286
J. DIAMOND
the usual trunk and tail responses, and their inhibition, even when the
brain is destroyed.
It may be assumed that the crossing inhibition, like the ipsilateral
excitation, is also organized at the segmental level.
B. The Spinal Responses
1. EXCITATION
At any level in the spinal cord the earliest response to excitation of
the Mauthner neuron is of course that in the Mauthner axon itself. The
cells which fire subsequently can be usefully divided into two main
groups (Fig. 13) which are distinguished by the latency of the onset
of activity in the individual cells. In the first group (group A) this
latency, measured from the steepest-rising portion of the Mauthner axon
spike in the same spinal segment, is usually only about 0.5-0.6 msec (at
12°-150C). Because of this extremely brief latency and for other reasons
which will become clear later, these cells may be confidently regarded
as monosynaptically activated by the Mauthner axon collaterals. However, in the second group of cells (group B), the response latency is
more variable and always longer, usually by at least 0.4 msec (Figs. 13
and 17). Even for the earliest activated cells in the B group therefore,
there is sufficient time for an interneuron to be involved in the reflex
pathway. Clearly, in such a fast and apparently direct reflex as that
with which we are dealing here, the group A cells are the more interesting ones and those which might be expected to provide the answers to
the questions posed above.
2. THE MONOSYKAPTICALLY ACTIVATED GROUP A CELLS
There are two sorts of units (this, as will be seen, is a more appropriate terminology than “cells”) in this group, which we can call A1
and A2 at this stage, and their relation to each other is of great interest.
The electrical response of an orthodromically excited (i.e., excited via
the Mauthner axon) A 1 unit shows typically two phases of activity; the
first, i.e., the earliest, appears to be an excitatory postsynaptic potential
(e.p.s.p.), which then fires one (occasionally two) all-or-none spikes
(Figs. 13-16). Group A2 records, however, show three (or even four)
distinct components of activity, and the spike proper arises not from
the first of these but always from the second; the first component can
usually be seen unequivocally only at high amplification (Figs. 14-16).
There is a possible explanation of this puzzling finding of two dis-
J. DIAMOND
the usual trunk and tail responses, and their inhibition, even when the
brain is destroyed.
It may be assumed that the crossing inhibition, like the ipsilateral
excitation, is also organized at the segmental level.
B. The Spinal Responses
1. EXCITATION
At any level in the spinal cord the earliest response to excitation of
the Mauthner neuron is of course that in the Mauthner axon itself. The
cells which fire subsequently can be usefully divided into two main
groups (Fig. 13) which are distinguished by the latency of the onset
of activity in the individual cells. In the first group (group A) this
latency, measured from the steepest-rising portion of the Mauthner axon
spike in the same spinal segment, is usually only about 0.5-0.6 msec (at
12°-150C). Because of this extremely brief latency and for other reasons
which will become clear later, these cells may be confidently regarded
as monosynaptically activated by the Mauthner axon collaterals. However, in the second group of cells (group B), the response latency is
more variable and always longer, usually by at least 0.4 msec (Figs. 13
and 17). Even for the earliest activated cells in the B group therefore,
there is sufficient time for an interneuron to be involved in the reflex
pathway. Clearly, in such a fast and apparently direct reflex as that
with which we are dealing here, the group A cells are the more interesting ones and those which might be expected to provide the answers to
the questions posed above.
2. THE MONOSYKAPTICALLY ACTIVATED GROUP A CELLS
There are two sorts of units (this, as will be seen, is a more appropriate terminology than “cells”) in this group, which we can call A1
and A2 at this stage, and their relation to each other is of great interest.
The electrical response of an orthodromically excited (i.e., excited via
the Mauthner axon) A 1 unit shows typically two phases of activity; the
first, i.e., the earliest, appears to be an excitatory postsynaptic potential
(e.p.s.p.), which then fires one (occasionally two) all-or-none spikes
(Figs. 13-16). Group A2 records, however, show three (or even four)
distinct components of activity, and the spike proper arises not from
the first of these but always from the second; the first component can
usually be seen unequivocally only at high amplification (Figs. 14-16).
There is a possible explanation of this puzzling finding of two dis-