300
J. DIAMOND
and that our findings are in general accord with those of Haller (1892),
Kolster ( 1898), Tagliani ( 1905), Beccari ( 1907), Tiegs ( 1931), Bodian
(1952), Leghissa (1956), and Barets (1961). The short dorsal collaterals
of the Mauthner axons run toward, and sometimes seem to join, the
long ventral processes of certain large motoneurons, which we shall
call the “primary” ones, in very close relation to the myelin sheath of
the Mauthner axon (although this is obscured by the shrinkage and lack
of myelin staining in these particular preparations ) . The cell bodies of
these motoneurons, which usually have one or two large lateral dendrites,
lie approximately in a column some 1W300 p dorsolateral to the Mauthner axon, up to 300p apart. The dorsal collaterals of the Mauthner
axon are associated with these primary motoneurons in a one-to-one
relationship in goldfish and tench.
The ventral extension of the primary motoneuron sweeps downward
to come into close apposition to the medial, or occasionally the lateral,
aspect of the Mauthner axon near where the dorsal collateral occurs,
and this feature largely defines this particular group of motoneurons.
The ventral process gives rise to the efferent motor axon, and it has
itself been variously described as axon, initial segment, and, by Tagliani
and by Tiegs, a dendrite. The latter interpretation is strongly supported
by its electron-microscopic appearance, and by the numerous nerve
endings which make synapses on it; we shall call it the ventral dendrite
of the primary motoneuron. It is not uncommon for an axon to arise
from a dendrite in fish (Nieuwenhuys, 1964). Occasionally a primary
motoneuron, its ventral dendrite, and the Mauthner axon collateral
running to it can be seen in a single l o p section, as in Figs. 3 and 23.
The motoneuron axon proper can be followed from the ventral extremity
of this dendrite, sweeping ventrolaterally to join the other axons of
the ventral root.
B. The Mauthner Collaterals and the Motoneurons
1. THE GROUP A CELLS
Figure 24 shows the distribution in goldfish and tench spinal cords
of the recording sites of a number of group A cells relative to the
Mauthner axon which was also recorded from in the same region of the
cord in each instance. All measurements were scaled to the size of cord
most commonly encountered, in which the Mauthner axons were 7 5 0 p
below the dorsal surface. The histological drawing on the left of each
diagram is based on an actual 10 p section, and the shaded areas indicate
the approximate variation we found in the position of the soma and
ventral dendrite of the primary motoneuron. It is clear that the distribu-
J. DIAMOND
and that our findings are in general accord with those of Haller (1892),
Kolster ( 1898), Tagliani ( 1905), Beccari ( 1907), Tiegs ( 1931), Bodian
(1952), Leghissa (1956), and Barets (1961). The short dorsal collaterals
of the Mauthner axons run toward, and sometimes seem to join, the
long ventral processes of certain large motoneurons, which we shall
call the “primary” ones, in very close relation to the myelin sheath of
the Mauthner axon (although this is obscured by the shrinkage and lack
of myelin staining in these particular preparations ) . The cell bodies of
these motoneurons, which usually have one or two large lateral dendrites,
lie approximately in a column some 1W300 p dorsolateral to the Mauthner axon, up to 300p apart. The dorsal collaterals of the Mauthner
axon are associated with these primary motoneurons in a one-to-one
relationship in goldfish and tench.
The ventral extension of the primary motoneuron sweeps downward
to come into close apposition to the medial, or occasionally the lateral,
aspect of the Mauthner axon near where the dorsal collateral occurs,
and this feature largely defines this particular group of motoneurons.
The ventral process gives rise to the efferent motor axon, and it has
itself been variously described as axon, initial segment, and, by Tagliani
and by Tiegs, a dendrite. The latter interpretation is strongly supported
by its electron-microscopic appearance, and by the numerous nerve
endings which make synapses on it; we shall call it the ventral dendrite
of the primary motoneuron. It is not uncommon for an axon to arise
from a dendrite in fish (Nieuwenhuys, 1964). Occasionally a primary
motoneuron, its ventral dendrite, and the Mauthner axon collateral
running to it can be seen in a single l o p section, as in Figs. 3 and 23.
The motoneuron axon proper can be followed from the ventral extremity
of this dendrite, sweeping ventrolaterally to join the other axons of
the ventral root.
B. The Mauthner Collaterals and the Motoneurons
1. THE GROUP A CELLS
Figure 24 shows the distribution in goldfish and tench spinal cords
of the recording sites of a number of group A cells relative to the
Mauthner axon which was also recorded from in the same region of the
cord in each instance. All measurements were scaled to the size of cord
most commonly encountered, in which the Mauthner axons were 7 5 0 p
below the dorsal surface. The histological drawing on the left of each
diagram is based on an actual 10 p section, and the shaded areas indicate
the approximate variation we found in the position of the soma and
ventral dendrite of the primary motoneuron. It is clear that the distribu-