2 Neuroscience2.7 Neural circuits
2.7.1.13 Memory, dementia and PTSD
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Written by Resident Doctor Committee · Published
Memory systems
Declarative memory, episodic and semantic, is consciously retrieved. Non-declarative memory is expressed through performance.
System | Content | Circuitry |
|---|---|---|
Episodic | Events in time and place | Hippocampus, entorhinal cortex, diencephalon |
Semantic | Facts and concepts | Temporal and association cortex |
Procedural | Motor skills, habits, sequences | Basal ganglia (striatum), cerebellum |
Procedural (skill) memory is implicit: a person shows it by performing the skill, not by consciously recalling it.
It depends on the striatum and cerebellum, not on the hippocampus or the medial temporal and diencephalic systems behind declarative memory, so skills can survive severe amnesia.
Long-term potentiation
Long-term potentiation (LTP) is Hebbian, lasting, activity-dependent strengthening of a synapse, and is the main cellular mechanism of long-term memory.
In its common form, glutamate binds and depolarisation removes the magnesium block from N-methyl-D-aspartate (NMDA) receptors, and the calcium that enters strengthens the synapse by adding glutamate receptors.
Amnesia and dementia
Lesion site | Causes | Profile |
|---|---|---|
Medial temporal | Herpes encephalitis, hypoxia, typical Alzheimer's disease | Poor new learning with little benefit from cues |
Diencephalic (mammillary bodies, thalamus) | Thiamine deficiency, as in Korsakoff syndrome | Anterograde and retrograde amnesia, sometimes confabulation |
Frontal and subcortical disorders impair speed, organisation and retrieval, and recall improves more with cues.
Vascular cognitive impairment depends on where the lesions are and often slows processing and retrieval.
Semantic dementia loses word and object knowledge while episodic memory is relatively preserved early.
Post-traumatic stress disorder
Intrusions are involuntary sensory and emotional recollections that carry a sense of present threat.
The vividness of a memory does not establish its accuracy, and incomplete recall does not disprove an event.