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A place cell is a type of pyramidal neuron within the hippocampus that is associated with spatial navigation and that fires to a particular place field, which acts as a cognitive representation of a specific location in space. Place cells are important for creating mental maps of our environments and in turn help organisms know where they are located. They are believed to work with other neurons in the hippocampus to help with spatial processing, but the ways in which they function within the hippocampus are still being researched. Studies with rats have show that place cells tend to fire quickly when a rat enters a new, open environment, however outside of a firing field, place cells tend to be relatively inactive. Together place cells are thought to form a “cognitive map” of the organism's environment, in which they have localized firing patterns within place fields. These firing patterns are often determined by external sensory information and the local environment. Place cells have proven to have the abilty to suddenly change their firing pattern from one pattern to another, a phenomenon known as “re-mapping” and though place cells do change according to the external environment, they are stabilized by attractor dynamics which “enable the system to resist small changes in sensory input but respond collectively and coherently to large ones".

Background
These cells were first discovered in the brain, and specifically in the hippocampus, by O’Keefe and Dostrovsky (1971). Though the hippocampus plays a role in learning and memory, the existence of place cells within the hippocampus demonstrates the role it plays with spatial adaptation and awareness. There have been recorded increases of firing patterns of rats in open environments and recorded spatial learning and awareness impairments after damage to the hippocampus and the place cells within. Studies with rats have shown that place cells are very responsive to spatial surroundings. For example a study by John O'Keefe and Lynn Nadel found that space cells would fire more rapidly when rats ran past places in the environment, when a new item was added to the environment, or when an item that is usually there is not present. Other studies have shown that place cells are thought to fire exclusively when a person or animal has entered a “limited” environment. For more information on studies with rats, see below.

Diseases Affiliated with Place Cells
Problems with spatial memory and navigation are thought to be one of the early indications of Alzheimer’s. Delpolyi and Rankin compared thirteen mild Alzheimer’s patients and twenty-one mild-cognitive impairment patients, with twenty-four subjects with normal brain functioning through a series of spatially related tasks. The first task entailed route memory and the study found that the non-control group could not find their location on the map, or recall the order in which they had seen landmarks. The overall results showed that only 10% of the control group got lost on the route while 50% of the non-control group got lost. The demonstrated issues with spatial navigation among Alzheimer’s and MCI patients indicates a malfunctioning with the firing of place cells and that abnormalities within the hippocampus may be an early indicatory of disease onset. O’Keefe who originally found the existence of place cells said that, “We suspect we’ll begin to see signs of changes in the functions of cells before we see changes in behavioral tasks."

Another study by Ming Yi looked at what contributes to the cognitive deficits often associated with Alzheimer’s disease and specifically how place cells at the hippocampal level look different from AD mice versus non-AD mice. The study demonstrated that physiological changes occurred at the hippocampal level of the tg2567 mice as early as 3 months. However, the changes were only visible at the hippocampal level, not at the neuronal level, and appeared before any changes in behavior or spatial orientation had occurred. The study then found that aged tg2567 mice at 16 months demonstrated spatial abilities and spatial recognition deficits.

Research with Rats
After O’Keefe and Dostrovsky first found the existence of place cells within the hippocampus in 1971, they conduced a study five years later with rats that demonstrated these place cells would fire whenever the rat was within a certain place in the environment. This was one of the first indications that place cells were related to spatial orientation. They also discovered that space cells fired in different areas of the hippocampus depending on where the rat went, and this whole firing network made up the rat’s environment (O’Keefe 1976, Wilson & McNaughton 1993). As environments changed, the same place cells would fire, but the relationship and dynamic between firing fields would change (O’Keefe & Conway 1978). Therefore place cells are thought to give humans and animals a guide to the environment it is navigating and its position in that environment. Place cells are generally observed through recorded action potentials. As humans or animals navigate large environments and then arrive at a particular location, there is a notable increase in the place cell firing rate once that specific location has been reached (Eichenbaum, Dudchencko Wood, Shaprio and Tanila, 1999).