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In associative learning, the activated neurones undergo a variety of concomitant functional alterations--increases or decreases of firing activity and modifications of membrane potential or resistance and of synaptic responsiveness. Synaptic transmission which can be strengthened only when there is paired activity in two pathways is of particular interest in relation to mechanisms for associative learning. For the neocortex, there are few observations of the plastic changes, induced by conditioning procedures, in the effectiveness of individual synapses. We now report that various regimes with joint stimulations of convergent excitatory pathways on to intracellularly recorded neurones in the motor cortex of the cat result in synaptic facilitation lasting for up to 30 min.

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Intracellular recordings were made from 117 neurons in the motor cortex of anesthetized cats. The pyramidal tract (PT) and VL nucleus of thalamus were stimulated in order to activate the neurons from two directions. 1. PT cells were conditioned by antidromic trains (10--50 cps for 4--15 s) and by paired PT and VL stimuli with different intervals and sequences. The VL-EPSPs were examined before and after conditioning, to find differences in efficacy in giving rise to spikes. The conditioning procedures resulted in a remarkable facilitation of VL-EPSPs, manifesting itself as a significant rise of efficacy in generating spikes, a shortening of peak latency and in some cases, an enhancement of background firing. 2. In non-PT neurons the same conditioning procedures elicited heterosynaptic facilitation and a rise in firing activity. 3. Intracellularly injected square wave pulses also resulted in facilitation of VL-EPSPs. 4. Pairings of PT and VL stimuli were more effective than trains in evoking conditioned changes. 5. Plastic modifications were observed in the 13.7% of the neurons subjected to conditioning procedures. 6. The authors assume that synchronous activity of the pre- and postsynaptic neurons is a highly important condition for plastic changes in the efficacy of synaptic transmission.