1. ANATOMY AND PHYSIOLOGY OF THE CENTRAL NERVOUS SYSTEM
61
order to bring the pH of the stimulating medium to comparable levels
in salt- and freshwater. Sodium chloride elicited a more gradual and
sustained nerve activity in both barbel preparations than did the acid.
In the marine species, increased discharges of taste receptors resulted
from an increase in the sodium chloride in the environment. However,
lowering of the sodium chloride resulted in an inhibition of the king
pattern in the marine species. When freshwater was applied there was
inhibition of discharge rate almost resulting in cessation of discharge.
This inhibition was followed by a rebound effect resulting in greater
activity upon restimulation with saltwater. In all species of fish studied
by Bardach et al. (1967), varying numbers of fibers responded to stimulation with quinine hydrochloride. Less than 10% of all the fibers tested
responded to sweet substances, and fibers from the marine sea robin
did not respond at all to sugar. This appears to be in keeping with
results of Konishi et d. (1966) on the barbel of sea catfish which also
did not respond to sugars. As shown by other investigators (Konishi
and Zotterman, 1963), Bardach et al. (1967) found that certain taste
fibers reacted selectively to different substances. Single nerve fibers
responded selectively to acid, salt, sugar, or quinine, respectively. In
addition, in the bullhead, catfish, and sea robin, there were fibers that
responded selectively to extracts of meat. Homocysteine produced no
responses in the sea robin taste receptors, and the bullhead and tomcod
tested with the same substance did not initially respond. However, after
rinsing the preparation, the tomcod and catfish receptors responded to
cysteine. Since homocysteine is only one carbon atom longer than cysteine
and will not produce responses in taste receptors, it appears that specific
compounds acting on specific response sites are responsible for the reaction of the taste buds. A series of amino acids were tested for their ability
to produce signals from chemoreceptors. Of interest is indole which is an
electrochemically active substance and leads to strong stimulation of the
preparation while indole acetic acid, although related to indole, failed
to elicit responses. Recordings made from the olfactory tract of brook
trout, Saluelinus fontinulis, demonstrated that amino acids perfused over
the olfactory organ produced discharges down the olfactory tract. These
substances also produce responses to stimulation of chemoreceptors in
fish, thus demonstrating olfactory and gustatory chemoreceptor overlap.
However, there are certain chemical sensitivities which appear to be
specific to one or the other of the chemoreceptive systems. Although fish
can both smell and taste certain amino acids, there are some organic
substances which can only be tasted or smelled (Bardach et al., 1967).
Konishi and Zotterman (1963) found an active lipid within substances
which stimulated carp palatal organs. Some of the lipids tested by
61
order to bring the pH of the stimulating medium to comparable levels
in salt- and freshwater. Sodium chloride elicited a more gradual and
sustained nerve activity in both barbel preparations than did the acid.
In the marine species, increased discharges of taste receptors resulted
from an increase in the sodium chloride in the environment. However,
lowering of the sodium chloride resulted in an inhibition of the king
pattern in the marine species. When freshwater was applied there was
inhibition of discharge rate almost resulting in cessation of discharge.
This inhibition was followed by a rebound effect resulting in greater
activity upon restimulation with saltwater. In all species of fish studied
by Bardach et al. (1967), varying numbers of fibers responded to stimulation with quinine hydrochloride. Less than 10% of all the fibers tested
responded to sweet substances, and fibers from the marine sea robin
did not respond at all to sugar. This appears to be in keeping with
results of Konishi et d. (1966) on the barbel of sea catfish which also
did not respond to sugars. As shown by other investigators (Konishi
and Zotterman, 1963), Bardach et al. (1967) found that certain taste
fibers reacted selectively to different substances. Single nerve fibers
responded selectively to acid, salt, sugar, or quinine, respectively. In
addition, in the bullhead, catfish, and sea robin, there were fibers that
responded selectively to extracts of meat. Homocysteine produced no
responses in the sea robin taste receptors, and the bullhead and tomcod
tested with the same substance did not initially respond. However, after
rinsing the preparation, the tomcod and catfish receptors responded to
cysteine. Since homocysteine is only one carbon atom longer than cysteine
and will not produce responses in taste receptors, it appears that specific
compounds acting on specific response sites are responsible for the reaction of the taste buds. A series of amino acids were tested for their ability
to produce signals from chemoreceptors. Of interest is indole which is an
electrochemically active substance and leads to strong stimulation of the
preparation while indole acetic acid, although related to indole, failed
to elicit responses. Recordings made from the olfactory tract of brook
trout, Saluelinus fontinulis, demonstrated that amino acids perfused over
the olfactory organ produced discharges down the olfactory tract. These
substances also produce responses to stimulation of chemoreceptors in
fish, thus demonstrating olfactory and gustatory chemoreceptor overlap.
However, there are certain chemical sensitivities which appear to be
specific to one or the other of the chemoreceptive systems. Although fish
can both smell and taste certain amino acids, there are some organic
substances which can only be tasted or smelled (Bardach et al., 1967).
Konishi and Zotterman (1963) found an active lipid within substances
which stimulated carp palatal organs. Some of the lipids tested by