Peripheral Nervous System

Sensory Receptors

• Specialized to respond to changes in environment (stimuli)

• Activation results in graded potentials that trigger nerve impulses

• Awareness of stimulus (sensation) and interprestation of meaning of stimulus (perception) occur in brain

• Three ways to classify receptors: by type of stimulus, body location and structural complexity

Mechanoreceptors

Respond to touch, pressure, vibration and stretch

Thermoreceptors

Sensitive to change in temperature

• cold receptors: located in superficial dermis

• heat receptors are activated from 32 to 48 C located in deeper dermis

• outside those temperature ranges, nociceptors are activated and interpreted as pain

Photoreceptors

respond to light energy (ex. retina)

Chemoreceptors

Respond to chemicals (ex. smell, taste, changes in blood chemistry)

Nociceptors

Sensitive to pain-causing stimuli (ex. extreme heat or cold, excessive pressure, inflammatory chemicals)

• pain receptors triggered by extreme temperature changes, pinch, or release of chemicals from damaged tissue

• Acts as ion channel that is opened by heat, low pH, chemicals (ex. capsaicin in red peppers)

• itch receptors in dermis: can be triggered by chemicals such as histamine

Exteroceptors (classification by location)

• Respond to stimuli arising outside body

• Receptors in skin for touch, pressure, pain and temperature

• Most special sense organs

Interoceptors (visceroceptors) (classification by location)

• respond to stimuli arising in internal viscera and blood vessels

• sensitive to chemical changes, tissue strentch and temperature changes

• sometimes cause discomfort but usually person is unaware of their workings

Propioceptors (classification by location)

• respond to stretch in skeletal muscles, tendons, joints, ligaments and connective tissue coverings of bones and muscles

• Inform brain of one’s movements

Simple receptors of general senses (classification by receptor structure)

• modified dendritic endings of sensory neurons

• are found throughout body and monitor most types of general sensory information

• general senses include tactile sensations (touch, pressure, stretch, vibration), temperature, pain and muscles sense

• no “one-receptor-one-function” relationship

• Receptors can respond to multiple stimuli

Receptors have either:

• nonencapsulated (free) nerve endings

• Encapsulated nerve endings

Receptors for special sense (classification by receptor structure)

• Vision, hearing, equilibrium, smell and taste

• all are housed in complex sense organs

Nonencapsulated (free) nerve endings

• Abundant in epithelia and connective tissues

• most are nonmyelinated, small-diameter group C fibers; distal terminals have knoblike swellings

• respond mostly to temperature, pain or light touch

Sensation

the awareness of changes in the internal and external environment

Perception

The conscious interpretation of those stimuli

Somatosensory System

• part of sensory system serving body wall and limbs

• receives inputs from:

  • Exteroceptors

  • proprioceptors

  • interoceptors

• input is relayed toward head, but processed along the way

Levels of neural integration in sensory systems

1. Receptor level: sensory receptors

2. Circuit level: processing in ascending pathways

3. Perceptual level: processing in cortical sensory areas

Nerve

• cordlike organs of PNS

• Bundle of myelinated and nonmyelinated peripheral axons enclosed by connective tissue

• two types of nerves: spinal or cranial, depending on where they originate

Structure and classification of nerves

• most nerves are mixtures of afferent and efferent fibers and somatic and autonomic (visceral) fibers

• Nerves are classified according to the direction they transmit impulses

  • Mixed nerves: contain both sensory an motor fibers. Impulses travel both to and from CNS

  • Sensory (afferent) nerves: impulses only toward CNS

  • Motor (efferent) nerves: impulses only away from CNS

• Pure sensory (afferent) or pure motor (efferent) nerves are rare; most nerves are mixed

Types of fibers in mixed nerves:

• Somatic afferent (sensory from muscle to brain)

• Somatic efferent (motor from brain to muscle)

• visceral afferent (sensory from organs to brain)

• Visceral efferent (motor from brain to organs)

Cranial nerves

• 12 pairs of cranial nerves are associated with brain

• two attached to forebrain, rest with brain stem

• Most are mixed nerves, but two pairs purely sensory

• Each numbered (I through XII) and named from rostral to caudal

Cranial Nerves - I. Olfactory nerves

• sensory nerves of smell

• run from nasal mucosa to olfactory bulbs

• fibers synapse in olfactory bulbs

• pathway terminates in primary olfactory cortex

• purely sensory (olfactory function)

Damage to the Olfactory Nerve:

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

Cranial Nerves - II. Optic Nerves

• Arise from retinas; really a brain tract

• Pass through optic canals, converge and partially cross over at optic chiasma

• optic tracts continue to thalamus, where they synapse

• Optic radiation fibers run to occipital (visual) cortex

• Purely sensory (visual) function

Optical Nerve Damage

Damage to optic nerve 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

Nerves involved in eye movement

• Oculomotor (III)

• Trochlear (IV)

• Abducens (VI)

Cranial Nerves - III. Oculomotor Nerves

• Fibers extend from ventral midbrain to four of six eye muscles

• Function in raising eyelid, directing eyeball, constricting iris (parasympathetic), and controlling lens shape

• Chiefly motor nerves (oculomotor = motor to the eye); contain a few proprioceptive afferents

• Somatic motor fibers = parasympathetic (autonomic) motor fibers to sphincter pupillae + sensory (proprioceptor) afferents

Damage to the Oculomotor Nerve

In oculomotor nerve 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 cranial nerves III are unopposed. Upper eyelid droops (ptosis) and the person has double vision and trouble focusing on close objects

Ptosis

Inactivation of the levator palpbrae

Mydraiasis

Decreased tone of the constrictor pupillae muscle

Down and out

Unopposed left superior oblique and lateral rectus muscles

Cranial Nerves - IV. Trochlear nerves

• Fibers from dorsal to innervate superior oblique muscle

• primarily motor nerve that directs eyeball

• Crosses the midline

Damage to the Trochlear Nerve

Damage to a trochlear nerve results in double vision (dilopia) and impairs ability to rotate eye inferolaterally.

Cranial Nerves - VI. Abducens Nerves

• Fibers from inferior pons enter orbits via superior orbital fissures

• Primarily a motor, innervating lateral rectus muscle

Damage to the Abducens Nerve

In abducens nerve paralysis, eye cannot be moved laterally. At rest, eyeball rotates medially (internal strabismus)

Cranial Nerves - V. Trigeminal Nerves

• Largest cranial nerves; fibers extend from pons to face

• three divisions

  • Opthalmic (V1)

  • Maxillary (V2)

  • Mandibular (V3)

• Convey sensory impulses from various areas of face

• Supply motor fibers (V3) for mastication

• As main general sensory nerves of face, transmit afferent impulses both touch, temperature and pain receptors. Cell bodies of sensory neurons of all three divisions are located in large trigeminal ganglion

• The mandibular division also contains motor fibers that innervate chewing muscles.

Damage to the Trigeminal Nerve

Trigeminal neuralgia, caused by inflammation of trigeminal nerve, is. widely considered to produce most excruciating pain known. The stabbing pain lasts for a few seconds to a minute, but it can be relentless, occuring 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 compesses the trigeminal nerve near its exit from the brain stem. Several drugs are used to treat this frustrating condition. In severe cases, trasitional 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.

Ophthalmic Division (V1)

Function:

Conveys snesory impulses from skin of anterior scalp, upper eyelid and nose and from nasal cavity mucosa, cornea and lacrimal glad

Clinical Testing:

Corneal reflex test - Touching cornea with wisp of cotton should elicit blinking

Maxillary Division (V2)

Function:

Conveys sensory impulses from nasal cavity mucose, palate, upper teeth, skin of cheek, upper lip, lower eyelid

Clinical Testing:

Test sensations of pain, touch and temperature with safety pin and hot and cold objects

Mandibular Division (V3)

Function:

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 masticatioin

Clinical Testing:

Assess motor branch by asking person to clench their teeth, open mouth against resistance and move jaw side to side

Cranial Nerves - VII. Facial Nerves

• Fibers from pons travel to lateral aspects of face

• Chief motor nerves of face with five major branches

• Motor functions include facial expression, eye closing, blinking, smile, inner ear response to loud noises

• Parasympathetic impulses to lacrimal and salivary glands

• Sensory function (taste) from anterior two-thirds of tongue

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 facial 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

Cranial Nerves - VIII. Vestibulocochlear Nerves

• Afferent fibers from hearing receptors (cochlear division) and equilibrium receptors (vestibular division) pass from inner ear to brain stem

• Mostly sensory function; small motor component for adjustment of sensitivity of receptors

• Formerly auditory nerve

Damage to the Vestibulocochlear Nerves

Lesion 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

Cranial Nerves - IX. Glossopharyngeal Nerves

• Fibers from medulla runs to throat

• Motor functions: innervate part of tongue and pharynx for swallowing

• Sensory functions: fibers conduct taste and general sensory impulses from pharynx and posterior tongue and impulses from carotid chemoreceptors (O2 and CO2 levels) and barorecptors (blood pressure)

• Parasympathetic motor fibers to parotid salivary glands

Damage to the Glossopharengeal Nerve

Injured or inflamed glossopharyngeal nerves impair swallowing and taste

Cranial Nerves - X. Vagus Nerve

• Only cranial nerves that extend beyond head and neck region

• FIbers originate from medulla

• Muscles of pharynx and larynx (involved in swallowing)

• Parasympathetic fibers that help regulate activities of heart, lungs and abdominal viscera

• Sensory fibers carry impulses from thoracic and abdominal viscera, baroreceptors, chemoreceptors and taste buds of posterior tongue and pharynx

Damage to the Vagus Nerve

Since laryngeal branched of the vagus innervate nearly all muscles of the larynx (“voice box”), vagal nerve 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

Cranial Nerves - XI. Accessory Nerves

• Formed from ventral rootlets from C1 to C5 region of spinal cord (not brain)

• Accessory nerves exit skull to innervate trapezius and sternocleidomastoid muscles which together move head and neck

• Formerly spinal accessory nerve

Damage to the Accessory Nerve

Injury to one accessory nerve causes head to turn toward the injured side as a result of sternocleidomastoid muscle paralysis. Shrugging that shoulder (role of trapezius muscle) becomes difficult

Cranial Nerves - XII. Hypoglossal Nerves

• Fibers from medulla

• Innervate extrinsic and intrinsic muscles of tongue that contribute to tongue movement during chewing, swallowing and speech

Damage to the Hypoglossal Nerve

Damage to hypoglossal nerves causes difficulties in speech and swallowing. If both nerves are imparied, the person cannot protrude tongue. If only one side is affected, tongue deviates (points) toward affected side; eventually paralysed side begins to atrophy

Cranial Nerves Part Two

Somatic, visceral, sensory and motor innervation for the head

4 supply parasympathetic autonomic input: oculomtor (III), facial (IV), glossopharyngeal (IX) and Vagus (X) nerves

Cranail nerves I and II enter the base of the forebrain, others the brainstem

3 oculomotor nerves: III, IV, VI

Spinal Nerves

• 31 pairs of spinal nerves

• All are mixed nerves named for point of issue from spinal cord

• Supply all body parts except head and part of neck

  • 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 (C0)

Ganglia

• contain neuron cell bodies associated with nerves in PNS

• Ganglia associated with afferent nerve fibers contain cell bodies of sensory neurons

• Dorsal root ganglia (sensory, somatic); Ganglia associated with efferent nerve fibers contain autonomic motor neurons

• autonomic ganglia (motor, visceral)