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

Dermatomes

• 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

Reflexes

• 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 (4^th week)

• Sensory receptors atrophy with age and muscle tone lessens

• Peripheral nerves remain viable throughout life unless subjected to trauma