🧠 Understanding the Hippocampus and the Trisynaptic Circuit: How We Form Memories

This article explains the structure of the hippocampus and the flow of information through the trisynaptic circuit, which are key components of how we process and store memory.

🧭 A Quick Look at Brain Anatomy

The brain is divided into four main lobes:

• Frontal lobe: involved in decision-making, movement, and planning
• Parietal lobe: integrates sensory input and helps with spatial awareness
• Occipital lobe: responsible for visual processing
• Temporal lobe: important for memory and auditory perception

Deep within the medial temporal lobe, you’ll find structures that make up the limbic system, which is associated with emotion, motivation, and memory. One of the most important structures in this system is the hippocampus, which is found on both sides of the brain.

🧠 What Is the Hippocampus?

The name hippocampus comes from the Greek word for “seahorse,” reflecting its curved shape. It is part of the hippocampal formation, which includes:

• The parahippocampal gyrus
• The entorhinal cortex
• The subiculum
• The dentate gyrus

The hippocampus plays a major role in:

• Declarative memory, which includes facts and events you can consciously recall
• Episodic memory, the ability to remember specific experiences
• Spatial navigation, such as remembering routes or locations

Research and clinical cases have shown that damage to the hippocampus often results in anterograde amnesia, where people lose the ability to form new long-term memories.

It is also important to remember that not all types of memory rely on the hippocampus. Non-declarative memory, such as habits and skills, is primarily supported by structures like the basal ganglia.

🔄 The Trisynaptic Circuit: How the Hippocampus Processes Information

The trisynaptic circuit describes how signals travel through different parts of the hippocampus. It involves three major synapses and helps explain how information is encoded and organized during learning.

1. Entorhinal Cortex to Dentate Gyrus (Perforant Pathway)

The entorhinal cortex (EC) acts as the main input hub, receiving information from the rest of the brain and sending it into the hippocampus. It transmits sensory and cognitive input through a bundle of fibers known as the perforant pathway to the granule cells of the dentate gyrus.

2. Dentate Gyrus to CA3 (Mossy Fiber Pathway)

The dentate gyrus sends projections to the CA3 region via the mossy fiber pathway. These mossy fibers connect to a relatively small number of pyramidal neurons but create strong and specific signals. Each mossy fiber can connect to around 35 CA3 cells, which supports pattern separation—the ability to distinguish between similar experiences.

The name CA stands for cornu ammonis, referencing the horned shape of the hippocampus, which reminded early anatomists of the Egyptian god Amun. The hippocampus is divided into subfields: CA1 through CA4.

3. CA3 to CA1 (Schaffer Collateral Pathway)

Neurons in CA3 project to CA1 via the Schaffer collateral pathway. This connection plays a central role in long-term potentiation (LTP), a process where synaptic strength increases with repeated use. CA1 contains a high concentration of NMDA receptors, which act as "coincidence detectors" and are essential for memory formation.

4. CA1 to Subiculum to Entorhinal Cortex (Loop Closure)

The output from CA1 continues to the subiculum, the main output zone of the hippocampus. The subiculum then sends information back to the entorhinal cortex, completing the circuit, and also sends outputs to other brain regions via the fornix.

The subiculum receives input from several areas of the cortex, including the entorhinal, perirhinal, and prefrontal cortices. It sends projections both back into the hippocampal loop and out toward cortical and subcortical regions for memory consolidation.

🧠 How the Hippocampus Supports Memory

The hippocampus acts like a memory index. Just as a computer index helps you retrieve the right file, the hippocampus helps access distributed memory traces stored throughout the brain. When you recall a memory, such as how to play a song on the piano, the hippocampus helps reactivate the different brain areas involved in that memory.

The stronger and more frequently used the neural connections are, the faster you can recall the information. This is why repeated learning or practice makes it easier to remember or perform tasks.

The cortical representation of a memory refers to how that memory is stored across multiple areas of the brain. The hippocampus serves as the central system that coordinates and reactivates this widespread network.

🧩 Summary of the Trisynaptic Circuit

Pathway:

1. Sensory input arrives at the entorhinal cortex
2. Information is passed to the dentate gyrus
3. Signals travel to CA3 via mossy fibers
4. CA3 connects to CA1 through Schaffer collaterals
5. CA1 projects to the subiculum
6. The subiculum sends information back to the entorhinal cortex and through the fornix to other brain areas

Key idea:
Repeated activation of this circuit strengthens connections through long-term potentiation (LTP). These stronger connections allow the hippocampus to help the cortex store and retrieve memories more efficiently.