How the Brain Stores Memory: LTP and the Synaptic Basis of Learning

Memory is not one system

Atkinson and Shiffrin's 1968 multi-store model divides memory by duration: sensory memory lasts only 200ms-4s and decays before it can all be processed; short-term memory holds roughly 7±2 items (Miller, 1956) for 15-30 seconds without rehearsal; and long-term memory splits into explicit memory available to conscious recall (episodic — "what happened to you," and semantic — "facts about the world") and implicit memory that operates without conscious access (procedural skills, priming, conditioning). Baddeley and Hitch's later working-memory model adds a central executive, a phonological loop, and a visuospatial sketchpad — the mental workspace behind mental arithmetic and reading comprehension.

The hippocampus and patient H.M.

The hippocampus, a seahorse-shaped structure in the medial temporal lobe, is essential for forming new explicit memories — a fact revealed most starkly by its damage. In 1953, Henry Molaison (patient H.M.) had both hippocampi removed to control epilepsy. He could never again form a new explicit memory, forgetting every conversation anew, yet his intelligence and personality stayed intact and he could still learn new motor skills — his mirror-drawing improved over days even though he had no memory of practising. This dissociation gave neuroscience its clearest proof that declarative and procedural memory are separate systems. The hippocampus performs pattern separation, telling similar memories apart, and pattern completion, reconstructing a full memory from a partial cue.

LTP: the synaptic basis of memory

Long-term potentiation (LTP), discovered by Bliss and Lømo in 1973, is a persistent increase in synaptic strength following high-frequency stimulation. During normal signalling, glutamate opens AMPA receptors while NMDA receptors stay blocked by a Mg²⁺ ion. Strong, repeated depolarisation expels that Mg²⁺ block, letting NMDA receptors admit Ca²⁺ as well as Na⁺. That calcium influx activates CaMKII, which boosts AMPA receptor conductance and traffics more receptors to the synapse — and, over hours, triggers new gene transcription and protein synthesis that physically enlarges the dendritic spine.

Hebb's rule (1949): "Cells that fire together, wire together"
  Δw_ij = η · x_i · x_j

LTP is the biological realisation of Hebbian learning — NMDA-receptor-
dependent coincidence detection requiring BOTH pre-synaptic glutamate
release AND post-synaptic depolarisation.

LTD (long-term depression): low-frequency stimulation → synaptic weakening,
complementary to LTP and essential for memory specificity and unlearning.

Consolidation, sleep, and forgetting

New memories are fragile and must be consolidated. Cellular consolidation, over minutes to hours, depends on new protein synthesis at the synapse. Systems consolidation, over weeks to years, gradually shifts memories out of the hippocampus into the neocortex — which is why old memories often survive hippocampal damage better than recent ones. Sleep is an active part of this process: during slow-wave sleep, hippocampal sharp-wave ripples replay recently encoded sequences in step with neocortical slow oscillations, effectively transferring the memory to longer-term storage. Forgetting itself follows Ebbinghaus's 1885 exponential curve — roughly 58% retained after 20 minutes, 33% after a day, 25% after a week — driven by trace decay, interference between similar memories, and simple retrieval failure. Spaced repetition (used by tools like Anki) counters this by reviewing material just before it would otherwise fade, achieving 90%+ retention with far fewer reviews than cramming.

When memory breaks down

In Alzheimer's disease, amyloid-β plaques and tau tangles progressively destroy episodic memory, starting in the entorhinal cortex before spreading through the hippocampus. In PTSD, traumatic memories become overly persistent and easily triggered, linked to amygdala hyperactivation and reduced hippocampal volume — reconsolidation-based therapies try to reactivate and then chemically blunt the emotional charge of a memory. For healthy people, the most reliable memory-enhancement strategies remain unglamorous: exercise (which raises BDNF and promotes hippocampal neurogenesis), 7-9 hours of sleep, spaced repetition, and retrieval practice — testing yourself, rather than simply rereading.

Frequently asked questions

Try it live

Everything above runs in your browser — open Long-Term Potentiation & LTD and watch Hebbian plasticity in action: synapse strength changes with correlated pre- and post-synaptic activity, following the BCM rule. Nothing is installed, nothing is uploaded.