The hippocampus is the cortical area concerned with formation and recall of memories. Memories can be divided into short-term and long term memories
Bilateral hippocampal damage results in a failure to remember recent events, but long term memories are relatively unaffected. The hippocampus appears to be directly involved in short-term memory and the process of converting short term memories into long term memories.
Short term cognitive memories are consolidated by repetition or rehearsal of a pattern of events. This pattern of inputs to neurones arises within different groups of synaptic inputs to pyramidal cells; changes in the power of each synaptic input increases with repetition.
Long term potentiation (LTP) is believed to be the mechanism whereby new neural pathways are created, or older ones are strengthened.
Emotionally-associated memories are frequently rehearsed during sleep and dreaming. The hippocampal involvement in emotional behaviour requires a rapid response to events, particularly unpleasant ones, and invokes short and long-term memory.
Long term memories involve similar biochemical changes within synapses, but within extensive networks of neurones throughout the cortex, including the pre-frontal cortex, which some scientists regard as a memory buffer. A consequence of continued use of these pathways is that synaptic efficacy can be more permanently achieved because gene expression of proteins that maintain these active synapses. It may take weeks or months to establish long-term memories.
Long Term Potentiation. The diagram shows one mechanism in LTP. The synaptic efficacy is increased by the increased presence of receptor molecules in the post-synaptic membrane.
Short-term memory lasts a few seconds, and the duration of the memory is increased by repetition.
Bilateral hippocampal damage results in a failure to remember recent events, but long term memories are relatively unaffected. These people have little recollection of things that happen moments previously, but can remember events in the past that were consolidated into long term memry beofe the hippocampal damage occurred.
The hippocampus appears to be directly involved in short-term memory and the process of converting short term memories into long term memories.
Short-term memories can be consolidated into long-term memories, a process that requires repetition and meaning. It is aided by breaking up the sequence of events into smaller groups; for example, a 16 digit number is more easily remembered if broken up into four groups of four digits.
Many scientists feel that the creation of a new neural pathway in a short time can only be done by strengthening existing synapses in some way.
The main way in which synaptic efficacy can be strengthened is by a process called long term potentiation. This increase in synatic strength is confined to synaptic inputs to neurones that are repeatedly activated.
The hippocampus is a site where cortical neurones are particularly prone to long term potentiation, and is seen as the site where memories are laid down by increasing the efficacy of synapses in pathways that are repetitively activated.
Image source: neuwritesd.org
Diagram of possible circuitry for long-term memory involving buffers in the pre-frontal cortex. LTP is though to be the synaptic mechanism involved throughout these pathways, and repetition of the memory trace reinforces the synaptic connection by the generation of proteins that maintain the strength of the synapses.
Long Term Memory
Long-term memory is the storage of information over a long period of time; these memories can generally be recalled easily, and can be updated as time goes by, so that previous memories may be forgotten. This process of supplanting memories that are later remembered differently is known as 'interference', and may be a means of forgetting. Items such as phone numbers may be remembered for daily use, but are easily forgotten when replaced by a new phone number.
Bilateral hippocampal damage leaves long term memories are relatively unaffected, and it seems that long term memories depend on enhancing the connectivity of networks of neurones throughout the cortex. the increased synaptic strength may be maintained by the expression of genes coding for factors that maintain these synapses.
This is probably achieved because the hippocampus is reciprocally connected with almost all areas of the neocortex, each of which has different functions, but contribute different concepts and contexts relevant to memory formation and recall.
The hippocampus appears to be directly involved in the consolidation of short-term memories; this incorporates the meaningful association of events, whch are rehearsed using many neuronal circuits.
The activity patterns in synapses induce local short-term memory as a result of local biochemical changes in the spine synapses involved. These include the generation of second messengers and the trafficking of receptors within spine synapses. However, stronger or repetitive activity patterns lead to changes of longer duration, because changes in gene expression can induce more permanent changes in the strength of these synapses, by producing proteins that stabilise the synaptic contacts.
It is likely that a network of cortical neurones is involved in the creation of a memory, including many connections to the primary visual, auditory and somatosensory cortices, as well as many areas of association cortex. The facilitation of a particular group of neural circuits by repetitive activity may become more permanent as a result of gene expression of proteins that have long term effects on the strength of synapses in these circuits.
So networks encompassing many areas of cortex may be facilitated by these changes in gene expression, and provide a more permanent memory trace.
Forgetting occurs in long-term memory when previously strengthened synaptic connections become inactive, or when interference updates an older memory trace. Long-Term Depression is a known mechanism and could also play a part in forgetting.
What is Long-Term Potentiation (LTP) ?
Image source: Purves: Neuroscience ncbi.nih.gov
Long Term Potentiation. The images show the sizes of the EPSPs in a cell in the hippocampal cortex. Two pathways are being stimulated, but Pathway 1 is stimulated repeatedly for a short period around time zero. It can be seen that the size of the EPSP increases about threefold and gradually declines over the following hour. However, pathways 2 that is not tetanised, shows no changes in the size of its EPSPs
Long Term Potentiation (LTP)
Repetition aids learning. Repeated inputs to a synapse can stengthen the function of a synapse, and the effects of repeated inputs are prolonged. This is known as Long Term Potentiation (LTP) and is seen by many as the neuronal basis of learning.
LTP is an increase in the response of a synapse following repeated stimulation, an effect that can last for hours or days or longer.
The diagram opposite shows the increase in the size of the EPSP in a pathway that has been repetitively stimulated; not that the changes considerably outlast the period of high frequency stimulation.
LTP has been studied at glutamatergic synapses in the hippocampus, and the phenomenon depends on activation of the NMDA receptor. Blocking the NMDA receptor interferes with LTP and learning and memory.
Synaptic strength can be increased in the short term by local changes within the spine synapses, such as trafficking of glutamate receptors from internal stores into the post-synaptic membrane.
Long-term memory involves consolidation of the re-inforced synaptic strength by the production of proteins that maintain each synapse throughout the network. Long-term lack of use can produce interference and forgetting.