Each sense mechanism has an arrangement more or less mechanical for modifying the stimulus, and another mechanism which transforms the stimulus into the nerve impulse, that message which is carried from sense organ to brain and there gives rise to consciousness. We know very little about the real nature of this nerve impulse transmitted from sense organ to brain, aside from its speed and a number of conditions affecting it. Some call it a chemical phenomenon, others an electrical phenomenon, and still others an electrochemical or physicochemical one. We know even less—in fact, nothing—about the change from nerve impulse to consciousness. But there are certain facts fairly well determined by microscopical examination of the sense organ about the nature of the mechanism which transforms the physical stimulus into a physiological one. In the preceding chapter the more mechanical features of the taste mechanism have been reviewed, and in this one we will describe the transforming mechanism.
About 1865 it was found that the coverings of the tongue of mammals and later the lining of the mouth cavity contained peculiar little bodies, which, on account of their apparent shape and their connection with the sense of taste, were called taste beakers. Small fissures called taste pores were discovered among the most superficial epithelial cells of the mucous membrane of the tongue, and these were thought to lead into the small, flask-like chambers. Further study showed that the beakers were not in reality hollow, but consisted of closely packed groups of modified epithelial cells. They were then given the name of taste buds, on account of their resemblance to a bud with its tightly folded petals. These have been spoken of frequently in the preceding pages as sensory ends, and the regions in which they are found were enumerated.
Each taste bud is set rather deeply in the epithelial layer of the mucous membrane covering the sides of the papillæ and communicates with the surface only by the above-mentioned taste pore. Thus the taste bud is projected from direct stimulation by all substances except those which can gain access through the narrow opening. Occasionally taste buds are found which lack this gustatory pore and communicate directly with the tongue surface. Such cases, however, are to be considered as exceptions. Likewise, twin taste buds are sometimes found, having a common base, but with peripheral ends distinct. These, too, are merely exceptions to the ordinary form and do not represent a type. The taste buds are globular in shape, measuring about 70 to 80 thousandths of a millimeter in length and about 50 thousandths of a millimeter in diameter. The taste pore averages only about 4 thousandths of a millimeter in diameter.
A microscopical examination of the taste bud shows it to be composed of three structures. First, there is a series of modified epithelial cells closely packed side by side and forming a kind of hollow shell and determining the limits of the taste bud. These are sometimes called marginal, or supporting, cells. They are very long and narrow and their thickest part is that occupied by the relatively large nucleus. The extremities of the cells directed toward the taste pore are quite long and thin and are gathered together so as to form a small circular opening, the neck of the taste beaker or bud. Within this hollow globe thus formed some of the same kind of cells are found, but it is almost entirely occupied by the second type of structure, the taste cell. These ordinarily range in number from 10 to 16 within any one bud, but sometimes as few as two or three have been found. These cells are longer and even more slender than the supporting cells and are very closely packed. The peripheral portion of the cell is very much elongated and ends in a hair-like filament which extends into the taste pore. All of these hair-like endings of the taste cells gather into a kind of brush within the taste pore. The centrally directed ends of these cells may have various forms—that is, there may be one long, thin branch or a number of branches, none of which, however, pass beyond the limits of the taste bud.
The third kind of structure found in a taste bud is perhaps the most important and consists of the fine branches of nerves which enter the taste bud at its base and twine around the taste cells. They do not actually grow into the cells, but pass through the taste bud and end very near to the taste pore in the form of small knobs or knots. Some of the fibrils, after reaching the peripheral end of the bud, turn back and really end in the more central portions of the bud.
A great deal of experimental work has been done to determine the relations among these three parts of the taste bud. It was at one time thought that the supporting cells were modified epithelial cells and only served as a structural support for the taste cells, which were real nerve cells. According to this view, the taste cell is a neurone[6] which is directly affected by the stimulus and forms the first link in the chain of neurones connecting the surface of the body with the brain. They were thought to be analogous to the olfactory cells, which are true nerve cells. But a number of facts more recently determined tend to minimize the distinction between supporting cells and taste cells. For instance, when certain stains are employed for isolating the different structures it is found that these two sorts of cells stain alike, while the nerve fibrils within the buds stain differently from them. This is taken as good evidence that there is a difference in the composition of the taste cells and the nerve fibrils, and a likeness between the taste and the supporting cells. In fact, both types of cells are now considered to be modified epithelial cells and to be, in a sense, supporting cells. The former serve as supports for the very delicate nerve fibrils as they pass through the taste bud, and the latter, in addition to acting as supports in this way, also form the walls of the taste bud and thus protect the nerve fibrils from undue compression. According to this view, then, the nerve fibrils themselves are directly affected by the stimuli. Their knob-like endings, which are found so closely associated with the taste pore in most cases, lend support to this view.
6. Neurone is the name given to a nerve cell, and includes the cell body with its nucleus, and all of its branches.
One further sort of evidence tends to show the merely secondary function of these two types of cells in the taste bud. Certain portions of the tongue, e.g., parts of the tip and sides, which are extremely sensitive to taste stimuli, have very few taste buds, or none at all. But there is a multitude of free nerve endings in these parts which are thought to function without the aid of the supporting, or taste, cells. It will be recalled that in Chapter V it was necessary to conclude that there seemed to be no absolute dependence of taste sensations upon the presence of taste buds.
The taste sense differs from most of the other special senses in a number of respects, some of which have been mentioned in the preceding chapter. Still another striking difference is to be found in the nerve supply for taste as compared with that of the senses of sight, hearing, and smell. In the latter group there is one nerve which carries the impulses from the sense organ to the brain. Thus, we have the optic nerve for vision, the auditory nerve for hearing, and the olfactory nerve for smell. In taste, however, no single cranial nerve can be called the nerve of taste or the gustatory nerve. There has always been a great deal of difference of opinion as to how the taste fibers are carried to the brain, no small part of which has been due to the seeming necessity for finding “a taste nerve.” As early as 1823 evidence began to accumulate to show that more than one nerve must be involved. Other contributing factors to this confusion are, first, the use of experimentation upon animals—the conclusions from which have been considered valid for man as well as animals, an assumption which is not necessarily correct; and, second, the fact that there seem to be rather pronounced individual differences in the course taken by the fibers from taste bud to brain center. Three of the cranial nerves are now generally conceded to carry taste fibers: the lingual, which is a branch of the trigeminus, or fifth, nerve; the glossopharyngeal, or the ninth, nerve, and the vagus, or the tenth, nerve. The first two of these are more important for taste than the third. The lingual carries the fibers from the anterior two-thirds of the tongue, the upper surface, and the sides of that portion of the tongue and the tip; the glossopharyngeal carries the fibers from the posterior third of the tongue, including the base of the tongue, the soft palate, and the papillæ foliatæ; the vagus carries fibers from the epiglottis and the larynx.
Although the above facts are well established, the more important question is, What is the origin of these fibers and how do they get into the nerve trunks in which they are found? To make this matter clear a word must be said about the general nature of sensory nerve paths. The connection between a sense organ and the brain consists of a series of separate links, which always remain distinct units. Each cell, or neurone, as one of these units is called, consists of a body, which contains the nucleus, and two sets of branches. In one set the branches are very short and numerous, called dendrites; in the other there is usually only one branch, called the axone, which is relatively long. A group of cell bodies is called a ganglion. In practically all sensory nerves these ganglia are situated outside of the central nervous system. The ganglia of the spinal nerves are located just outside of the spinal cord and are called simply spinal ganglia. The ganglia of the cranial nerves usually receive special names. Thus, the ganglion for the great trigeminal, or fifth, nerve is called the gasserian ganglion. Of the two for the glossopharyngeal, or ninth, nerve that one of interest to us is called the petrosal ganglion. The vagus has two ganglia, an upper and a lower; the former, or jugular, ganglion is involved in the sense of taste.
From these ganglia the branches called the dendrites pass to the sense organ and are found twining around the taste cells in the taste buds. The other branches, the axones, pass into the brain stem and finally end in some brain center. The question, then, is, Do the taste fibers which are found in the lingual branch of the fifth nerve have their cell bodies in the gasserian ganglion, do those found in the ninth nerve have their cell bodies in the petrosal ganglion and those found in the tenth nerve have their cell bodies in the jugular ganglion? These are indeed difficult questions to answer. Reliance must be placed largely upon the results of surgical operations upon these nerves and ganglia, with their resulting effects upon the sense of taste. Earlier reports of surgical operations in which the gasserian ganglion was removed were that the taste sense on the tongue was completely destroyed, suggesting that all of the gustatory fibers of the tongue had their origin in the gasserian ganglion. Later reports, however, were that only the taste sense of the anterior two-thirds of the tongue was destroyed by this operation, suggesting that the fibers found in the glossopharyngeal nerve were independent of the gasserian ganglion and really had their origin in the petrosal ganglion, these fibers getting into the lingual branch of the fifth nerve by a circuitous route.
The experiments of Cushing indicate, further, that operations with removal of the gasserian ganglion produced a dulling or complete loss of the sense of taste in the anterior two-thirds of the tongue, but this loss was followed later by complete recovery. The interpretation of these facts must be that the temporary effect upon the taste sense is only an incidental result of the operation and that the taste fibers for this part of the tongue originate in the geniculate ganglion, which is the ganglion for the facial, or the seventh, cranial nerve, and only get into the lingual branch of the fifth by way of the chorda tympani nerve. Consequently, the chorda tympani, a connecting link between the seventh and the lingual branch of the fifth, is extremely important for the taste sense, since it carries all of the fibers concerned in the taste function of the anterior two-thirds of the tongue. This nerve passes across the cavity of the middle ear, and when stimulated mechanically, chemically, or electrically at this point is said to arouse taste sensations variously described as sour or metallic, sometimes as sweet and bitter. Salt tastes alone have never been reported as resulting from such stimulation.
Fig. 3.—Diagram showing some of the various courses which
have been advocated for the taste fibers in man. (Courtesy W.
B. Saunders Company.)
The taste fibers of the vagus seem to have their cells of origin in the jugular ganglion of that nerve. Thus, while taste fibers are found in the fifth, ninth, and tenth cranial nerves, it seems that the fibers really originate in the seventh, ninth, and tenth nerves. The accompanying figure (Fig. 3.), after Cushing, will show the facts about the distribution of the gustatory fibers.
The first unit in the path connecting sense organ of taste and brain, represented by the neurones having their cell bodies in the ganglia described above, all end in the medulla oblongata, in the neighborhood of the fourth ventricle. These terminals, called the primary sensory nuclei for the seventh, ninth, and tenth nerves, are all included in the nucleus of the solitary bundle.
From these primary sensory nuclei a second series of conducting units begins, sending their fibers in two directions, downward into the spinal cord and upward toward the higher brain centers. Those fibers passing downward make connections in the medulla with the motor centers controlling mastication and swallowing and in the cord with the various motor centers. These connections make possible the reflex responses to taste stimuli, such as secretion of gastric and the other juices of the alimentary canal. Little is known about the course taken by the fibers which must carry the gustatory impulses from the primary sensory nuclei to the higher brain centers. It is quite likely that this second conduction unit is represented by fibers which mingle with those carrying impulses from the sense organs of skin and muscles to the higher centers, and which are found in the median fillet, or the pathway in the brain stem for body sensations. These fibers end in the thalamus.[7] In the thalamus a third conducting unit begins and carries the impulses to a still higher center in the cortex of the cerebrum. Exactly what course the fibers take from the thalamus, or just where the cortical center for taste is, has not been definitely determined for human beings.
7. The thalamus is a center in the upper part of the brain stem, where connections are made between the cortex or outer layer of the brain proper and lower centers of the nervous system.
Indeed, less is known about the localization of the taste function in the cortex than about any of the other special senses. This is not due to lack of interest, for a large number of researches have been reported in this field. The reason is, rather, that there are certain difficulties in the way of its solution. First, the help to be got from physiological experiments upon animals is limited, because of the close relation between the senses of taste and smell, with the consequent uncertainty in interpreting behavior after surgical operations. And, second, since the exploration of the most accessible portions of the cortex has not revealed a taste center, it is probably located somewhere upon the ventral, or mesial, surfaces of the cerebrum, where experimental work is practically impossible. Clinical and physiological investigations have furnished conflicting results. A survey of recent work suggests that the most probable center for taste is in the hippocampal gyre near the anterior portion of the temporal lobe. No more definite localization is at present possible.