302
J. DIAMOND
the dorsal collaterals of the Mauthner axon and the ventral dendrites
of the primary motoneurons (Fig. 24). The dorsal collateral, though
often short and straight, can be relatively tortuous and long, and
occasionally it forms two branches, each supplying a primary motoneuron. As we have seen, the A1 unit appears to be chemically excited
from the Mauthner axon and therefore cannot be the dorsal collateral
itself. But we ourselves have never observed any cells (or fibers) connecting the Mauthner axon, or its collaterals, to the primary motoneuron,
nor do the descriptions of any of the other workers mentioned above
include any suggestion of such intermediary cells. Nevertheless, physiologically the A1 units appear to function in some intermediary capacity.
One solution to this intriguing situation would be that the chemically
transmitting synapses of an ipsilateral Mauthner axon collateral are
located on a branch of the primary motoneuron. This branch (the A1
unit) would need to be connected to the rest of the primary motoneuron
(the A2 unit) by a transitional region of relatively high resistance. Our
own light microscopy has occasionally suggested this possibility of a
dendritic branch or protuberance in the junctional region (Figs. 23b
and 25). Tiegs, in a most interesting paper (1931), described a “transitional tissue,” a morphologically curious intervening region between the
Mauthner axon collateral and the ventral dendrite of the motoneuron.
The electron microscope, as we shall see, has been extremely helpful
on this question.
3. THE GROUP B MOTONEURONS
In approximately the same region (though covering a larger area)
as that of the primary motoneurons, there are other fairly large cells
which send their axons directly toward the ventral root without passing
especially close to the Mauthner axon. Such motoneurons (though not
their axons) are seen in Fig. 3. The distribution of the group B cells
of the kind which gave records like those of Figs. 13 and 17a corresponded to that of these other motoneurons, at least some of which,
therefore, may be presumed to give rise to the later firing components
of the all-or-nothing response to Mauthner cell excitation (see Section
E below).
C. Evidence from Electron Microscopy
Figures 26 and 27 summarize the evidence from both electron and
light microscopy (Diamond and Yasargil, 1969; Diamond et al., 1969,
1970; Gray, 1969).
J. DIAMOND
the dorsal collaterals of the Mauthner axon and the ventral dendrites
of the primary motoneurons (Fig. 24). The dorsal collateral, though
often short and straight, can be relatively tortuous and long, and
occasionally it forms two branches, each supplying a primary motoneuron. As we have seen, the A1 unit appears to be chemically excited
from the Mauthner axon and therefore cannot be the dorsal collateral
itself. But we ourselves have never observed any cells (or fibers) connecting the Mauthner axon, or its collaterals, to the primary motoneuron,
nor do the descriptions of any of the other workers mentioned above
include any suggestion of such intermediary cells. Nevertheless, physiologically the A1 units appear to function in some intermediary capacity.
One solution to this intriguing situation would be that the chemically
transmitting synapses of an ipsilateral Mauthner axon collateral are
located on a branch of the primary motoneuron. This branch (the A1
unit) would need to be connected to the rest of the primary motoneuron
(the A2 unit) by a transitional region of relatively high resistance. Our
own light microscopy has occasionally suggested this possibility of a
dendritic branch or protuberance in the junctional region (Figs. 23b
and 25). Tiegs, in a most interesting paper (1931), described a “transitional tissue,” a morphologically curious intervening region between the
Mauthner axon collateral and the ventral dendrite of the motoneuron.
The electron microscope, as we shall see, has been extremely helpful
on this question.
3. THE GROUP B MOTONEURONS
In approximately the same region (though covering a larger area)
as that of the primary motoneurons, there are other fairly large cells
which send their axons directly toward the ventral root without passing
especially close to the Mauthner axon. Such motoneurons (though not
their axons) are seen in Fig. 3. The distribution of the group B cells
of the kind which gave records like those of Figs. 13 and 17a corresponded to that of these other motoneurons, at least some of which,
therefore, may be presumed to give rise to the later firing components
of the all-or-nothing response to Mauthner cell excitation (see Section
E below).
C. Evidence from Electron Microscopy
Figures 26 and 27 summarize the evidence from both electron and
light microscopy (Diamond and Yasargil, 1969; Diamond et al., 1969,
1970; Gray, 1969).