sensory reception

1. RECEPTOR PHYSIOLOGY: TRANSDUCTION, SENSORY CODING, RANGE
FRACTIONATION, SENSORY ADAPTATION
Receptor cells
Cell that are specialized for sensory reception and respond to particular kinds of stimuli.
Afferent neurons.
(Neurons that carry information from the periphery toward and into the central nervous system are called
afferent neurons.
Efferent neurons
Neurons that carry information away from the central nervous system are called efferent neurons.
Qualities
The features that characterize stimuli are called qualities.
For example, the mechanical stimulation that produces the sensation of touch is different from the light
that produces a visual response. In addition, stimuli of a particular type may differ in some features. Light
can be red or blue; sounds can be high or low.
Sensory modalities
Types of sensory information that we can distinguish, typically includes vision, hearing, touch, taste, and
smell
Interoceptive(internal) receptors
Respond to signals from within the body and communicate the information to the brain by pathways that
often are not brought into consciousness. For example, proprioceptors monitor the position of muscles
and joints, and other receptors monitor the orientation and chemical and thermal state of the body.
Importance
These internal receptor systems play crucial roles in providing information to the brain about the state of
the body and its position in space, but we normally are not consciouslyaware of these signals. Imagine
how complicated walking would be if we had to pay conscious attention to the position of every muscle
and joint taking part in the process.
Many species of animals use sensory modalities that are unavailable to human beings. For example, some
species ofsnakes, the pit vipers, can detect emitted heat energy (infrared radiation), which they can use to
locate their mammalian prey because these warm-bodied animals stand out against a cold background.
The fish species that are called "weakly electric fish" (to distinguish them from the electric fish that can
stun or kill prey by using electric shocks) use very low frequency electric signals to communicate in
murky water, allowing them to find one another and to negotiate with one another regarding reproduction
and territory. Some animals appear to sense Earth's magnetic field and use it as a navigational guide.
GENERAL PROPERTIES OF SENSORY RECEPTION
Sensation begins in receptor cells, or more exactly at the specialized membranes of these cells (Figure 7-
1).
General features of receptor cells
Two general features are common to sensory receptor cells.
1. First, each kind of receptor cell is highly selective for a specific kind of energy.
2. Second, the receptors are exquisitely sensitive to their selected stimuli because they can amplify
the signal that is being received.

2. The form of energy (e.g., light, sound, mechanical pressure, etc.) to which a sensory receptor is most
sensitive is called its sensory modality, and receptor cells are said to transduce the sensory input, because
they change the stimulus energy into the energy of a nerve impulse.
Properties of Receptor Cells
1. Sensory receptor cells are selective because their membranes- or structures that are associated
with their membranes- respond differentially to different types of energy.
Example External energy, such as light, may strike any part of the external anatomy; but, in the
mammals, only the eyes and the pineal gland (a small gland located in the brain) contain sensory
cells that can transduce photons into neuronal energy.
2. Transduction depends on a conformational change in particular receptor molecules, which
typically are proteins.
For example,
i. the cell membrane of a photoreceptor cell contains a visual pigment based on protein
molecules called opsins. A functional pigment molecule, called rhodopsin, consists of an
opsin plus an organic, light-absorbing molecule. Molecules of rhodopsin absorb photons,
capturing their energy. When a photon is captured, it produces a transient structural
change that activates a cascade of associated molecules, ultimately changing the
functional state of ion channels in the receptor cell membrane.
ii. Similarly, the membranes of mechanoreceptors contain molecules that respond to slight
distortions in the cell membrane. Evidence from the molecular biological study of these
receptor molecules has indicated that many of them have related structures, and they may
have evolved from common ancestors.
3. Receptor cells can receive very weak signals of their selected form of energy and transduce these
signals into nerve signals that contain much larger amounts of energy, because receptor cells
contain intracellular machinery that amplifies weak stimuli.
i. The initial activation of receptor , molecules leads to different types of intracellular
events, depending on the receptor type. In some receptors, activation of receptor
molecules in the membrane initiates a cascade of chemical reactions in the cell that
effectively amplifies the signal by many orders of magnitude.
ii. The final step in all receptor cells is the opening (or closing) of ion channels, which
changes the amount of ionic current crossing the cell membrane and potentially modifies
the number of APs produced in the receptor cells.

3. Figure 7-1 Sensory receptors are specialized to respond to only certain stimuli. Although many forms of energy may impinge on
a receptor (represented by arrows A, B, and C), only one form-in this case, stimulus B-effectively activates the receptor at weak
to moderate levels of stimulus energy. Other kinds of stimuli fail to activate the receptor at such low energy levels. Often, the
signal is chemically amplified within the receptor cell and, in order for it to be effective, the intracellular chemical signal must
cause membrane channels to open (or, in some cases, to close), producing a neuronal signal that can travel to the central
nervous system (CNS).
In summary, each receptor cell transduces a particular form of stimulus energy into a membrane
current that produces a change in membrane potential of the receptor cell. In this way, receptors are
analogous to common electrical devices-for example, a microphone or a photocell. A microphone
transduces the mechanical energy of sound into modulated electrical signals, which can then be
amplified. Similarly, a photocell converts light into an electrical signal. Once again, vertebrate
photoreceptor cells provide a convenient example. One photon of red light contains about 3 X 190 -19
joules (J) of radiant energy, but capture of a single photon by a receptor cell has been found to
produce a receptor current equivalent to about 5 x 10-l4
J of electrical energy. The cell amplifies the
signal by a factor of 1.7 x l05
. The exquisite sensitivity of human photoreceptor cells allows a dark-
adapted human subject to detect a flash containing as few as 10 photons delivered simultaneously
over a small region of the retina, a feat that is equivalent to being able to see the light from a candle
flame that is 19 miles away.

4. Common Mechanisms and Molecules of Sensory Transduction
All sensory transduction systems perform the same basic operations of detection, amplification, and
transmission; it is now clear that many types of sensory receptors operate through similar cellular
mechanisms and contain related molecules. Table 7-1 summarizes typical events in sensory transduction
as it is carried out by many different kinds of receptors. Some of the processes occur within single
receptor cells, whereas others depend on interactions among many cells. The basic events in a receptor
cell are detection, amplification, and encoding of the sensory stimulus.
1. Detection
The initial event in all sensory transduction is detection, and the smallest amount of stimulus energy that
will produce a response in a receptor 50% of the time is called the threshold of detection. Many sensory
receptors are capable of detecting inputs that are very near the theoretical limits of the stimulus energy:
 photoreceptors can be activated by single photons
 mechanoreceptor hair cells by displacements equal to the diameter of a hydrogen atom
 odor receptors by binding only a few molecules of the correct sort.
The time constant of sensory reception is important because, in order for a sensory system to convey
accurate information about rapidly changing stimuli, the receptors must be able to respond quickly and
repeatedly. Alternatively, the receptors must be interconnected in a way that allows the population of
receptors to extract information about very rapid events on the basis of their collective activity.
Interestingly, the response latencies of the various known receptor cells vary over five log units. Hair cells
in the auditory system respond within several microseconds; olfactory receptors respond only after
several hundred milliseconds. It is intriguing to speculate about how such large differences in time
constants might reflect, fundamental differences in the roles played in the life of an animal by different
sensory modalities.
Common structural motif for different senses
Recent evidence indicates that in vertebrates the receptors for three of the senses-vision, olfaction, and,
probably, sweet and bitter tastes-have in their cell membranes receptor proteins with a common structural
motif. The secondary structure of these membrane proteins includes seven transmembrane a helix
domains, and transduction in all three senses requires G proteins as ~ntermediaries (Figure 7-2). This
pattern is also found in several neurotransmitter receptor molecules, including the muscarinic
acetylcholine receptor .
Detection in sense of vision and olfaction
By far, the most detailed knowledge available is about the molecules responsible for detecting photons:
the protein opsin and its associated molecules (Figure 7-3), However, the close relation among sensory
receptors was recently underscored when the DNA sequence that codes for opsin was used to identify
putative olfactory receptor molecules (Chess et al., 1992). The sequences in this new family of olfactory
receptor molecules are expressed only in the cells of the olfactory epithelium, and the family appears to
be very large (containing several hundred different gene products- perhaps even as many as a thousand).
The sequences of these molecules differ significantly from one another only in a particular region that is
thought to be the site at which stimulus molecules bind.
Detection in sense of taste
Detection of salt and sour tastes occurs by way of much simpler mechanisms than those underlying vision
and olfaction. Detection of these tastes depends on ion channels that are found on cells throughout the

5. body. Sour sensation is mediated by a common pH-sensitive K+ channel, and the salt response is caused
by passive movement of Na+ across the cell membrane, depolarizing the taste cell directly. In both of
these cases, because the stimuli are themselves ions and are in great abundance, no intermediate
amplification step is needed.
2. Amplification
In some sensory systems, amplification of sensory signals occurs within the receptor cells, mediated by a
number of different intracellular mechanisms.
Amplification is best understood for vertebrate photoreceptors.
1. When a photon is captured by a visual pigment molecule, the net effect is to activate
transducin, which is a GTP-binding protein, or G protein .
2. Transducin, in turn, activates a phosphodiesterase to hydrolyze cyclic guanosine
monophosphate a (cGMP), which in turn leads to modification of conductance through ion
channels.
3. Each photon captured leads to the hydrolysis of many cGMP molecules, producing a huge
amplification in the signal.
Although these steps have not yet been positively identified in the detection of odor and taste, several
facets of the transduction cascade are probably similar. In each case, the energy available from a unitary
stimulus at the receptor site is so low that amplification within the receptor cell is required to generate
neuronal impulses that can carry the signal into the central nervous system.
3. Encoding
Encoding sensory information into a neuronal signal to be transmitted to the brain depends on changes in
the conductance through membrane ion channels.When channel conductance changes, it can shift the
probability that the neuron will produce an AP, although it must be remembered that not all receptors
transmit information through the use of APs. In photoreceptors, cGMP may act directly on a class of
membrane channels to increase their conductance. The corresponding mechanisms for olfaction and taste
(also called gustation) are still unknown, although channels that respond to cyclic nucleotides have been
recently found in the olfactory system.
Responses within a single receptor neuron encode information about the intensity of a stimulus, but they
cannot directly report the quality of the stimulus. For example, a single photoreceptor cannot report
whether a stimulating light is red or blue. Additional information, such as the wavelength of light or the
frequency of a sound, is conveyed by activity patterns within combinations of receptor cells that are

6. activated by the stimulus. Typically, sensory organs contain a variety of receptor cells that respond
differentially to stimuli with different qualities. For example, certain photoreceptors respond maximally to
red light, whereas others respond maximally to blue light. Thus, when receptor cells are grouped into
organs, significantly more information about the stimulus can be conveyed, including its absolute
intensity, its spatial distribution, and other characteristics such as quality.
Each sensory system must be able to detect stimuli that persist in time, while at the same time retaining
the ability to respond to further changes. From the perspective of an organism, adaptation allows
detection of new sensory stimuli in the presence of ongoing stimulation, and it thus makes the sensory
system much more useful. For example, wearing clothing stimulates touch receptors at all points where
our garments touch the skin, and we typically adapt to the touch input from our garments. Yet, we can
easily detect any new touch stimuli that impinge on our skin, even at locations covered by our clothing.
Many mechanisms underlying adaptation take place within individual receptor cells, and several of them
appear to depend on Ca2+ (e.g., in vision, olfaction, and mechanoreception). In addition, some adaptation
depends on negative feedback from higher brain centers.
Control of Sensory Sensitivity
Adaptation takes place in the receptor cells, some as a result of time-dependent changes in accessory
tissues, and some in the central nervous system.
Mechanisms of adaptation
Different classes of receptors exhibit different degrees of adaptation. Tonic receptors continue to fire
steadily in response to a constant stimulus, which shows a receptor that responds to the displacement of a
hair; this receptor produces APs at an almost constant frequency when the hair is displaced and held in the
new position. In contrast, phasic receptors adapt quickly. In one class of phasic receptors, for example,
APs occur only during changes in the strength of the stimulus, by a mechanoreceptor that fires only when
the displacement of the hair changes, and the frequency of APs depends on the rate of change. Adaptation

7. can take place at any of a number of different stages in the processes that link a stimulus to the output of a
sensory neuron
1. The mechanical properties of the receptor cell may act as a filter that preferentially passes
transient, rather than sustained, stimuli. This mechanism is common among mechanoreceptors.
2. The transducer molecules themselves may "run down" during a constant stimulus. For example, a
significant percentage of visual pigment molecules can become bleached when exposed to
continuous light and must be regenerated metabolically before they can again respond to
illumination.
3. The enzyme cascade activated by a transducer molecule may be inhibited by the accumulation of
a prod axons,, uct or an intermediate substance.
4. The electrical properties of the receptor cell may - activation of receptor channels diminishes
intracellular free Ca" increases during sustained stimulation. Accumulation of intracellular free
Ca2+ can also activate Ca-dependent K+ channels, producing a shift in membrane potential, Vm,
back toward the resting potential.
5. The membrane of the spike-initiating zone (see Figure 7-4) may become less excitable during
sustained stimuli.
6. Sensory adaptation can also take place in higher-order cells in the central nervous system (which
includes the vertebrate retina).
Both the first and fifth of these mechanisms of adaptation are illustrated by the muscle stretch receptors of
crayfish and lobsters. These receptors are present in pairs in the abdominal musculature, each pair
consisting of one phasic receptor and one tonic receptor. A stretch of the muscle fiber produces a transient
response in the phasic receptor and a sustained response in the tonic receptor. When these receptors are
stimulated by direct injection of a depolarizing current through a microelectrode, rather than by stretching
the muscle fiber; each cell retains some of its characteristic properties. That is, when the stimulating
current is prolonged, the tonic receptor produces a longer train of APs than does the phasic receptor.
A change in the filtering that is produced by accessory structures (mechanism 1) contributes importantly
to the rapid adaptation of the Pacinian corpuscle, a pressure and vibration receptor found in the skin,
muscles, mesentery, tendons, and joints of mammals (Figure 7-14A). Each Pacinian corpuscle contains a
region of receptor membrane that is sensitive to mechanical stimuli and that is surrounded by concentric
lamellae of connective tissue resembling the layers of an onion. When something presses on the
corpuscle, deforming it, the disturbance is transmitted mechanically through the layers to the sensitive
membrane of the receptor neuron. The receptor membrane normally responds with a brief, transient
depolarization at both the onset and the offset of the deformation (Figure 7-14B). However, when the
layers of the corpuscle are peeled away, permitting a mechanical stimulus to be applied directly to the
naked axon, the receptor potential obtained is sustained much longer, producing a more accurate
representation of the stimulus (Figure 7-14C). Although the receptor potential still shows some degree of
adaptation (there is sag in the record shown in Figure 7-14C), there is no distinctive response at the offset
of the stimulus. The mechanical properties of the intact corpuscle, which preferentially pass rapid changes
in pressure, confer on the receptor neuron its normally phasic response. This behavior explains, in part,
why we quickly lose awareness of moderate, sustained pressures, such as the stimuli that wearing clothing
produces on our skin.
Regardless of its site or mechanism of origin, adaptation plays a major role in extending the dynamic
range of sensory reception. Together with the logarithmic nature of the primary transduction process,
sensory adaptation allows an animal to detect changes in stimulus energy against background intensities
that range over many orders of magnitude.

8. Figure 7-12 Sensory adaptation can take place at any of several stages in information processing. The
dashed lines indicate that, in some systems, filtering or the modulation of AP frequency, or both, take
place in the receptor cell itself; whereas, in other systems, these functions take place outside the receptor
cell.
Mechanisms that enhance sensitivity
Many receptor cells produce APs-or release neurotransmitter independent of APs-spontaneously in the
absence of stimuli. (The amount of transmitter released from nonspiking receptors varies with the
membrane potential, vm.) When these spontaneously active receptors are stimulated, the frequency of
their APs-or their non spiking release of transmitter-is increased or decreased above its baseline level.
Several mechanisms enhance the sensitivity of receptors to sustained stimuli, and one important
mechanism modifies the properties of the receptor's on-going spontaneous activity. The spontaneous
release of transmitter from receptor cells-whether it is mediated by APs or by graded changes in Vm-has
two important consequences. First, any small increase in stimulus energy will produce an increase in the
rate of firing above the spontaneous level. Small receptor currents in response to weak stimuli modulate
the impulse frequency by shortening the intervals between impulses (Figure 7-15). This modulation of
impulse frequency allows the receptors to be much more sensitive to changes in stimuli than would be
feasible if the receptor current had to bring a completely quiescent spike-initiating zone to threshold. The
input-output relations of such a sensory fiber are described by the sigmoid curve in Figure 7-16. In the
unstimulated condition, the firing frequency is already on the steep part of the curve, so even a small input
will produce a significant increase in firing frequency. Second, in some spontaneously active sensory
neurons, stimuli can either increase or decrease impulse frequency, permitting the receptor to convey
information about the polarity or direction of a stimulus. For example, in some mechanoreceptors, such as
hair cells, movement of the hair in one direction increases the rate of firing in the sensory fiber, whereas
movement in the other direction decreases the rate of firing. If these receptors were silent when they were

9. not stimulated, it would be impossible to encode information about movement in the second direction.
The existence of numerous parallel sensory pathways provides another mechanism for enhancing the
distinction between a signal and ongoing background noise. In this situation, signals from many receptor
cells can be summed by the central nervous system. All of the signals produced by the stimulus will arrive
at central neurons nearly simultaneously, whereas noise will be random and will tend to be canceled out at
central synapses. By reducing noise, this arrangement allows small changes in input to be detected. For
example, a human observer cannot reliably perceive a single photon absorbed by a single receptor cell;
but, if each of several receptors simultaneously absorbs a single photon, the observer experiences the
sensation of light.
Efferent control of receptor sensitivity
The responsiveness of some sense organs is influenced by the central nervous system through efferent
axons that innervate the sense organ itself. For example, the muscles to which muscle stretch receptors in
skeletal muscles of vertebrates and crustaceans are attached are innervated by efferent fibers. By
controlling the length of the receptor muscles, this efferent innervation sets the sensitivity of the stretch
receptor to changes in overall muscle length. In crayfish and lobsters, when the tail extensor muscle
shortens, the receptor muscles (which run parallel to the extensor muscle) shorten too, driven by efferent
neurons. If there were no such mechanism, when the tail extensor muscle shortened, the stretch receptors
would go slack and would be unable to detect any further change in the length of the extensor muscle.
Instead, contraction of the receptor muscles, in response to the efferent input, maintains a fairly constant
tension on the sensory parts of the receptor, and the receptor retains its sensitivity to flexion of the tail,
regardless of the tail's position in space. In addition to this mechanism by which the receptors maintain
high sensitivity, the abdominal stretch receptors are innervated by efferent neurons that form inhibitory
synapses directly on the stretch receptor cells (see Figure 7-13). When the inhibitory efferent neuron is
active, the size of the receptor potential in the stretch receptor is diminished, reducing the frequency of
APs in the axon or even abolishing them altogether. The interplay of these two mechanisms-one that
enhances the responsiveness and the other that inhibits it-allows activity in the central nervous system
either to increase or to decrease the sensitivity of these stretch receptors.
Feedback inhibition of receptors
The sensitivity of sensory receptors is also controlled through feedback inhibition. In this mechanism,
activity in the receptors produces signals that are sent more or less directly back to the receptors,
inhibiting them.
Example
The crustacean abdominal stretch receptors provide one example of this (Figure 7-17). Activation of the
sensory neuron by stretch produces a reflex output-initiated in the central nervous system-that travels in
the efferent inhibitory nerves leading to the stimulated sensory neuron (auto inhibition) and to its anterior
and posterior neighbors (lateral inhibition). At low stimulus intensities, the feedback plays little or no
role, because it takes a relatively strong sensory signal to evoke the reflex activation of the inhibitory
neurons. However, stronger stimuli produce stronger inhibitory feedback; as a result, strong stimuli are
preferentially inhibited. This mechanism acts to keep the receptor within its operating range (i.e., it keeps
the frequency of APs less than the maximum frequency possible in the cell), and the net effect of the
inhibitory feedback is to extend the dynamic range of the receptor. When receptors produce signals that
inhibit their neighbors, as do the crayfish stretch receptors, this mutual inhibition between neighboring
receptors can strongly influence sensory reception. For example, this lateral inhibition can enhance the
contrast between activity in neighboring receptors (Figure 7-18). Although this phenomenon was first
discovered in visual systems, it occurs in a number of sensory systems. The net effect of the interaction
between neighboring cells is that the differences in activity levels found in weakly and strongly
stimulated receptors are exaggerated, producing an increase in the perceived contrast between regions of
weak and strong stimulation.

10. Range Fractionation
The dynamic range of a multineuronal sensory system is typically much broader than is the range of any
single receptor or afferent sensory fiber. The extended dynamic range of the entire system is possible
because individual afferent fibers of a sensory system cover different parts of the full spectrum of
sensitivity. The most sensitive receptors produce a maximal response at stimulus intensities that are below
threshold or only slightly above threshold for other, less sensitive receptors in the population. Above that
intensity, the most sensitive receptors become saturated, but the less sensitive receptors can take over at
the higher intensities. Thus, at the lowest stimulus energies, a few especially sensitive sensory fibers will
respond weakly. If the stimulus energy is increased a little, their discharge frequencies will increase,
whereas new, less sensitive fibers will join in weakly. With still greater stimulus intensities, another,
formerly quiescent lower-sensitivity population of afferents will join in. As the stimulus intensity is
increased, receptors that are less and less sensitive will become active, a phenomenon called recruitment,
until the least sensitive sensory fibers will finally be recruited, and all receptors will respond maximally.
At that point, the system will be saturated and therefore unable to detect further increases in intensity.
This range fractionation, in which individual receptors or sensory afferents cover only a fraction of the
total dynamic range of the sensory system (Figure 7-10), enables the sensory processing centers of the
central nervous system to discriminate stimulus intensities over a range much greater than that of any
single sensory receptor.
Example
One example of range fractionation are the photoreceptors of the vertebrate eye. Rod photoreceptors are
more sensitive to light and respond to dimmer stimuli; cones respond to bright light that saturates the
rods.
Figure 7-10 Range fractionation extends the dynamic range of sets of sensory receptors Each curve In
thls a b c graph represents the dlschargefrequency of an indlvidual sensory afferent, plotted as a function
of stlmulus Intensity In thls hypothetical example, each of the four sensory fibers, labeled a through d, has
a dynamic range of about three to four log units of stimulus intensity, whereas the overall dynailc range
of the four neurons taken together covers seven log unlts of intenslty.