Hippocampus, memory, and synaptic plasticity Flashcards
Hippocampus in rats
- very large relative to brain size
Made of three main regions
- Dendate Gyrus - receives info
- sends axons to CA3
- sent to CA1 (output neurons)
You can place electrons into it and record results
John O’Keef - electrodes into ca1 region, found that certain place cells would only fire at a high rate when a rat was in a specific part of the cage. They would also track rotation when areas were moved.
Therefore implied that this structure was involved in encoding information about the rats broader spatial environment
Hippocampus supports COGNITIVE MAP
Spatial memory in rodents
Water maze: Richard morris
Cloudy liquid used with rats- maze, platform
Rats learn very quickly to swim towards platform, removed platform to see if they would swim over the remembered location
compared rats with removal of hippocampus cells to control groups and found rats with hippocampal leisions struggled greatly to find the platform. They learned to swim a fixed distance around the platform edge but did not know where in the circle the platform was located.
Donald Hebb
when an axon of cell A in near enough to excite cell b, and repeatedly takes part in its firing some growth process or change occurs in one of both cells so that cell A’s efficiency in firing cell B is increased.
“cells that fire together wire together”
= learning and memory are a result of connections between neurons and pathways becoming stronger.
Cells form a unit which represents sensory information coming in when they are co-activated.
Evidence for wiring together firing together
Tim bliss placed a stimulating electrode placed in the PERFORANT pathway.
Sends an electrical pulse/ action potential which stimulate cells in the dentate gyrus.
When he stimulated this pathway he saw an increase of neurons in the dentate gyrus.
Excitedry post synaptic potential - where neurons begin to increase their activity closer towards a threshold for firing.
you can take various measures of this potential that measures the efficiency of neurons to fire.
Population spike- cells actually firing
He found increased efficiency when many high frequency signals were sent into pathwayby accident.
He found that neurons showed potentiation and an increased efficiency at firing.
Highly replicable!!!
demonstrated longevity in the increase in efficiency as it lasts for months.
this is known as LTP long term potentiation.
Process for change in efficiency?
Post synaptic component of the dentate gyrus has two types of receptors.
Receptors are collections of proteins in the membrane that bind certain chemicals/ neurotransmitters.
Neurotransmitters are released whenever an action potential arises.
The two different receptors bind to glutimate
1. AMPA receptor - mediates normal transmission, whole is opened and sodium and potassium ions pass through to change properties of membrane to become closer to firing an action potential
2. NMDA receptor - also bind glutamate they are blocked normally by magnesium ion
HOWEVER block is removed if post synaptic membrane is strongly depolarized, or ‘becoming close to the firing threshold.’
Now when glutimate binds there is a huge increase of calcium flooding into post synaptic receptor.
This leads to an increase of AMDA receptors, this means that when baseline returns, the threshold for action potential is reached much faster as more chemicals can bind
For cells that fire together to wire together, magnesium blocks need to be removed in order for glutimate to bind and create a surge of calcium which results in more AMPA receptors therefore the threshold for action potential is reached more easily.
Inhibiting NMDA receptors
Morris et al used DLAP5 which blocks the activation of NMDA receptors to inhibit Long Term Potentiation.
Found that using his watermaze that animals who had consumed the active version of the drug showed no bias to where the platform was located.
Shows that blocking nmda receptors blocks spatial memory
DLAP5 did not inhibit performance in visual discrimination task
Synaptic plasticity underlies memory.
Donald hebb
NMDA receptors allow cells to wire together
Animals process information and this information is represented by cell assemblies
“grandmother sells”
Reverberating circuit - cells continue to become active through recall, and through that you get consolidation of memory. The more its activated, the stronger the links.
Conclusions
- hippocampus vital for spatial learning and memory, forms the “where” component of episodic memory
- Hippocampus first to show LTP
- Reliant on calcium surge to increase AMDA
