, and sacral regions

•      Each resulting branch of a plexus contains fibers from several spinal nerves

•      Fibers travel to the periphery via several different routes

•      Each muscle receives a nerve supply from more than one spinal nerve

•      Therefore damage to one spinal segment cannot completely paralyze a muscle

Spinal Nerve Innervation: Back, Anterolateral Thorax, and Abdominal Wall

•      The back is innervated by dorsal rami via several branches

•      The thorax is innervated by ventral rami T1-T12 as intercostal nerves

•      Intercostal nerves supply muscles of the ribs, anterolateral thorax, and abdominal wall

Spinal Nerve Innervation: Back, Anterolateral Thorax, and Abdominal Wall

Cervical Plexus

•      The cervical plexus is formed by ventral rami of C1-C4

•      Most branches are cutaneous nerves of the neck, ear, back of head, and shoulders

•      The most important nerve of this plexus is the phrenic nerve

•      The phrenic nerve is the major motor and sensory nerve of the diaphragm

Brachial Plexus

•      Is formed by C5-C8 and T1 (C4 and T2 may also contribute to this plexus)

•      It gives rise to the nerves that innervate the upper limb

•      There are four major branches of this plexus

•    Roots – five ventral rami (C5-T1)

•    Trunks – upper, middle, and lower, which form divisions

•    Divisions – anterior and poster serve the front and back of the limb

•    Cords – three fiber bundles: lateral, medial, and posterior

Brachial Plexus: Nerves

•      Axillary – innervates the deltoid and teres minor

•      Musculocutaneous – sends fibers to the biceps brachii and brachialis

•      Median – branches to most of the flexor muscles of arm

•      Ulnar – supplies the flexor carpi ulnaris and part of the flexor digitorum profundus

•      Radial – innervates essentially all extensor muscles

Lumbar Plexus

•      Arises from
L1-L4 and innervates the thigh, abdominal wall, and psoas muscle

•      The major nerves are the femoral and the obturator

Sacral Plexus

•      Arises from L4-S4 and serves the buttock, lower limb, pelvic structures, and the perineum

•      The major nerve is the sciatic, the longest and thickest nerve of the body

•      The sciatic is actually composed of two nerves: the tibial and the common fibular (peroneal) nerves

Innervation of Joints

•      Hilton’s law: any nerve serving a muscle that produces movement at a joint also innervates the joint itself and the skin over the joint


•      A dermatome is the area of skin innervated by the cutaneous branches of a single spinal nerve

•      All spinal nerves except C1 participate in dermatomes


•      A reflex is a rapid, predictable motor response to a stimulus

•      Reflexes may:

•    Be inborn or learned (acquired)

•    Involve only peripheral nerves and the spinal cord

•    Involve higher brain centers as well

Reflex Arc

•      There are five components of a reflex arc

•    Receptor – site of stimulus

•    Sensory neuron – transmits the afferent impulse to the CNS

•    Integration center – either monosynaptic or polysynaptic region within the CNS

•    Motor neuron – conducts efferent impulses from the integration center to an effector

•    Effector – muscle fiber or gland that responds to the efferent impulse

Stretch and Deep Tendon Reflexes

•      For skeletal muscles to perform normally:

•    The Golgi tendon organs (proprioceptors) must constantly inform the brain as to the state of the muscle

•    Stretch reflexes initiated by muscle spindles must maintain healthy muscle tone

Muscle Spindles

•      Are composed of 3-10 intrafusal muscle fibers that lack myofilaments in their central regions, are noncontractile, and serve as receptive surfaces

•      Muscle spindles are wrapped with two types of afferent endings: primary sensory endings of type Ia fibers and secondary sensory endings of type II fibers

•      These regions are innervated by gamma (g) efferent fibers

•      Note: contractile muscle fibers are extrafusal fibers and are innervated by alpha (a) efferent fibers

Operation of the Muscle Spindles

•      Stretching the muscles activates the muscle spindle

•    There is an increased rate of action potential in Ia fibers

•      Contracting the muscle reduces tension on the muscle spindle

•    There is a decreased rate of action potential on Ia fibers

Stretch Reflex

•      Stretching the muscle activates the muscle spindle

•      Excited g motor neurons of the spindle cause the stretched muscle to contract

•      Afferent impulses from the spindle result in inhibition of the antagonist

•      Example: patellar reflex

•    Tapping the patellar tendon stretches the quadriceps and starts the reflex action

•    The quadriceps contract and the antagonistic hamstrings relax

Deep Tendon Reflex

•      The opposite of the stretch reflex

•      Contracting the muscle activates the Golgi tendon organs

•      Afferent Golgi tendon neurons are stimulated, neurons inhibit the contracting muscle, and the antagonistic muscle is activated

•      As a result, the contracting muscle relaxes and the antagonist contracts

Flexor and Crossed Extensor Reflexes

•      The flexor reflex is initiated by a painful stimulus (actual or perceived) that causes automatic withdrawal of the threatened body part

•      The crossed extensor reflex has two parts

•    The stimulated side is withdrawn

•    The contralateral side is extended

Superficial Reflexes

•      Initiated by gentle cutaneous stimulation

•      Example: 

•    Plantar reflex is initiated by stimulating the lateral aspect of the sole

•    The response is downward flexion of the toes

•    Indirectly tests for proper corticospinal tract functioning

•    Babinski’s sign: abnormal plantar reflex indicating corticospinal damage where the great toe dorsiflexes and the smaller toes fan laterally

Developmental Aspects of the PNS

•      Spinal nerves branch from the developing spinal cord and neural crest cells

•    Supply motor and sensory function to developing muscles

•      Cranial nerves innervate muscles of the head

•      Distribution and growth of spinal nerves correlate with the segmented body plan (4th week)

•      Sensory receptors atrophy with age and muscle tone lessens

•      Peripheral nerves remain viable throughout life unless subjected to trauma