chapter 13- The peripheral Nervous system

Peripheral nervous system (PNS)

Links CNS to body and to external environment

Sensory division

consists of sensory (afferent) neurons that detect and transmit sensory stimuli to CNS

Somatic sensory division

detects both internal and external stimuli

Visceral sensory division

relays internal information (like blood pressure) from organs of abdominopelvic and thoracic cavities

Motor division

consists of motor (efferent) neurons; carry out motor functions of nervous system

Somatic motor divison

responsible for voluntary motor functions; composed of lower motor neurons which directly trigger skeletalmuscle contractions

Visceral motor divison

responsible for maintaining many aspects of homeostasis by controlling involuntary motorfunctions in body

Sympathetic nervous system (fight or flight division)

involved in homeostasis activities surrounding physicalwork and visceral responses of emotions

Parasympathetic system (rest and digest division)

involved in digestion and maintaining body’s homeostasis at rest

Peripheral nerves

main organs of PNS; consist of axons of many neurons bound together by connective tissue

Mixed nerves

contain both sensory and motor neurons

Sensory nerves

contain only sensory neurons

motor nerves

contains mostly motor neurons

Spinal nerves

originate from spinal cord and innervate structures below head and neck

Anterior root

consists of motor neurons from anterior horn

Posterior root

consists of sensory neurons from posterior horn

Posterior root ganglion

Posterior root features a swollen area that houses cell bodies of sensory neurons

Epineurium

outermost layer of connective tissue that holds motor and sensory axons together

Fascicles

small groups of bundled axons surrounded by connective tissue called perineurium

endoneurium

Each individual axon within a fascicle is surrounded by its own connective tissue

Cranial nerves

attach to brain and mostly innervate structures in head and neck

Bell’s palsy

Common problem associated with facial nerve

Spinal nerve

short and divides into following 2 mixed nerves; both carry both somatic motor and sensory information

Posterior ramus

travels to posterior side of body

Anterior ramus

travels to anterior side of body and/or to an upper or lower limb

Nerve plexuses

Anterior rami of cervical, lumbar, and sacral spinal nerves each merge to form complicated networks of nerves

Hiccups

annoying spasms of diaphragm that cause a forcefulinhalation of air

Sciatic nerve

longest and largest nerve in body; contains axons from both anterior and posterior divisions of sacral plexus

Exteroceptors

usually close to body’s surface; detect stimuli originating from outside body

Interoceptors

usually found within body’s Interior ; detect stimuli originating from within body itself

Mechanoreceptors

depolarize in response to anything that mechanically deforms tissue where receptors are found

Thermoreceptors

exteroceptors, most of which are slowly adapting receptors

Chemoreceptors

can be either interoceptors or exteroceptors; depolarize in response to binding to specific chemical

Photoreceptors

special sensory exteroceptors found only in eye

Nociceptors

usually slowly adapting exteroceptors

Capsaicin

Hot peppers can make your mouth feel like it’s on fire; result of chemical in peppers

neuropathies

capsaicin may be applied as a topical cream to relieve pain of peripheral nerve disorders

Shingles

a viral infection caused by chickenpox virus, and other conditions

Types of thermoreceptors

usually small knobs at end of free nerve endings in skin

“Cold” receptors

respond to temperatures between
10 C and 40 C (50–104 F); in superficial dermis

“Hot” receptors

respond to temperatures between
32 C and 48 C (90–118 F); deep in dermis

Dermatomes

Skin can be divided into different segments

dermatome map

Dermatomes can be combined to assemble a

Referred pain

phenomenon whereby pain that originates in an organ is perceived as cutaneous pain

Upper motor neurons

neurons of primary motor cortex make decision to move and initiate that movement

Lower motor neurons

receive messages from upper motor neurons; in contact with skeletal muscle fibers

Reflexes

programmed, automatic responses to stimuli

Reflex arch

Occur in a three-step sequence of events; usually protective negative feedback loops

Monosynaptic reflexes (simple reflex)

involve only a single synapse within spinal cord between a sensory and motor neuron

Polysynaptic reflexes

involve multiple synapses

Patellar (knee-jerk) reflex and jaw-jerk reflex

are examples of simple stretch reflexes

Golgi tendon reflexes

polysynaptic reflexes; protect muscles and tendons from damaging forces

Flexion or withdrawal reflex

involves rapidly conducting nociceptive afferents and multiple synapses in spinal cord

Crossed -extension reflex

occurs simultaneously on opposite side of body for balance and postural support while other limb is withdrawn from a painful stimulus

Cranial nerve reflexes

polysynaptic reflex arcs that involve cranial nerves

Gag reflex

triggered when visceral sensory nerve endings of glossopharyngeal nerve in posteriorthroat are stimulated

Corneal blink reflex

triggered when a stimulus reaches somatic sensory receptors of trigeminal nerve in thin outer covering of eye (cornea)

Plantar reflex

Healthy adult will flex toes, a response known as

Amyotrophic lateral sclerosis (ALS) also known as Lou Gehrig’s disease

involves degeneration of cell bodies of motor neurons in anterior horn of spinal cord as well as upper motor neurons in cerebral cortex