DETERMINATION OF NEURAL CONNECTIONS
209
remain in or return to it. The dendrites of the deeper lying, large, secondary sensory neurons are embedded in the dense matrix of a longitudinal axon system of the substantia gelatinosa, just as the Purkinje cell
dendrites are embedded in the matrix of parallel fibers. The physiological
significance of this structure may be the modification of the conditions
for impulse transmission to the secondary sensory neurons of the dorsal
horn. Electrophysiological studies made by Wall (1962) support this
postulated function of the substantia gelatinosa. Speaking in an unsophisticated manner, the substantia gelatinosa may be the first "decision
relay" for determining which, and how many, of the secondary sensory
neurons are going to discharge to a particular afferent impulse pattern.
Since a substantia gelatinosa-like structure is present in the spinal cord
of lower vertebrates as well, the existence of a complicated system for the
analysis of impulse patterns seems to be established. In their interesting
paper, Melzack and Wall (1962) have put forward arguments about the
possibility of such a nervous mechanism. The information generated by
receptors are coded in the form of spatial and temporal patterns of
nerve impulses. On the basis of different reactivity of various neurons,
synapses, and fibers to different stimuli, these patterns become analyzed
and transmitted toward different loci of the central nervous system. A
detailed description of their paper is not within the scope of the present
discussion.
As an alternative to the concept of neuronal specificity, it is suggested
that the grafted limb may have altered the quality and quantity of the
inflowing impulses, and thereby established an impulse pattern characteristic to the graft. The central nervous system, in turn, is capable
of analyzing the impulse patterns, and this analysis becomes apparent
in the activity of the effector center which is most sensitive to that
particular pattern. The same may hold true for the establishment of the
corneal reflex from an extra eye or from a regeneration blastema. It will
be necessary to employ electrophysiological techniques to obtain information about the nature of this neural mechanism.
IV. Conclusion
From the foregoing considerations it appears that the interpretation
of these experiments is rather ambiguous. Only in the visual system does
it seem to be true that the best possible explanation for the observed
phenomena lies in the assumption of selective neuronal connection; even
here it is questionable that this selectivity can act at the level of the
individual neuron. In other cases, the results can more easily be interpreted by abandoning this assumption, and some results (e.g., the corneal
209
remain in or return to it. The dendrites of the deeper lying, large, secondary sensory neurons are embedded in the dense matrix of a longitudinal axon system of the substantia gelatinosa, just as the Purkinje cell
dendrites are embedded in the matrix of parallel fibers. The physiological
significance of this structure may be the modification of the conditions
for impulse transmission to the secondary sensory neurons of the dorsal
horn. Electrophysiological studies made by Wall (1962) support this
postulated function of the substantia gelatinosa. Speaking in an unsophisticated manner, the substantia gelatinosa may be the first "decision
relay" for determining which, and how many, of the secondary sensory
neurons are going to discharge to a particular afferent impulse pattern.
Since a substantia gelatinosa-like structure is present in the spinal cord
of lower vertebrates as well, the existence of a complicated system for the
analysis of impulse patterns seems to be established. In their interesting
paper, Melzack and Wall (1962) have put forward arguments about the
possibility of such a nervous mechanism. The information generated by
receptors are coded in the form of spatial and temporal patterns of
nerve impulses. On the basis of different reactivity of various neurons,
synapses, and fibers to different stimuli, these patterns become analyzed
and transmitted toward different loci of the central nervous system. A
detailed description of their paper is not within the scope of the present
discussion.
As an alternative to the concept of neuronal specificity, it is suggested
that the grafted limb may have altered the quality and quantity of the
inflowing impulses, and thereby established an impulse pattern characteristic to the graft. The central nervous system, in turn, is capable
of analyzing the impulse patterns, and this analysis becomes apparent
in the activity of the effector center which is most sensitive to that
particular pattern. The same may hold true for the establishment of the
corneal reflex from an extra eye or from a regeneration blastema. It will
be necessary to employ electrophysiological techniques to obtain information about the nature of this neural mechanism.
IV. Conclusion
From the foregoing considerations it appears that the interpretation
of these experiments is rather ambiguous. Only in the visual system does
it seem to be true that the best possible explanation for the observed
phenomena lies in the assumption of selective neuronal connection; even
here it is questionable that this selectivity can act at the level of the
individual neuron. In other cases, the results can more easily be interpreted by abandoning this assumption, and some results (e.g., the corneal