484
UNIT 3
Regulation and Integration of the Body
13
Classification by Stimulus Type
Tese categories are easy to remember because the name usu-
ally indicates the stimulus that activates the receptor.
Mechanoreceptors
respond to mechanical force such as
touch, pressure (including blood pressure), vibration, and
stretch.
Termoreceptors
respond to temperature changes.
Photoreceptors
, such as those of the retina of the eye, re-
spond to light.
Chemoreceptors
respond to chemicals in solution (mol-
ecules smelled or tasted, or changes in blood or interstitial
fluid chemistry).
Nociceptors
(no
0
se-sep
9
torz;
noci
5
harm) respond to po-
tentially damaging stimuli that result in pain. For example,
searing heat, extreme cold, excessive pressure, and inflam-
matory chemicals are all interpreted as painful. Tese signals
stimulate subtypes of thermoreceptors, mechanoreceptors,
and chemoreceptors.
Classification by Location
Receptors can be grouped into three receptor classes according
to either their location or the location of the activating stimulus:
exteroceptors, interoceptors, and proprioceptors.
Exteroceptors
(ek
0
ster-o-sep
9
torz) are sensitive to stimuli
arising outside the body (
extero
5
outside), so most extero-
ceptors are near or at the body surface. Tey include touch,
pressure, pain, and temperature receptors in the skin and
most receptors of the special senses (vision, hearing, equilib-
rium, taste, smell).
Interoceptors
(in
0
ter-o-sep
9
torz), also called
visceroceptors
,
respond to stimuli within the body (
intero
5
inside), such
as from the internal viscera and blood vessels. Interoceptors
monitor a variety of stimuli, including chemical changes,
needs. Te
peripheral nervous system (PNS)
provides these
links from and to the world outside our bodies. Ghostly white
nerves thread through virtually every part of the body, enabling
the CNS to receive information and carry out its decisions.
Te PNS includes all neural structures outside the brain and
spinal cord, that is, the
sensory receptors
, peripheral
nerves
and
their associated
ganglia
, and efferent
motor endings
.
Figure 13.1
diagrams its basic components.
In the first portion of this chapter we deal with the func-
tional anatomy of each PNS element. Ten we consider the
components of reflex arcs and some important somatic reflexes,
played out almost entirely in PNS structures, that help maintain
homeostasis.
PART 1
Sensory Receptors
and Sensation
Sensory Receptors
Classify general sensory receptors by structure, stimulus
detected, and body location.
Sensory receptors
are specialized to respond to changes in
their environment, which are called
stimuli
. ±ypically, activa-
tion of a sensory receptor by an adequate stimulus results in
graded potentials that in turn trigger nerve impulses along the
afferent PNS fibers coursing to the CNS.
Sensation
(awareness
of the stimulus) and
perception
(interpretation of the meaning
of the stimulus) occur in the brain. But we are getting ahead of
ourselves here.
For now, let’s just examine how sensory receptors are classi-
fied. Basically, there are three ways to classify sensory receptors:
(1) by the type of stimulus they detect; (2) by their body loca-
tion; and (3) by their structural complexity.
Central nervous system (CNS)
Peripheral nervous system (PNS)
Motor (efferent) division
Sensory (afferent)
division
Somatic nervous
system
Autonomic nervous
system (ANS)
Sympathetic
division
Parasympathetic
division
Figure 13.1
Place of the PNS in the structural organization of the nervous system.
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