Place cell
A place cell is a kind of pyramidal neuron in the hippocampus that becomes active when an animal enters a particular place in its environment, which is known as the place field. Place cells are thought to act collectively as a cognitive representation of a specific location in space, known as a cognitive map.[1] Place cells work with other types of neurons in the hippocampus and surrounding regions to perform this kind of spatial processing.[2] They have been found in a variety of animals, including rodents, bats, monkeys and humans.
Place-cell firing patterns are often determined by stimuli in the environment such as visual landmarks, and olfactory and vestibular stimuli. Place cells have the ability to suddenly change their firing pattern from one pattern to another, a phenomenon known as remapping.[3] This remapping may occur in either some of the place cells or in all place cells at once. It may be caused by a number of changes, such as in the odor of the environment.
Place cells are thought to play an important role in episodic memory. They contain information about the spatial context a memory took place in. And they seem to perform consolidation by exhibiting replay – the reactivation of the place cells involved in a certain experience at a much faster timescale. Place cells show alterations with age and disease, such as Alzheimer's disease, which may be involved in a decrease of memory function.
The 2014 Nobel Prize in Physiology or Medicine was awarded to John O'Keefe for the discovery of place cells, and to Edvard and May-Britt Moser for the discovery of grid cells.[4][5]
Properties[edit]
Place fields[edit]
Place cells fire in a specific region of an environment, known as a place field. Place fields are roughly analogous to the receptive fields of sensory neurons, in that the firing region corresponds to a region of sensory information in the environment. However, unlike receptive fields, place cells show no topography, meaning that two neighboring cells do not necessarily have neighboring place fields.[22] Place cells fire spikes in bursts at a high frequency inside the place field, but outside of the place field they remain relatively inactive.[23] Place fields are allocentric, meaning that they are defined with respect to the outside world rather than the body. By orienting based on the environment rather than the individual, place fields can work effectively as neural maps of the environment.[24] A typical place cell will have only one or a few place fields in a small laboratory environment. However, in larger environments, place cells have been shown to contain multiple place fields which are usually irregular.[25] Place cells may also show directionality, meaning they will only fire in a certain location when travelling in a particular direction.[8][26][27]
Disturbances to place cell function[edit]
Alzheimer's disease[edit]
Problems with spatial memory and navigation are thought to be one of the early indications of Alzheimer's disease.[79] Place cells have been shown to degenerate in Alzheimer's mouse models, which causes such problems with spatial memory in these mice.[80] Furthermore, the place cells in these models have unstable representations of space,[81] and cannot learn stable representations for new environments as well as place cells in healthy mice.[82] The hippocampal theta waves, as well as the gamma waves, that influence place cell firing, for example through phase precession, are also affected.[81]
Aging[edit]
Place field properties, including the rate of firing and spike characteristics such as width and amplitude of the spikes, are largely similar between young and aged rats in the CA1 hippocampal region. However, while the size of place fields in the hippocampal CA3 region remains the same between young and aged rats, average firing rate in this region is higher in aged rats. Young rats exhibit place field plasticity: when they are moving along a straight path, place fields are activated one after another. When young rats repeatedly traverse the same straight path, connection between place fields are strengthened due to plasticity, causing subsequent place fields to fire more quickly and causing place field expansion, possibly aiding young rats in spatial memory and learning. However, this observed place field expansion and plasticity is decreased in aged rat subjects, possibly reducing their capacity for spatial learning and memory.[83]
This plasticity can be rescued in aged rats by giving them memantine, an antagonist that blocks the NMDA receptors which is known to improve spatial memory, and was therefore used in an attempt to restore place field plasticity in aged subjects. NMDA receptors, which are glutamate receptors, exhibit decreased activity in aged subjects. The application of memantine leads to in increase in place field plasticity in aged rat subjects.[84] Although memantine aids in the encoding process of spatial information in aged rat subjects, it does not help with the retrieval of this information later in time.
Aged rats further show a high instability in their place cells in the CA1 region. When introduced to the same environment several times, the hippocampal map of the environment changed about 30% of the time, suggesting that the place cells are remapping in response to the exact same environment.[84] Contrarily, the CA3 place cells are show increased plasticity in aged subjects. The same place fields in the CA3 region to activate in similar environments, whereas different place fields in young rats would fire in similar environments because they would pick up on subtle differences in these environments.[84] One possible cause of these changes in plasticity may be increased reliance on self-motion cues.[84]
