Supporting Cells, Gustatory Cells, and Nerve Filaments
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.
Relations Among the Structures Within the Taste 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 Sensory Nerves of Taste
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.