Heterosynaptic facilitation in neurones of the abdominal ganglion of Aplysia depilans.

The cellular mechanisms whereby a neural network undergoes a longlasting change in its response to a given stimulus, when the latter is repeated or combined with other stimuli, are still mainly unexplored. An important beginning has been the investigation of the effects of physiological use and disuse and of tetanic nerve stimulation on the monosynaptic spinal reflex (Eccles, 1958, 1964). This work has shown that limited potentialities for synaptic plasticity can be demonstrated, even in this simple system, with repeated homosynaptic stimulation. The plastic changes so produced manifest themselves as an alteration in the amplitude of the excitatory post-synaptic potential. The neurophysiological application of complex intermittent and heterosynaptic stimulus patterns, modelled on those used in psychological learning experiments, have usually been limited to experiments in which behavioural conditioning was established in the whole animal. While some important electrical correlates and neural determinants of the conditioning process have been described in these studies, it seems unlikely that the electro-encephalographic methods employed will yield much information about the mechanisms underlying the observed electrical changes (see reviews by Morrell, 1961; Doty & Guirgea, 1961). A more detailed analysis of the long-term neural consequences of these complex stimulus sequences would seem to require the use of analytic techniques capable of distinguishing between different cellular mechanisms as well as the selection of simpler preparations than those currently used in conditioning the whole animal (Eccles, 1958; Spencer, Thompson & Neilson, 1964). This paper and the one that follows (Kandel & Tauc, 1965) describe an attempt to apply a heterosynaptic stimulus sequence, analogous to that used in psychological learning experiments, to a numerically simple and