Peripheral Nervous System

= Structures of the nervous system peripheral to the brain and spinal cord

Sensory (afferent‡ toward brain) fibers and motor (efferent‡ away from brain) fibers run in the same nerves (mixed nerves)

Cranial nerves – 12 pairs, directly from the brain
ie) optic, vagus ‡ down neck to heart (sleeper hold = wrestling move that stimulates the vagus and slows down heart rate)

Spinal nerves – 31 pairs, from the spinal cord
8 cervical to go with 7 vertebrae ‡ merge with each other and mix = plexus

Plexus – network of nerves near the spinal cord where branches from different spinal nerves combine to carry fibers to specific body parts
3 plexuses: cervical (@ top), brachial and lumbosacral @ btm (no "thoracic plexus")
ß if 10 mnth old falls, pull arm, can tear their brachial plexis and cause peralis
ß cut spinal cord ‡ where cut determines paralysis
parapa = below T2
quadra = up in cervical
ß if accident, don’t move someone! Could cut the spinal cord if vertebrae broken!

TWO PARTS of the PNS
1. Somatic – sensory input from skin and muscles, motor to the voluntary (skeletal) muscles
ß Nociceptors (pain) – free nerve endings
stimulated by chemicals released by damaged tissue
Acute = fast message (myelinated), sharp pain
chronic fibers –prolonged dull ache – persistent message sent
ß Thermoreceptors – respond to hot or cold but at extremes the sensation turns to pain
ß Touch and Pressure – encapsulated nerve endings (less sensitive)
Meissner’s = closer to surface, less covering
Pacinian corpuscles = deeper down, more stimuls needed
ß Proprioceptor – monitors muscle length, supplies position sense (position)

2. Autonomic – involuntary, control internal organs (viscera), motor to smooth and cardiac (involuntary) muscles; has a ganglion external to the spinal cord

Two parts with opposite effects: sympathetic and parasympathetic
A. Sympathetic – "fight, flight, fright" – prepares for strenuous physical exertion
Uses norepinephrine as the post-ganglionic neurotransmitter (NE also released by adrenal medulla above the kidney into bloodstream)
Effects:
o increases heart rate,
o raises blood pressure,
o dilates pupil (see better),
o dilates airways of the lung (need more O2 for muscles),
o converts glycogen (storage) to glucose (use),
o shunts blood from skin and intestines to muscles, brain and heart (where needs it!)
o inhibits intestinal and urinary functions (stop to pee while running from knife?!)
Ie: taking test, heart beats faster & harder to supply muscular exersion, but just sitting (no exersion) = stress! Imagine all these things happening and not moving! Day to day basis wears on system!

B. Parasympathetic– relax (take down from sympathetic)
uses acetylcholine as neurotransmitter
Reverses effects of sympathetic nervous system (dec h.r. dec b.p.)