Historically, it was generally thought that the role of the synapse was to simply transfer information between one neuron and another neuron or between a neuron and a muscle cell. In addition, it was thought that these connections, once established during development, were relatively fixed in their strength, much like a solder joint between two electronic components. One exciting development in neurobiology over the past forty years is the realization that most synapses are extremely plastic; they are able to change their strength as a result of either their own activity or through activity in another pathway. Many think that this synaptic plasticity is central to understanding the mechanisms of learning and memory.
There are two general forms of synaptic plasticity, intrinsic and extrinsic. Intrinsic mechanisms, also known as homosynaptic mechanisms, refer to changes in the strength of a synapse that are brought about by its own activity. (Homo from the Greek meaning the same.) Extrinsic plasticity, or heterosynaptic plasticity, is a change in the strength of a synapse brought about by activity in another pathway.