9. THE MAUTHNER CXLL
281
tail, is completely absent (Fig. &). All the other components of the
response, however, appear unchanged (Figs. 9c and 1Oc). The proof
that the stimulus, applied at the lower end of the spinal cord, did excite
an impulse in each of the Mauthner axons is given by the recordings
made simultaneously at the respective axon hillocks in the medulla.
It should be noted that in an investigation of this sort only responses which
occur in constant association with the firing of one or both Mauthner cells
are being considered. It is possible to produce muscular responses by the coincidental excitation of other (non-Mauthner) fibers, and these can be distinguished by careful observation of effects caused by stimuli just at threshold
for excitation of one or both Mauthner axons. The difficulties are greater
with orthodromic stimulation because it is almost impossible then to avoid
activating other cells in the medulla which are also associated with reflex
movements.
What is revealed here, then, is the existence in the spinal cord of a
mutual inhibitory system whose activation by an impulse in one Mauthner axon prevents the impulse in the opposite Mauthner axon from
causing the usual excitation of spinal motoneurons. The inhibitory system, however, does not operate on the cranial component of the total
reflex (Figs. 9 and 10).
C. Time-Course of the Inhibition
Since the crossing inhibition owing to activation of one Mauthner
axon can so totally and dramatically suppress the most striking excitatory
response to the impulse in the other, it seems likely to be of fundamental
importance in Mauthner cell function. It is surprising that, coming from
the other side of the spinal cord, it acts as quickly as the ipsilaterally
evoked excitation. In fact, the inhibition can be delayed slightly behind
the excitation and still be as effective as when the two Mauthner axon
impulses are synchronous (this is considered below).
Figure 11 shows the EMGs recorded simultaneously on each side
of the trunk when both Mauthner axons were stimulated. The moment
of excitation of one Mauthner axon was kept constant and that of the
opposite one was progressively delayed. Except at very small separation
times, which are not shown in this figure but are considered in detail
below, there was always the same response to the impulse in the first
excited Mauthner axon, being larger on the ipsilateral side and smaller
and slightly delayed on the opposite side. But a response to the second
excited axon did not appear until the sepxation between the Mauthner
impulses (which were simultaneously recorded and were always used
for measuring time relations) was about 11 msec; the response was then
a very small one. As the delay was increased this response became pro-
281
tail, is completely absent (Fig. &). All the other components of the
response, however, appear unchanged (Figs. 9c and 1Oc). The proof
that the stimulus, applied at the lower end of the spinal cord, did excite
an impulse in each of the Mauthner axons is given by the recordings
made simultaneously at the respective axon hillocks in the medulla.
It should be noted that in an investigation of this sort only responses which
occur in constant association with the firing of one or both Mauthner cells
are being considered. It is possible to produce muscular responses by the coincidental excitation of other (non-Mauthner) fibers, and these can be distinguished by careful observation of effects caused by stimuli just at threshold
for excitation of one or both Mauthner axons. The difficulties are greater
with orthodromic stimulation because it is almost impossible then to avoid
activating other cells in the medulla which are also associated with reflex
movements.
What is revealed here, then, is the existence in the spinal cord of a
mutual inhibitory system whose activation by an impulse in one Mauthner axon prevents the impulse in the opposite Mauthner axon from
causing the usual excitation of spinal motoneurons. The inhibitory system, however, does not operate on the cranial component of the total
reflex (Figs. 9 and 10).
C. Time-Course of the Inhibition
Since the crossing inhibition owing to activation of one Mauthner
axon can so totally and dramatically suppress the most striking excitatory
response to the impulse in the other, it seems likely to be of fundamental
importance in Mauthner cell function. It is surprising that, coming from
the other side of the spinal cord, it acts as quickly as the ipsilaterally
evoked excitation. In fact, the inhibition can be delayed slightly behind
the excitation and still be as effective as when the two Mauthner axon
impulses are synchronous (this is considered below).
Figure 11 shows the EMGs recorded simultaneously on each side
of the trunk when both Mauthner axons were stimulated. The moment
of excitation of one Mauthner axon was kept constant and that of the
opposite one was progressively delayed. Except at very small separation
times, which are not shown in this figure but are considered in detail
below, there was always the same response to the impulse in the first
excited Mauthner axon, being larger on the ipsilateral side and smaller
and slightly delayed on the opposite side. But a response to the second
excited axon did not appear until the sepxation between the Mauthner
impulses (which were simultaneously recorded and were always used
for measuring time relations) was about 11 msec; the response was then
a very small one. As the delay was increased this response became pro-