What do the CA1 through CA4 regions do in the hippocampus?

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Patient H.M. and short-term memory loss explained — hippocampus Learn about Henry Molaison (better known as “patient H.M.”), whose short-term memory loss following a lobotomy helped researchers to understand how brain structures are related to brain function.

The CA1 through CA4 regions, collectively known as the cornu ammonis, are subregions of the hippocampus that serve as critical components of the brain's memory and spatial navigation circuitry.

These regions function primarily through two neural pathways:

Trisynaptic Circuit: Information flows from the dentate gyrus to CA3 via the mossy fibre pathway. Subsequently, information is transmitted from CA3 to CA1 along bundles of axons called Schaffer collaterals.
Monosynaptic Circuit: This pathway allows information to bypass the dentate gyrus and CA3, transmitting data directly from the entorhinal cortex to CA1.

Morphologically, these fields are distinguished by their three-layered structure and the use of pyramidal cells as their principal excitatory neurons. Notably, the CA3 region contains a large excitatory recurrent collateral network, where axons loop back to input fibers, representing the largest input source to that specific area. Ultimately, these circuits facilitate the hippocampus's role in binding experiences, supporting spatial mapping, and enabling the formation of long-term memories.1