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Conditioning of Neurons

In the 1960s and 1970s scientists found they could carry out classical conditioning with neurons surgically removed from simple animals such as sea slugs. The clear implication is that classical conditioning is a very basic form of learning not even requiring a complete nervous system or a brain. That helps to explain why classical conditioning can be involved in all sorts of unconscious biological processes.

How was classical conditioning demonstrated in a single neuron?

Toledo-Morrell and colleagues (1979) did one such experiment. In their case, the signal (conditional stimulus) was activation of an incoming synapse, not strong enough by itself to make the cell fire a nerve impulse. The researchers simply followed the signal with an input that was strong enough to trigger a nerve impulse. After repeated pairings, activation of the incoming synapse was enough to trigger a nerve impulse by itself.

What changes were observed at synapses involved in conditioning?

Similarly, Farley and colleagues (1983) showed classical conditioning in a single photoreceptor cell of a mollusk. Kelso and Brown (1986) showed classical conditioning of individual neurons called pyramidal cells from the rat hippocampus. Other researchers showed that classical conditioning produces noticeable changes at the junction between neurons, the synapse. A synapse involved in successful conditioning often showed a thickening on the presynaptic side (before the synapse). Eric Kandel of Columbia University won a Nobel Prize in 2000 (shared with Arvid Carlsson and Paul Greengard) for his decades-long research program on this and related topics. The main laboratory task Kandel used to study neural changes induced by learning was classical conditioning of the sea slug or Aplysia.

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