Spatial Orientation Of Signals In Somatosensory Area

signals received from the sensory portions of the cortex, which keep the motor cortex informed at each instant about the positions and motions of the different body parts.

Somatosensory Areas I and II. Figure 47-6 shows two separate sensory areas in the anterior parietal lobe called somatosensory area I and somatosensory area II. The reason for this division into two areas is that a distinct and separate spatial orientation of the different parts of the body is found in each of these two areas. However, somatosensory area I is so much more extensive and so much more important than somatosensory area II that in popular usage, the term "somatosensory cortex" almost always means area I.

Somatosensory area I has a high degree of localization of the different parts of the body, as shown by the names of virtually all parts of the body in Figure 47-6. By contrast, localization is poor in somatosensory area II, although roughly, the face is represented anteriorly, the arms centrally, and the legs posteriorly.

Little is known about the function of somatosensory area II. It is known that signals enter this area from the brain stem, transmitted upward from both sides of the body. In addition, many signals come secondarily from somatosensory area I as well as from other sensory areas of the brain, even from the visual and auditory areas. Projections from somatosensory area I are required for function of somatosensory area II. However, removal of parts of somatosensory area II has no apparent effect on the response of neurons in somatosensory area I. Thus, much of what we know about somatic sensation appears to be explained by the functions of somatosensory area I.

Spatial Orientation of Signals from Different Parts of the Body in Somatosensory Area I. Somatosensory area I lies immediately behind the central fissure, located in the

Figure 47-7

Representation of the different areas of the body in somatosensory area I of the cortex. (From Penfield W, Rasmussen T: Cerebral Cortex of Man: A Clinical Study of Localization of Function. New York: Hafner, 1968.)

postcentral gyrus of the human cerebral cortex (in Brodmann's areas 3,1, and 2).

Figure 47-7 shows a cross section through the brain at the level of the postcentral gyrus, demonstrating representations of the different parts of the body in separate regions of somatosensory area I. Note, however, that each lateral side of the cortex receives sensory information almost exclusively from the opposite side of the body.

Some areas of the body are represented by large areas in the somatic cortex—the lips the greatest of all, followed by the face and thumb—whereas the trunk and lower part of the body are represented by relatively small areas. The sizes of these areas are directly proportional to the number of specialized sensory receptors in each respective peripheral area of the body. For instance, a great number of specialized nerve endings are found in the lips and thumb, whereas only a few are present in the skin of the body trunk.

Note also that the head is represented in the most lateral portion of somatosensory area I, and the lower part of the body is represented medially.

Layers of the Somatosensory Cortex and Their Function

The cerebral cortex contains six layers of neurons, beginning with layer I next to the brain surface and extending progressively deeper to layer VI, shown in Figure 47-8. As would be expected, the neurons in each layer perform functions different from those in other layers. Some of these functions are:

Structure of the cerebral cortex, showing I, molecular layer; II, external granular layer; III, layer of small pyramidal cells; IV, internal granular layer; V, large pyramidal cell layer; and VI, layer of fusiform or polymorphic cells. (From Ranson SW, Clark SL [after Brodmann]: Anatomy of the Nervous System. Philadelphia: WB Saunders, 1959.)

Structure of the cerebral cortex, showing I, molecular layer; II, external granular layer; III, layer of small pyramidal cells; IV, internal granular layer; V, large pyramidal cell layer; and VI, layer of fusiform or polymorphic cells. (From Ranson SW, Clark SL [after Brodmann]: Anatomy of the Nervous System. Philadelphia: WB Saunders, 1959.)

1. The incoming sensory signal excites neuronal layer IV first; then the signal spreads toward the surface of the cortex and also toward deeper layers.

2. Layers I and II receive diffuse, nonspecific input signals from lower brain centers that facilitate specific regions of the cortex; this system is described in Chapter 57. This input mainly controls the overall level of excitability of the respective regions stimulated.

3. The neurons in layers II and III send axons to related portions of the cerebral cortex on the opposite side of the brain through the corpus callosum.

4. The neurons in layers V and VI send axons to the deeper parts of the nervous system. Those in layer V are generally larger and project to more distant areas, such as to the basal ganglia, brain stem, and spinal cord where they control signal transmission. From layer VI, especially large numbers of axons extend to the thalamus, providing signals from the cerebral cortex that interact with and help to control the excitatory levels of incoming sensory signals entering the thalamus.

The Sensory Cortex Is Organized in Vertical Columns of Neurons; Each Column Detects a Different Sensory Spot on the Body with a Specific Sensory Modality

Functionally, the neurons of the somatosensory cortex are arranged in vertical columns extending all the way through the six layers of the cortex, each column having a diameter of 0.3 to 0.5 millimeter and containing perhaps 10,000 neuronal cell bodies. Each of these columns serves a single specific sensory modality, some columns responding to stretch receptors around joints, some to stimulation of tactile hairs, others to discrete localized pressure points on the skin, and so forth. At layer IV, where the input sensory signals first enter the cortex, the columns of neurons function almost entirely separately from one another. At other levels of the columns, interactions occur that initiate analysis of the meanings of the sensory signals.

In the most anterior 5 to 10 millimeters of the postcentral gyrus, located deep in the central fissure in Brodmann's area 3a, an especially large share of the vertical columns respond to muscle, tendon, and joint stretch receptors. Many of the signals from these sensory columns then spread anteriorly, directly to the motor cortex located immediately forward of the central fissure. These signals play a major role in controlling the effluent motor signals that activate sequences of muscle contraction.

As one moves posteriorly in somatosensory area I, more and more of the vertical columns respond to slowly adapting cutaneous receptors, and then still farther posteriorly, greater numbers of the columns are sensitive to deep pressure.

In the most posterior portion of somatosensory area I, about 6 per cent of the vertical columns respond only when a stimulus moves across the skin in a particular direction. Thus, this is a still higher order of interpretation of sensory signals; the process becomes even more complex as the signals spread farther backward from somatosensory area I into the parietal cortex, an area called the somatosensory association area, as we discuss subsequently.

Functions of Somatosensory Area I

Widespread bilateral excision of somatosensory area I causes loss of the following types of sensory judgment:

1. The person is unable to localize discretely the different sensations in the different parts of the body. However, he or she can localize these sensations crudely, such as to a particular hand, to a major level of the body trunk, or to one of the legs. Thus, it is clear that the brain stem, thalamus, or parts of the cerebral cortex not normally considered to be concerned with somatic sensations can perform some degree of localization.

2. The person is unable to judge critical degrees of pressure against the body.

3. The person is unable to judge the weights of objects.

4. The person is unable to judge shapes or forms of objects. This is called astereognosis.

Figure 47-9

5. The person is unable to judge texture of materials because this type of judgment depends on highly critical sensations caused by movement of the fingers over the surface to be judged. Note that in the list nothing has been said about loss of pain and temperature sense. In specific absence of only somatosensory area I, appreciation of these sensory modalities is still preserved both in quality and intensity. But the sensations are poorly localized, indicating that pain and temperature localization depend greatly on the topographical map of the body in somatosensory area I to localize the source.

Somatosensory Association Areas

Brodmann's areas 5 and 7 of the cerebral cortex, located in the parietal cortex behind somatosensory area I (see Figure 47-5), play important roles in deciphering deeper meanings of the sensory information in the somatosensory areas. Therefore, these areas are called somatosensory association areas.

Electrical stimulation in a somatosensory association area can occasionally cause an awake person to experience a complex body sensation, sometimes even the "feeling" of an object such as a knife or a ball. Therefore, it seems clear that the somatosensory association area combines information arriving from multiple points in the primary somatosensory area to decipher its meaning. This also fits with the anatomical arrangement of the neuronal tracts that enter the somatosensory association area because it receives signals from (1) somatosensory area I, (2) the ven-trobasal nuclei of the thalamus, (3) other areas of the thalamus, (4) the visual cortex, and (5) the auditory cortex.

Effect of Removing the Somatosensory Association Area— Amorphosynthesis. When the somatosensory association area is removed on one side of the brain, the person loses ability to recognize complex objects and complex forms felt on the opposite side of the body. In addition, he or she loses most of the sense of form of his or her own body or body parts on the opposite side. In fact, the person is mainly oblivious to the opposite side of the body—that is, forgets that it is there. Therefore, he or she also often forgets to use the other side for motor functions as well. Likewise, when feeling objects, the person tends to recognize only one side of the object and forgets that the other side even exists. This complex sensory deficit is called amorphosynthesis.

Overall Characteristics of Signal Transmission and Analysis in the Dorsal Column-Medial Lemniscal System

Basic Neuronal Circuit in the Dorsal Column-Medial Lemniscal System. The lower part of Figure 47-9 shows the basic

Transmission of a pinpoint stimulus signal to the cerebral cortex.

organization of the neuronal circuit of the spinal cord dorsal column pathway, demonstrating that at each synaptic stage, divergence occurs. The upper curves of the figure show that the cortical neurons that discharge to the greatest extent are those in a central part of the cortical "field" for each respective receptor. Thus, a weak stimulus causes only the centralmost neurons to fire. A stronger stimulus causes still more neurons to fire, but those in the center discharge at a considerably more rapid rate than do those farther away from the center.

Two-Point Discrimination. A method frequently used to test tactile discrimination is to determine a person's so-called "two-point" discriminatory ability. In this test, two needles are pressed lightly against the skin at the same time, and the person determines whether two points of stimulus are felt or one point. On the tips of the fingers, a person can distinguish two separate points even when the needles are as close together as 1 to 2 millimeters. However, on the person's back, the needles must usually be as far apart as 30 to 70

Figure 47-10

Transmission of signals to the cortex from two adjacent pinpoint stimuli. The blue curve represents the pattern of cortical stimulation without "surround" inhibition, and the two red curves represent the pattern when "surround" inhibition does occur.

millimeters before two separate points can be detected. The reason for this difference is the different numbers of specialized tactile receptors in the two areas.

Figure 47-10 shows the mechanism by which the dorsal column pathway (as well as all other sensory pathways) transmits two-point discriminatory information. This figure shows two adjacent points on the skin that are strongly stimulated as well as the areas of the somatosensory cortex (greatly enlarged) that are excited by signals from the two stimulated points. The blue curve shows the spatial pattern of cortical excitation when both skin points are stimulated simultaneously. Note that the resultant zone of excitation has two separate peaks. These two peaks, separated by a valley, allow the sensory cortex to detect the presence of two stimulatory points, rather than a single point. The capability of the sensorium to distinguish this presence of two points of stimulation is strongly influenced by another mechanism, lateral inhibition, as explained in the next section.

Effect of Lateral Inhibition (Also Called Surround Inhibition) to Increase the Degree of Contrast in the Perceived Spatial Pattern. As pointed out in Chapter 46, virtually every sensory pathway, when excited, gives rise simultaneously to lateral inhibitory signals; these spread to the sides of the excitatory signal and inhibit adjacent neurons. For instance, consider an excited neuron in a dorsal column nucleus. Aside from the central excitatory signal, short lateral pathways transmit inhibitory signals to the surrounding neurons. That is, these signals pass through additional interneurons that secrete an inhibitory transmitter.

The importance of lateral inhibition is that it blocks lateral spread of the excitatory signals and, therefore, increases the degree of contrast in the sensory pattern perceived in the cerebral cortex.

In the case of the dorsal column system, lateral inhibitory signals occur at each synaptic level—for instance, in (1) the dorsal column nuclei of the medulla, (2) the ventrobasal nuclei of the thalamus, and (3) the cortex itself. At each of these levels, the lateral inhibition helps to block lateral spread of the excitatory signal. As a result, the peaks of excitation stand out, and much of the surrounding diffuse stimulation is blocked. This effect is demonstrated by the two red curves in Figure 47-10, showing complete separation of the peaks when the intensity of lateral inhibition is great.

Transmission of Rapidly Changing and Repetitive Sensations.

The dorsal column system also is of particular importance in apprising the sensorium of rapidly changing peripheral conditions. Based on recorded action potentials, this system can recognize changing stimuli that occur in as little as 1/400 of a second.

Vibratory Sensation. Vibratory signals are rapidly repetitive and can be detected as vibration up to 700 cycles per second. The higher-frequency vibratory signals originate from the pacinian corpuscles in the skin and deeper tissues, but lower-frequency signals (below about 200 per second) can originate from Meissner's corpuscles as well. These signals are transmitted only in the dorsal column pathway. For this reason, application of vibration (e.g., from a "tuning fork") to different peripheral parts of the body is an important tool used by neurologists for testing functional integrity of the dorsal columns.

Interpretation of Sensory Stimulus Intensity

The ultimate goal of most sensory stimulation is to apprise the psyche of the state of the body and its surroundings. Therefore, it is important that we discuss briefly some of the principles related to transmission of sensory stimulus intensity to the higher levels of the nervous system.

The first question that comes to mind is, how is it possible for the sensory system to transmit sensory experiences of tremendously varying intensities? For instance, the auditory system can detect the weakest possible whisper but can also discern the meanings of an explosive sound, even though the sound intensities of these two experiences can vary more than 10 billion times; the eyes can see visual images with light intensities that vary as much as a half million times; and the skin can detect pressure differences of 10,000 to 100,000 times.

As a partial explanation of these effects, Figure 46-4 in the previous chapter shows the relation of the receptor potential produced by the pacinian corpuscle to the intensity of the sensory stimulus. At low stimulus intensity, slight changes in intensity increase the potential markedly, whereas at high levels of stimulus intensity, further increases in receptor potential are slight. Thus, the pacinian corpuscle is capable of accurately measuring extremely minute changes in stimulus at low-intensity levels, but at high-intensity levels, the change in stimulus must be much greater to cause the same amount of change in receptor potential.

The transduction mechanism for detecting sound by the cochlea of the ear demonstrates still another method for separating gradations of stimulus intensity. When sound stimulates a specific point on the basilar membrane, weak sound stimulates only those hair cells at the point of maximum sound vibration. But as the sound intensity increases, many more hair cells in each direction farther away from the maximum vibratory point also become stimulated. Thus, signals are transmitted over progressively increasing numbers of nerve fibers, which is another mechanism by which stimulus intensity is transmitted to the central nervous system. This mechanism, plus the direct effect of stimulus intensity on impulse rate in each nerve fiber, as well as several other mechanisms, makes it possible for some sensory systems to operate reasonably faithfully at stimulus intensity levels changing as much as millions of times.

Importance of the Tremendous Intensity Range of Sensory Reception. Were it not for the tremendous intensity range of sensory reception that we can experience, the various sensory systems would more often than not be operating in the wrong range. This is demonstrated by the attempts of most people, when taking photographs with a camera, to adjust the light exposure without using a light meter. Left to intuitive judgment of light intensity, a person almost always overexposes the film on bright days and greatly underexposes the film at twilight. Yet, that person's own eyes are capable of discriminating with great detail visual objects in bright sunlight or at twilight; the camera cannot do this without very special manipulation because of the narrow critical range of light intensity required for proper exposure of film.

Judgment of Stimulus Intensity

Weber-Fechner Principle—Detection of "Ratio" of Stimulus Strength. In the mid-1800s,Weber first and Fechner later proposed the principle that gradations of stimulus strength are discriminated approximately in proportion to the logarithm of stimulus strength. That is, a person already holding 30 grams weight in his or her hand can barely detect an additional 1-gram increase in weight. And, when already holding 300 grams, he or she can barely detect a 10-gram increase in weight. Thus, in this instance, the ratio of the change in stimulus strength required for detection remains essentially constant, about 1 to 30, which is what the logarithmic principle means. To express this mathematically.

Interpreted signal strength = Log (Stimulus) + Constant

More recently, it has become evident that the Weber-Fechner principle is quantitatively accurate only for higher intensities of visual, auditory, and cutaneous sensory experience and applies only poorly to most other types of sensory experience. Yet the Weber-

Fechner principle is still a good one to remember, because it emphasizes that the greater the background sensory intensity, the greater an additional change must be for the psyche to detect the change.

Power Law. Another attempt by physiopsychologists to find a good mathematical relation is the following formula, known as the power law.

Interpreted signal strength = K • (Stimulus - k)y

In this formula, the exponent y and the constants K and k are different for each type of sensation.

When this power law relation is plotted on a graph using double logarithmic coordinates, as shown in Figure 47-11, and when appropriate quantitative values for the constants y, K, and k are found, a linear relation can be attained between interpreted stimulus strength and actual stimulus strength over a large range for almost any type of sensory perception.

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