General nature of sensation
Sensory organs are important devices for receiving information in animals. The stimulus to which the particular sensory organ shows strongest sensitivity is called adequate stimulus. Sensation such as olfaction, vision, hearing, equilibrium and taste, which are sensed by specialized sensory organs are called special senses. All the sensory organs that sense special senses are located in the head of animals. In addition to external stimuli, internal information related to the animal’s own body also has important meanings. For example, somatic sensation that senses the joint position, the load placed on the joint, pressure on skin, pain and temperature plays an important role in physical posture and movement control. Moreover, organic sensation such as hunger, thirst, defecation desire, uresiesthesia, nausea, and visceral sensation such as abdominal pain and stomach pain are sensed through a path called autonomic nerve afferent pathway. This mechanism sends out a physically recognizable warning in times of emergencies that cannot be handled merely by homeostasis which is normally an automatic and unrecognized process.
Sensory organs can be divided into the two types shown in Figure 20-1. In one type, part of the neuron (see Chapter 17) serves as the receptor of stimulation. Generally, the receptor triggers changes in the membrane potential (receptor potential) according to the strength of the stimulus received, and depolarizes the cell membrane (changes so that the membrane potential approaches 0 mV). If the stimulus is strong, the depolarization is large. This generates an electrical stimulus, opening Na+ channels located elsewhere on the neuron, thereby triggering an action potential. While the receptor potential threshold is met, this action potential will be triggered repeatedly. As a result of this strong, continuous stimulation, a continuous, frequent excitation pulse is generated. The other kind of sensory organs include specialized receptor cells that are different from the sensory nerves. Similar to the type explained above, these receptor cells also generate receptor potential according to the strength of the stimulus. This signal is then transmitted to a neuron across the synapse. In the case of auditory cells or taste cells, this signal will be transmitted as an excitatory signal across an excitatory synapse. Here, the action potential whose frequency corresponds to the intensity of the stimulus on the receptor cells is generated in the adjoining neurons. In some sensory cells such as visual cells, receptor cells carry out inhibitory signaling through an inhibitory synapse. In response to intense light stimuli, the receptor potential hyperpolarizes (reverse of depolarization) to inhibit the excitation of visual cells. As a result, inhibition of adjoining neurons (interneurons) is minimized, and therefore, nerve ganglion cells are excited (pulse is generated).
Figure 20-1 Two types of sensory organs in humans
A) The ending of a neuron (nerve fiber) or the cell body is specialized for receiving stimuli. Electrical signals (receptor potential) with an amplitude corresponding to the intensity of the received stimulus are generated. When the receptor potential increases, the current occurring there triggers excitation of the nerve axon at the afferent side (side near the central nervous system) of the neuron. Example sensory organs of this type include pain, touch (nerve ending is the receptor), and olfactory cells (cell body is the receptor). B) Receptor cells that have originated from other types of cells than neurons receive external signals and send signals to sensory nerves via synapses. Example sensory organs of this type include visual, gustation, and hair cells.
Let us look at another example. The frequencies of the genes for blood types in the human population is A = 0.22, B = 0.16, and O = 0.62. If the abovementioned principle is applied, then the distribution of blood types can be calculated. Therefore, the frequencies obtained by calculating (0.22A + 0.16B + 0.62O)2 are [A] = AA + AO = 0.32, [B] = BB + BO = 0.22, [AB] = 0.07, and [O] = 0.39. These frequencies are almost completely consistent with those for the distribution of blood types worldwide. Thus, based on the distribution of ABO blood types, it can be concluded that human mating is quite random.
Figure 20-2 Example of Weber’s law in vision
The two circles at the top and those at the bottom identically share different tones of gray. When the contrast is high with a bright background, the difference in brightness of the two gray circles is difficult to discriminate (on top), whereas with the dark background it is easier to discriminate because the stimulus entering the retina is weak (at bottom).
Figure 20-3 Relation between intensity of stimulus and sensory organ response
Generally, the intensity of sensory response is a parameter that is difficult to quantify. It is estimated from psychological experiments and electrical records of sensory receptor cells. It is known that in most cases, the curve to saturation can be expressed by an equation in terms of K･Sn, where A is the threshold intensity of stimulus, the lower limit at which a response is observed, and B is the saturation point.
Figure 20-4 Response of optical receptor cells of horseshoe crab
The electrical response (frequency of action potential) to the intensity of stimulus was investigated immediately after start of light stimulus (A) and 3.5 seconds later (B). Responses are apparently reduced gradually. Both stimulus threshold intensity and saturation point increase, and the curve slope decreases. These results suggest that different stimulus information may be sent to the central nervous system of the horseshoe crab according to the light intensity and elapsed time (Source: Hartline, 1934 data).
Receptor potentials are analog signals whose amplitude is correlated with the intensity of stimuli, whereas action potentials are excitation pulses transmitted to the central nervous system. The amplitude of all action potentials is the same, following the all-or-none law. Thus, information concerning the intensity of a stimulus is not transmitted by the size of the amplitude. Instead, it is transmitted to the central nervous system in the form of the frequency of the excitation pulse. This is referred to as coding of stimulus intensity information. The intensity at which we sense stimulation can be understood by studying the relationship between stimulus intensity and excitation pulse frequency. In animal experiments, this can be verified by directly recording the membrane potential of receptor cells. When it cannot be recorded electrically, it is analogized from behavioral experiments in animals. In humans, it can also be investigated with psychological experiments.
Generally in psychological experiments, two different stimuli of slightly different intensity (I and I') are applied to the subject. The intensity of stimulus I is gradually changed to identify the limit value for discriminating the difference between the two stimuli (absolute value of ΔI = I-I'). For example, a study subject is blindfolded and asked to hold objects of the same shape and material but of different weights, one in each hand. Then, the accuracy rate at which the subject is able to discriminate the difference in the weights of the two objects (ΔI) is investigated. In a certain stimulus intensity range, the ΔI/I' ratio will generally be constant. This is called Weber's law, and is a nature common to many sensations. The ΔI/I' ratio is about 3% for weight comparison, as well as for visual sensation and touch; in olfaction (smell), the ratio is 20–40% (Table 20-1). When this value is small, the discrimination ability is high, meaning that subtle differences can be discriminated (Figure 20-2).
Figure 20-3 shows a curve of the frequency of action potentials actually triggered by the sensory receptor cells of animals in response to different intensities of stimuli. Generally, if the stimulus on a receptor is weaker than the stimulus threshold intensity, the frequency of the action potential of a receptor does not change. If this threshold is exceeded, the response gradually increases (the frequency of generated pulses increases). In the sensory receptor cells of most animals, including humans, this change may be approximated by a curve expressed by K･Sn, where n is a constant and the greater n is, the more rapid the changes are in response to changes in the stimulus intensity (Table 20-1). However, as stimulus intensity increases, there is a saturation point at which the frequency of the generated pulse will not grow any higher.
Adaptation*1 is also a common characteristic of sensory receptor cells. Figure 20-4 shows an experiment using the light receptor (eyes) of the horseshoe crab. Here, when a light stimulus is continuously applied to the light receptor for a certain period of time, the pulse frequency of the action potential actually decreases over time. This phenomenon, in which the stimulus threshold intensity increases or the excitation pulse generation frequency decreases in response to continuous or repeated stimulation, is called adaptation. The rate of adaptation differs considerably across types of sensation. For example, hair cells of the cochlear duct, which are auditory receptors, and pacinian corpuscles (Figure 20-5), which are cutaneous touch and pressure receptors, demonstrate fast adaptation in which responses to mechanical stimuli decrease rapidly. In contrast, adaptation to pain is slow and the sensation does not weaken even if the same stimulus continues over a long time. Adaptation is not limited to sensory receptor cells and peripheral neurons, but is also a phenomenon which can be observed in other processes including sophisticated information processing by the central nervous system (Figure 20-6).
*1 Adjustment to environmental stimuli on an individual basis is also called adaptation. Although adjustments such as visual accommodation are usually distinguished from adaptation, they can be broadly defined as a type of adaptation.
Figure 20-5 Pacinian corpuscle in human skin
Onion-shaped touch receptors with a diameter of 1 mm and layered structure, visible to the naked eye. Characterized by very fast adaptation, they react with high sensitivity to vibrations of as low as 200 Hz. They are present extensively in subcutaneous tissue, internal organs, and periostea.
Figure 20-6 Example of adaptation in high visual functions
Adaptation can be observed not only in sensory organs but in the central nervous system as well. When the black cross in the figure is stared at continuously, the gray pattern surrounding the cross with low contrast in the background will gradually become unrecognizable. It indicates that rough low contrast patterns are adapted to and become unrecognizable than high contrast precise patterns. This may be due not only to the nature of the optical receptors of the retina, but also to the adaptation process of the visual cortex in the cerebrum (see main text).
Spatial information processing
Stimulus intensity is encoded as excitation pulse frequency of the sensory nerve, and then sent to the central nervous system. Additionally, when several sensory receptor cells are excited by the same type of stimulus, the number of cells responding (number of sensory nerves sending excitation signals) also serves as important information conveying stimulus intensity, because the threshold stimulus intensity slightly differs across individual receptor cells.
Signals transmitted to the central nervous system by sensory nerves are pulse-form action potentials. They do not contain information on the type or quality of sensation. The type of stimulus is registered according to which area of the central nervous system (sensory cortex) the sensory nerves transmit the information to, and sensation is born there, at the sensory cortex.
During the transfer of signals from several receptor cells, signals are processed in various ways until reaching the sensory cortex. One representative example is seen in visual cells and auditory hair cells. This is a case when information on which sensory cells were stimulated includes important information (such as visual field position or sound frequency). Positional information regarding which sensory cells were stimulated is sent accurately to the sensory cortex. In vision, stimulation of specific visual cells in the visual field is projected to a specific region of the sensory cortex called primary visual area (VI area, see Selection 2 of Chapter 20, Figure 20-12) as if it were projected on a screen. During the projection process, in most cases, contrast is automatically increased by lateral inhibition (Figure 20-7). This mechanism enables differences in subtle stimulus intensities to be detected sharply. In vision, neuron connections seen in the horizontal cells of the retina and ganglion cells (see Column Selection 2 of Chapter 20) make up such processing circuits.
Figure 20-7 Example of lateral inhibition in vision
The left shows reed shaped patterns with different brightness on a gray background. The ones at the top are drawn adjoining each other at the sides. Near the borderlines, the brighter side appears brighter while the darker side appears darker. When the reed patterns are drawn apart, however, it can be seen that each pattern has a consistent brightness. The mechanism by which the contrast of such position information appears enhanced can be explained by the nerve circuit on the right. This lateral inhibition occurs when there is a circuit which inhibits the adjoining parts (connection indicated by “—⃓”) existing in parallel to the circuit conveying excitation (connection indicated by “→”).
Another example is the sorting of signals in olfaction (Figure 20-8). In this case, the type and number of sensory receptor cells receiving the stimulus have significance, rather than the position of the sensory receptor cells. Olfactory epithelial cells express different odorant receptors (one to several types) on the cellular surface of the nasal mucosa. The binding of chemical substances to this surface triggers receptor potentials through amplification actions using G proteins (see Chapter 14). As olfactory cells with the same type of odorant receptors are scattered across the olfactory epithelium, there is a need to separately establish correct neuron connections at a different site to accurately sort the signal information. Sensory nerve fibers from these olfactory cells are reconnected to another kind of neurons (secondary neurons) that are located at designated positions in the olfactory glomerulus of the olfactory bulb according to the type of odorant receptors. The signals are then sent to the central nervous system. Thus, information regarding odorant type is replaced by positional information of the secondary neuron. This secondary neuron has a connection circuit for lateral inhibition mentioned earlier. Secondary neurons are responsible for the accurate sorting of information, signal accumulation according to the intensity of the stimulus, as well as contrast amplification processing between different odorants.