somatosensory system overview
cutaneous nerves
A-delta fibers: pain and temp
C fibers: pain and temp
A-beta and A-alpha fibers: touch
increase in diameter of nerve fiber corresponds to increase in conduction velocity
inactivation and activation depends on neuron fiber diameter
local anesthetics inactivate C fibers first
compression/ischemia will inactivate A-alpha and A-beta nerve fibers first
electrical stimulation activates A-alpha and A-beta nerve fibers before it activates C fibers
skin receptors
epidermis (superficial): smaller receptive field and more sensitive
dermis (deep): larger receptive field and less sensitive
meissner corpuscle: rapidly adapting, senses light touch, numerous in the pads of the fingers
merkel cell: slowly adapting, pressure, deep static touch, position sense, fingertips and superficial skin
pacinian corpuscle: rapidly adapting, sense vibration and pressure
ruffini endings: slowly adapting, respond to pressure
free nerve endings are present in
joints: detect extreme range of motion
hair: they detect bending of the hair
skin: they detect crude touch, pain and temp
two point discrimination improves as innervation density increases (fingertips are more sensitive, your arm isn't as sensitive)