444
UNIT THREE
Proprioception
Proprioceptors are mechanoreceptors that send information to
the CNS about body position and the length and tension of
skeletal muscles. The role of lamellated corpuscles as pressure
receptors in joints has been mentioned. The other main prop-
rioceptors are stretch receptors: muscle spindles and Golgi ten-
don organs. No sensation results when they are stimulated.
Muscle spindles
are in skeletal muscles near their junc-
tions with tendons. Each spindle consists of several small,
modiF ed skeletal muscle F bers (intrafusal F bers) enclosed
in a connective tissue sheath. Near its center, each intrafusal
F
ber has a specialized nonstriated region with the end of a
sensory nerve F
ber wrapped around it
(f
g. 12.4
a
)
.
The striated portions of the intrafusal F ber contract to keep
the spindle taut at different muscle lengths. Thus, if the whole
muscle is stretched, the muscle spindle is also stretched, trig-
gering sensory nerve impulses on its nerve F ber. These sensory
F bers synapse in the spinal cord with lower motor neurons
leading back to the same muscle. Thus, impulses triggered by
stretch of the muscle spindle contract the skeletal muscle of
which it is a part. This action, called a
stretch refl
ex,
opposes
the lengthening of the muscle and helps maintain the desired
position of a limb despite gravitational or other forces tending
to move it (see chapter 11, pp. 390–391).
Golgi tendon organs
are in tendons close to their attach-
ments to muscles. Each is connected to a set of skeletal mus-
Another group of neuropeptides with pain-suppressing,
morphinelike actions are the
endorphins.
They are found in
the pituitary gland and in regions of the nervous system, such
as the hypothalamus, that transmit pain impulses. Enkephalins
and endorphins are released in response to extreme pain
impulses, providing natural pain control. Clinical Application
12.1 discusses treatments for severe pain.
PRACTICE
6
Describe three types of touch and pressure receptors.
7
Describe thermoreceptors.
8
What types of stimuli excite pain receptors?
9
What is referred pain?
10
Explain how neuropeptides control pain.
Cannabinoids are substances in the plant
Cannibus sativa,
the source of
marijuana, that may relieve pain. Anecdotal evidence for such an eF
ect
dates to
A
.
D
. 315. Neurons in areas of the brain, brainstem, and periph-
eral nervous system have receptors for cannabinoids. A synthetic ver-
sion of the compound in marijuana responsible for most of its eF
ects
(delta-9-tetra-hydrocannabinol), marketed as the prescription drug
Marinol, is used to treat nausea and vomiting that occurs with cancer
chemotherapy and to boost appetite in people who have AIDS.
(a)
Sensory
nerve fiber
Sensory
nerve endings
Intrafusal
fiber
Skeletal muscle
fiber
Muscle spindle
Connective
tissue sheath
Sensory
nerve fiber
Skeletal muscle fiber
Golgi tendon organ
Tendon
(b)
FIGURE 12.4
Stretch receptors maintain posture. (
a
) Increased muscle length stimulates muscle spindles, which stimulate muscle contraction.
(
b
) Golgi tendon organs occupy tendons, where they inhibit muscle contraction.
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