439
CHAPTER TWELVE
Nervous System III
potential must be transferred to a neuron to trigger an action
potential. Peripheral nerves transmit sensory impulses to the
central nervous system (CNS), where they are analyzed and
interpreted in the brain.
Sensation and Perception
A sensation occurs when the brain becomes aware of sen-
sory impulses. A perception occurs when the brain interprets
those sensory impulses. All the nerve impulses that travel
away from sensory receptors into the CNS are alike, so the
resulting sensation depends on which region of the cerebral
cortex receives the impulse. For example, impulses reach-
ing one region are always interpreted as sounds, and those
reaching another are always sensed as touch. (Some recep-
tors, such as those that measure oxygen levels in the blood,
do not trigger sensations.)
Sensory receptors are specialized to respond to spe-
cific stimuli, but they may respond to other stimuli that
are strong enough, in which case the sensations will be
the same. Pain receptors, for example, can be stimulated
by heat, cold, or pressure, but the sensation is always the
same because, in each case, the same part of the brain
interprets the resulting nerve impulses as pain. Similarly,
stimuli other than light, such as a sharp blow to the head,
may trigger nerve impulses in visual receptors. When this
happens, the person may “see stars,” even though no light
is entering the eye, because any impulses reaching the
visual cortex are interpreted as light. Normally receptors
only respond to speci±
c stimuli, so the brain creates the
correct sensation for that particular stimulus.
At the same time that a sensation forms, the cerebral
cortex interprets it to seem to come from the receptors being
stimulated. This process is called
projection
because the
brain projects the sensation back to its apparent source.
Projection allows a person to pinpoint the region of stimula-
tion. Thus, we perceive that the eyes see an apple, the nose
smells it, and the ears hear the teeth crunch into it.
Receptor Types
Five types of sensory receptors are recognized, based on
their sensitivities to speci±
c stimuli:
1.
Chemoreceptors
(ke
mo-re-sep
torz) respond to
changes in the concentration of chemicals. Receptors
associated with the senses of smell and taste are of this
type. Chemoreceptors in internal organs detect changes
in the blood concentrations of oxygen, hydrogen ions,
glucose, and other chemicals.
2.
Pain receptors,
also called nociceptors (no
se-sep
torz),
respond to tissue damage. Triggering stimuli include
exposure to excess mechanical, electrical, thermal, or
chemical energy.
3.
Thermoreceptors
(ther
mo-re-sep
torz) are sensitive to
temperature change.
4.
Mechanoreceptors
(mek
ah-no re-sep
torz) are of
several types and sense mechanical forces by detecting
changes that deform the receptors.
Proprioceptors
(pro
pre-o-sep
torz) sense changes in the tensions of
muscles and tendons,
baroreceptors
(bar
o-re-sep
torz),
also called pressoreceptors, in certain blood vessels
detect changes in blood pressure, and
stretch receptors
in the lungs sense degree of infl
ation.
5.
Photoreceptors
(fo
to-re-sep
torz) in the eyes respond
to light energy of suf±
cient intensity.
Sensory Impulses
Sensory receptors can be ends of neurons or other types of
cells close to them. In either case, stimulation causes local
changes in their membrane potentials (receptor potentials),
generating a graded electric current that refl
ects the intensity
of stimulation (see chapter 10, p. 368).
If a receptor is a neuron and the change in membrane
potential reaches threshold, an action potential is generated,
and a sensory impulse is propagated along the afferent ±
ber.
However, if the receptor is another type of cell, its receptor
TABLE
12.1
|
Information Flow from the Environment Through the Nervous System
Information Flow
Smell
Taste
Sight
Hearing
Sensory receptors
Olfactory cells in nose
Taste bud receptor cells
Rods and cones in retina
Hair cells in cochlea
↓↓
Impulse in sensory F
bers
Olfactory nerve F
bers
Sensory F
bers in various cranial nerves
Optic nerve F
bers
Auditory nerve F
bers
↓↓
Impulse reaches CNS
Cerebral cortex
Cerebral cortex
Midbrain and cerebral cortex
Midbrain and cerebral cortex
↓↓
Sensation (new experience,
recalled memory)
A pleasant smell
A sweet taste
A small, round, red object
A crunching sound
↓↓
Perception
The smell of an apple
The taste of an apple
The sight of an apple
The sound of biting into an apple
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