478
UNIT THREE
c. Golgi tendon organs are stimulated when muscle
tension increases, and they initiate a refl
ex that
relaxes the muscle.
5. Visceral receptors include lamellated corpuscles and
free nerve endings.
12.4
SPECIAL SENSES (PAGE 446)
Special senses have receptors in complex sensory organs
of the head.
1. Sense of smell
a. Olfactory receptors
(1) Olfactory receptors are chemoreceptors
that chemicals dissolved in nasal secretions
stimulate.
(2) Olfactory receptors function with taste
receptors and aid in food selection.
b. Olfactory organs
(1) The olfactory organs consist of receptors and
supporting cells in the nasal cavity.
(2) Olfactory receptors are neurons with cilia.
c. Olfactory nerve pathways.
(1) Nerve impulses travel from the olfactory
receptors through the olfactory nerves,
olfactory bulbs, and olfactory tracts.
(2) They go to interpreting centers in the limbic
system.
d. Olfactory stimulation
(1) Olfactory impulses may result when an
odorant molecule stimulates a distinct set of
receptor cells.
(2) Olfactory receptors adapt rapidly.
(3) Olfactory receptors are often damaged by
environmental factors and are replaced from a
pool of stem cells.
2. Sense of taste
a. Taste receptors
(1) Taste buds consist of receptor cells and
supporting cells.
(2) Taste cells have taste hairs that are sensitive to
particular chemicals dissolved in water.
(3) Taste hair surfaces have receptor sites to which
chemicals combine and trigger impulses to the
brain.
b. Taste sensations
(1) The F
ve primary taste sensations are sweet,
sour, salty, bitter, and umami.
(2) Various taste sensations result from the
stimulation of one or more sets of taste
receptors.
(3) Each of the F
ve primary types of taste cells
is particularly sensitive to a certain group of
chemicals.
c. Taste nerve pathways
(1) Sensory impulses from taste receptors travel
on F
bers of the facial, glossopharyngeal, and
vagus nerves.
(2) These impulses are carried to the medulla
and ascend to the thalamus and then to the
gustatory cortex in the parietal lobes.
12.3
GENERAL SENSES (PAGE 440)
General senses receive information from receptors in
skin, muscles, joints, and viscera. They can be grouped as
exteroceptive, visceroceptive, and proprioceptive senses.
1. Touch and pressure senses
a. ±ree ends of sensory nerve F
bers are the receptors
for the sensations of touch and pressure.
b. Tactile corpuscles are the receptors for the
sensations of light touch.
c. Lamellated corpuscles are the receptors for the
sensations of heavy pressure and vibrations.
2. Temperature senses
a. Thermoreceptors include two sets of free nerve
endings that are heat and cold receptors.
b. Combinations of input from both receptor types
are interpreted as intermediate temperatures.
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.
(1) These receptors are most sensitive to certain
chemicals and lack of blood fl ow.
(2) The sensations they produce may 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 F
bers are acute
pain F
bers and chronic pain F
bers.
(2) Acute pain F
bers are fast conducting; chronic
pain F
bers 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 CNS
inhibit pain impulses.
4. Proprioception
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 refl
ex that contracts the
muscle.
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