DETERMINATION OF NEURAL CONNECTIONS
195
a lot of poorly arborizing optic terminals in the newt which extend
over about one-quarter of the area reported in the frog. These endings
make contact with the distal ends of apical dendrites of cells lying in the
stratum granulosum. The fine detail of these synapses are unknown.
Although the early state of our histological investigation does not
allow a more elaborate description of the structure of the optic tectum,
nevertheless this quick glance at the histological picture suggests that
such a complicated structure cannot be regarded as a simple reflex relay.
The wide arborization of the optic terminals leads us to assume that
each afferent fiber makes connection with at least a few hundred tectal
cells, and this figure may very well be larger because of the arborization
of the apical dendrites. Maturana (1959) has found about twice as many
optic fibers as cells in the tectum of frogs, and this finding indicates at
least twice as great a degree of convergence of fibers upon tectal elements
than the above divergence deduced from the size of the terminals. Taking
into account the immense wealth of intrinsic interconnections of the
tectal neurons, the difficulty of trying to organize this system merely
on the basis of selective interneuronal affinity is immediately apparent.
We have not the slightest idea of the probable functional significance of
this peculiar structure. It suggests, however, that the optic tectum is a
highly complex system within which the pathway of impulses cannot be
given by the usual blueprints of nerve centers, and the various physiological qualities of afferent impulses may play just as instrumental a
role in determining the output pattern of the system as the selective
connections of the participating elements.
The other important reason for collecting more precise data about the
organization of the optic tectum is to enable us to determine the origin
of evoked potentials set up by stimulating the retina. Grafstein and
Burgen (1964) and Gaze and Jacobson (1963) did not specify the tectal
structure giving rise to the recorded potentials, but Lettvin et al. (1959)
and Maturana et al. (1960) have given reasonable arguments that they
recorded from the bushy terminal arborization of optic fibers. Their
arguments are very convincing in the case of a normal frog; it is, however, doubtful whether or not such elaborated terminals were present in
the case of an early stage of regeneration of optic fibers, when Gaze and
Jacobson recorded random responses from the surface of the tectum.
Since the dendrites of some neurons have been reported to be capable of
producing spike responses similar to presynaptic spikes (Cragg and
Hamlyn, 1955; Andersen, 1960; Spencer and Kandel, 1961; Wall, personal
communication, 1964), the possibility that spike responses from the apical
dendrites of tectal neurons might contribute to the establishment of an
electrophysiological mapping pattern should not be forgotten.
195
a lot of poorly arborizing optic terminals in the newt which extend
over about one-quarter of the area reported in the frog. These endings
make contact with the distal ends of apical dendrites of cells lying in the
stratum granulosum. The fine detail of these synapses are unknown.
Although the early state of our histological investigation does not
allow a more elaborate description of the structure of the optic tectum,
nevertheless this quick glance at the histological picture suggests that
such a complicated structure cannot be regarded as a simple reflex relay.
The wide arborization of the optic terminals leads us to assume that
each afferent fiber makes connection with at least a few hundred tectal
cells, and this figure may very well be larger because of the arborization
of the apical dendrites. Maturana (1959) has found about twice as many
optic fibers as cells in the tectum of frogs, and this finding indicates at
least twice as great a degree of convergence of fibers upon tectal elements
than the above divergence deduced from the size of the terminals. Taking
into account the immense wealth of intrinsic interconnections of the
tectal neurons, the difficulty of trying to organize this system merely
on the basis of selective interneuronal affinity is immediately apparent.
We have not the slightest idea of the probable functional significance of
this peculiar structure. It suggests, however, that the optic tectum is a
highly complex system within which the pathway of impulses cannot be
given by the usual blueprints of nerve centers, and the various physiological qualities of afferent impulses may play just as instrumental a
role in determining the output pattern of the system as the selective
connections of the participating elements.
The other important reason for collecting more precise data about the
organization of the optic tectum is to enable us to determine the origin
of evoked potentials set up by stimulating the retina. Grafstein and
Burgen (1964) and Gaze and Jacobson (1963) did not specify the tectal
structure giving rise to the recorded potentials, but Lettvin et al. (1959)
and Maturana et al. (1960) have given reasonable arguments that they
recorded from the bushy terminal arborization of optic fibers. Their
arguments are very convincing in the case of a normal frog; it is, however, doubtful whether or not such elaborated terminals were present in
the case of an early stage of regeneration of optic fibers, when Gaze and
Jacobson recorded random responses from the surface of the tectum.
Since the dendrites of some neurons have been reported to be capable of
producing spike responses similar to presynaptic spikes (Cragg and
Hamlyn, 1955; Andersen, 1960; Spencer and Kandel, 1961; Wall, personal
communication, 1964), the possibility that spike responses from the apical
dendrites of tectal neurons might contribute to the establishment of an
electrophysiological mapping pattern should not be forgotten.