What is synaptic plasticity?

Synaptic plasticity is the ability of synapses — the connections between neurons — to strengthen or weaken over time in response to activity. It is widely considered to be the cellular basis of learning and memory.

The foundational insight came from Donald Hebb in 1949. Hebb proposed that when two neurons fire together repeatedly, the connection between them is strengthened. This principle — often summarised as 'neurons that fire together wire together' — remains one of the most important ideas in neuroscience.

The best-studied form of synaptic plasticity is long-term potentiation (LTP). LTP is a persistent increase in synaptic strength that follows high-frequency stimulation. It was first described in the hippocampus — a brain region critical for memory — and has since been observed throughout the cortex. LTP depends on NMDA receptors, which act as coincidence detectors: they require both the presynaptic neuron to release glutamate and the postsynaptic neuron to be depolarised before they activate.

The opposite process — long-term depression (LTD) — involves a persistent decrease in synaptic strength. LTD is thought to be important for eliminating unnecessary connections, refining neural circuits, and allowing the brain to 'forget' or update outdated information.

Together, LTP and LTD are thought to underlie Hebbian learning at the synaptic level. When you learn a new fact, practise a skill, or form a memory, the pattern of synaptic weights in relevant neural circuits is being modified.

Synaptic plasticity also has significant clinical implications. Disrupted plasticity mechanisms have been implicated in Alzheimer's disease, schizophrenia, addiction, and PTSD. Understanding how plasticity works — and how it goes wrong — is central to developing treatments for these conditions.

If you're studying neuroscience at university, synaptic plasticity will appear in your exams. More importantly, it is genuinely one of the most elegant ideas in biology — the mechanism by which experience shapes the physical structure of the brain.