The most commonly invoked distinction among memories, however, is the split between short and long-term memories. Short term memory immediately follows exposure to an input and decays rapidly, while long term memory has both unlimited capacity and slow decay. In the former case, no long-term changes occur, while in the latter, there is a fundamental change in neural structure to correspond with the changes in input. Any temporal increase in synaptic potential is the result of changes in the presynaptic transmitter release, although additional messengers are active in inducing long-term change through phosphorylation and the synthesis of new proteins. The causes of neurobiological changes associated with learning are divided into several types.
Habituation refers to the development of pathways among neurons.
It comes about through an influx of calcium from the presynaptic side, resulting in a reduction of the membrane potential of the post-synaptic neuron. Habituation weakens synaptic connections, requiring less input to transmit messages, while Sensitization strengthens them. Neurons subjected to repeated inputs can result in Long-Term Potentiation, a phenomenon which allows neurons to fire for long periods of time. It occurs in the hippocampus, as well as in the cerebral cortex, and both are thought to be necessary for memory formation, retention, and recall. In contrast to sensitization and habituation, LTP affects the post-synaptic neuron and uses glutamate as a neurotransmitter, which are received by NMDA receptors.