9. THE MAUTHNER CELL
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would of course be that on the opposite side to the vestibular system
most affected by the stimulus. Even so, the chances of causing a delay
as great as 0.15-0.2 msec must be sometimes rather slim. It may be
necessary to involve yet another factor, to be discussed now, which we
propose can operate even when the stimulus is entirely symmetrical
with respect to the two vestibular systems, or when the stimulus is
transmitted via a swim bladder which affects equally the vestibular
systems on either side.
E. The Symmetrical Stimulus
1. THRESHOLD FLUCTUATIONS IN THE Two CELLS
From all that has been said above it is clear that in many situations
the mechanical stimulus will be transmitted equally to each vestibular
system, and then often the output will be equal in both VIIIth nerves.
It would seem that under these conditions the Mauthner cells would
fire simultaneously, and as we already know this will cause no reflex
movement of the trunk and tail. Are there other factors which could
make this undesirable result unlikely? Consider the fish in its normal
environment. Here is a system which is rarely bilaterally symmetrical.
The fish undulates as it swims. The water turbulence on either side
of the animal is unlikely to be identical in all respects, nor is the illumination presented to the two eyes; in particular, the muscular activity will
not be identical on the two sides at any given instant. From considerations
such as these we may suppose that in the central nervous system the
representation of the two sides of the fish at any instant of time will
differ. Now it is known that the Mauthner cells receive an enormous
neural input, which has origins in very many parts of the brain (Fig. 4);
presumably this input presents to each Mauthner cell information about
the state of the animal and its environment, very probably with reference
to the same side as the cell. If the state is different on the two sides, then
the pattern of impulses arriving at the cells will also be different on the
two sides. In this context it is worth mentioning that the sudden shadowing caused by passing the hand between a direct light source and the fish
tank can produce a startle-response extremely similar to that caused by
a sudden blow to the tank. The indirect optical pathway to the Mauthner
cells could be involved in this response.
Doubtless there are other conditions which could produce or contribute to the production of a startle-response which are dependent on
inputs other than the VIIIth nerve ones on to the Mauthner cells. During
normal “unstartled” behavior the effect of this information will be to
329
would of course be that on the opposite side to the vestibular system
most affected by the stimulus. Even so, the chances of causing a delay
as great as 0.15-0.2 msec must be sometimes rather slim. It may be
necessary to involve yet another factor, to be discussed now, which we
propose can operate even when the stimulus is entirely symmetrical
with respect to the two vestibular systems, or when the stimulus is
transmitted via a swim bladder which affects equally the vestibular
systems on either side.
E. The Symmetrical Stimulus
1. THRESHOLD FLUCTUATIONS IN THE Two CELLS
From all that has been said above it is clear that in many situations
the mechanical stimulus will be transmitted equally to each vestibular
system, and then often the output will be equal in both VIIIth nerves.
It would seem that under these conditions the Mauthner cells would
fire simultaneously, and as we already know this will cause no reflex
movement of the trunk and tail. Are there other factors which could
make this undesirable result unlikely? Consider the fish in its normal
environment. Here is a system which is rarely bilaterally symmetrical.
The fish undulates as it swims. The water turbulence on either side
of the animal is unlikely to be identical in all respects, nor is the illumination presented to the two eyes; in particular, the muscular activity will
not be identical on the two sides at any given instant. From considerations
such as these we may suppose that in the central nervous system the
representation of the two sides of the fish at any instant of time will
differ. Now it is known that the Mauthner cells receive an enormous
neural input, which has origins in very many parts of the brain (Fig. 4);
presumably this input presents to each Mauthner cell information about
the state of the animal and its environment, very probably with reference
to the same side as the cell. If the state is different on the two sides, then
the pattern of impulses arriving at the cells will also be different on the
two sides. In this context it is worth mentioning that the sudden shadowing caused by passing the hand between a direct light source and the fish
tank can produce a startle-response extremely similar to that caused by
a sudden blow to the tank. The indirect optical pathway to the Mauthner
cells could be involved in this response.
Doubtless there are other conditions which could produce or contribute to the production of a startle-response which are dependent on
inputs other than the VIIIth nerve ones on to the Mauthner cells. During
normal “unstartled” behavior the effect of this information will be to