[Sossin 2008] discusses the problem of timescales of memory at the neuron level, where the human “short term” and “long term” notions don’t map cleanly to neurons and synapses. Instead, tagging by the biochemical process is more useful.
One dividing line between short term memory and long term memory is new protein synthesis. In this model, short term uses materials and proteins already available to the cell, and long term uses new proteins or new structural changes. In the diagram above, the blue items are new proteins.
Short term memory also covers very short, including the 20-500ms range of slower neurotransmitters. While that timeframe might not seem like memory, compared to the 5ms of fast neurotransmitters, it is storing a memory of the faster responses. As a comparison to electrical circuits, the 20-500ms range is like an electrical latch, which is a memory circuit for very short time periods, typically one clock cycle. The 20-500ms range in neural circuits covers both the common gamma and theta clock cycles in the vertebrate brain, and may serve a functional role similar to electronic latches.
References
Sossin, Wayne S. “Defining memories by their distinct molecular traces.” Trends in neurosciences 31.4 (2008): 170-175.