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 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

Functional Organization of the PNS Figure 14.1 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

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 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

Structural Organization of PNS in Region of a Spinal Nerve Peripheral Nerve Endings: Afferent: Sensory Receptors Efferent: Somatic Motor Efferent: Autonomic Nervous System

Peripheral Sensory Receptors Structures that pick up sensory stimuli Initiate signals in sensory axons 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 Sensory receptors can be classified in three ways: Type of stimulus detected Location Structural Complexity Monitor most types of special sensory information

Classification by Type of Stimuli 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 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

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 within the body internal viscera & blood vessels Monitor a variety of stimuli such as chemical changes, stretching of tissue & temperature Proprioceptors: – monitor degree of stretch Located in musculoskeletal organs

Classification by Structure Simple: Vast majority Modified dendritic extensions of sensory neurons Found in skin, mucous membranes, muscles & connective tissue Complex: Associated with the special senses (vision, hearing, smell & taste) Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

General Sensory Receptors (simple receptors) Widely distributed Nerve endings of sensory neurons monitor: Touch, pressure, vibration, stretch Pain, temperature, proprioception Divided into two groups Free dendritic (nerve) endings Encapsulated dendritic (nerve) endings Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

Abundant in epithelia and underlying connective tissue Free Nerve Endings Abundant in epithelia and underlying connective tissue Most are unmyelinated with a small diameter 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 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

Encapsulated Nerve Endings Consist of one or more end fibers of sensory neurons Enclosed in connective tissue Mechanoreceptors Include four main types: Meissner’s corpuscles Pacinian corpuscles Ruffini’s corpuscles Proprioceptors – monitor stretch in locomotory organs Muscle spindles Golgi tendon organs Joint kinesthetic receptors

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 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

Structure of Receptors in Skin Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

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 Motor unit – a motor neuron and all the muscle fibers it innervates

The Neuromuscular Junction Figure 14.5a, b Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

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

Attach to the brain and pass through foramina of the skull 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 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

The 12 Pairs of Cranial Nerves Figure 14.8 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

Sensory nerves of smell CN I: Olfactory Nerves Sensory nerves of smell Table 14.2 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

Sensory nerve of vision CN II: Optic Nerve Sensory nerve of vision Table 14.2 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

CN III: Oculomotor Nerve Innervates four of the extrinsic eye muscles Table 14.2 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

Innervates an extrinsic eye muscle CN IV: Trochlear Nerve Innervates an extrinsic eye muscle Table 14.2 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

Provides sensory innervation to the face CN V: Trigeminal Nerve Provides sensory innervation to the face Motor innervation to chewing muscles

CN VI: Abducens Nerve Abducts the eyeball Table 14.2 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

Innervates muscles of facial expression Sensory innervation of face CN VII: Facial Nerve Innervates muscles of facial expression Sensory innervation of face Taste Table 14.2 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

CN VIII: Vestibulocochlear Nerve Sensory nerve of hearing and balance Table 14.2 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

CN IX: Glossopharyngeal Nerve Sensory and motor innervation of structures of the tongue and pharynx Taste Table 14.2 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

CN X: Vagus Nerve (pneumogastric nerve) A mixed sensory and motor nerve Main parasympathetic nerve “Wanders” into thorax and abdomen Table 14.2 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

An accessory part of the vagus nerve CN XI: Accessory Nerve An accessory part of the vagus nerve Somatic motor function of pharynx, larynx, neck muscles Table 14.2 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

CN XII: Hypoglossal Nerve Runs inferior to the tongue Innervates the tongue muscles Table 14.2 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

31 pairs – contain thousands of nerve fibers 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) Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

Spinal Nerves Posterior View Figure 14.9 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

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 & Posterior View

A reflex is a rapid, predictable motor response to a stimulus Reflex Activity A reflex is a rapid, predictable motor response to a stimulus The basic reflex is unlearned, unpremeditated and involuntary, Some reflexes we are aware of (pulling away from hot pot) and others, usually visceral, we are not In addition to basic reflexes there are also learned or acquired reflexes (automatic actions of an experienced driver) Reflexes occur in highly specific neural paths called reflex arcs Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

The Reflex Arc Receptor: site of stimulus action Sensory Neuron: transmits afferent impulses to the CNS Integration Center: synapse between sensory & motor neuron or multiple synapses within a chain of interneurons Motor Neuron: conducts the afferent impulses from integration center to effector organ Effector: the muscle fiber or gland cell that response to the efferent impulses in a characteristic way

Spinal reflexes are somatic reflexes mediated by the spinal cord In the stretch reflex the muscle spindle is stretched and excited by either an external stretch or an internal stretch The Golgi tendon reflex produces muscle relaxation and lengthening in response to contraction The flexor, or withdrawal, reflex is initiated by a painful stimulus and causes automatic withdrawal of the threatened body part from the stimulus They crossed extensor reflex is a complex spinal reflex consisting of an ipsilateral withdrawal reflex and a contralateral extensor reflex Superficial reflexes are elicited by gentle cutaneous stimulation

The Stretch Reflex

Deep Golgi Tendon Reflex

Disorders: Anencephaly Various degrees of absence of the various of the skull bones, cerebrum and the brain stem One of the most common neural tube defects. Neural tube. Occurs early in the development when the upper part of the neural tube fails to close. Possible causes include environmental toxins and low intake of folic acid by the mother during pregnancy. Often results in miscarriage, if child survives to birth he/she will be totally vegetative and unable to see, hear or process sensory input. Death will occur soon after birth.

Disorders: Shingles Viral infection Childhood chicken pox dormant in spinal ganglia Often brought on by stress Mostly experienced by those over 50 Unilateral patch of skin blisters and discoloration along the path of one or more spinal nerves Most commonly around the waist. The symptoms may persist for 3 to 6 months.

Disorders: Cerebal Palsy Group of neuromuscular disorders in which the voluntary muscles are poorly controlled or paralyzed due to brain damage Can result in spasticity, speech difficulties, seizures, deafness or reduced cognitive ability Can be due to lack of oxygen during delivery, smoking or drug use, or maternal infection during pregnancy

Disorders: Spina Bifida & Lumbar Myelomeningocele Incomplete formation of vertebral arches, usually in lumbosacral region Laminae & spinal processes missing in at least one vertebrae In the severe form, spina bifida cyctica, it can result in a sac-like cyst protruding dorsally The cyst may contain meninges and CSF and possibly portiosns of the spinal cord and spinal root nerves primary functional deficits are lower limb paralysis and sensory loss, bladder and bowel dysfunction, and cognitive dysfunction

Disorders :Migraine Headache Relates to sensory innervation of cerebral arteries Arteries dilate Compresses and irritates sensory nerve endings Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

Disorders: Poliomyelitis “polio” means gray matter caused by Poliovirus hominis. CNS infection, but its major effect is on the peripheral nerves and the muscles they supply. virus infects the motor neurons in the anterior horn of the central gray matter of the spinal cord. The infection causes degeneration of the motor neurons, which results in paralysis and atrophy of the muscles innervated by those nerves.

Disorders: Anesthetic leprosy Bacterial infection of the peripheral nerves caused by Mycobacterium leprae. Infection results in anesthesia, paralysis, ulceration, and gangrene

Disorders :Myasthenia Gravis Progressive weakening of the skeletal muscles An autoimmune disorder Antibodies destroy acetylcholine receptors Ptosis due to weakness of eyelid muscles Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings