Chapter 13
The Peripheral Nervous System and Reflex Activity
485
13
tissue stretch, and temperature. Sometimes their activity
causes us to feel pain, discomfort, hunger, or thirst. However,
we are usually unaware of their workings.
Proprioceptors
(pro
0
pre-o-sep
9
torz), like interoceptors, re-
spond to internal stimuli. However, their location is much
more restricted. Proprioceptors occur in skeletal muscles,
tendons, joints, and ligaments and in connective tissue cov-
erings of bones and muscles. (Some authorities include the
equilibrium receptors of the inner ear in this class.) Proprio-
ceptors constantly advise the brain of our body movements
(
propria
5
one’s own) by monitoring how much the organs
containing these receptors are stretched.
Classification by Receptor Structure
Te overwhelming majority of sensory receptors belong to the
general senses
and are simply the modified dendritic endings
of sensory neurons. Tey are found throughout the body and
monitor most types of general sensory information.
Receptors for the
special senses
(vision, hearing, equilib-
rium, smell, and taste) are housed in complex
sense organs
. For
example, the sense organ we know as the eye is composed not
only of sensory neurons but also of nonneural cells that form its
supporting wall, lens, and other associated structures.
Tough the special senses are most familiar to us, the simple
sensory receptors associated with the general senses are no less
important, and we will concentrate on their structure and func-
tion in this chapter. Te special senses are the topic of Chapter 15.
Simple Receptors of the General Senses
Te widely distributed general sensory receptors are involved in
tactile sensation (a mix of touch, pressure, stretch, and vibration),
temperature monitoring, and pain, as well as the “muscle sense”
provided by proprioceptors. As you read about these receptors,
notice that there is no perfect “one-receptor–one-function” re-
lationship. Instead, one type of receptor can respond to several
different kinds of stimuli. Likewise, different types of receptors
can respond to similar stimuli. Anatomically, general sensory
receptors are nerve endings that are either
nonencapsulated
(
free
)
or
encapsulated
.
Table 13.1
illustrates the general sensory recep-
tors. You may find it helpful to refer to this table throughout the
discussion that follows.
Nonencapsulated (Free) Nerve Endings
Present nearly every-
where in the body,
nonencapsulated (free) nerve endings
of
sensory neurons are particularly abundant in epithelia and con-
nective tissues. Most of these sensory fibers are nonmyelinated,
small-diameter group C fibers, and their distal endings (the sen-
sory terminals) usually have small knoblike swellings.
Free nerve endings respond chiefly to temperature and
painful stimuli, but some respond to tissue movements
caused by pressure as well. Nerve endings that respond to
Table 13.1
General Sensory Receptors Classified by Structure and Function
STRUCTURAL CLASS
ILLUSTRATION
FUNCTIONAL CLASSES
ACCORDING TO LOCATION (L)
AND STIMULUS TYPE (S)
BODY LOCATION
Nonencapsulated
Free nerve endings of sensory
neurons
 
L: Exteroceptors, interoceptors,
and proprioceptors
S: Thermoreceptors (warm and
cool), chemoreceptors (itch,
pH, etc.), mechanoreceptors
(pressure), nociceptors
(pain, hot, cold, pinch, and
chemicals)
Most body tissues; most dense
in connective tissues (ligaments,
tendons, dermis, joint capsules,
periostea) and epithelia
(epidermis, cornea, mucosae,
and glands)
Modified free nerve endings:
Tactile (Merkel) discs
 
L: Exteroceptors
S: Mechanoreceptors (light
pressure); slowly adapting
Basal layer of epidermis
Hair follicle receptors
 
L: Exteroceptors
S: Mechanoreceptors (hair
deflection); rapidly adapting
In and surrounding hair follicles
Tactile
cell
Tactile
disc
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