The Peripheral Nervous System

The Peripheral Nervous System Nervous structures outside the brain and spinal cord Nerves allow the CNS to receive information and take action Functional components of the PNS Sensory inputs and motor outputs Categorized as somatic or visceral Sensory inputs also classified as general or special

Functional Organization of the PNS

Basic Structural Components of the PNS Sensory receptors – pick up stimuli from inside or outside the body Motor endings – axon terminals of motor neurons Innervate effectors (muscle fibers and glands) Nerves and ganglia Nerves – bundles of peripheral axons Ganglia – clusters of peripheral neuronal cell bodies

Structural Organization of PNS in Region of a Spinal Nerve

Peripheral Endings Afferent: Sensory Receptors Efferent: Somatic Motor Efferent: Autonomic Nervous System

Peripheral Sensory Receptors Afferent: Sensory Peripheral Sensory Receptors Structures that pick up sensory stimuli Initiate signals in sensory axons

Peripheral Sensory Receptors Two main categories of sensory receptors Special nerve endings of sensory neurons Monitor general sensory information Independent receptor cells – specialized epithelial cells or small neurons Monitor most types of special sensory information

Sensory Receptors

Peripheral Sensory Receptors Sensory receptors also classified according to: Location Type of stimulus detected Structure

Classification by Location Exteroceptors – sensitive to stimuli arising from outside the body Located at or near body surfaces Include receptors for touch, pressure, pain, and temperature Interoceptors – (visceroceptors) receive stimuli from internal viscera Monitor a variety of stimuli Proprioceptors – monitor degree of stretch Located in musculoskeletal organs

Classification by Modality Mechanoreceptors – respond to mechanical forces Thermoreceptors – respond to temperature changes Chemoreceptors – respond to chemicals in solution Photoreceptors – respond to light – located in the eye Nociceptors – respond to harmful stimuli that result in pain

Classification by Structure General sensory receptors Widely distributed Nerve endings of sensory neurons monitor: Touch, pressure, vibration, stretch Pain, temperature, proprioception Divided into two groups Free nerve endings Encapsulated nerve endings

Free Nerve Endings Abundant in epithelia and underlying connective tissue Respond to pain and temperature Monitor affective senses Two specialized types of free nerve endings Merkel discs – lie in the epidermis Slowly adapting receptors for light touch Hair follicle receptors – wrap around hair follicles Rapidly adapting receptors

Unencapsulated Nerve Endings

Encapsulated Nerve Endings Consist of one or more end fibers of sensory neurons Enclosed in connective tissue Mechanoreceptors Include four main types

Encapsulated Nerve Endings Encapsulated nerve endings: dendrites with special supporting structures (mechanoreceptors and proprioceptors)

Encapsulated Nerve Endings Meissner’s corpuscles Pacinian corpuscles Ruffini’s corpuscles Proprioceptors

Encapsulated Receptors

Proprioceptors Monitor stretch in locomotory organs Three types of proprioceptors

Three Types of Proprioceptors Muscle spindles – measure the changing length of a muscle Imbedded in the perimysium between muscle fascicles Golgi tendon organs – located near the muscle-tendon junction Monitor tension within tendons Joint kinesthetic receptors Sensory nerve endings within the joint capsules

Proprioceptors

Structure of Receptors in Skin

Somatic Efferent: Innervation of Skeletal Muscles

Innervation of Skeletal Muscle Motor axons innervate skeletal muscles Neuromuscular junctions (motor end plates) Similar to synapses between neurons Acetylcholine diffuses across the synaptic cleft Binds with molecules on the sarcolemma Motor axons branch to innervate muscle fibers

The Neuromuscular Junction

Innervation of Skeletal Muscle Motor unit – a motor neuron and all the muscle fibers it innervates

Efferent: Autonomic Nervous System

Innervation of Visceral Muscle and Glands Simpler than neuromuscular junctions of skeletal muscle Near the smooth muscle or gland it innervates Visceral motor axon swells into a row of varicosities Visceral motor responses Slower than somatic motor reflexes

Innervation of Smooth Muscle

Cranial Nerves

Cranial Nerves Attach to the brain and pass through foramina of the skull Numbered from I–XII 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) extends into the abdomen

The 12 Pairs of Cranial Nerves

CN I: Olfactory Nerves Sensory nerves of smell Sense of smell Damage causes impaired sense of smell

CN II: Optic Nerve Sensory nerve of vision Provides vision Damage causes blindness in visual field

CN III: Oculomotor Nerve Innervates four of the extrinsic eye muscles Somatic and Autonomic motor function Eye movement (Superior, inferior, medial rectus muscles and inferior oblique muscle), opening of eyelid (levator palpebrae superioris), constriction of pupil (circular muscle), focusing (ciliary muscle and accomodation) Damage causes drooping eyelid, dilated pupil, double vision, difficulty focusing and inability to move eye in certain directions

CN IV: Trochlear Nerve Innervates an extrinsic eye muscle Eye movement (superior oblique muscle) Damage causes double vision and inability to rotate eye inferolaterally

CN V: Trigeminal Nerve Provides sensory innervation to the face Motor innervation to chewing muscles Ophthalmic branch – sensations from nasal cavity, skin of forehead, upper eyelid, eyebrow, nose Maxillary branch – sensations from lower eyelid, upper lips and gums, teeth of the maxilla, cheek, nose, palate, pharynx Mandibular branch – sensations from teeth of the mandible, lower gums and lips, palate, tongue. Motor function of temporalis and masseter muscles. Damage produces loss of sensation and impaired chewing

Trigeminal Nerve

CN VI: Abducens Nerve Abducts the eyeball Provides eye movement (lateral rectus m.) Damage results in inability to rotate eye laterally and at rest eye rotates medially

CN VII: Facial Nerve Innervates muscles of facial expression Sensory innervation of face Taste Somatic Motor - facial expressions Autonomic Motor - salivary and lacrimal glands, mucous membranes of nasal and palatine mucosa Special Sensory - taste on anterior 2/3’s of tongue Damage produces sagging facial muscles and disturbed sense of taste (no sweet and salty)

Branches of Facial Nerve Clinical test: Test anterior 2/3’s of tongue with substances such as sugar, salt, vinegar, and quinine; test response of tear glands to ammonia fumes; test motor functions by asking subject to close eyes, smile, whistle, frown, raise eyebrows, etc.

CN VIII: Vestibulocochlear Nerve Sensory nerve of hearing and balance Special Sensory Provides hearing (cochlear branch) and sense of balance (vestibular branch) Damage produces deafness, dizziness, nausea, loss of balance and nystagmus

CN IX: Glossopharyngeal Nerve Sensory and motor innervation of structures of the tongue and pharynx Taste Somatic motor – Swallowing and voice production via pharyngeal muscles Autonomic motor - salivation, gagging, control of BP and respiration Sensations from posterior 1/3 of tongue including taste Sensations from baroreceptors and chemoreceptors Damage results in loss of bitter and sour taste and impaired swallowing, blood pressure anomalies (with CN X).

CN X: Vagus Nerve A mixed sensory and motor nerve Main parasympathetic nerve “Wanders” into thorax and abdomen

Vagus Nerve X Sensations from skin at back of ear, external acoustic meatus, part of tympanic membrane, larynx, trachea, espophagus, thoracic and abdominal viscera Sensations from bararoceptors and chemoreceptors Special sensory – taste from epiglottis and pharynx Somatic motor – Swallowing and voice production via pharyngeal muscles Autonomic motor – smooth muscle of abdominal viscera, visceral glands secretions, relaxation of airways, and normal or decreased heart rate. Damage causes hoarseness or loss of voice, impaired swallowing, GI dysfunction, blood pressure anomalies (with CN IX), fatal if both are cut

CN XI: Accessory Nerve An accessory part of the vagus nerve Somatic motor function of pharynx, larynx, neck muscles Swallowing, head, neck and shoulder movement via trapezius and sternocleidomastoid and pharyngeal muscles Damage causes impaired head, neck, shoulder movement

CN XII: Hypoglossal Nerve Runs inferior to the tongue Innervates the tongue muscles Tongue movements for speech, food manipulation and swallowing If both are damaged – can’t protrude tongue If one side is damaged – tongue deviates towards injured side

Cranial Nerve Disorders Trigeminal neuralgia (tic douloureux) recurring episodes of intense stabbing pain in trigeminal nerve area (near mouth or nose) pain triggered by touch, drinking, washing face treatment may require cutting nerve Bell’s palsy disorder of facial nerve causes paralysis of facial muscles on one side may appear abruptly with full recovery within 3-5 weeks

Spinal Nerves

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 Posterior View

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 (cell bodies in gray matter of spinal cord – no ganglia)

Spinal Nerves Branch into dorsal ramus and ventral ramus Rami communicantes connect to the base of the ventral ramus Lead to the sympathetic chain ganglia (gray and white ramus) Dorsal and ventral rami contain sensory and motor fibers

Innervation of the Skin: Dermatomes Dermatome – an area of skin Innervated by cutaneous branches of a single spinal nerve Upper limb – skin is supplied by nerves of the brachial plexus Lower limb Lumbar nerves – anterior surface Sacral nerves – posterior surface

Map of Dermatomes – Anterior View

Map of Dermatomes – Posterior View

Disorders of the PNS: Shingles Shingles (herpes zoster) Viral infection Stems from childhood chicken pox Often brought on by stress Mostly experienced by those over 50

Disorders of the PNS: Migraine Headache Relates to sensory innervation of cerebral arteries Arteries dilate Compresses and irritates sensory nerve endings

Disorders of the PNS: Myasthenia Gravis Progressive weakening of the skeletal muscles An autoimmune disorder Antibodies destroy acetylcholine receptors Ptosis due to weakness of eyelid muscles