The human brain is a complex network of billions of neurons that orchestrate our thoughts, emotions, and actions. It allows us to perceive the world, learn new information, and recall past experiences. Different regions of the brain work in concert, each contributing specialized functions that enable our cognitive abilities. Understanding how these areas interact provides insight into the mechanisms underlying our mental lives.
The Entorhinal Cortex: A Gateway to Memory
The entorhinal cortex (EC) is a region of the brain’s allocortex, situated in the medial temporal lobe. It serves as a primary interface between the neocortex and the hippocampus. This area processes and relays various types of information, including sensory and spatial data, making it a hub for memory, navigation, and even the perception of time.
The entorhinal cortex contains specialized neurons called grid cells. These cells fire in a repeating hexagonal pattern as an animal navigates an open environment, creating a coordinate system for spatial understanding. Grid cells are important for path integration, allowing the brain to track position by integrating movement, distance, and direction. Their discovery in 2005, alongside place cells, contributed to the Nobel Prize in Physiology or Medicine in 2014.
The Hippocampus: The Brain’s Memory Hub
The hippocampus, named for its seahorse-like shape, is deeply embedded within the medial temporal lobe of each brain hemisphere. It plays a significant role in consolidating information, transforming short-term into long-term memories. It is involved in forming new declarative memories, such as facts and events.
Beyond its role in memory, the hippocampus is also involved in spatial navigation. It contains specialized neurons called place cells, which activate when an animal enters a specific location, defining a “place field.” These place cells form a cognitive map of our surroundings, helping us remember where we are and find our way around. While the hippocampus helps form and consolidate memories, long-term memories are ultimately stored in other cortical regions.
Working Together for Memory and Navigation
The entorhinal cortex and hippocampus engage in an intricate, synergistic relationship that underpins our capacity for memory and navigation. The entorhinal cortex is the primary source of input to the hippocampus, with neural projections forming the perforant path. Axons from entorhinal cortex layers II and III form indirect and direct pathways, respectively, to the hippocampus.
This circuit allows for the encoding, processing, and consolidation of spatial and episodic memories. Grid cells in the entorhinal cortex provide a metric, grid-like representation of space, while place cells in the hippocampus respond to specific locations. The collaboration between these cell types creates a “cognitive map” of our environment, enabling us to navigate effectively. This combined function allows us to remember where events occurred and to navigate through familiar or new surroundings.
When Things Go Wrong
Dysfunction in the entorhinal cortex and hippocampus can have significant consequences, particularly in neurodegenerative conditions. These regions are among the earliest affected in Alzheimer’s disease, leading to significant memory loss and disorientation. Pathological changes, such as the accumulation of tau proteins and amyloid-beta, often begin in the entorhinal cortex and then spread to the hippocampus.
Early degeneration of neurons and synapses in these regions contributes to the initial cognitive decline observed in Alzheimer’s disease. This damage can disrupt network activity within these regions, impacting memory formation and spatial navigation. While Alzheimer’s disease is the most prominent example, other conditions, such as oxygen deprivation or encephalitis, can also damage the hippocampus, resulting in anterograde amnesia—the inability to form new memories.