Synaptic Plasticity: Short Term

Chemical synapses are rarely static transmitters of information between neurons. Their effectiveness waxes and wanes, depending on the frequency of stimulation and the history of prior activity. At most synapses, repetitive highfrequency action potentials (often called a tetanus) initially result in a rapid growth of successive postsynaptic potential (PSP) amplitudes, called synaptic facilitation (or sometimes frequency facilitation). This process builds to a steady state within about 1 s, and decays equally rapidly when stimulation stops. Decay is measured by single test stimuli given at various intervals after a conditioning train. Facilitation can often be divided into two exponential phases, lasting tens and hundreds of milliseconds, respectively, and are called the first and second components of facilitation. It may reach appreciable levels, as high as a doublingormoreof thePSPamplitude in the secondof two responses at short intervals, and as much as nearly a 100fold increase in PSP amplitude in a high-frequency train in some synapses. If presynaptic activity is sustained, facilitation is followed at most synapses by a slower phase of increased efficacy called augmentation, with a characteristic time constant of several seconds. Finally, prolonged activity results in a final phase of PSP growth, called potentiation, which lasts several minutes. Each of these processes can further increase transmission several-fold over initial baseline levels. The combination of all three processes, at least some of which (facilitation and potentiation in particular) appear to interact multiplicatively, could in principle elevate synaptic transmission several 100-fold, but in practice, saturation of the release mechanisms usually limits the overall enhancement to less than 100-fold. See also: Chemical synapses; Synapses