W.S. McCulloch


Conduction in the reflex arc is said to be irreversible; but, inasmuch as no nerve cell has yet been found to have but a single synapse upon it, nor has a nervous impulse ever been shown to be effective across a single synapse in either direction, there is neither logical nor empirical justification for inferring any irreciprocity of the individual synapse.

There is good evidence that a synapse is, as Keith Lucas suggested, a region of decrement which, in the cases so far reported, reduces the impulse to subthreshold value and thus, for transmission, requires summation. Lorente de Nó’s recent measurements of the period of latent addition at the synapse have shown it to be of the same order of magnitude as the refractory phase. Hence, temporal summation at a single synapse is extremely improbablewhereas spatial summation from neighboring synapses is possible anatomically and known to occur physiologically.

Because subthreshold stimulation does not set up a propagated disturbance, spatial summation can only occur from synapses close together on the cell receiving the impulses; and, because the separation of synapses, measured along that axon whose endings they are, is great in comparison with the separation of synapses, measured along the surface of that cell body and its dendrites, to which they are applied, spatial summation will occur mainly in one direction onlyand this will obtain though many synapses upon a single cell be terminations of a single axon.

If, then, for any reason, summation is required for transneuronal conduction and temporal summation at the individual synapse does not occur while spatial summation occurs from many axonal terminations upon one cell but not in the reverse direction, transneuronal conduction can only occur in the direction in which it does occur in the reflex arc.

In brief, even though there be no irreciprocal property of the individual synapse, there may still be irreciprocity of transneuronal conduction.

Three characteristics of such conduction have in times past been commonly explained by attributing special properties to synapses. Lorente de Nó has demonstrated that the prolonged period of temporal summation in such conduction depends upon delay paths (Forbes) and reverberating chains of internuncial neurons and not on any protraction of the period of latent addition at the synapse itself. Gasser has already accounted for the inhibition in reciprocal innervation in terms of threshold changes in necessary internuncial neurons, instead of in terms of inhibitory synapses. Thus the explanation here offered for the irreversibility of conduction in the reflex arc, without the assumption of irreciprocity of the synapse itself, renders it now unnecessary to attribute to any individual synapse any property except that of a region of decrement.



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Keywords: Synapse, Conduction, Cell, Impulse, Arc, Memory, Summation, Body, Conn

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1 Reprinted from Science, January 21, 1938, Vol. 87, No. 2247, pp. 65-66.
2 From the Laboratory of Neurophysiology of the Yale University School of Medicine, New Haven, Conn.