A physiological mechanism for Hebb's postulate of learning.

Hebb's postulate of learning envisages that activation or inactivation of extant synaptic contacts in plastic neural networks depends on the synchronous impulse activity of pre- and postsynaptic nerve cells. The physiological mechanism proposed here for this process posits that at synapses acting according to Hebb's postulate, the receptors for the neurotransmitter are eliminated from the postsynaptic membrane by the transient reversals of the sign of membrane polarization that occur during action potential impulses in the postsynaptic cell. But, since the release of neurotransmitter drives the membrane potential of the synaptic zone towards a level about half-way between the negative-inside resting potential and the positive-inside action potential, it would follow that the membrane patches surrounding the receptors of a synapse whose activity has contributed to setting off the postsynaptic impulse would be spared the full extent of the noxious polarity reversal. This mechanism can account for a neurophysiologically documented example of the operation of Hebb's postulate, namely the plasticity of the connections between fourth- and fifth-order neurons in the visual cortex of cats.