Learning and Memory Physiology
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Direct answer
H.M., the most famous patient in neuroscience, could recall his childhood but after bilateral medial temporal lobe surgery in 1953 could form no new conscious memory — yet he learned mirror drawing better each day without recognising the task. His case split memory into dissociable systems: declarative (facts and events), dependent on the hippocampus for consolidation, and procedural (skills and habits), dependent on the striatum and cerebellum. The cellular engine of storing information is long-term potentiation, an NMDA receptor-dependent, calcium-triggered strengthening of synapses first characterised in the hippocampal CA1 region.
What you must remember
- Stages: sensory memory (milliseconds), working or short-term memory (seconds to minutes, capacity about seven items), long-term memory (effectively unlimited, consolidated over weeks to years).
- Declarative (explicit) memory — facts and events — needs the medial temporal lobe, hippocampus and entorhinal cortex; non-declarative (procedural skill, priming, conditioning) survives hippocampal damage.
- Working memory lives in the prefrontal cortex; Baddeley's model divides it into the phonological loop, visuospatial sketchpad, central executive and episodic buffer.
- Bilateral hippocampal damage causes anterograde amnesia with intact working memory, remote memory and procedural learning — the H.M. phenotype.
- Korsakoff psychosis, from thiamine deficiency in alcoholism, damages the mammillary bodies and dorsomedial thalamus and adds confabulation.
- Long-term potentiation at the Schaffer collateral–CA1 synapse: strong presynaptic glutamate plus postsynaptic depolarisation removes the magnesium block of NMDA receptors, calcium enters, CaMKII and protein kinases insert more AMPA receptors — late-phase LTP needs new protein synthesis through CREB.
- Ribot's gradient: memories closest to the injury are most vulnerable, remote memories are most durable, because consolidation slowly transfers storage to distributed neocortex.
- Alzheimer disease begins in the entorhinal cortex and hippocampus, so recent-memory loss is its earliest symptom; the amygdala tags memories with emotion.
From synapse to cortex — how today's fact becomes permanent
Learn a physiology concept tonight and it is first held in working memory, active and fragile, gone if you are distracted. Within minutes to hours, synaptic strengthening in the hippocampus encodes it: high-frequency input releases glutamate on CA1 pyramidal dendrites, depolarisation expels the magnesium plug from NMDA receptor channels, and the calcium influx activates CaMKII, which phosphorylates targets and drives more AMPA receptors into the postsynaptic membrane. The same input now yields a bigger response — long-term potentiation, the closest thing neuroscience has to a memory molecule.
The trace then spends years migrating. Hippocampal sharp-wave ripples during subsequent sleep replay the day's sequences to neocortex, and gradually the cortex stores the information independently — hippocampal damage years later no longer erases it, which is exactly why H.M.'s childhood was intact while his last 50 years were blank. Separate systems run in parallel: the striatum lays down the habit of tying a shoelace, the cerebellum the timing of a tennis stroke, the amygdala the fear attached to the dog that bit you. Clinically this parcel explains why a patient with advanced Alzheimer disease still cycles to the market — cortical declarative stores destroyed, basal ganglia habit intact.
High-yield viva angles
Panels ask for contrasts, so rehearse three. Declarative versus procedural: reciting versus riding — hippocampus versus striatum-cerebellum. Anterograde versus retrograde amnesia: cannot make new memories versus cannot retrieve old ones; ECT and head injury classically produce transient retrograde loss with a temporal gradient. And the emergency-medicine favourite: why give thiamine before glucose in an alcoholic — giving glucose first consumes the remaining thiamine and can precipitate acute Wernicke encephalopathy, so thiamine precedes dextrose in every ward protocol.
Frequently asked questions
Which structure converts short-term into long-term declarative memory?
The hippocampus, which binds distributed cortical representations during consolidation until the neocortex can support retrieval independently.
What is the ionic mechanism of long-term potentiation?
Coincident glutamate release and postsynaptic depolarisation remove the magnesium block of NMDA receptors; calcium entry then triggers kinases that strengthen the synapse.
Which memory types are preserved after bilateral hippocampal damage?
Working memory, remote memories already consolidated, and all procedural learning such as mirror-drawing, which H.M. demonstrated.
What is Ribot's gradient?
Recent memories are lost more readily than remote ones after injury, because older traces have been consolidated into neocortex and no longer depend on the hippocampus.
Why does recent memory fail first in Alzheimer disease?
The entorhinal cortex and hippocampus, the gateway and engine of new declarative memory formation, are the earliest sites of neurofibrillary degeneration.