Modulation of Neurotransmission
As discussed earlier, modulation of synaptic activity occurs in the form of potentiation, an increase in the synaptic responsivity (excitability/sensitivity) of one cell communicating with another, or inhibition, the reduction of synaptic activity between one cell and another. In the spectrum of potentiation, posttetanic potentiation (PTP) is the shortest-lived event, lasting only a minute or two. Short-term potentiation (STP) lasts somewhat longer than PTP. Ultimately, long-term potentiation (PTP), lasts quite a bit longer and may actually change the physical structure of the synaptic connection. On the other end of the spectrum, long-term depression (LTD) is the inhibitory counterpart of long-term potentiation. This process is important as it is believed that while LTP is the mechanism through which memories are established, LTD may work to help us forget some things we don't really need so that the brain does not become overloaded attempting to "remember everything." This is also related to the difference in how the brain creates short-term, as opposed to long-term memories. Everything that is needed to establish and store short-term memories is already in synaptic abundance: proteins, receptors, ion channels, enzymes and transporters, but only for a short time. Long-term memories are dependent on creating something--synthesizing new proteins or increasing the synthesis of already-existing proteins.