Neurobiology of Memory

The hippocampus has long been considered the central hub of all memory, and therefore responsible for a large majority of learning. Located in the ventral-medial temporal area of the brain, its importance regarding the consolidation of new memories, and thus the learning of new things, was demonstrated by the infamous case of HM (patient), a man who had both medial temporal regions of his brain removed. This resulted in his inability to form new long-term memories.

1. The hippocampus belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation.
2. Engrams are means by which memory traces are stored in the brain in response to external stimuli. They are sometimes thought of as neural networks and sometimes conceptualized using a hologram analogy in light of results showing that specific memories appear not to be localized.


3. Long-term potentiation is a persistent strengthening of synapses based on recent patterns of activity. These are patterns of synaptic activity that produce a long-lasting increase in signal transmission between two neurons.




Long term potentiation: first stage. A synapse is repeatedly stimulated, sending neurotransmitters from the axon terminal (left) across the synapse to the dendrites of a second neuron (right).



Second stage. More receptors are found on the dendrite.





Third stage. More neurotransmitters are produced.



A stronger link between neurons.



4. Memory consolidation is the category of processes that stabilize a memory trace after its initial acquisition.
5. The initial process of memory consolidation, synaptic consolidation, occurs within the first few hours after learning. This involves the strengthening of synapses based on recent patterns of activity and is a form of long-term potentiation.
6. Systems consolidation is the process of memory consolidation through which hippocampus-dependent memories become independent of the hippocampus over a period of weeks to years.
7. A type of memory process which is amenable to detailed cellular analysis with certain animal models such as Aplysia, sensitization is a non-associative learning process in which repeated administrations of a stimulus results in the progressive amplification of a response.