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GEORGE SZÉKELY
pulses may reach the center from the graft than from the surroundings.
However, a mere increase of sensory impulses is not enough to elicit
a corneal reflex, since a response never could be evoked from an intact
limb graft even applying very strong stimuli. The presence of cornealtype endings, apparently, is also necessary, which may result in a qualitative difference between the afferent impulses arising from the graft and
from its surroundings. Thus a certain type of sensory inflow may develop,
and this can discharge the abducens nucleus without the establishment
of specific reflex connections. The possibility of such a neural mechanism
will be discussed later.
An interesting suggestion was made by Kornacker (1963) from his
electrophysiological study of the corneal reflex. As mentioned before,
following electrical stimulation of the cornea, he recorded postsynaptic
potentials in the lateral part of the medulla at the level of the abducens
nucleus. From analysis of the records he concluded that impulses carried by the finest fibers of the trigeminal were monosynaptically transmitted to the distal end of the abducens dendrites. Similar potentials
were recorded following two stimuli applied within short intervals to
the snout nerve. A temporal summation was thus essential for the response to stimulation of the snout nerve which contained fewer fine
fibers than the ciliary nerve. He thought that the activity of a sufficient
number of fine fibers was essential to evoke a corneal reflex and assumed
that the diameter of fibers innervating an extra cornea or a regeneration
blastema, because of the poor blood supply of the innervated periphery,
became reduced and resulted, therefore, in a corneal reflex. The finding of J. E. Brown (personal communication, 1963) that a corneal
reflex could be elicited from the freshly cut surface of a limb grafted
into the head region of Ambystoma
is not in favor of the assumption
which presupposes the elapse of a time interval between the cut and the
appearance of the reflex. At the same time, it excludes the possibility
that the establishment of the reflex from eye or blastema grafts is the
result of breaking down the old and building up new synaptic connections
under the influence of the periphery. Further studies are required to clear
up this problem.
B. Limb Specificity
The selective establishment of central connections of sensory fibers
was also demonstrated by limb-specific reflexes from extra limbs (Miner,
1951). A hindlimb was grafted into the trunk region of tadpoles, and this
limb became innervated by thoracic nerves that otherwise would have
connected with trunk skin. After metamorphosis stimulation of the transplanted limb elicited reflex responses in the ipsilateral hindlimb similar
GEORGE SZÉKELY
pulses may reach the center from the graft than from the surroundings.
However, a mere increase of sensory impulses is not enough to elicit
a corneal reflex, since a response never could be evoked from an intact
limb graft even applying very strong stimuli. The presence of cornealtype endings, apparently, is also necessary, which may result in a qualitative difference between the afferent impulses arising from the graft and
from its surroundings. Thus a certain type of sensory inflow may develop,
and this can discharge the abducens nucleus without the establishment
of specific reflex connections. The possibility of such a neural mechanism
will be discussed later.
An interesting suggestion was made by Kornacker (1963) from his
electrophysiological study of the corneal reflex. As mentioned before,
following electrical stimulation of the cornea, he recorded postsynaptic
potentials in the lateral part of the medulla at the level of the abducens
nucleus. From analysis of the records he concluded that impulses carried by the finest fibers of the trigeminal were monosynaptically transmitted to the distal end of the abducens dendrites. Similar potentials
were recorded following two stimuli applied within short intervals to
the snout nerve. A temporal summation was thus essential for the response to stimulation of the snout nerve which contained fewer fine
fibers than the ciliary nerve. He thought that the activity of a sufficient
number of fine fibers was essential to evoke a corneal reflex and assumed
that the diameter of fibers innervating an extra cornea or a regeneration
blastema, because of the poor blood supply of the innervated periphery,
became reduced and resulted, therefore, in a corneal reflex. The finding of J. E. Brown (personal communication, 1963) that a corneal
reflex could be elicited from the freshly cut surface of a limb grafted
into the head region of Ambystoma
is not in favor of the assumption
which presupposes the elapse of a time interval between the cut and the
appearance of the reflex. At the same time, it excludes the possibility
that the establishment of the reflex from eye or blastema grafts is the
result of breaking down the old and building up new synaptic connections
under the influence of the periphery. Further studies are required to clear
up this problem.
B. Limb Specificity
The selective establishment of central connections of sensory fibers
was also demonstrated by limb-specific reflexes from extra limbs (Miner,
1951). A hindlimb was grafted into the trunk region of tadpoles, and this
limb became innervated by thoracic nerves that otherwise would have
connected with trunk skin. After metamorphosis stimulation of the transplanted limb elicited reflex responses in the ipsilateral hindlimb similar