Download Types of Receptors

Types of Receptors
Sensory receptors enable us to learn about the environment around us
or about the state of our internal environment. Stimuli from varying
sources, and of different types, must be received and changed into the
electrochemical signals of the nervous system represented by changes in
the membrane potential. The sensory information is relayed to the
central nervous system where it is integrated with other sensory
information, or sometimes-higher cognitive functions, to become a
conscious perception of that stimulus. The central integration may then
lead to a motor response.
Sensory Receptors
Stimuli in the environment activate specialized receptors in the
peripheral nervous system. The classification of receptors into types can
be based on three different criteria: structure of the receptors, location
of the receptors relative to the stimuli they sense, and by the types of
stimuli to which they respond. Regardless of type, the function of these
receptors is to transduce a stimulus from one form of energy
(chemical, physical, etc.) into a change in the cell membrane potential
that may or may not create an action potential.
Structural Receptor Types
The cells that detect a change in the environment can be neurons
with free nerve endings, where the dendrites are exposed to the
surrounding tissue; neurons with encapsulated endings, where
supporting cells aid in the reception of stimuli; or specialized receptor
cells, which have specific structural components for detecting stimuli.
Examples of neurons with free nerve endings are the pain and
temperature receptors in the dermis of the skin. Also in the dermis are
encapsulated nerve endings such as the lamellar corpuscle that senses
pressure. The cells in the retina that receive light stimuli are an example
of specializedphotoreceptor cells, not neurons, that in turn can
stimulate an associated sensory neuron.
Receptors can also be classified based on their location relative to the
stimuli. Exteroceptors are receptors that receive input from the
external environment, such as the lameller corpuscles of the dermis and
photoreceptors
of
the
eye
that
have
already
been
mentioned. Interoceptors are those that sense stimuli from the
internal organs. Examples would include a stretch receptor in the wall of
an organ, such as those that sense the increase in blood pressure in the
aorta or carotid artery or detects stretch as the bladder fills with urine.
Finally, proprioceptors are widely distributed receptors in muscles,
tendons and joint capsules that the body uses to determine position and
movement of structures, such as its limbs and fingers. Proprioceptors
allow you to touch your finger to your nose, even with your eyes closed.
Functional Receptor Types
Lastly, receptors can be classified by the types of signals they transduce
into changes in membrane potential.
Chemoreceptors sense chemical stimuli, examples being taste, smell
and the osmotic pressure of the body’s extracellular fluids (the latter
sensed by osmoreceptors).
Nociceptors are pain receptors. Although pain is primarily a chemical
sense that detects the presence of chemicals released during tissue
damage, nociceptors are typically considered in a functional category of
their own. Nociceptors are found in most tissues throughout the body,
exceptions being the brain and possibly certain internal structures of
organs.
Mechanoreceptors sense physical stimuli, such as pressure and
vibration, as well as the sensation of sound and pull of gravity. A specific
example of a mechanoreceptor is the baroreceptors (pressure receptors)
found in the carotid arteries, which sense blood pressure.
Thermoreceptors are specific to sensing temperature and changes in
temperature. Thermoreceptors are found in two forms, those that
respond most strongly to temperatures below normal body temperature
(cold thermoreceptors), and those that respond most strongly at
temperatures above normal body temperature (warm thermoreceptors).
At normal body temperature, both types of receptors are active, but
there is generally no awareness of cold or warmth.
Photoreceptors respond to electromagnetic radiation (light). Humans
have the ability to sense electromagnetic waves at wavelengths between
400 and 700 nanometers, with different wavelengths corresponding to
different colors.