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Brain Essay- the Hippocampus

The hippocampus is a mirrored pair of curved structures located in the medial temporal lobe of the brain, largely beneath the temporal and parietal lobes. It belongs to the limbic system and serves a critical purpose in mammals and other species, regulating formation and upkeep of memory; particularly long term and has been shown to have some link to the process of spatial navigation. The word Hippocampus derives from the Greek for `seahorse` because of its similar shape, other anatomists have compared its shape to the horns of a ram or a mountain goat. Initially the hippocampus was thought to be involved with the process of olfaction with many scientists believing that it received neuronal input from the olfactory bulb, this theory along with others such as the Jeffery Gray inhibition/ anxiety theory are generally disregarded as there is overwhelming data to disprove/ refute these theories. Current research involving techniques such as animal models of hippocampal damage, functional brain imaging and direct recording of neuronal firing patterns has helped to indicate some of the fundamental cognitive processes the hippocampus is responsible for and the neural representations which underpin them.

The first and probably most significant research into the hippocampus' role in memory comes from the experiments performed on Henry Gustav Molaison known to the scientific world until his death on December 02 2008 simply as HM. In 1953 HM underwent surgery to treat intractable epileptic seizures, during the surgery parts of his medial temporal lobe were removed on both sides of his brain including two thirds of his hippocampus and amygdala. His entire entorhinal cortex, the major sensory input to the hippocampus was also destroyed, along with some of his parahippocampal gyrus and anterolateral temporal cortex. The surgery was a success in alleviating the epileptic seizures however HM now suffered from anterograde amnesia; he was unable to form new long term semantic memories. There was also some retrograde amnesia where HM was unable to remember events from a few days to ten years before the surgery however this retrograde amnesia was temporally graded, more interestingly HM's procedural memory system was still intact and although he was unable to remember it he was capable of learning new motor skills. These observations suggested that firstly the hippocampus was involved in mediating memory processing and secondly this involvement continues in some cases for many years after the memory is initially formed. Further studies involving other amnesiac patients have confirmed the critical role the hippocampus plays in declarative memory; our capacity for remembering unique biographical experiences and in the processes of semantic or factual memory. The use of neuronal recording in animal models of selective hippocampal damage has revealed some of the methods by which these complex memory functions are mediated. Through the course of this essay I will discuss several of these neuronal processes of memory mediated by the hippocampus,

The cognitive processes of memory

Baddeley (2000) describes the construction of perceived reality as an accumulation of episodic events within a mental buffer, in which episodic memories of discreet events are constructed sequentially from a series of associative representations; relevant people/ objects, actions or events and the place where the actions or events occurred, in order to give a chronological "flow" of events that comprise the memory of that experience/event. Cohen (1984) hypothesised that these episodic memories may be encoded along with semantic memories the common features of which forming a relational network such that the common features become encoded as timeless semantic cues which are then linked to the episodic memories which are encoded with time references. This model explains the flexibility of memory in its ability to link together non related semantic memories which are related to a single semantic memory encoded alongside a copy of itself in the chronological progression of an episodic memory. This property of relational memory formation involving association and interleaving of memories has been proposed as an explanation for the extensive period of consolidation some memories undergo, perhaps explaining the temporal graduation of memory loss in cases such as HM.

Many studies infer the neuronal circuitry and layered cell structure of the hippocampus as being responsible for mediating the overall process of declarative memory including its relational components, the hippocampus is composed of a series of narrow bands of cells. The first of these collections of cells is the dentate gyrus which is comprise of a tightly packed layer of granule cells, after the DG there are the Cornu Ammonis areas (CA4-CA1) comprised of tightly packed bands of pyramidal cells. After the CA regions of the hippocampus come the subiculum, the pre and parasubiculum then the entorhinal cortex, the term "hippocampus proper" is used to refer the Cornu Ammonis areas only. The major neuronal pathways within the hippocampus form loops, one such loop is transduction of external inputs coming from the perforant pathway of the layer 2 neurones in the entorhinal cortex to the mossy cells in the hilus of the dentate gyrus and the CA3 layer of the hippocampus proper. These CA3 regions combine the EC layer 2 inputs with those from the dentate gyrus and relay the information to CA1 through the Shaffer collateral fibres and the entorhinal cortex. The CA1 region receives these inputs from the CA3 region and EC layer 3 sending projections to the subiculum which combines information from CA1 and EC layer 3 to form the final signal projected through the output pathways of the hippocampus.

CA3 neurones receives convergent afferent neurones from virtually all cortical areas including information about time and location which are obviously important in the processing of memories to form relational networks. Interestingly many of the CA3 neurones are interconnected with excitatory synapses which may play an important role in the method of long term potentiation via rapid neuronal plasticity.