306
J. DIAMOND
appear, as a consequence of shrinkage, to be broken off the main part
of what is described above as the dendritic spine of the primary motoneuron. The spine bears a series of fingers at its extremity which reach
out (as indicated in Fig. 27) toward the region of synaptic contact.
Typical membrane thickenings can sometimes be seen along the synaptic
clefts where the spine fingers invaginate the dome of the Mauthner
collateral.
D. Conclusion on the Identity of the A1
(“Connecting”) Unit
The physiological evidence, the location of the A1 recording sites,
and the evidence from both light and electron microscopy suggest that
the A1 unit is indeed a dendritic branch or spine of the primary
motoneuron, and that the neck of the spine near its origin from the
main dendrite is presumably the transitional region of high resistance
between the synaptic region of the spine and the parent (ventral)
dendrite of the motoneuron. The morphological situation is diagrammed
in Fig. 27. This is, of course, an hypothesis [which has in fact been
extended to a general hypothesis on the function of dendritic spines
in central neurons ( Diamond et nl., 1969, 1970) 1. Nonetheless, whether
the A1 unit is a dendritic spine or branch, or a very small separate intermediary or connecting cell, we believe that there is an important functional significance of the arrangement, which relates to the circuit discrimination achieved in the Mauthner reflex.
E. An Excitatory Cascade?
The primary motoneurons, which the physiological and anatomical
evidence indicates are the only ones directly supplied by the ipsilateral
(excitatory) Mauthner axon collaterals, form only a small proportion of
the total niotoneuron population (see also Barets, 19Sl). The powerful
tail flip caused by excitation of a Mauthner cell must involve a good
proportion of the muscle of the trunk and tail, but we are not sure
exactly which muscles are activated. However, it seems almost certain
that some of the tail muscle must be supplied by group B motoneurons.
(We do not know which motoneurons suppIy fin muscles.) The ventral
root response (Fig. 28) shows that there is an apparently sequential
activation of two or three groups of motoneurons; presumably the first
of these is the primary motoneuron group (comprised of only a few
cells) while the later responses result from the group B cells. What is
J. DIAMOND
appear, as a consequence of shrinkage, to be broken off the main part
of what is described above as the dendritic spine of the primary motoneuron. The spine bears a series of fingers at its extremity which reach
out (as indicated in Fig. 27) toward the region of synaptic contact.
Typical membrane thickenings can sometimes be seen along the synaptic
clefts where the spine fingers invaginate the dome of the Mauthner
collateral.
D. Conclusion on the Identity of the A1
(“Connecting”) Unit
The physiological evidence, the location of the A1 recording sites,
and the evidence from both light and electron microscopy suggest that
the A1 unit is indeed a dendritic branch or spine of the primary
motoneuron, and that the neck of the spine near its origin from the
main dendrite is presumably the transitional region of high resistance
between the synaptic region of the spine and the parent (ventral)
dendrite of the motoneuron. The morphological situation is diagrammed
in Fig. 27. This is, of course, an hypothesis [which has in fact been
extended to a general hypothesis on the function of dendritic spines
in central neurons ( Diamond et nl., 1969, 1970) 1. Nonetheless, whether
the A1 unit is a dendritic spine or branch, or a very small separate intermediary or connecting cell, we believe that there is an important functional significance of the arrangement, which relates to the circuit discrimination achieved in the Mauthner reflex.
E. An Excitatory Cascade?
The primary motoneurons, which the physiological and anatomical
evidence indicates are the only ones directly supplied by the ipsilateral
(excitatory) Mauthner axon collaterals, form only a small proportion of
the total niotoneuron population (see also Barets, 19Sl). The powerful
tail flip caused by excitation of a Mauthner cell must involve a good
proportion of the muscle of the trunk and tail, but we are not sure
exactly which muscles are activated. However, it seems almost certain
that some of the tail muscle must be supplied by group B motoneurons.
(We do not know which motoneurons suppIy fin muscles.) The ventral
root response (Fig. 28) shows that there is an apparently sequential
activation of two or three groups of motoneurons; presumably the first
of these is the primary motoneuron group (comprised of only a few
cells) while the later responses result from the group B cells. What is