Deep within the brain lies a navigation system, a biological GPS that allows us to understand our place in the world. Central to this system are specialized neurons known as grid cells. Located in the entorhinal cortex, these cells form a coordinate system that the brain uses to track our position. This internal map functions like a sheet of graph paper overlaid on our surroundings, providing a framework for knowing where we are, where we have been, and how to get to our destination.
The Discovery of the Brain’s GPS
The discovery of this internal positioning system was a multi-decade scientific journey. The first piece of the puzzle emerged in the 1970s when John O’Keefe discovered “place cells” in the hippocampus. These neurons become active only when an animal is in a specific location, acting as a “you are here” signal.
Decades later, in 2005, a team in Norway led by Edvard and May-Britt Moser made another discovery while studying rats. They found neurons in the entorhinal cortex that fired in a different pattern from place cells. Instead of firing in one location, these cells became active at multiple points, forming a regular, repeating pattern across the environment. This work identified grid cells, revealed the brain’s coordinate system, and earned the three scientists the 2014 Nobel Prize in Physiology or Medicine.
How Grid Cells Map Space
A grid cell’s defining characteristic is its precise firing pattern. As an animal explores an area, a single grid cell fires at multiple locations. When mapped, these firing locations form the vertices of a grid of equilateral triangles, creating a hexagonal pattern similar to a honeycomb. This arrangement is consistent across any open environment, providing a universal metric for space.
Spatial mapping is achieved by many grid cells organized into modules. Within a module, grid cells share the same spacing and orientation but have offset grids. Different modules also have different scales; some grids are fine-grained with closely spaced firing fields, while others are coarse with widely spaced fields. This variation allows the brain to map both small and large environments. The system functions independently of landmarks, operating even in darkness by integrating information about the body’s own movement.
The Brain’s Full Navigation System
Grid cells do not operate in isolation but are part of an integrated network that creates a comprehensive sense of our surroundings. This system combines the coordinate information from grid cells with signals from other navigational neurons to form a rich and dynamic map.
Working alongside the place cells that signal a specific location, this network also includes other specialized neurons. Head direction cells function like an internal compass, firing only when the head points in a particular direction. Border cells, found in the entorhinal cortex, become active when an animal is near a physical boundary like a wall. The continuous interplay between these cell types allows the brain to compute position, direction, and distance, enabling us to navigate complex environments and plan new routes.
Grid Cells Beyond Navigation
Research suggests the function of grid cells extends beyond mapping physical space. The brain appears to repurpose this organizational mechanism to chart non-spatial, conceptual landscapes, using the same neural code to organize thoughts and memories.
Evidence indicates that grid cells play a role in episodic memory, which involves remembering the “what, where, and when” of past events. By providing a scaffold, grid cells may help structure memories by placing events along a timeline. Studies also suggest this system helps organize abstract knowledge, allowing us to understand relationships between concepts. The brain’s grid-like coding may be a method for organizing many forms of information.
Implications for Neurological Disorders
Grid cell function has implications for understanding neurological conditions, particularly Alzheimer’s disease. One of the earliest symptoms of Alzheimer’s is spatial disorientation, where individuals become easily lost in familiar surroundings. This symptom is linked to the health of the entorhinal cortex, one of the first brain regions damaged by the disease.
Research shows the grid cell network is disrupted in the early stages of Alzheimer’s, sometimes before the formation of amyloid plaques. Studies using mouse models of the disease found that deficits in grid cell activity correlate with impairments in spatial navigation. This connection suggests that evaluating grid cell function could offer a way to detect Alzheimer’s earlier than current methods. Understanding why these cells malfunction may also open new avenues for therapies aimed at mitigating the disease’s effects on memory and navigation.