CHAPTER 6 SUMMARY
Introduction
•Sensory cells have ion channels that are opened in response to external stimuli. Some
versions of these stimulus-sensitive channels are found in all types of life including microorganisms.
•In animals, interoreceptors respond to factors in the internal environment, exteroreceptors to
those in the external environment, and proprioceptors to body and limb position and motion.
Receptor Physiology
•Receptors are specialized peripheral endings of afferent neurons; they respond to particular
stimuli, translating the energy forms of the stimuli into electrical signals, the language of the
nervous system.
•In vertebrates, there are discrete labeled-line pathways from the receptors to the CNS so that
information about the type and location of the stimuli can be deciphered by the CNS, even though
all the information arrives in the form of action potentials. What the brain perceives from its
input, however, is an abstraction and not reality. The only stimuli that can be detected are those
for which receptors are present. Furthermore, as sensory signals ascend through progressively
more complex processing, some of the information may be suppressed, whereas other parts of it
may be enhanced.
•Stimulation of a receptor produces a graded receptor potential. The strength and rate of change
of the stimulus are reflected in the magnitude of the receptor potential, which in turn determines
the frequency of action potentials generated in the afferent neuron.
•The magnitude of the receptor potential is also influenced by the extent of receptor adaptation,
which refers to a reduction in receptor potential in spite of sustained stimulation. Tonic receptors
adapt slowly or not at all and thus provide continuous information about the stimuli they monitor.
Phasic receptors adapt rapidly and frequently exhibit off responses, thereby providing information
about changes in the energy form they monitor.
Photoreception: Eyes and Vision
•The vertebrate eye is a camera-type eye, specialized structure housing the light-sensitive
receptors essential for vision perception—namely, the rods and cones found in its retinal layer.
The iris controls the size of the pupil, thereby adjusting the amount of light permitted to enter the
eye. The cornea and lens are the primary refractive structures that bend the incoming light rays to
focus the image on the retina. The cornea contributes most to the total refractive ability of the
eye.
•Rods and cones are activated when the photopigments they contain differentially absorb various
wavelengths of light. Light absorption causes a biochemical change in the photopigment that is
ultimately converted into a change in the rate of action potential propagation in the visual
pathway leaving the retina. The visual message is transmitted to the visual cortex in the brain for
perceptual processing.
•Cones display high acuity but can be used only for day vision because of their low sensitivity to
light. Different ratios of stimulation of the various cone types by varying wavelengths of light
lead to color vision.
•Rods provide only indistinct vision in shades of gray, but because they are very sensitive to light,
they can be used for night vision.
•Advanced camera-type eyes are also found in cephalopods; their structures show that they
evolved independently of the vertebrate eye.
•The functional unit of the arthropod faceted, or compound, eye is termed the ommatidium. Each
ommatidium consists of an optical, light-gathering part as well as a sensory portion, which
transduces light into an action potential.
Mechanoreception: Touch and Pressure
•Mechanically gated channels transduce touch and pressure into electrical signals.
•Channel proteins have external protein fibers attached to them, and when these fibers are
stretched or distorted, they essentially pull open the gate of the ion channel.
•Cations entering the sensory dendrite create a receptor potential and, if the signal is strong
enough, action potentials.
Mechanoreception: Proprioceptor Organs
•Numerous classes of animals have gravity receptors, known as statocysts, which are considered
the simplest organs of equilibrium. A statocyst is essentially a hollow chamber lined with ciliated
mechanoreceptors that contain dense, moveable objects, the statoliths.
•This sense organ is particularly important in animals that are essentially neutrally buoyant such
as fish, for they lack information about gravity from other sensory sources.
•In fish, mechanoreceptor systems extend the length of the animal’s body. Information about the
fish’s orientation with respect to gravity, its swimming velocity and details about water currents
and vibrations are collected by a string of neuromast cells, the basic sensory unit of the lateral line
system. Each neuromast is dome-shaped, with up to several hundred mechanoreceptor sensory
hair cells clustered at its base.
•Microvillar processes called stereocilia are the actual sensory transducers that protrude from the
sensory hair cells into a jellylike substance (at the base of the dome). Stereocilia are arranged
together into ciliary bundles and are orientated according to size.
•Neuromasts continually send out bursts of nerve impulses: when pressure waves cause the
gelatinous caps of the neuromasts to move, the enclosed hairs are bent. If the ciliary bundle is
bent in the direction of the tallest row of stereocilia, this results in an excitatory depolarization of
the hair cell whereas bending in the opposite direction generates an inhibitory hyperpolarization.
The actual magnitude of the response depends on the degree of bending.
Mechanoreception: Ears and Hearing
•The mammalian ear performs two unrelated functions: (1) hearing, which involves the external
ear, middle ear, and cochlea of the inner ear; and (2) sense of equilibrium, which involves the
vestibular apparatus of the inner ear.
•In contrast to the photoreceptors of the eye, the ear receptors located in the inner ear—the hair
cells in the cochlea and vestibular apparatus—are mechanoreceptors. Hearing depends on the
ear’s ability to convert airborne sound waves into mechanical deformations of receptive hair cells,
thereby initiating neural signals.
•Sound waves are funneled through the mammalian external ear canal to the tympanic membrane,
which vibrates in synchrony with the waves. Middle ear bones bridging the gap between the
tympanic membrane and the inner ear amplify the tympanic movements and transmit them to the
oval window, whose movement sets up traveling waves in the cochlear fluid. These waves, which
are at the same frequency as the original sound waves, set the basilar membrane in motion.
Various regions of this membrane selectively vibrate more vigorously in response to different
frequencies of sound.
•On top of the basilar membrane are the receptive hair cells of the organ of Corti, whose hairs are
bent as the basilar membrane is deflected up and down in relation to the overhanging stationary
tectorial membrane in which the hairs are embedded. This mechanical deformation of specific
hair cells in the region of maximal basilar membrane vibration is transduced into neural signals
that are transmitted to the auditory cortex in the brain for sound perception.
•The vestibular apparatus in the mammalian inner ear consists of (1) the semicircular canals,
which detect rotational acceleration or deceleration in any direction, and (2) the utricle and
saccule, which detect changes in the rate of linear movement in any direction and provide
information important for determining head position in relation to gravity. Neural signals are
generated in response to mechanical deformation of hair cells caused by specific movement of
fluid and related structures within these sense organs. This information is important for the sense
of equilibrium and for maintaining posture.
Chemoreception: Taste and Smell
•Taste and smell are chemical senses. In both cases, attachment of specific dissolved molecules to
binding sites on the receptor membrane causes receptor potentials that, in turn, set up neural
impulses that signal the presence of the chemical.
•Taste receptors are housed in taste buds on the mammalian tongue; olfactory receptors are
located in the mucosa in the upper part of the nasal cavity. Both sensory pathways include two
routes: one to the limbic system for emotional and behavioral processing and one through the
thalamus to the cortex for conscious perception and fine discrimination. Taste and olfactory
receptors are continuously renewed, unlike visual and hearing receptors, which are irreplaceable.
Thermoreception
•Vertebrates have at least two kinds of thermoreceptors: cold sensors and warmth sensors. These
inform the brain about skin and internal temperatures to aid in thermoregulation.
•At least two groups of animals, the pit vipers, and some pythons and boas, have a third type, an
extraordinarily sensitive receptor that responds to infrared radiation, the low-energy radiation
that carries heat energy. The receptor cells are simply branched dendrites of neurons, located in
small pits in the skin, on either side of the head in pit vipers (anterior to and below the eyes), and
along the jaws of pythons.
Nociception
•Painful experiences are elicited by noxious mechanical, thermal, or chemical stimuli and consist
of two components: the perception of pain coupled with behavioral responses to it.
•Pain signals are transmitted over two afferent pathways in vertebrates: a fast pathway that carries
sharp, prickling pain signals; and a slow pathway that carries dull, aching, persistent pain signals.
•Descending pathways from the brain use endogenous opiates to suppress the release of substance
P, the neurotransmitter from the afferent pain fiber terminal. This blocks further transmission of
the pain signal and serves as a built-in analgesic system.
Electroreception and Magnetoreception
•Active electroreception (in some fishes) resembles echolocation in that the animal assesses its
environment by actively emitting signals and receiving the feedback signal. An electric organ in
the tail of the fish discharges a current field, which emanates from its anterior body and then
converges on the tip of the tail. Passive electroreception, as in sharks, detects only fields produced
by other animals (such as prey). Ampullary and tuberous electroreceptors on the anterior body
surface in the lateral line system monitor changes in external current flow.
•Several fish species, such as the electric eel, can produce discharges up to several hundred volts
outside their bodies, whereas in the majority of species the discharge is limited in range to
millivolts to volts. Strong voltages are emitted to stun or kill prey, while weak discharges are
emitted continually for electrolocation and social communication.
•The highly sensitive electroreceptors of elasmobranchs are also capable of detecting the earth’s
magnetic field. Movement of the fish across magnetic field lines produces distortions in the
electric currents, which are monitored by the electroreceptors in the ampullae of Lorenzini.
•A variety of other organisms, including some bacteria, insects, birds and fish can also detect
magnetic fields and are capable of using this information for orientation. These contain chains of
magnetite crystals.
•Individual crystals of magnetite do not interact strongly enough with the earth’s magnetic field
to overcome the randomizing effects of thermal buffeting. However, once arranged in chains their
individual movements add together such that they are capable of aligning with a magnetic field
and interacting with a receptor.