Sensory Receptors, Neuronal Circuits for Processing Information Flashcards
lists and classifies five basic types of sensory
receptors:
(1) mechanoreceptors, which detect mechanical compression or stretching of the receptor or of tissues
adjacent to the receptor; (2) thermoreceptors, which
detect changes in temperature, with some receptors
detecting cold and others warmth; (3) nociceptors (pain
receptors), which detect physical or chemical damage
occurring in the tissues; (4) electromagnetic receptors,
which detect light on the retina of the eye; and (5) chemoreceptors, which detect taste in the mouth, smell in the
nose, oxygen level in the arterial blood, osmolality of the body fluids, carbon dioxide concentration, and other factors that make up the chemistry of the body
modality of sensation
Each of the principal types of sensation that we can
experience—pain, touch, sight, sound, and so forth—is
called a modality of sensation
receptor potential
Each of the principal types of sensation that we can
experience—pain, touch, sight, sound, and so forth—is
called a modality of sensation
RECEPTOR POTENTIAL OF THE
PACINIAN CORPUSCLE—AN EXAMPLE
OF RECEPTOR FUNCTION
Observe the small area of the terminal fiber that has been deformed by compression of the corpuscle, and note that ion channels have opened in the membrane, allowing
positively charged sodium ions to diffuse to the interior of the fiber. This action creates increased positivity inside the fiber, which is the “receptor potential.” The receptorpotential in turn induces a local circuit of current flow, shown by the arrows, that spreads along the nerve fiber. At the first node of Ranvier, which lies inside the capsule of the Pacinian corpuscle, the local current flow depolarizes the fiber membrane at this node, which then sets off typical action potentials that are transmitted along the nerve fiber toward the central nervous system.
mechanism by which receptors adapt
First, the Pacinian corpuscle is a viscoelastic structure,
so that when a distorting force is suddenly applied to
one side of the corpuscle, this force is instantly transmitted by the viscous component of the corpuscle directly to
the same side of the central nerve fiber, thus eliciting a
receptor potential. However, within a few hundredths
of a second, the fluid within the corpuscle redistributes
and the receptor potential is no longer elicited f by chance the central core fiber should
continue to be distorted, the tip of the nerve fiber gradually becomes “accommodated” to the stimulus. This probably results from progressive “inactivation” of the sodium
channels in the nerve fiber membrane, which means that
sodium current flow through the channels causes them
gradually to close
slow adapting receptors
(1) receptors
of the macula in the vestibular apparatus, (2) pain receptors, (3) baroreceptors of the arterial tree, and (4) chemoreceptors of the carotid and aortic bodies
tonic receptors
Because the slowly adapting receptors can continue to
transmit information for many hours, or even days, they
are called
rapidly adapting receptors detect change when
there is a change in the stimulus
predictive function of rate receptors
For instance, the receptors of the semicircular canals in the vestibular apparatus of the ear detect the rate at which the head begins to turn when one runs around a curve. Using this information, a person can predict how much he or she will turn within the next 2 seconds and can adjust the motion of the legs ahead of time to keep from losing balance For instance, when one is running,
information from the joint rate receptors allows the
nervous system to predict where the feet will be during
any precise fraction of the next second
general classification and sensory classification of nerve fibers
general classification” and a “sensory
nerve classification” of the different types of nerve fibers.
In the general classification, the fibers are divided into
types A and C, and the type A fibers are further subdivided
into α, β, γ, and δ fibers
Type A fibers are the typical large and medium-sized
myelinated fibers of spinal nerves. Type C fibers are the
small unmyelinated nerve fibers that conduct impulses at
low velocities. The C fibers constitute more than one half
of the sensory fibers in most peripheral nerves, as well as
all the postganglionic autonomic fibers.
examples of neuronal pools
cerebral cortex
dorsal gray matter of spinal cord
different nuclei in thalamus, pons medulla mesencephlon
stimulatory field
The neuronal area stimulated by each incoming nerve
fiber is called its stimulatory field
suprathreshold stimulus
Note that input
fiber 1 has more than enough terminals to cause neuron
a to discharge. The stimulus from input fiber 1 to this
neuron is said to be an excitatory stimulus; it is also called
a suprathreshold stimulus because it is above the threshold required for excitation
e subthreshold, and the neurons are
said to be facilitated.
Input fiber 1 also contributes terminals to neurons b
and c, but not enough to cause excitation. Nevertheless,
discharge of these terminals makes both these neurons
more likely to be excited by signals arriving through other
incoming nerve fibers. Therefore, the stimuli to these
neurons are said to be subthreshold, and the neurons are
said to be facilitated.
divergence
Often it is important for weak signals entering a neuronal
pool to excite far greater numbers of nerve fibers leaving
the pool. This phenomenon is called divergence.
