Figure 21-1 The sensory systems encode four elementary attributes of stimulimodality, location, intensity, and timingwhich are manifested in sensation. The four attributes of sensation are illustrated in this figure for the somatosensory modality of touch.
A. In the human hand the submodalities of touch are sensed by four types of mechanoreceptors. Specific tactile sensations occur when distinct types of receptors are activated. Firing of all four receptors produces the sensation of contact with an object. Selective activation of Merkel cells and Ruffini endings produces sensations of steady pressure on the skin above the receptor. When the same patterns of firing occur only in Meissner's and Pacinian corpuscles, the tingling sensation of vibration is perceived.
B. Location and other spatial properties of a stimulus are encoded by the spatial distribution of the population of activated receptors. Each receptor fires action potentials only when the skin close to its sensory terminals is touched, ie, when a stimulus impinges on the receptor's receptive field (see Figure 21-5). The receptive fields of mechanoreceptorsshown as red areas on the finger tipdiffer in size and response to touch. Merkel cells and Meissner's corpuscles provide the most precise localization of touch, as they have the smallest receptive fields and are also more sensitive to pressure applied by a small probe.
C. The intensity of stimulation is signaled by the firing rates of individual receptors, and the duration of stimulation is signaled by the time course of firing. The spike trains below each finger indicate the action potentials evoked by pressure from a small probe at the center of the receptive field. Two of these receptors (Meissner's and Pacinian corpuscles) adapt rapidly to constant stimulation, while the other two adapt slowly (see Figure 21-8).
Table 21-1 Sensory Systems and Modalities
Figure 21-2 Mechanoreceptors are depolarized by stretch of the cell membrane and the depolarization is proportional to the stimulus amplitude.
A. The spindle organ in skeletal muscle mediates limb proprioception. These receptors signal muscle length and the speed at which the muscle is stretched. The receptor consists of a bundle of specialized (intrafusal) muscle fibers enclosed by a capsule. The sensory nerve endings respond to stretch of the muscle fibers. Stretch-sensitive ion channels in the nerve membrane are linked to the cytoskeleton by the protein spectrin. Mechanical deformation of the membrane opens these cation-selective channels. The influx of Na+ and possibly Ca2+ depolarizes the nerve ending, producing the receptor potential. (Adapted from Sachs 1990.)
B. Response of an isolated muscle spindle to stretch. Upper records show the depolarizing receptor potentials recorded from the sensory axon when the muscle spindle is stretched to different lengths. Lower records show the amplitude and rate of stretch. Action potentials in this nerve have been blocked with tetrodotoxin to allow analysis of the receptor potentials. The initial depolarization of the muscle spindle in response to change in muscle length (dynamic response) is proportional to both the rate and amplitude of stretch. When stretch is maintained at a fixed length, the receptor potential decays to a lower value proportional only to the amount of stretch (static response). (Adapted from Ottoson and Shepherd 1971.)
C. Patch clamp records of a single stretch-sensitive channel recorded from skeletal myocytes. Pressure is applied to the receptor cell membrane by suction. At rest (top record) the stretch-sensitive channel opens sporadically for short time intervals, producing a transient depolarizing current. As the pressure on the membrane is increased (lower records), the channel opens more often and remains in the open state for longer time intervals (indicated by the bar above the channel openings). Each channel opening increases the membrane conductance to cations. The increase in the probability of opening and open time produces longer and larger depolarizations. (Adapted from Sachs 1990.)
Figure 21-3 Transduction of stimulus energy into neural activity by chemoreceptors and photoreceptors requires intracellular second messengers. (Adapted from Shepherd 1994.)
A.1. The olfactory hair cell is a chemoreceptor that mediates the sense of smell. The olfactory cilia on the mucosal surface bind specific odorant molecules and depolarize the sensory nerve via a second-messenger system. The firing rate signals the concentration of odorant in the inspired air. 2. Chemoelectric transduction is produced when the appropriate odorant binds to a receptor protein on the cell membrane, which activates G proteins linked to the receptor. Channel opening and depolarization in olfactory receptors and certain gustatory receptors are mediated by a second messenger (cAMP) stimulated by G protein activation. 3. Receptor currents evoked by the appropriate odorant. (Reproduced with permission from Maue and Dionne 1987).
B.1. Rod and cone photoreceptors are the sensory receptors of the retina. The outer segment of both receptors contains the photopigment rhodopsin, which changes configuration when it absorbs light. 2. Stimulation of the chromophore by light reduces the concentration of cGMP in the cytoplasm. This hyperpolarizes the photoreceptor by closing cation channels, decreasing the transmitter released by the photoreceptor terminals in the inner segment. 3. Receptor currents evoked by light flashes. (Reproduced with permission from Baylor et al. 1979.)
Figure 21-4 Tuning curves of sensory receptors measure the minimum amplitude of stimulation needed to activate a sensory receptor over a range of stimulus energies. Each sensory receptor responds optimally to a narrow range of intensities of a single type of energy. The tuning curve shown here is for an auditory receptor most sensitive to sound at 2.0 kHz. Higher and lower frequencies require stronger amplitude stimuli to evoke a response from the receptor. The tuning curve also illustrates the range of stimulus energies that can excite the receptor when presented at a given intensity. In this example, as the loudness of the tone rises, the receptor responds to a greater range of auditory frequencies. However, the receptor provides a stronger response at the preferred frequency than at other frequencies. Graded responses over the energy bandwidth provide a mechanism for sensory neurons to signal the particular type of stimulus energy that is presented. The auditory system tunes receptors in distinct parts of the sensory epithelium to different frequencies of sound. The relative response amplitude of these receptors to tones signals the sound frequency.
Figure 21-5 Structural basis of the receptive field of receptors for the sense of touch. The receptive field of a touch-sensitive neuron in the skin includes the sensory transduction apparatus in the nerve terminals and the surrounding skin in which the terminals are located. A patch of skin contains many overlapping receptive fields innervated by individual sensory nerve fibers. When this region is touched, spikes are initiated at the node of Ranvier closest to the nerve terminals in the skin. They are conducted past the cell body, located in the dorsal root ganglion, to the synaptic terminals in the spinal cord or medulla.
Figure 21-6 The density of sensory receptors in the retina and the size of the receptive field for each receptor determine the resolution of a visual image. Each square or pixel in these images represents a receptive field. The gray scale is proportional to the average light intensity in that region of the image. White pixels represent receptors with the highest firing rate, while black pixels represent receptors with the lowest firing rate. If there are a small number of receptors and each spans a large area of the scene, the result is a fuzzy, very schematic representation of the scene (A). There is no cue from this representation what the picture actually shows. As the density of receptors increases, and the size of the receptive field of each receptor decreases, the spatial detail becomes clearer (B-D). Clouds, mountains, trees, grasslands, and water emerge, until the scenery is identifiable as Yosemite valley. However, the increased resolution comes at the cost of enlarging the total size of the receptor population.
The brain resolves the conflict between information overload from a huge number of receptors and the need for resolution of spatial detail by having a higher density of receptors in regions of the body where high resolution of detail is behaviorally important and using progressively lower numbers of receptors in surrounding regions. Spatial resolution for vision and touch parallels the density of receptors in the retina and skin. Spatial resolution on the fingertips approaches that of the image in D. Receptor density and tactile sensitivity on the palm is similar to the resolution in C. Resolution of spatial detail on the forearm approaches that in image B, while on the trunk it is similar to that in image A. (Photographs courtesy of Daniel Gardner.)
Figure 21-7 Sensory thresholds and the just noticeable difference (JND) between stimuli that differ in intensity, frequency, or other parametric features are quantifiable.
A. The psychometric function plots the percentage of stimuli detected by a human observer as a function of stimulus intensity. Threshold is defined as the stimulus intensity detected on 50% of the trials.
B. The absolute sensory threshold (curve b) is an idealized relationship between stimulus intensity and the probability of stimulus detection. If the sensory system's ability to detect the stimulus is increased or the subject's response criterion is decreased, curve a would be observed; curve c illustrates the converse.
Figure 21-8 The firing rates of sensory nerves encode the stimulus magnitude. (Adapted from Mountcastle et al. 1966.)
A. The number of action potentials per second in a slowly adapting mechanoreceptor action the amount of skin indentation. This receptor required a minimum indentation of 80 µm to respond. The relationship between increases in frequency of firing and pressure on the skin is linear.
B. Estimates made by a human subject of the magnitude of sensation produced by pressure on the hand increase linearly as a function of skin indentation. The relation between a subject's estimate of the intensity of the stimulus and its strength resembles the relation between the discharge frequency of a sensory neuron and the stimulus strength. These data suggest that the neural coding of stimulus intensity is faithfully transmitted from the peripheral receptors to the cortical centers that mediate sensation.
Figure 21-9 Measurements of firing rates quantify how sensory neurons represent the intensity of stimulation over time.
A. Slowly adapting mechanoreceptors respond throughout a continuous stimulus. Each successive trace illustrates the response to increases in the pressure applied to the skin; the trace below each spike record illustrates the amplitude and time course of the stimulus. As the pressure increases, the total number of action potentials discharged rises, leading to higher firing rates. The firing rate is higher at the beginning of skin contact than during steady pressure, as these receptors also sense how rapidly pressure is applied to the skin. When the probe is removed from the skin, the spike activity ceases. (Adapted from Mountcastle et al. 1966.)
B. Rapidly adapting mechanoreceptors respond only at the beginning and end of the stimulus, signaling the rate at which the stimulus is applied or removed. The slope of the pressure pulse indicates the speed of skin indentation in millimeters per second; all the stimuli have the same final amplitude. Slowly applied pressure evokes a long-lasting burst of low frequency firing; rapid indentation produces a very brief burst of high frequency firing. Motion of the probe against the skin is signaled by both the rate and duration of firing of this receptor. The receptor is silent when the indentation is maintained at a fixed amplitude and fires again when the probe is removed from the skin. (Adapted from Talbot et al. 1968.)
Figure 21-10 Receptor morphology influences adaptation in rapidly adapting mechanoreceptors. The Pacinian corpuscle is a rapidly adapting mechanoreceptor located in the skin, in joint capsules, and in the mesentary of the abdominal wall. The receptor consists of concentrically arranged, fluid-filled lamellae of connective tissue that form a capsule surrounding the sensory nerve terminal. Because of this capsule, the sensory endings specialize in the detection of motion.
A. The capsule of the Pacinian corpuscle deflects steady pressure. The receptor responds with one or two action potentials at the beginning and end of a pressure stimulus but is silent when the stimulus is constant in intensity. When a stimulus first impinges on the skin, the capsule is deformed, compressing the nerve terminal. The pressure pulse activates stretch-sensitive channels in the nerve terminal, producing the response to stimulus onset. During steady pressure the capsule changes shape, reducing stretch of the nerve membrane. The outer lamellae of the capsule are compressed, absorbing the static load and preventing the deformation from being transmitted to the inner core of the capsule and the nerve terminal. When the pressure is removed, the capsule resumes its initial shape, and the resultant tissue movement stimulates the nerve terminal again, producing an off response.
B. Pacinian corpuscles are sensitive to vibration. Rapid movements are transmitted through the lamellae to the nerve terminal, generating a receptor potential and action potential for each vibratory cycle.
Figure 21-11 The functional and anatomical organization of sensory processing networks is hierarchical. Stimulation of a population of receptors initiates signals that are transmitted through a series of relay nuclei to higher centers in the brain (only one relay is shown). At each processing stage the signals are integrated into more complex sensory information. (Adapted from Dudel 1983.)
A. In the somatosensory system excitatory synaptic connections from each receptor in the skin are widely distributed to a large group of postsynaptic neurons at each relay nucleus. 1. Each relay neuron receives sensory input from a large group of receptors and therefore has a bigger receptive field than any of the input neurons. 2. Receptors closest to the stimulus respond more vigorously than distant receptors.
B.1. The addition of inhibitory interneurons (gray) narrows the discharge zone. 2. On either side of the excitatory region the discharge rate is driven below the resting level by feedback inhibition.
Figure 21-12 Inhibition of selected projection neurons in a sensory relay nucleus enhances the contrast between stimuli. The illustration shows three inhibitory pathways in the circuitry of the dorsal column nuclei, the first relay in the system for touch. The projection (or relay) cells (brown) send their axons to the thalamus. They receive excitatory input from touch receptor axons traveling in the dorsal columns. These afferent fibers also excite inhibitory interneurons (gray) that make feed-forward inhibitory connections onto adjacent projection cells. In addition, activity in the projection cells can inhibit surrounding cells by means of feedback connections. Finally, neurons in the cerebral cortex can modulate the firing of projection cells by distal inhibition of either the terminals of primary sensory neurons or the cell bodies of projection neurons.