26710.1 Characteristics of Sensory Receptors
Each type of sensory receptor responds to a particular
modality of stimulus by causing the production of action
potentials in a sensory neuron. These impulses are con-
ducted to parts of the brain that provide the proper inter-
pretation of the sensory information when that specific
neural pathway is activated.
narrow range of energy. Our vision, for example, is limited to
light in a small range of electromagnetic wavelengths known
as the visible spectrum. Ultraviolet and infrared light, x-rays
and radio waves, which are the same type of energy as vis-
ible light, cannot normally be seen. The perception of cold
is entirely a product of the nervous system—there is no such
thing as cold in the physical world, only varying degrees of
heat. The perception of cold, however, has obvious survival
value. Although filtered and distorted by the limitations of sen-
sory function, our perceptions of the world allow us to interact
effectively with the environment.Categories of Sensory Receptors
Sensory receptors can be categorized on the basis of structure
or various functional criteria. Structurally, the sensory recep-
tors may be the dendritic endings of sensory neurons. These
dendritic endings may be free, such as those that respond to
pain and temperature, or encapsulated within nonneural struc-
tures, such as those that respond to pressure (see fig. 10.4 ). The
photoreceptors in the retina of the eyes (rods and cones) are
highly specialized neurons that synapse with other neurons in
the retina. In the case of taste buds and of hair cells in the inner
ears, modified epithelial cells respond to an environmental
stimulus and activate sensory neurons.Functional Categories
Sensory receptors can be grouped according to the type of
stimulus energy they transduce. These categories include
(1) chemoreceptors, which sense chemical stimuli in the envi-
ronment or the blood (e.g., the taste buds, olfactory epithelium,
and the aortic and carotid bodies); (2) photoreceptors —the
rods and cones in the retina of the eye; (3) thermoreceptors,
which respond to heat and cold; and (4) mechanoreceptors,
which are stimulated by mechanical deformation of the recep-
tor cell membrane (e.g., touch and pressure receptors in the
skin and hair cells within the inner ear).
Nociceptors are pain receptors that depolarize in res-
ponse to stimuli that accompany tissue damage. These stimuli
include noxiously high heat or pressure, acid, and a variety
of chemicals such as bradykinin, prostaglandins, nitric oxide,
adenosine, and ATP. Depolarization can stimulate the produc-
tion of action potentials in sensory neurons, which enter the
spinal cord in the dorsal roots of spinal nerves and then relay
information (via the neurotransmitters glutamate and sub-
stance P) to the brain. However, the actual perception of the
pain is enhanced or reduced by a person’s emotions, concepts,
and expectations. This involves various brain regions that acti-
vate descending pathways in the spinal cord. Analgesia (pain
reduction) depends to a large degree on the endogenous opi-
oid neurotransmitters (including b -endorphin: chapter 7), but a
nonopioid mechanism also functions to reduce the perception
of pain.Susan, a 28-year-old woman, was experiencing a ring-
ing in her ears, hearing loss, and an occasional feeling
that the room was spinning. She went to a specialist and,
after ruling out otitis media and Meniere’s disease, was
told that she had otosclerosis. The specialist said that
she would need a hearing aid and likely surgery. After
leaving this physician, she went to an optometrist who
told her that she needed new glasses for her myopia, or
alternatively she could go in for LASIK surgery. Susan
laughed ruefully and agreed to seek that procedure.
Some of the new terms and concepts you will
encounter include:- Middle-ear and inner-ear structures involved in
hearing and equilibrium - Eye structure and function in focusing and vision
Clinical Investigation
LEARNING OUTCOMES
After studying this section, you should be able to:- Explain how the stimulus modality is perceived, and
how phasic receptors relate to sensory adaptation. - Describe the nature and significance of the receptor
(generator) potential.
Our perceptions of the world—its textures, colors, and sounds;
its warmth, smells, and tastes—are created by the brain from
electrochemical nerve impulses delivered to it from sensory
receptors. These receptors transduce (change) different forms
of energy in the “real world” into the energy of nerve impulses
that are conducted into the central nervous system by sensory
neurons. Different modalities (forms) of sensation—sound,
light, pressure, and so forth—result from differences in neural
pathways and synaptic connections. The brain thus interprets
impulses arriving from the auditory nerve as sound and from
the optic nerve as sight, even though the impulses themselves
are identical in the two nerves.
We know, through the use of scientific instruments, that
our senses act as energy filters that allow us to perceive only a