Hippocampus Damage

Therefore, a damaged hippocampus makes itself even more susceptible to a metabolic challenge than a healthy hippocampus, creating a downward spiral of increased chances of potentiated hippocampal damage with each metabolic episode and this risk increases with age. Tiger street data. In Alzheimer's disease the hippocampus is one of the first regions of the brain to suffer damage; memory problems and disorientation appear among the first symptoms. Damage to the hippocampus can also result from oxygen starvation (hypoxia), encephalitis, or medial temporal lobe epilepsy.

Memory formationThe hippocampus is the structure of the brains that is most closely associated with memory formation. It is known as the repository for long-term and is involved in storing memories from long-term memory to permanent memory. The hippocampus also plays an important role in spatial orientation.Context of memories and experiencesThe hippocampus is closely involved in organizing that which you imagine with something in your mind. That is, the context of memories like they have been experienced.

The hippocampus is very important to be able to connect experiences with other experiences in a kind of network of memories.

Module 11 Module 11: Hippocampus and Memory.Dramatic evidence for the localization of at least some aspect of memory came in 1953when surgeons performed an operation called a bilateral resection of the medial temporallobe on a patient by the name of HM. The purpose of this operation was to alleviate theintractable epilepsy which plagued him; in this matter, at least, it was a success.Unfortunately, the operation left HM with severe anterograde amnesia, which means that hecould not form new memories. His retrograde amnesia, or loss of memory for events prior tothe surgery, was only mild. Clearly some mechanism which helps create memories wasdamaged.

In later years, researchers discovered that the hippocampus, which had beenremoved in HM's surgery, is primarily responsible for the consolidation of short-term intolong-term memories. This process can take up to three years, hence HM's retrograde amnesiafor events close to the surgery.The hippocampus is not the only structure involved in memory, however. There are manydifferent types of memory, and the hippocampus is involved in declarative, spatial,contextual, episodic, and working memories, as well as in the detection of novel stimuli.Non-declarative memories, such as motor skills, are dealt with in the neostriatum.Although testing declarative memory in animals is problematic, evidence for spatialmemory in them has been found. One example of the type of task used to test spatial memoryis the Morris Water Maze.In this task, a tank is filled with milky water with a platform concealed somewherebeneath the surface. When a rat is placed in the tank, he will swim around until he findsthe platform. When this task is repeated many times, he learns to use cues from outsidethe tank and can find the platform almost immediately from any spot in the tank. He can dothis because he has created a spatial map of the tank and its surroundings.If the rat's hippocampus is lesioned after he learns the task, there will be no effect;if it is lesioned before (when he is naive), however, he is unable to learn it.Clearly, the hippocampus is involved in creating the spatial map, but not in storing itfor long periods of time.

This is also evident in the fact that migratory birds have largehippocampi and as adults still experience neurogenesis in the hippocampal region.The hippocampus is not fully developed at birth; that takes about two and one halfyears. An interesting effect of this is infantile amnesia-most people do not havedeclarative memories from their first couple years of life.The hippocampus is a looped structure; information is funneled into it, then is sentback. It is a cylindrical structure around the thalamus. CA4 and CA2 are both quite small,and really are parts of CA3 and CA1, respectively. The hippocampal formation also includesthe entorhinal cortex and a few other parts.The main circuit is the trisynaptic pathway, which includes the perforant pathway,mossy fibers, and Schaffer collaterals.

A more modern view of the connections includesstaggered projections, by which each area except the dentate gyrus projects to the nexttwo.Back in 1949, Donald Hebb defined a type of learning which he thought occurred incells, Hebbian Learning. It is a use-dependent mechanism which produces synapticfacilitation when pre- and post-synaptic activity is correlated.When evidence for LTP in the hippocampus was found in 1973 by Bliss and Lomo, theprocess of LTP followed Hebb's principles. The mechanisms by which synaptic facilitationcan occur include: an increase in the number of postsynaptic receptors or amount ofneurotransmitter released, sprouting of a new terminal bouton, and structural changeswhich lead to less electrical resistance in the dendrite of the postsynaptic cell.In the hippocampus, different types of structural changes occur in different areas,depending on the type of receptor involved. In the perforant pathway to the dentate gyrus,where NMDA receptors are used, they include a larger spine head, shorter neck, and achange from a convex to a concave. Between the dentate gyrus and CA3, where there areAdrenergic receptors, they include a swelling of the spine head and a wider neck. LocationReceptorBeforeAfterDG/CA1NMDACA3AdrenergicGases such as NO (nitric oxide) are released by the postsynaptic cell for thepresynaptic cell when activity is correlated. Gasses can diffuse over larger areas,affecting nearby synapses as well.Long Term Potentiation (LTP) depends on, among other things, the presence of threetypes of glutamate receptors: K, Q, and NMDA.

When glutamate is released by thepresynaptic cell, it binds to all three types of receptors, but only the K and Q channelsopen to allow sodium into the cell, because the NMDA channel is blocked by a magnesiumion.When the resulting depolarization reaches a threshold level, the magnesium block isreleased, opening the NMDA channel to calcium ions. Because of their size, calcium isusually prevented from entering the cell.

The calcium influx then triggers a long-lastingincrease in the amounts of two kinases, calimodulin kinase and protein kinase, whicheventually leads to the structural changes associated with LTP.The opposite of LTP occurs when pre- and post-synaptic activity is uncorrelated; thisis called Long Term Depression (LDP).

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