486
UNIT 3
Regulation and Integration of the Body
13
Intrafusal
fibers
STRUCTURAL CLASS
ILLUSTRATION
FUNCTIONAL CLASSES
ACCORDING TO LOCATION (L)
AND STIMULUS TYPE (S)
BODY LOCATION
Encapsulated
Tactile (Meissner's) corpuscles
 
L: Exteroceptors
S: Mechanoreceptors (light
pressure, discriminative touch,
vibration of low frequency);
rapidly adapting
Dermal papillae of hairless skin,
particularly nipples, external
genitalia, fingertips, soles of
feet, eyelids
Lamellar (Pacinian) corpuscles
 
L: Exteroceptors, interoceptors,
and some proprioceptors
S: Mechanoreceptors (deep
pressure, stretch, vibration
of high frequency); rapidly
adapting
Dermis and hypodermis;
periostea, mesentery, tendons,
ligaments, joint capsules; most
abundant on fingers, soles of
feet, external genitalia, nipples
Bulbous corpuscles
(Ruffini endings)
 
L: Exteroceptors and
proprioceptors
S: Mechanoreceptors (deep
pressure and stretch); slowly or
nonadapting
Deep in dermis, hypodermis,
and joint capsules
Muscle spindles
 
L: Proprioceptors
S: Mechanoreceptors (muscle
stretch, length)
Skeletal muscles, particularly in
the extremities
Tendon organs
 
L: Proprioceptors
S: Mechanoreceptors (tendon
stretch, tension)
Tendons
Joint kinesthetic receptors
 
L: Proprioceptors
S: Mechanoreceptors and
nociceptors
Joint capsules of synovial joints
cold (10–40°C, or 50–104°F) are located in the superficial
dermis. Tose responding to heat (32–48°C, or 90–120°F) are
deeper in the dermis.
Heat or cold outside the range of thermoreceptors activates
nociceptors and is perceived as painful. Nociceptors also re-
spond to pinch and chemicals released from damaged tissue.
A key player in detecting painful stimuli is a plasma membrane
protein called the
vanilloid receptor
. Tis protein is an ion chan-
nel that is opened by heat, low pH, and various chemicals in-
cluding capsaicin, the substance found in red peppers.
Another sensation mediated by free nerve endings is itch.
Located in the dermis, the
itch receptor
escaped detection for
years because of its thin diameter. A number of chemicals—
notably histamine—present at inflamed sites activate these
nerve endings.
Other nonencapsulated nerve endings include:
Tactile (Merkel) discs
, which lie in the deepest layer of the
epidermis, function as light touch receptors. Certain free
nerve endings associate with enlarged, disc-shaped epider-
mal cells (
tactile
or
Merkel cells
) to form tactile discs.
Hair follicle receptors
, free nerve endings that wrap basket-
like around hair follicles, are light touch receptors that detect
bending of hairs. Te tickle of a mosquito landing on your
skin is mediated by hair follicle receptors.
Encapsulated Nerve Endings
All
encapsulated nerve end-
ings
consist of one or more fiber terminals of sensory neurons
enclosed in a connective tissue capsule. Virtually all encapsu-
lated receptors are mechanoreceptors, but they vary greatly in
shape, size, and distribution in the body. Tey include tactile
Table 13.1
General Sensory Receptors Classified by Structure and Function
(continued)
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