Somatic senses receive information from receptors in skin, muscles, joints, and viscera. They can be grouped as exteroceptive, proprioceptive, and visceroceptive senses.
1. Touch and pressure senses a. Free ends of sensory nerve fibers are the receptors for the sensations of touch and pressure.
b. Meissner's corpuscles are the receptors for the sensations of light touch.
c. Pacinian corpuscles are the receptors for the sensations of heavy pressure and vibrations.
2. Thermoreceptors include two sets of free nerve endings that are heat and cold receptors.
3. Sense of pain a. Pain receptors
(1) Pain receptors are free nerve endings that tissue damage stimulates.
(2) Pain receptors provide protection, do not adapt rapidly, and can be stimulated by changes in temperature, mechanical force, and chemical concentration.
b. The only receptors in viscera that provide sensations are pain receptors. These receptors are most sensitive to certain chemicals and lack of blood flow. The sensations they produce feel as if they come from some other part of the body (referred pain).
c. Pain nerve pathways
(1) The two main types of pain fibers are acute pain fibers and chronic pain fibers.
(2) Acute pain fibers are fast conducting; chronic pain fibers are slower conducting.
(3) Pain impulses are processed in the dorsal horn of the spinal cord, and they ascend in the spinothalamic tracts.
(4) Within the brain, pain impulses pass through the reticular formation before being conducted to the cerebral cortex.
d. Regulation of pain impulses
(1) Awareness of pain occurs when impulses reach the thalamus.
(2) The cerebral cortex judges the intensity of pain and locates its source.
(3) Impulses descending from the brain cause neurons to release pain-relieving substances, such as enkephalins and serotonin.
(4) Endorphin is a pain-relieving biochemical produced in the brain.
e. Certain neuropeptides synthesized in the brain and spinal cord inhibit pain impulses.
4. Stretch receptors a. Stretch receptors provide information about the condition of muscles and tendons.
b. Muscle spindles are stimulated when a muscle is relaxed, and they initiate a reflex that contracts the muscle.
c. Golgi tendon organs are stimulated when muscle tension increases, and they initiate a reflex that relaxes the muscle.
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