ANP 1106: The Peripheral Nervous System and Reflex Activity

peripheral nervous system (PNS)

provides links from and to the world outside our bodies; includes all neural structures outside the brain and spinal cord: sensory receptors, afferent and efferent nerves and their associated ganglia, and motor endings

sensory receptors

specialized to respond to changes in their environment (stimuli); its activation by anadequate stimulus results in graded potentials that in turn trigger nerve impulses along the afferent PNS fibers coursing to the CNS; can be classified by the type of stimulus they detect, their body location, and their structural complexity

sensation

awareness of the stimulus

perception

interpretation of the meaning of the stimulus

mechanoreceptors

respond to mechanical force such as touch, pressure (including blood pressure), vibration, and stretch

thermoreceptors

respond to temperature changes

photoreceptors

respond to light; e.g., those of the retina of the eye

chemoreceptors

respond to chemicals in solution (molecules smelled or tasted, or changes in blood or interstitial fluid chemistry)

nociceptors

respond to potentially damaging stimuli that result in pain (e.g., searing heat, extreme cold, excessive pressure, inflammatory chemicals); these signals stimulate subtypes of thermoreceptors, mechanoreceptors, and chemoreceptors

exteroceptors

sensitive to stimuli arising outside the body; most are near or at the body surface; include touch, pressure, pain, and temperature receptors in the skin and most receptors of the special senses (vision, hearing, equilibrium, smell, and taste)

interoceptors (visceroceptors)

respond to stimuli within the body, such as from the internal viscera and blood vessels; monitor a variety of stimuli, including chemical changes, tissue stretch, and temperature; sometimes their activity causes us to feel pain, discomfort, hunger, or thirst; usually unaware of their workings

proprioceptors

resopnd to internal stimuli; location is much more restricted; occur in skeletal muscles, tendons, joints, and ligaments and in connective tissue coverings of bones and muscles; constantly advise the brain of our body movements by monitoring how much the organs containing these receptors are stretched

simple receptors of the general senses

majority; modified dendritic endings of sensory neurons; found throughout the body and monitor most types of general sensory information; involved in tactile sensation (a mix of touch, pressure, stretch, and vibration), temperature monitoring, and pain, as well as the "muscle sense" provided by proprioceptors; one type of receptor can respond to several different kinds of stimuli and different types of receptors can respond to similar stimuli

receptors for special senses

housed in complex sense organs

nonencapsulated (free) nerve endings

present nearly everywhere in the body; particularly abundant in epithelia and connective tissues; nonmyelinated, small-diameter group C fibers; their distal endings (the sensory terminals) usually have small knoblike swellings; respond chiefly to painful stimuli and changes in temperature, but some respond to tissue movements caused by pressure as well

cold receptors

located in the superficial dermis

heat receptors

located deeper in the dermis

vanilloid receptor

plasma membrane protein; key player in detecting painful stimuli; ionchannel that is opened by heat, low pH, and various chemicals including capsaicin, the substance found in chili peppers

itch receptor

located in the dermis; thin diameter; activated by a number of chemicals--notably histamine--present at inflamed sites

epithelial tactile complexes (Merkel cells and discs)

lie at the junction of the epidermis and dermis; function as light pressure receptors; certain disclike free nerve endings (Merkel discs) associate with a spiky, hemispherical epidermal cell (tactile epithelial cell/Merkel cell), forming a complex

hair follicle receptors

free nerve endings that wrap basket-like around hair follicles; light touch receptors that detect bending of hairs

encapsulated nerve endings

consist of one or more fiber terminals of sensory neurons enclosed in a connective tissue capsule; virtually all are mechanoreceptors; vary greatly in shape, size, and distribution in the body; include tactile corpuscles, lamella corpuscles, bulbous corpuscles, muscle spindles, tendon organs, and joint kinesthetic receptors

tactile corpuscles (Meissner's corpuscles)

small receptors in which a few spiraling sensory terminals are surrounded by Schwann cells and then by a thin egg-shaped connective tissue capsule; found just beneath the epidermis in the dermal papillae and are especially numerous in sensitive and hairless skin areas such as the nipples, fingertips, and soles of the feet; receptors for discriminative touch; play the same role in sensing light touch in hairless skin that hair follicle receptors do in hairy skin

lamellar corpuscles (Pacinian corpuscles)

scattered deep in the dermis and in subcutaneous tissue underlying the skin; mechanoreceptors stimulated by deep pressure; respond only when the pressure is first applied; best suited to monitoring vibration (an "on/off" pressure stimulus); largest corpusclar receptors; visible to the naked eye as white, egg-shaped bodies; its single dendrite is surrounded by a capsule containing up to 60 layers of collagen fibers and flattened supporting cells

bulbous corpuscles (Ruffini endings)

lie in the dermis, subcutaneous tissue, and joint capsules; contain a spray of receptor endings enclosed by a flattened capsule; bear a striking resemblance to tendon organs (which monitor tendon stretch) and probably play a similar role in other dense connective tissues where they respond to deep and continuous pressure

muscle spindles

fusiform (spindle-shaped) proprioceptors found throughout the perimysium that wraps individual fascicles of skeletal muscle; each consists of a bundle of modified skeletal muscle fibers, called intrafusal fibers, enclosed in a connective tissue capsule; detect muscle stretch and initiate a reflex that resists the stretch

tendon organs

proprioceptors located in tendons, close to the junction between the skeletal muscle and the tendon; consist of small bundles of tendon (collagen) fibers enclosed in a layered capsule, with sensory terminals coiling between and aroundthe fibers; when muscle contraction stretches the tendon fibers, the resulting compression of the nerve fibers activates them; this initiates a reflex that causes the contracting muscle to relax

joint kinesthetic receptors

proprioceptors that monitor stretch in the articular capsules that enclose synovial joints; contains at least four receptor types: lamellar corpuscles, bulbous corpuscles, free nerve endings, and receptors resembling tendon organs; together these receptors provide information on joint position and motion, a sensation of which we are highly conscious

somatosensory system

part of the sensory system serving the body walland limbs; receives input from exteroceptors, proprioceptors, and interoceptors; transmits information about several different sensory modalities, or types of sensation

receptor level

sensory receptors

circuit level

processing in ascending pathways

perceptual level

processing in cortical sensory areas

nerve

cordlike organ that is part of the PNS; classified as cranial or spinal depending on whether they arise from the brain or spinal cord; vary in size, but every one consists of parallel bundles or peripheral axons (some myelinated and some not) enclosed by successive wrappings of connective tissue

endoneurium

surrounds each axon/nerve fiber; delicate layer of loose connective tissue that also encloses the fiber's associated Schwann cells

perineurium

coarser connective tissue wrapping; binds groups of axons into fascicles

fascicles

groups of axons bundled together by the perineurium

epineurium

tough fibrous sheath; encloses all the fascicles to form the nerve

mixed nerves

contain both sensory and motor fibers; transmit impulses both to and from the CNS; common

sensory (afferent) nerves

carry impulses only toward the CNS; rare

motor (efferent) nerves

carry impulses only away from the CNS; rare

somatic afferent nerves

carry impulses from muscle to brain

somatic efferent nerves

carry impulses from brain to muscle

visceral afferent nerves

carry sensory impulses from organs to brain

visceral efferent nerves

carry motor impulses from brain to organs

cranial nerves

12 pairs; associated with the brain; first two pairs attach to the forebrain, and the rest are associated with the brain stem; other than the vagus nerves, which extend into the abdomen, they serve only head and neck structures; numbered (I through XII) from the most rostral to the most caudal

olfactory nerves

I; arise from olfactory sensory neurons located in olfactory epithelium of nasal cavity and pass through cribriform plate of ethmoid bone to synapse in olfactory bulb; fibers of olfactory bulb neurons extend posteriorly as olfactory tract, which runs beneath frontal lobe to enter cerebral hemispheres and terminates in primary olfactory cortex; purely sensory function; carry afferent inpulses for sense of smell

damage to the olfactory nerve

fracture of ethmoid bone or lesions of olfactory fibers may result in partial or total loss of smell, a condition known as anosmia

optic nerves

II; fibers arise from retina of eye to form it, which passes through optic canal of orbit; converge to form the optic chiasma where fibers partially cross over, continue on as optic tracts, enter thalamus, and synapse there; thalamic fibers run (as the optic radiation) to occipital (visual) cortex, where visual interpretation occurs; purely sensory function; carry afferent impulses for vision

damage to the optic nerve

damage results in blindness in eye served by nerve; damage to visual pathway beyond the optic chiasma results in partial visual losses; visual defects are called anopsias

oculomotor nerves

III; fibers extend from ventral midbrain (near its junction with pons) and pass through bony orbit, via superior orbital fissure, to eye; chiefly motor nerves; contain a few proprioceptive afferents; each contains: (1) somatic motor fibers to four of the six extrinsic eye muscles (inferior oblique and superior, inferior, and medial rectus muscles) that help direct eyeball, and to levator palpebrae superioris muscle, which raises upper eyelid; (2) parasynpathetic (autonomic) motor fibers to sphincter pupillae (circular muscles of iris), which cause pupil to constrict, and to ciliary muscle, controlling lens shape for visual focusing; some parasympathetic cell bodies are in the ciliary ganglia; (3) sensory (proprioceptor) afferents, which run from the skeletal muscles served to the midbrain

damage to the oculomotor nerve

in its paralysis, eye cannot be moved up, down, or inward; at rest, eye rotates laterally (external strabismus) because the actions of the two extrinsic eye muscles not served by it are unopposed; upper eyelid droops (ptosis), and the person has double vision and trouble focusing on close objects

trochlear nerves

IV; fibers emerge from dorsal midbrain and course ventrally around midbrain to enter orbit through superior orbital fissure along with oculomotor nerves; primarily motor nerves; supply somatic motor fibers to (and carry proprioceptor fibers from) one of the extrinsic eye muscles, the superior oblique muscle, which passes through the pulley-shaped nerve

damage to the trochlear nerve

damage results in double vision (diplopia) and impairs ability to rotate eye inferolaterally

abducens nerves

VI: fibers leave inferior pons and enter orbit via superior orbital fissure to run to eye; primarily motor function; supply somatic motor fibers to lateral rectus muscle, an extrinsic muscle of the eye; convey proprioceptor impulses from same muscle to brain

damage to the abducens nerve

in its paralysis, eye cannot be moved laterally; at rest, eye rotates medially (internal strabismus)

trigeminal nerves

V; largest cranial nerves; fibers extend from pons to face, and form three divisions: ophthalmic (V1), maxillary (V2), and mandibular (V3) divisions; as mani general sensory nerves of face, transmit afferent impulses from touch, temperature, and pain receptors; cell bodies of sensory neurons of all three divisions are located in large trigeminal ganglion; mandibular division also contains motor fibers that innervate chewing muscles

ophthalmic division of trigeminal nerve

V1; fibers run from face to pons via superior orbital fissure; conveys sensory impulses from skin of anterior scalp, upper eyelid, and nose, and from nasal cavity mucosa, cornea, and lacrimal gland; tested by corneal reflex test: touching cornea with wisp of cotton should elicit blinking

maxillary division of trigeminal nerve

V2; fibers run from face to pons via foramen rotundum; conveys sensory impulses from nasal cavity mucosa, palate, upper teeth, skin of sheek, upper lip, lower eyelid; test sensations of pain, touch, and termperature with safety pin and hot and cold objects

mandibular division of trigeminal nerve

V3; fibers pass through skull via foramen ovale; conveys sensory impulses from anterior tongue (except taste buds), lower teeth, skin of chin, temporal region of scalp; supplies motor fibers to, and carries proprioceptor fibers from, muscles of mastication; assess motor branch by asking person to clench their teeth, open mouth against resistance, and move jaw side to side

damage to the trigeminal nerve

trigeminal neuralgia, caused by its inflammation, is widely considered to produce most excrutiating pain known; the stabbing pain leasts for a few seconds to a minute, but it can be relentless, occurring a hundred times a day; usually provoked by some sensory stimulus, such as brushing teeth or even a passing breeze hitting the face; thought to be caused by a loop of artery or vein that compresses the nerve near its exit from the brain stem; several drugs are used to treat this frustrating condition;in severe cases, traditinoal or gamma knife surgery relieves the agony--either by moving the compressing vessel or by destroying the nerve; nerve destruction results in loss of sensation on that side of face

facial nerves

VII; fibers issue from pons, just lateral to abducens nerves, enter temporal bone via internal acoustic meatus, and run within bone (and through inenr ear cavity) before emerging through stylomastoid foramen; nerve then courses to lateral aspect of face; mixed nerves that are the chief motor nerves of face; five major branches: temporal, zygomatic, buccal, mandibular, and cervical; convey motor impulses to skeletal muscles of face (muscles of facial expression), except for chewing muscles served by trigeminal nerves, and transmit proprioceptor impulses from same muscles to pons; transmit parasympathetic (autonomic) motor impulses to lacrimal (tear) glands, nasal and palatine glands, and submandibular and sublingual salivary glands; some of the cell bodies of these parasympathetic motor neurons are in pterygopalatine and submandibular ganglia on the trigeminal nerve; convey sensory impulses from taste buds of anterior two-thirds of tongue; cell bodies of these sensory neurons are in geniculate ganglion

damage to the facial nerve

Bell's palsy is characterized by paralysis of facial muscles on affected side and partial loss of taste sensation; may develop rapidly (often overnight); caused by inflamed and swollen nerve, possibly due to herpes simplex 1 viral infection; lower eyelid droops, corner of mouth sags (making it difficult to eat or speak normally), tears drip continuously from eye and eye cannot be completely closed (conversely, dry-eye syndrome may occur); treated with corticosteroids; recovery is complete in 70% of cases

vestibulocochlear nerves

VIII; fibers arise from hearing and equilibrium apparatus located within inner ear of temporal bone and pass through internal acoustic meatus to enter brain stem at pons-medulla border; afferent fibers from hearing receptors in cochlea form the cochlear division (cochlear nerve); those from equilibrium receptors in semicircular canals and vestibule form the vestibular division (vestibular nerve); the two divisions merge to form nerve; mostly sensory function; vestibular division transmits afferent impulses for sense of equilibrium, and sensory nerve cell bodies are located in vestibular ganglia; cochlear division transmits afferent impulses for sense of hearing, and sensory nerve cell bodies are located in spiral ganglion within cochlea; small motor component adjusts the sensitivity of sensory receptors

damage to the vestibulocochlear nerve

lesions of cochlear nerve or cochlear receptors result in central, or nerve, deafness; damage to vestibular division produces dizziness, rapid involuntary eye movements, loss of balance, nausea, and vomiting

glossopharyngeal nerves

fibers emerge from medulla and leave skull via jugular foramen to run to throat; mixed nerves that innervate part of tongue and pharynx; provide somatic motor fibers to, and carry proprioceptor fibers from, a superior pharyngeal muscle called the stylopharyngeus, which elevates the pharynx in swallowing; provide parasympathetic motor fibers to parotid salivary glands (some of the nerve cell bodies of these parasympathetic motor neurons are located in otic ganglion); sensory fibers conduct taste and general sensory (touch, pressure, pain) impulses from pharynx and posterior tongue, impulses from chemoreceptors in the carotid body (which monitor oxygen and carbon dioxide levels in the blood and help regulate respiratory rate and depth), and impulses from baroreceptors of carotid sinus (which monitor blood pressure); sensory neuron cell bodies are located in superior and inferior ganglia

damage to the glossopharyngeal nerve

injured or inflamed nerves impair swallowing and taste

vagus nerves

X; the only cranial nerves to extend beyond head and neck region; fibers emerge from medulla, pass through skull via jugular foramen, and descend through neck region into thorax and abdomen; mixed nerves; nearly all motor fibers are parasympathetic efferents, except those serving skeletal muscles of pharynx and larynx (involved in swallowing); parasympathetic motor fibers supply heart, lungs, and abdominal viscera and are involved in regulating heart rate, breathing, and diggestive system activity; transmit sensory impulses from thoracic and abdominal viscera, from the aortic arch baroreceptors (for blood pressure) and the carotid and aortic bodies (chemoreceptors for respiration), and taste buds on the epiglottis; also carry general somatic sensory information from small area of skin on external ear; carry proprioceptor fibers from muscles of larynx and pharynx

damage to the vagus nerve

since laryngeal branches of the nerve innervate nearly all muscles of the larynx, its paralysis can lead to hoarseness or loss of voice; other symptoms are difficulty swallowing and impaired digestive system motility; these parasympathetic nerves are important for maintaining the normal state of visceral organ activity; without their influence, the sympathetic nerves, which mobilize and accelerate vital body processes (and shut down digestion), would dominate

accessory nerves

XI; unique in that they form from rootlets that emerge from the spinal cord, not the brain stem; these rootlets arise laterally from superior region (C1-C5) of spinal cord, pass upward along spinal cord, and enter the skull as nerves via foramen magnum; exit from skull through jugular foramen together with the vagus nerves, and supply two large neck muscles; mixed nerves, but primarily motor in function; supply motor fibers to trapezius and sternocleidomastoid muscles, which together move head and neck, and convey proprioceptor impulses from same muscles

damage to the accessory nerve

injury to once causes head to turn toward the injured side as a result of sternocleidomastoid muscle paralysis; shrugging that shoulder (role of trapezius muscle) becomes difficult

hypoglossal nerves

XII; mainly serve the tongue; fibers arise by a series of roots from medulla and exit from skull via hypoglossal canal to travel to tongue; mixed nerves, but primarily motor in function; carry somatic motor fibers to intrinsic and extrinsic muscles of tongue, and proprioceptor fibers from same muscles to brain stem; nerve control allows tongue movements that mix and manipulate food during chewing, and contribute to swallowing and speech

damage to the hypoglossal nerve

damage causes difficulties in speech and swallowing; if both nerves are impaired, the person cannot protrude tongue; if only one side is affected, tongue deviates (points) toward affected side; eventually paralyzed side begins to atrophy

spinal nerves

31 pairs; arise from the spinal cord and supply all parts of the body except the head and some areas of the neck; all are mixed nerves; named according to where they emerge 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 tiny coccygeal nerves (Co1)

dermatome

area of skin supplied by a single spinal nerve

epidural space

space between the dura mater and the wall of the vertebral canal; contains fat

subdural space

space between dura mater and arachnoid mater

subarachnoid space

space in the meninges beneath the arachnoid membrane and above the pia mater that contains the cerebrospinal fluid

pia mater

thin, delicate inner membrane of the spinal meninges

arachnoid mater

weblike middle layer of the spinal meninges

dura mater

thick, outermost layer of the spinal meninges

dorsal root ganglion

sensory; somatic; ganglia associated with efferent nerve fibers containing autonomic motor neurons

dorsal root

the sensory branch of each spinal nerve

somatic sensory neuron

nerve cell that carries messages from receptors in the skin, muscles, bones and joints into the brain and spinal cord

visceral sensory neuron

detects stimuli associated with blood vessels and the viscera

visceral motor neuron

innervates glands and muscles (involuntary)

somatic motor neuron

nerve cell that stimulates the contraction of skeletal muscle fibers

ventral root

the motor branch of each spinal nerve

dorsal horn

region at the rear of the spinal cord that receives inputs from receptors in the skin; contains interneurons

ventral horn

somatic motor neurons whose axons exit the cord via ventral roots

ganglia

contain neuron cell bodies associated with nerves in PNS; those associated with afferent nerve fibers contain cell bodies of sensory neurons

autonomic ganglia

motor; visceral