FUNCTION OF THE TASTE MECHANISM
_Function of Tongue and Salivary Glands_
WHEN sapid substances are taken into the mouth as solids, liquids, or
gases they either become dissolved in the saliva or mixed with it. The
glandular activity, with the resulting secretion of the saliva, as
described in chapter V, may begin at the sight of the objects or may not
begin until the substances have come into contact with the linings of
the mouth cavity or tongue. The breaking down of the solid substances
and their mixture with saliva is facilitated by chewing movements and by
the movements of the tongue. When the substances have been transformed
into the liquid state they move toward the back part of the mouth, from
which the swallowing reflex movements will carry them into the gullet
and stomach. In the course of this movement the fluids will come into
contact with the tip, the superior surface, and sides of the tongue, and
with portions of the mucous linings of the mouth. And it is just in
these regions that we find that the taste organs are located.
The uneven surface of the tongue, due to the presence of the papillæ,
tends to retard the movement of the fluid substances and to give them
time to affect the taste organs. It will be recalled that on the tip and
the superior surface of the front part of the tongue there are few taste
buds found, even where the papillæ of the filiform and fungiform type
are numerous, but a tremendous number of free nerve endings are found
close to the surface of the epithelial covering of the tongue. They can
be affected by the fluids without passing through a gustatory pore into
the taste bud. Now, it happens that the latent time of the sweet sense
is very short compared with that for bitter. And since it is known that
the bitter sensations are aroused especially by stimulating the
circumvallate papillæ, which contain real taste buds, it seems quite
probable that the free nerve endings in the forward part of the tongue
are real sensory ends of taste and are directly affected by the fluid
stimuli. It was at one time supposed that sweet tastes could not be
aroused on this part of the tongue without the aid of tongue movements.
Although this is no longer believed, it is, nevertheless, likely that
tongue movements which would press its surfaces against neighboring
parts of the mouth cavity would bring the sapid substances into contact
with the free nerve endings, and that more quickly than in the absence
of any movement.
As the fluids pass over the sides and superior surface of the tongue
still farther back they meet the foliate and the circumvallate papillæ.
The character of these papillæ is well adapted to retard the fluids in
their passage and give ample time for stimulating the taste nerves. The
former does this by holding the fluid in its long folds, or ditches, and
the latter by collecting it in the circular ditches surrounding the
papillæ proper. In these two types of papillæ real taste buds are found,
with their taste pores leading from the surface into the interior of the
taste bud. It is necessary, then, that the fluid be retained long enough
to reach these hidden parts. As might be expected, there is a rather
long latent time for the sensations aroused in these parts, namely, sour
and bitter.
Tongue movements would be of service here, perhaps even more than in the
forward portion of the tongue, in forcing the fluids more rapidly
through the taste pore. But the tongue movements are said to be of use
in still another way. The bases of the papillæ beneath the epithelial
layer are supplied with a rich network of small veins. Now, tongue
movements increase the flow of blood to the tongue and these veins
become congested with blood. Thus, the veins form a kind of erectile
mechanism through which the papillæ become swollen, and at the same time
the crevices in the epithelial tissue are opened wider, and easier
access to the taste buds results. This hypothesis of the erectility of
the papillæ is not generally accepted.
The devices in connection with the circumvallate and foliate papillæ,
the circular and linear ditches, for retarding the fluid, may account
for certain other characteristics of taste sensations, namely, the
difference in duration of the taste sensations. Since the depressions
and the taste pores become filled with the sapid substances, the taste
sensations ought to last as long as the fluid remains, or until the
taste organs become adapted to them, and thus interfere with the
production of new and different sensations. But a corrective device for
this defect has been assumed by certain investigators in the form of the
numerous secretory glands found in the mucous membrane of this part of
the tongue. These glands are said to pour their secretions through ducts
into these depressions and flush them out, thus removing stimulating
fluids and making way for new ones. This mechanism would prevent the
confusion which would necessarily occur from the mixture of old and new
sapid solutions in the depressions of these papillæ.
_The Function of the Taste Buds_
Real difficulties and differences of interpretation come when
explanation of what takes place in the taste bud is attempted. Such
questions as the following arise, none of which has received a perfectly
definite answer. Theories of various sorts are all that can be offered
in this connection:
1. Does the sapid substance affect the taste-bud cells, or only the
nerve fibrils that twine around these cells?
2. If it affects these cells, does it affect the gustatory cells only,
or both these and the supporting cells?
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.
Sternberg, whose work was mentioned in chapter VII, recognized only two
elementary taste sensations, sweet and bitter, and he found no
differences in the molecule of substances producing these two
sensations. He concludes that it is the intramolecular vibrations that
form the real taste stimuli. By transferring the seat of the activity
from molecular vibration to intramolecular vibration the whole matter
becomes more difficult of solution and verification. Still, the
modification of the theory of Ramsay, made necessary by the work of
Sternberg, would leave its essentials, namely, that vibrations of some
sort form the stimulus and that the sense organ of taste is tuned to
receive only a limited range of vibration rates.
Granting that a dynamic theory as outlined above, with possible
modifications to meet new discoveries, is correct, still another
question requires an answer. In the visual organ affected by ether
vibrations of different rates every theory assumes certain parts of the
mechanism sensitized especially to certain vibration rates. To take the
theory of Hering, for example, there are three photochemical substances,
one decomposed by a relatively slow vibration rate, another by a more
rapid vibration rate, and so on. And in the auditory mechanism there is
the _basilar membrane_, capable of vibration in parts in sympathy with
different rates of vibration of the air. The answer to this question
concerning the taste sense is peculiarly difficult, in that it is
impossible to stimulate individual taste buds, and even difficult to
stimulate a single papilla, which may contain many buds. And then it
must be further borne in mind that each bud contains many nerve endings,
so that what is stimulated in experimental work is really a very complex
portion of the sense organ.
As mentioned in another connection, certain papillæ when stimulated with
various substances respond to but one, e.g., sweet, sour, bitter, or
salt, while others respond to two or three stimuli. Further, it is well
known that certain portions of the tongue when stimulated produce a
predominance of certain kinds of taste sensations. On the other hand, an
examination of the taste buds or of the nerve endings within the taste
buds shows no differences among them. Still, in this respect taste does
not differ from sight. The cones of the retina, which are sensory ends
of vision, show no differences in structure, and it is generally
believed that all of the elementary sensations can be produced by the
stimulation of one single cone. If it be true that the nerve fibrils in
the taste buds themselves receive the stimulus, then any specific
characteristics of the receiving mechanism would have to be looked for
in these nerve endings.
Oehrwall has accounted for the facts of the specific sensitivity of
different parts of the tongue and of different individual papillæ
mentioned above by the assumption that each taste bud has a specific
function and that certain papillæ of the tongue have buds all of one
sort, other of two sorts, and so on. Nagel prefers to modify this view
to the extent that each taste bud is capable of arousing every
elementary sensation, but is adapted to respond best and easiest to a
certain specific stimulus with a certain elementary sensation. This view
is analogous to that of the visual photochemical substances which,
according to the Helmholtz theory, were each sensitive to all wave
lengths of light, but not equally.
The previous review of theories does not consider the possibility of the
specific energy of brain centers, rather than of the peripheral sense
mechanism—that is, the possibility that the sensation qualities, sweet,
sour, etc., may be due to structural characteristics of brain centers,
rather than of sense organs. This is a question which is unsolved for
other senses and consequently is not peculiar to taste. The present
tendency seems to be toward attributing the different sensation
qualities to the sensitivity of the sense organ, or else to divide it
between brain center and sense organ. The following facts are considered
as indirect evidence of the specific energy of the sensory ends of
taste: (1) The distribution of taste sensitivity over the tongue. (2)
The effect of certain drugs, e.g., cocaine, which destroys the taste
sensations one at a time. This suggests that the sensory ends that have
to do with the different sensations are differently affected by the
drug. (3) The fact that the same substance as it passes over the surface
of the tongue arouses different sensations, e.g., sodium sulphate, which
is sweet on the tip and bitter on the back of the tongue.
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