322
J. DIAMOND
D. An Asymmetrically Effective Vibrational Stimulus?
1. THE EXCITATORY INPUTS
Consider the situations when ( a ) a sudden vibrational stimulus
affecting the vestibular systems directly is located nearer one side
of the head than the other, and when ( b ) the stimulus acts via the
swim bladder but the linkage of this to the two vestibular systems is
sufficiently asymmetrical for one (say the ipsilateral) to be affected
more than the other. In each case the excitatory VIIIth nerve volley to
the ipsilateral Mauthner cell might be expected to be very slightly
larger than that to the contralateral cell; furthermore, it would also be
set up slightly in advance of that in the contralateral VIIIth nerve,
although for moderate-sized goldfish the time difference could hardly
be more than a very few microseconds (see Section IX, B). The inference
from these considerations however is that unless the asymmetry involved
is quite gross, the e.p.s.p. in the ipsilateral Mauthner cell would at
best begin fractionally earlier, and rise slightly more steeply than
that in the contralateral. Without the involvement of other factors, it
seems highly improbable that, with a vibrational stimulus, asymmetrical
effects alone could cause differences in the excitatory inputs to the
two Mauthner cells sufficient to result in the 0.15-0.20 msec separation
in the times of their firing needed for the production of the reflex trunk
and tail movement.
2. THE CROSSED INHIBITION OF THE MAUTHNER CELL
There is onc potentially useful system here which must now be
considered, a crossed inhibition acting on the opposite Mauthner cell,
which can be observed when an VIIIth nerve is excited electrically
( Furukawa arid Furshpan, 1963). Figure 36 shows how this crossed
inhibition can canccl out the effects of ipsilateral VIIIth nerve excitation
and how these mutually opposed VIIIth nerve influences can be balanced
experimentally to control the firing of the Mauthner cell.
Is it possible, again considering the asymmetrical situations describcd
above, that this inhibition could be responsiblc for producing the necessary time separation between the firing of the two Mauthner cells?
By the same reasoning as was used above for the excitatory VIIIth nerve
activity, the inhibitory volley derived from the ipsilateral vestibular
apparatus would be expected to be slightly greater, and initiated slightly
earlier, than that from the contralateral. That is, when both vestibular
J. DIAMOND
D. An Asymmetrically Effective Vibrational Stimulus?
1. THE EXCITATORY INPUTS
Consider the situations when ( a ) a sudden vibrational stimulus
affecting the vestibular systems directly is located nearer one side
of the head than the other, and when ( b ) the stimulus acts via the
swim bladder but the linkage of this to the two vestibular systems is
sufficiently asymmetrical for one (say the ipsilateral) to be affected
more than the other. In each case the excitatory VIIIth nerve volley to
the ipsilateral Mauthner cell might be expected to be very slightly
larger than that to the contralateral cell; furthermore, it would also be
set up slightly in advance of that in the contralateral VIIIth nerve,
although for moderate-sized goldfish the time difference could hardly
be more than a very few microseconds (see Section IX, B). The inference
from these considerations however is that unless the asymmetry involved
is quite gross, the e.p.s.p. in the ipsilateral Mauthner cell would at
best begin fractionally earlier, and rise slightly more steeply than
that in the contralateral. Without the involvement of other factors, it
seems highly improbable that, with a vibrational stimulus, asymmetrical
effects alone could cause differences in the excitatory inputs to the
two Mauthner cells sufficient to result in the 0.15-0.20 msec separation
in the times of their firing needed for the production of the reflex trunk
and tail movement.
2. THE CROSSED INHIBITION OF THE MAUTHNER CELL
There is onc potentially useful system here which must now be
considered, a crossed inhibition acting on the opposite Mauthner cell,
which can be observed when an VIIIth nerve is excited electrically
( Furukawa arid Furshpan, 1963). Figure 36 shows how this crossed
inhibition can canccl out the effects of ipsilateral VIIIth nerve excitation
and how these mutually opposed VIIIth nerve influences can be balanced
experimentally to control the firing of the Mauthner cell.
Is it possible, again considering the asymmetrical situations describcd
above, that this inhibition could be responsiblc for producing the necessary time separation between the firing of the two Mauthner cells?
By the same reasoning as was used above for the excitatory VIIIth nerve
activity, the inhibitory volley derived from the ipsilateral vestibular
apparatus would be expected to be slightly greater, and initiated slightly
earlier, than that from the contralateral. That is, when both vestibular