9. THE MAUTHNER CELL
283
istic powerful movement which may be described as the Mauthner reflex
response. One of the these features [point (4) below] embodies a principle of general interest, which will be returned to later.
(1) The muscular response to a single Mauthner axon impulse is
effectively all or nothing.
(2) For a period, which varies from fish to fish, of 8-15 msec after
an impulse in one Mauthner axon, an impulse in the opposite Mauthner
axon leads to no muscle response at all in the trunk and tail.
( 3 ) Following this period of absolute suppression of the response,
there is a period which may last as long as 50 msec, during which the
impulse in the contralateral Mauthner axon evokes a muscle response
whose size depends on the interval between the two Mauthner spikes.
As this interval lengthens the response becomes larger until it reaches
its full control value (i.e., the value obtained in the absence of the
ipsilateral Mauthner impulse). Individual motoneurons, therefore, are
freed of the crossing inhibition at times which vary from 8 to 60 msec
after the impulse in the opposite Mauthner axon, which causes it.
(4) When two impulses, one in each Mauthner axon, are produced
within 0.15-0.20 msec of one another (the exact time varies from one
animal to the other) there is no muscle response on either side insofar
as trunk and tail are concerned. This time can be called the minimum
“discrimination time” of the system (inset Fig. 12). In our experiments,
unless obvious deterioration of the preparation occurred, the time was
fairly constant in any one fish (goldfish and tench) during repeated
testing. When the two impulses are separated by times less than the
minimum discrimination time, there is no indication from the spinal
musculature that there were indeed any Mauthner axon impulses at
all. At this separation interval, or greater ones, there will always be a
unilateral movement of the trunk and tail. This movement reaches a
maximum (on the side of the leading Mauthner axon spike) when the
interval is more than 0.5-0.8 msec, and it then equals the response in
the absence of the opposite Mauthner axon impulse.
However, the response to the later spike (in the opposite Mauthner
axon) is always zero in the trunk and tail muscle until the period of total
inhibition is over.
We can suppose that were it not for the inhibition described above,
synchronous impulses in both Mauthner axons would lead to a calamitous
situation for the fish, namely, the simultaneous activation of the two
effectively opposed muscle masses on either side of the body. This is
happily disallowed by the reciprocal inhibitory systems. But what is
the basis of the remarkably low value of the minimum discrimination
283
istic powerful movement which may be described as the Mauthner reflex
response. One of the these features [point (4) below] embodies a principle of general interest, which will be returned to later.
(1) The muscular response to a single Mauthner axon impulse is
effectively all or nothing.
(2) For a period, which varies from fish to fish, of 8-15 msec after
an impulse in one Mauthner axon, an impulse in the opposite Mauthner
axon leads to no muscle response at all in the trunk and tail.
( 3 ) Following this period of absolute suppression of the response,
there is a period which may last as long as 50 msec, during which the
impulse in the contralateral Mauthner axon evokes a muscle response
whose size depends on the interval between the two Mauthner spikes.
As this interval lengthens the response becomes larger until it reaches
its full control value (i.e., the value obtained in the absence of the
ipsilateral Mauthner impulse). Individual motoneurons, therefore, are
freed of the crossing inhibition at times which vary from 8 to 60 msec
after the impulse in the opposite Mauthner axon, which causes it.
(4) When two impulses, one in each Mauthner axon, are produced
within 0.15-0.20 msec of one another (the exact time varies from one
animal to the other) there is no muscle response on either side insofar
as trunk and tail are concerned. This time can be called the minimum
“discrimination time” of the system (inset Fig. 12). In our experiments,
unless obvious deterioration of the preparation occurred, the time was
fairly constant in any one fish (goldfish and tench) during repeated
testing. When the two impulses are separated by times less than the
minimum discrimination time, there is no indication from the spinal
musculature that there were indeed any Mauthner axon impulses at
all. At this separation interval, or greater ones, there will always be a
unilateral movement of the trunk and tail. This movement reaches a
maximum (on the side of the leading Mauthner axon spike) when the
interval is more than 0.5-0.8 msec, and it then equals the response in
the absence of the opposite Mauthner axon impulse.
However, the response to the later spike (in the opposite Mauthner
axon) is always zero in the trunk and tail muscle until the period of total
inhibition is over.
We can suppose that were it not for the inhibition described above,
synchronous impulses in both Mauthner axons would lead to a calamitous
situation for the fish, namely, the simultaneous activation of the two
effectively opposed muscle masses on either side of the body. This is
happily disallowed by the reciprocal inhibitory systems. But what is
the basis of the remarkably low value of the minimum discrimination