Structure:
Sensory organs are somatic, when they are in skin,
muscle, tendon, or bones,
and are visceral when they are attached to sensory nerve fibers in visceral
organs. Most of the sensory neurons
are dendrites, and most cell bodies also have an axon that links to another
part of the nervous system. Three types of receptor organs are described;
free, encapsulated, and associated. Those that inform about the external
environment are called exteroceptors; those related to the internal environment,
interoceptors. Special sensory receptors in tendons are called proprioceptors,
because they inform about the position of
body parts.
Function:
Sensory organs are designed to magnify, or amplify a stimulus. Free sensory
receptors report about painful sensations, touch, and sometimes, hot and
cold; encapsulated receptors inform about pressure, hot and cold, and also
touch; associated nerve receptors refer to nerve endings on other structures,
like a hair follicle. As a stimulus overcomes a threshold level, as the
result of a graded potential,
the sensation travels toward the spinal cord, and then to the brain, where
it arrives, if it was large enough initially. If the signal was small,
it may end at the spinal cord, producing a reflex
action, like a jerk away from a hot object.
