440
UNIT THREE
soles, nipples, and external genital organs. They provide
f
ne touch, such as distinguishing two points on the skin
where an object touches, to judge its texture (f
g. 12.1
b
).
3.
Lamellated (Pacinian) corpuscles.
These sensory
bodies are relatively large, ellipsoidal structures
composed oF connective tissue f
bers and cells. They are
common in the deeper dermal tissues oF the hands, Feet,
penis, clitoris, urethra, and breasts and also in tendons
oF muscles and ligaments oF joints (f
g. 12.1
c
). Heavier
pressure and stretch stimulate lamellated corpuscles.
They also detect vibrations in tissues.
Temperature Senses
Temperature receptors (thermoreceptors) include two
groups oF Free nerve endings in the skin. Those that respond
to warmer temperatures are
warm receptors,
and those that
respond to colder temperatures are
cold receptors.
The warm receptors are most sensitive to temperatures
above 25°C (77°±) and become unresponsive at tempera-
tures above 45°C (113°±). As 45°C is approached, pain
receptors are also triggered, producing a burning sensation.
Cold receptors are most sensitive to temperatures
between 10°C (50°±) and 20°C (68°±). IF the temperature
drops below 10°C, pain receptors are stimulated, and the
person Feels a Freezing sensation.
At intermediate temperatures, the brain interprets sen-
sory input From diFFerent combinations oF these receptors
as a particular temperature sensation. Both warm and cold
receptors rapidly adapt, so within about a minute oF con-
tinuous stimulation, the sensation oF warm or cold begins
to Fade. This is why we quickly become comFortable aFter
jumping into a cold swimming pool or submerging into a
steaming hot tub.
Sense of Pain
Pain receptors (nociceptors) consist oF Free nerve endings.
These receptors are widely distributed throughout the skin
and internal tissues, except in the nervous tissue oF the brain,
which lacks pain receptors. Pain receptors protect in that
they are stimulated when tissues are damaged. Pain sensa-
tion is usually perceived as unpleasant, signaling that action
be taken to remove the source oF the stimulation.
Most pain receptors can be stimulated by more than one
type oF change. However, some pain receptors are most sen-
sitive to mechanical damage. Others are particularly sensitive
to extremes in temperature. Some pain receptors are most
responsive to chemicals, such as hydrogen ions; potassium
ions; or specif
c breakdown products oF proteins, histamine,
and acetylcholine. A def
ciency oF blood (ischemia) and thus
a deFiciency oF oxygen (hypoxia) in a tissue also triggers
pain sensation. ±or example, pain elicited during a muscle
cramp results From interruption oF blood fl
ow that occurs
as the sustained contraction squeezes capillaries, as well as
From the stimulation oF mechanoreceptors. Also, when blood
ow is interrupted, pain-stimulating chemicals accumulate.
Sensory Adaptation
The brain must prioritize the sensory input it receives, or
it would be overwhelmed by unimportant inFormation. ±or
example, until this sentence prompts you to think about it,
you are probably unaware oF the pressure oF your clothing
against your skin, or the background noise in the room. This
ability to ignore unimportant stimuli is called
sensory adap-
tation
(sen
so-re ad
ap-ta
shun). It may refl ect a decreased
response to a particular stimulus From the receptors (periph-
eral adaptation) or along the CNS pathways leading to the
sensory regions oF the cerebral cortex (central adaptation).
As adaptation occurs, sensory impulses become less Frequent
until they may cease. Once adaptation occurs, impulses are
triggered only iF the strength oF the stimulus changes.
PRACTICE
1
Distinguish between general and special senses.
2
List the f
ve general types oF sensory receptors.
3
What do all types oF receptors have in common?
4
Explain how a sensation occurs.
5
What is sensory adaptation?
12.3
GENERAL SENSES
General senses are those whose sensory receptors are wide-
spread, associated with the skin, muscles, joints, and vis-
cera. These senses can be divided into three groups:
1.
Exteroreceptive senses
are associated with changes
at the body surFace. They include the senses oF touch,
pressure, temperature, and pain.
2.
Visceroreceptive
(interoceptive)
senses
are associated
with changes in viscera (blood pressure stretching blood
vessels, an ingested meal stimulating pH receptors in
the small intestine, and so on).
3.
Proprioceptive senses
are associated with changes in
muscles and tendons and in body position.
Touch and Pressure Senses
The senses oF touch and pressure derive From three types
oF receptors
(f g. 12.1)
. As a group, these receptors sense
mechanical Forces that deForm or displace tissues. The touch
and pressure receptors include the Following:
1.
Free nerve endings.
These simplest receptors are
common in epithelial tissues, where they lie between
epithelial cells. They are responsible For the sensation oF
itching (f
g. 12.1
a
).
2.
Tactile (Meissner’s) corpuscles.
These are small, oval
masses oF fl
attened connective tissue cells in connective
tissue sheaths. Two or more sensory nerve f
bers branch
into each corpuscle and end within it as tiny knobs.
Tactile corpuscles are abundant in the hairless
portions oF the skin, such as the lips, f
ngertips, palms,
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