310
J. DIAMOND
It is temptins to relate the excitation of an impulse in the cell body
of the primary motoneuron to the excitation of group B motoneurons.
The effects of the progressively delayed crossed inhibition are consistent
with this (cf. Fig. 28), and so are those of fatigue (cf. Fig. 42). Limitation of the respcinse to the ventral dendrite can be more confidently correlated with the excitation of the first-activated motoneuron group, which
ought of course to be the primary motoneurons.
There is, as yet, no direct evidence to support this suggestion of
an excitatory cascade, in which the primary motoneuron also acts in
the capacity of an interneuron for excitation of other motoneurons (or
interneurons ) . We may note, however, the electron microscopic evidence
of the presence of tight junctions between the primary motoneuron
ventral dendrite and other processes. It is possible that here is a basis
for the transmission of excitation from the primary motoneuron to other
cells, perhaps leading to the crossed excitation. (Since the crossed inhibition occurs when neither A1 nor A2 units is excited, as happens with
synchronous Mauthner axon impulses, it must require quite distinct
collateral branches of its own.) We may note here that the motoneurons
activated by excitation of the opposite Mauthner axon (the crossed
excitation) are quite distinct from those involved in the ipsilateral
excitation. This follows from the experiment of Fig. 20, in which block
of the crossed inhibition by strychnine allowed the algebraic summation of the two excitatory responses in the ventral root; there was no
“occlusion” as would have occurred if the same motoneurons had been
excited as a consequence of the ipsilateral and contralateral Mauthner
axon impulses.
VII. THE PRECISION AND CONSTANCY OF THE
MINIMUM DISCRIMINATION TIME
A. The Function of the A1 Unit
We are now in a position to suggest a likely function for the A1 unit
and indeed to propose a basis upon which the most important function
of the Mauthner cell system probably depcnds. The proposition is that
the Mauthner axon collaterals act on a special postsynaptic region (the
A1 unit) uhich is reserved exclusively for them, and this is the principal
reason for their having the property of remaining uniformly excitable
at all times. Becausc of this property, the effects of the impulses
arriving along the ipsilateral and contralateral Mauthner collaterals
J. DIAMOND
It is temptins to relate the excitation of an impulse in the cell body
of the primary motoneuron to the excitation of group B motoneurons.
The effects of the progressively delayed crossed inhibition are consistent
with this (cf. Fig. 28), and so are those of fatigue (cf. Fig. 42). Limitation of the respcinse to the ventral dendrite can be more confidently correlated with the excitation of the first-activated motoneuron group, which
ought of course to be the primary motoneurons.
There is, as yet, no direct evidence to support this suggestion of
an excitatory cascade, in which the primary motoneuron also acts in
the capacity of an interneuron for excitation of other motoneurons (or
interneurons ) . We may note, however, the electron microscopic evidence
of the presence of tight junctions between the primary motoneuron
ventral dendrite and other processes. It is possible that here is a basis
for the transmission of excitation from the primary motoneuron to other
cells, perhaps leading to the crossed excitation. (Since the crossed inhibition occurs when neither A1 nor A2 units is excited, as happens with
synchronous Mauthner axon impulses, it must require quite distinct
collateral branches of its own.) We may note here that the motoneurons
activated by excitation of the opposite Mauthner axon (the crossed
excitation) are quite distinct from those involved in the ipsilateral
excitation. This follows from the experiment of Fig. 20, in which block
of the crossed inhibition by strychnine allowed the algebraic summation of the two excitatory responses in the ventral root; there was no
“occlusion” as would have occurred if the same motoneurons had been
excited as a consequence of the ipsilateral and contralateral Mauthner
axon impulses.
VII. THE PRECISION AND CONSTANCY OF THE
MINIMUM DISCRIMINATION TIME
A. The Function of the A1 Unit
We are now in a position to suggest a likely function for the A1 unit
and indeed to propose a basis upon which the most important function
of the Mauthner cell system probably depcnds. The proposition is that
the Mauthner axon collaterals act on a special postsynaptic region (the
A1 unit) uhich is reserved exclusively for them, and this is the principal
reason for their having the property of remaining uniformly excitable
at all times. Becausc of this property, the effects of the impulses
arriving along the ipsilateral and contralateral Mauthner collaterals