Synaptic strength can be modulated by parallel mechanisms that affect the probability of transmitter release and that may thus provide a basis for memory.

released from specialized presynaptic structures synaptic terminals (Fig. l), and interact with receptors on postsynaptic neurons. Clas sical studies by Katz 111 showed that neurotransmitter release is preceded by an essential influx of Caz + , and that neurotransmitter molecules are released in packets, or quanta. With the advent of electron microscopy, which revealed the existence of small vesicles in synaptic ter- minals, the notion that one synaptic vesicle could be the morphological equivalent of one quantum became widely accepted. Chemical synapses are now known to be dynamic struc- tures subject to a variety of controls. As memory ap- pears to be at least pay more recent results imply that the quantaf release process itself can be directly modulated. When an action potential invades the presynaptic term- naf it causes a Ca2+ influx through specific Ca2+ char- nels in the presynaptic membrane. As Ca*+ is