The Sense of Taste

by Harry L. Hollingworth; Albert T. Poffenberger

3. Does the sapid substance really enter the taste bud at all, or only affect the ends of the cells which form the so-called entrance to the bud?

4. Regardless of what portion of the taste bud is affected by the stimulus, what is the character of the effect produced? Is it mere contact or mechanical stimulation, or is it a chemical process which is set up?

5. Must different types of receiving structures, of whatever form they may be, be assumed for each type of elementary taste sensation?

The structural relations among the parts of the taste bud were discussed in the chapter on Sensory Elements. There it was concluded that the analogy between the sense of taste and that of certain of the other senses, especially sight and smell, is not so close as it has seemed to be. It will be recalled that in these sensory mechanisms there are modified nerve structures, rods and cones in the eye, and the olfactory cells in the nose, which are affected directly by the stimuli, and in which a transformation of the stimulus takes place, with the resultant nerve impulse. This transformation accompanies a chemical change within these structures, hence vision and smell are called chemical senses. In the taste mechanism, also considered a chemical sense, it was natural to see in the gustatory cells of the taste bud structures with functions similar to that of the rods, cones, and olfactory cells. But the analogy between these types of structures breaks down because the gustatory cells do not have the characteristics of nerve tissue, as revealed especially by the use of differential stains. In fact, as has been said above, there seems to be no fundamental difference between supporting and taste cells. Two further facts seem to indicate that the supporting and gustatory cells take no primary part in the taste function. First, there is no more intimate connection between these cells and the nerve fibrils than that of contact, and the contact seems only incidental to the supporting function. The endings of the nerve fibrils are free from the cells. And, second, those free nerve endings in the anterior portion of the tongue seem to arouse taste sensations without the intervention of any structures resembling the taste buds or their cells. Thus, the evidence to date leads to the conclusion that the nerve fibrils alone are the parts affected by the taste stimulus.

If this hypothesis be correct, is it necessary that the sapid substances should actually enter the taste bud, or only affect its peripheral end? Many of the nerve fibrils entering the taste bud pass through its whole length and end quite near the mouth of the gustatory pore. These might be stimulated without the entrance of the stimulus within the taste bud. But there are many of these fibrils which do not reach to the peripheral end of the bud, but stop far short of this point, and then there are others that reach the entrance of the taste pore but turn back and end in the characteristic knob-like formation within the taste bud. In order that these fibrils may be stimulated upon their ends the stimuli would have to enter the taste bud.

The answer to the fourth question is indeed the most difficult of all. What is the nature of this stimulation by which a fluid substance shall start an impulse along the nerve paths to the brain, which shall there produce sensation? About this last stage of the process nothing is known either about taste or any of the other senses. But very well-developed theories exist to account for the transformation of the physical stimulus into physiological nerve impulse. For instance, in the case of vision, the stimulus for which consists of ether vibrations, these ether waves cause chemical changes in certain hypothetical substances within the rods and cones of the retina. It is this chemical change which creates the nerve impulse. In the case of hearing, for which the stimulus consists of air vibrations, these waves, being slightly modified by the more superficial portions of the auditory mechanism, finally cause vibrations of the basilar membrane, which, in turn, produces the impulse in the auditory nerve.

One of the earliest and simplest conceptions of the nature of the process in the taste organ was a mechanical theory proposed by Boyle, about 1675. He thought that the particles of various sapid substances differed in size and shape and that on account of these differences they produced different effects in their simple contact with the sensory ends of taste.

According to Graham, who announced his theory in 1889, sapidity of substances depends on their chemical constitution, colloids being generally insipid and crystalloids being sapid, hence this has been known as a chemical theory. This difference of chemical structure, discussed on page 94, was made to account for the contact, or the lack of it, between the substances and the sensory ends, but does not account for the effect produced upon the sensory ends by the substances reaching them.

Sir William Ramsay prepared an explanation quite analogous to the theories of color vision and called it a dynamic theory. According to him, the real stimulus to the taste organs is molecular vibration, the different taste sensations being due to stimulation by different rates of molecular movement. Just as in the case of the luminiferous ether or of the air there is quite a range of molecular vibration rates, from exceedingly slow to exceedingly rapid. And, just as in the case of vision and hearing, so is the taste mechanism tuned to respond to only the middle range of these molecular vibration rates. Substances may then be insipid, either because their molecular movements are too slow or too fast to affect the receiving mechanism. In vision we have the analogous case of the infra-red and the ultra-violet rays not producing visual sensations, because they are beyond the range of sensitivity of the eye. Yet the effect of these rays can be recorded by other means. The rate of molecular movement depends on the weight of the molecule, so that very heavy or very light molecules would not produce taste sensations.

About the same time Richet and Gley performed a series of experiments which seemed to show that the molecular weight of the substance was an important factor in producing taste sensations. They found that the intensity of the effect produced by different salts was in proportion to their molecular weight; that if account was taken of the different molecular weights of the salts used as stimuli the threshold stimulus would be the same for all of the salts. But if solutions were prepared according to the absolute weight of the salts these threshold stimuli appeared to be quite different for the different salts. Later experiments have shown that the same relation does not hold for sour-producing substances, certain sours of very small molecular weight having the sourest taste.