summary
section 14.1 A stimulus is a form of
energy that the body detects by means of
sensory receptors. A sensation is a conscious
awareness that stimulation has occurred.
Perception is understanding what the
sensation means.
Some sensory receptors are free nerve
endings. Others are encapsulated organs or
are formed by a cell that synapses with a
sensory neuron. Sensory receptors respond to stimuli, which
are specific forms of energy, such as mechanical pressure and
light (Table 14.3).
Mechanoreceptors detect mechanical energy that is asso-
ciated with changes in pressure (e.g., sound waves), changes
in position, or acceleration.
Thermoreceptors detect the presence of or changes in
radiant energy from heat sources.
Nociceptors (pain receptors) detect tissue damage. Their
signals are perceived as pain.
Chemoreceptors detect chemical substances that are
dissolved in the body fluids around them.
Osmoreceptors detect changes in water volume (hence
solute concentrations) in the surrounding fluid.
Photoreceptors detect light.
A sensory system has receptors for specific stimuli and
nerve pathways from those receptors to processing centers in
the brain. The brain assesses each stimulus based on which
nerve pathway is delivering the signals, how often signals are
traveling along each axon of the pathway, and the number of
axons that were recruited into action. In sensory adaptation,
the response to a stimulus decreases.
The special senses include taste, smell, hearing, balance,
and vision. The receptors associated with these senses are in
sense organs or another specific body region.
section 14.2 Somatic sensations
include touch, pressure, pain, temperature,
and muscle sense. Receptors associated
with these sensations occur in various parts
of the body. Their signals are processed
in the somatosensory cortex of the brain.
The simplest receptors, including those for
temperature and pain, are free nerve endings in the skin
or internal tissues. Some somatic sensations arise when
encapsulated receptors respond to stimuli.
As Section 14.10 described, there are various forms of color blindness.
Figure 14.24 shows simple tests, called Ishihara plates, which are standardized tests for
different forms of color blindness. For instance, you may have one form of red–green color
blindness if you see the numeral “7” instead of “29” in the circle in Figure 14.24A. You may
have another form if you see a “3” instead of an “8” in the circle in Figure 14.24B.
If you do this exercise and have questions about your color vision, visit your doctor to
determine whether additional testing is in order.
Figure 14.24 Color blindness tests.
(© Cengage Learning)A b282 Chapter 14
expLoreon your oWn
section 14.3 Taste and smell are
chemical senses. Their sensory pathways
travel from chemoreceptors to processing
regions in the cerebral cortex and limbic
system. Taste buds in the tongue and mouth
contain the taste receptors. The sense of
smell relies on olfactory receptors in patches
of epithelium in the upper nasal passages.
sections 14.5, 14.6 The sense of
hearing requires parts of the outer, middle,
and inner ear that collect, amplify, or respond
to sound waves that vibrate the tympanic
membrane (eardrum). The vibrations are
transferred to fluid in the cochlea of the
inner ear, where they in turn vibrate the
tectorial membrane. The moving fluid bends sensory hair cells
in the organ of Corti. The bending triggers nerve impulses that
travel to the brain via the auditory nerve.
Balance organs are located in the vestibular apparatus of
the inner ear. Sensory receptors in these semicircular canals
(including hair cells) respond to gravity, velocity, acceleration,
and other factors that affect body positions and movements.
section 14.8 Eyes are the sensory
organs associated with the sense of vision.
Key eye structures include the cornea and
lens, which focus light; the iris, which adjusts
incoming light; and the retina, which
contains photoreceptors (rods and cones). The optic nerve at
the back of the eyeball transmits visual signals to the visual
cortex in the brain.
section 14.9 The rod cells and
cone cells detect dim and bright light,
respectively. Light detection in rods
depends on changes in the shape of the
visual pigment rhodopsin. The visual
pigments in cones respond to colors. Visual
signals are processed in the retina before being sent on to the
brain. In the retina, abundant receptors in the fovea provide
sharp visual acuity.reVieW Questions
- When a receptor cell detects a specific kind of stimulus,
what happens to the stimulus energy?
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