DETERMINATION OF NEURAL CONNECTIONS
201
plain the curious gill reflex from the eye in animals with two trigeminal
ganglia and the corneal reflex from the gill region in animals with two
vagus ganglia. One possibility is that the grafted ganglia impart their
specificity to the central cells on which they terminate, as was proposed
by Sperry (1955) and Weiss (1961). Thus, the vagus motor cells could
take up an "abducens character" under the influence of the grafted
trigeminal ganglion, and they could make connections also with the
normal trigeminal fibers; similarly, the abducens nucleus is "respecified"
by the grafted vagus ganglion. On the other hand, these two motor
nuclei innervate gill and eye muscles, and muscles are also assumed to
influence the specificity of the innervating motoneurons. By following
this train of thought, it would be easy to develop the concept to the
point of absurdity. So the question emerges, "Were these ganglia functionally specified at all or was there some peculiar nervous mechanism
that could simulate the effect of specific synaptic linkages?"
In discussing this problem one should bear in mind that there are only
these two reflexes in the head region accessible for behavioral test in the
newt, and only one of them can be investigated at the one time: the
gill reflex during the larval period, the corneal reflex after metamorphosis.
This means that some other possible connections of the heterotopic
ganglia do not become apparent in the form of a reflex, and any sufficiently strong afferent inflow may evoke one of these reflexes. Although
nothing is known of the physiology of these reflexes in the newt, there
are reasons for assuming that a quantitative relation of afferent impulses
may be instrumental in their establishment. Not every stimulus elicited
the reflexes ; repeated strokes proved to be most effective, whereas simple
touch seldom produced a response. Great individual variety was found
in the intensity of the responses; even the same individual showed different behavior from one day to the next. From the electrophysiological
analysis of the corneal reflex in the frog, Kornacker (1963) described a
monosynaptic reflex arc in which afferent impulses were conveyed by the
finest fibers of the trigeminal, and impulse transmission took place at the
distal end of dendrites of the abducens motoneurons. It is obvious that
such unfavorable synaptic connections require a large number of active
afferent fibers to evoke a reflex; also, the activity of motoneurons largely
depends on their functional state being influenced from many other
sources. If the situation is the same in newts, then it is possible that
nociceptive sensory impulses from the head, if they are strong enough,
may irradiate and discharge both the abducens and vagus nuclei, but
evoke only that reflex which is present at the time of observation. This
supports the possibility that there is a quantitative aspect to the establishment of these reflexes, and, if we assume that the functional specificity
201
plain the curious gill reflex from the eye in animals with two trigeminal
ganglia and the corneal reflex from the gill region in animals with two
vagus ganglia. One possibility is that the grafted ganglia impart their
specificity to the central cells on which they terminate, as was proposed
by Sperry (1955) and Weiss (1961). Thus, the vagus motor cells could
take up an "abducens character" under the influence of the grafted
trigeminal ganglion, and they could make connections also with the
normal trigeminal fibers; similarly, the abducens nucleus is "respecified"
by the grafted vagus ganglion. On the other hand, these two motor
nuclei innervate gill and eye muscles, and muscles are also assumed to
influence the specificity of the innervating motoneurons. By following
this train of thought, it would be easy to develop the concept to the
point of absurdity. So the question emerges, "Were these ganglia functionally specified at all or was there some peculiar nervous mechanism
that could simulate the effect of specific synaptic linkages?"
In discussing this problem one should bear in mind that there are only
these two reflexes in the head region accessible for behavioral test in the
newt, and only one of them can be investigated at the one time: the
gill reflex during the larval period, the corneal reflex after metamorphosis.
This means that some other possible connections of the heterotopic
ganglia do not become apparent in the form of a reflex, and any sufficiently strong afferent inflow may evoke one of these reflexes. Although
nothing is known of the physiology of these reflexes in the newt, there
are reasons for assuming that a quantitative relation of afferent impulses
may be instrumental in their establishment. Not every stimulus elicited
the reflexes ; repeated strokes proved to be most effective, whereas simple
touch seldom produced a response. Great individual variety was found
in the intensity of the responses; even the same individual showed different behavior from one day to the next. From the electrophysiological
analysis of the corneal reflex in the frog, Kornacker (1963) described a
monosynaptic reflex arc in which afferent impulses were conveyed by the
finest fibers of the trigeminal, and impulse transmission took place at the
distal end of dendrites of the abducens motoneurons. It is obvious that
such unfavorable synaptic connections require a large number of active
afferent fibers to evoke a reflex; also, the activity of motoneurons largely
depends on their functional state being influenced from many other
sources. If the situation is the same in newts, then it is possible that
nociceptive sensory impulses from the head, if they are strong enough,
may irradiate and discharge both the abducens and vagus nuclei, but
evoke only that reflex which is present at the time of observation. This
supports the possibility that there is a quantitative aspect to the establishment of these reflexes, and, if we assume that the functional specificity