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Introduction: One of the fundamental functions of brain is to navigate in space. This ability depends on the ability to have a sense of position which in turn is interlinked with the sense of direction, distance, and knowledge of the earlier positions through which a person has travelled. A man depends on it for everyday activities ranging from finding his car key to location of his house. In case of animal they do not passively react to the external stimuli. Rather they learn facts about the world and use it when it is required. This was discovered by Edward Tolman.
Hippocampus and its function: Hippocampus is the greek name for a mythical sea monster, hippo means ‘horse’ and kampos means ‘sea monster’. When a hippocampus is cut in cross section, it resembles a ram;s horn. For a long time investigations were directed towards understanding the mechanism and the role of the hippocampus in the formation of memory.
Discpovery of place cell in the hippocampus: In the late 1960’s single cell neuronal recording in awake rats was cutting-edge technology. With the help of the brain atlas, tiny wire electrodes were guided to specific area of interest in the brain. When the electrode tip is close to a neuron the electrode can record the action potentials from that neuron. During an experiment with rat Jhon O;Keefe an expert at recording neurons using this techniques, recorded the hippocampus neuronal activity when the animal was doing a variety of behaviours. This approach proved quite faithful because they discovered neurons in the rat hippocampus that showed activity correlated to the rat’s location within its environment. They called these neurons ‘place cell’. Place cells fire when the animal is moving in a specific location in the environment, which corresponds to the place field of that particular neuron. This place cells were first found in the pyramidal cell layer of area CA1 of the hippocampus. Later, other groups show that area CA3 of the hippocampus also has place cells. Place cells firing in a part of the animal’s environment is not because of something the animal does there or because of its motivation for going to that place. Rather, they appear to be a cognitive process, signalling the animal’s position within an environment irrespective of its behaviour and motivational state or the reward properties of that place. The discovery of place cells by John O’Keefe in the hippocampus created a new surge of interest in the field in spite of the skepticism from a few investigators.
Place cell remapping: In 1987 Bob Muller and colleagues discovered that place cell maps are completely different in two environments, knowing the location of the firing field in the first environment will not predict the location of the firing field in the second environment. Changing the shape of an enclosure invariably induces full remapping. To be brief, the hippocampal map of space is sensitive to non-spatial factors and understanding these factor will be essential in decoding the relationship between hippocampal maps and memory.
Head direction cells and Boundary cells: In order to maintain the spatial orientation and to guide navigation, an animal must have knowledge of its location, displacement or distance and direction from that location. By now, we know that place cells could be used to identify familiar environmental locations. Also, therewas a speculation that the oscillations of the theta rhythm1 of the LFP2 (local field potentials) that occurred during running could measure the distance. The missing piece in the puzzle is that if the hippocampal system were to guide navigation, it needed a sense of direction, which was predicted byn O’Keefe and Nadel in their 1978 book. In 1984, Jim Ranck reported that cells in the post-subiculum of the rat brain discharges whenever the animal’s head points to a specific direction, independent of their location or behaviour, thus providing a compass signal. Head direction cells are found in many brain regions like the anterior thalamic nuclei, post and parasubiculum and also in the entorhinal cortex.
Hippocampal Circuitry and the Entorhinal cortex: The neuronal antecedents to the place cells were not understood clearly. The obvious line of investigation was whether the map is located within the hippocampus or it is constructed elsewhere and merely transferred to the hippocampus. Anatomically entorhinal cortex is the major source of cortical input to the hippocampal place cells. This is substantiated by the findings that entorhinal damage causes serious deficits in spatial problem-solving tasks as hippocampal lesions do. Discovery of the grid cell in the Entorhinal cortex: In order to have a better understanding of the spatial organization of the firing fields, the same cells were subsequently recorded in larger environments, including boxes with surface areas that were more than three times larger than those of previous studies.
The firing fields of the neurons in the layer II of dMEC formed a beautiful grid like structure of tessellating equilateral triangles; in other words the firing locations form the vertices of a hexagonal grid, representing the entire space traversed by the animal. The basic unit of the tessellated spatial firing pattern is called a grid and the cells that elicit such a firing pattern are called ‘grid cells’. The characteristic features of the grids are the spatial phase, spacing of the grid and their orientation (Figure 4). The temporal and spatial auto correlation analysis suggested that the recurring peaks in the auto-correlograms did not reflect artefacts in the analytical procedures.
Conclusion: The discovery of place cells and grid cells by John O’Keefe, Edvard Moser and May-Britt Moser has clearly revolutionized our understanding of the cognitive functions of the brain, especially the spatial navigation. These findings open a lot of avenues for future research in both memory and spatial navigation, since the link between the two are yet to be understood
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