Recent studies have shown that spatial memories are encoded as sparse populations of cells that are activated during learning and are necessary for the retrieval of specific memories.
We refer to these cells as "memory engram cells" and the focus of Tomás J Ryan and his team is to understand how engram cells are able to store specific memories as information.
Here is a quick summary/breakdown of the video above.
- The old storage theory of amnesia is WRONG.
- The problem is the ability to retrieve the memory.
- Short term memory lasts a few hours. It is very disruptable (long term is not).
- Long term memory lasts your entire lifetime. It requires transcription and translation of many different genes.
- We have many different genes that regulate memory.
- They are different for short term and long term - playing roles at different stages.
- Most popular was CREB (turns on other genes).
- Engrams cells are structures in the brain where memories are stored. R
- They used optigenetics to prove engram cells were important for memory.
- Engram cells only account for 4-6% of the brain cells in the hippocampus (of mice). V
- Consolidation idea - a short term memory (unstable) forms into a long term memory (stable), thus stabilizing the engram.
- Optogenetics - let's us express photoactivatable opsins in whatever region of the brain we want.
- The brain is electrically active.
- They put certain opsins into brain cells and then stimulate them with light.
- These opsins can conduct ions - putting positively charged cations from the external environment of the cell into the neuron.
- This depolarizes the cell, which results in an action potential (creating neuronal activity).
- What they did at MIT was integrate optogenetics and memory engram cells.
- They tested parts of the brain using a transgenetics - they took immediate/early genes (genes that show activity as a function of neuronal spike) and used opsins to only activate those genes.
- Amnesia and Memory
- Learning is the enhancement of synaptic connections.
- Memory is a stable thing in the brain and can last for your entire life.
- Learning and recall are not "things", they are activities.
- Learning = process of making a memory. Recall = process of retrieving a memory.
- So the problem is not memory itself.
- Amnesic engram cells have a loss of synaptic strength.
- Increasing the synaptic strength helps get the memory (which is commonly seen in Alzheimer's (AD), Huntington's (HD)).
- If you increase the synaptic potentiation (increasing the dendritic spine density), esp. of the amnesic engram cells, you can get natural access to those memories.
- Neurotrophic factors (BDNF, NGF, CNTF, GDNF) do this, but are not localized nor memory specific.
- Learning something new (about something you are amnesic about) will train your brain to those particular engram cells.
- Once you activate the engram cells (with optogenetics), you can restore access to old information and add new information on those amnesic memories.
- Without dealing with the cause, you probably won't be able to really restore memories (in diseases like AD, HD, etc.)
- In late stages of neurodegeneration, you may actually lose memories.
- Actions to take for prevention
- Stay mentally active
- Engage in physical exercise
- Actions to take for amnesia (acute memory loss)
- Do everything to remind yourself of those experiences
- Retrain on the "forgotten" pieces
- We briefly discussed gene expression and histone acetylation to enhance memory.
There are 4 ways to examine neural mechanisms of encoding memory in mice:
- Transgene expression - Genes that are artificially introduced into or deleted from (brain) cells. R
- Activity-dependent expression - limit the expression of the transgene, such as using immediate early genes (IEGs), which naturally turn on immediately after neuronal activity. It shows shich neurons are being fired in the cells. R
- Time-dependent expression - limiting the expression of the transgene within a specific time window. R
- Non-electrical control of neural firing - activate neurons with light (optogenetics). Ion channels control neuronal firing. Ion gates admit charged molecules (mostly sodium, potassium, and chloride) under the direction of certain amino acids and their derivatives (usually glutamate or GABA) acting on synaptic receptors. R
Memory Engram Cells
The term engram was first invented as "the enduring though primarily latent modification in the irritable substance produced by a stimulus (from an experience)". R
Interaction with the world leaves a physical residue within the excitable substance of the nervous system. R
Some internal modifications may manifest directly as a behavioral change or memory; others may be accumulated over time into a learned behavior. R
Increasing synaptic strength is important for memories because the brain may adaptively modulate the accessibility of stored information by altering the strength of transmission at synapses. R
Permanently enhanced synaptic transmission does not and cannot store the memory information itself. R
Synaptic strength delegates engram retrieval. R
More Research - Tomás J Ryan's Studies
- What is memory? The present state of the engram
- NMDA receptors are selectively partitioned into complexes and supercomplexes during synapse maturation
- Memory retrieval by activating engram cells in mouse models of early Alzheimer’s disease
- Rehebbilitating Memory
- Memory engram storage and retrieval
- Memory. Engram cells retain memory under retrograde amnesia
- Evolution of GluN2A/B cytoplasmic domains diversified vertebrate synaptic plasticity and behavior
- The Subtype of GluN2 C-terminal Domain Determines the Response to Excitotoxic Insults
- The origin and evolution of synapses
- Evolution of NMDA receptor cytoplasmic interaction domains: Implications for organisation of synaptic signalling complexes
- The bacterium that got infected by a cow!: Horizontal gene transfer and evolution
- Creating a False Memory in the Hippocampus