Cortical area · one in each half of the brain
Cornu Ammonis (CA fields)
Also called Ammon's horn, CA fields, Hippocampus proper
Ammon's horn: the curled band of the hippocampus where memories take shape.
The Cornu Ammonis is the main curled strip of the hippocampus, packed with neurons called pyramidal cells. It is split into four zones, named CA1, CA2, CA3 and CA4. According to one influential theory, CA3 links the pieces of a memory together, so that a small part can bring back the whole memory. CA1 then passes the signal on to the subiculum. In rats, many of its pyramidal cells are "place cells" that fire when the animal is in a particular spot.
Where it is
Inside the hippocampus, forming a C-shaped band that hooks into the dentate gyrus, like two letter Cs interlocked. CA1 lies next to the subiculum, and CA4 sits tucked inside the curve of the dentate gyrus.
What it does
Recalling from a clue
CA3 cells are richly connected to one another. According to one influential theory, this lets a small part of a memory call back the whole thing.
Sending memories out
CA1 collects the processed signal and passes it on to the subiculum, and from there back to the entorhinal cortex.
Mapping places
In rats, many of its pyramidal cells are "place cells" that fire when the animal is in a particular spot. They help it recall places and the routes between them.
Replaying the day
In rats, firing sequences seen in CA1 while the animal ran on a wheel were later replayed, sped up, during deep sleep. This replay may help lock memories in.
Remembering others
In mice, switching off the small CA2 zone caused a big loss of social memory, the ability to remember another mouse, while memory for places stayed normal.
Think of it as an assembly line with four stations, where the raw details of an experience are linked, checked and packaged before being sent out as a memory.
Works with
- Dentate gyrusThe dentate gyrus feeds CA3 through mossy fibers, sparse but powerful connections that form the middle step of the hippocampus's classic three-step pathway.
- SubiculumCA1 hands its output to the subiculum, the hippocampus's main exit.
- Entorhinal cortexThe entorhinal cortex sends signals both to the dentate gyrus and directly to CA3 and CA1, and gets the hippocampus's output back through the subiculum.
- Pyramidal neuronIts main workers are excitatory pyramidal neurons, each with a pointed tip and a base, packed into one band 10 to 30 cells deep.
- FornixSome of its output leaves through the fornix toward deeper parts of the brain.
When it goes wrong
Oxygen shortage
The large neurons of CA1 are extremely sensitive to a lack of oxygen. If their blood supply fails, they can die within minutes, and the person may lose memories of the few hours before.
Epilepsy scarring
The most common form of temporal lobe epilepsy usually comes with hippocampal sclerosis, a kind of scarring. Many neurons die, especially in CA1 and CA4, and support cells called glia multiply in their place, while the CA2 zone is often spared.
Alzheimer's disease
In Alzheimer's disease, CA1 loses many of its pyramidal neurons. Together with early damage to the hippocampus's connections, this may help cause the memory loss.
For learning only, not medical advice. If you have a health concern, talk to a doctor.
Surprising facts
- The dentate gyrus talks to CA3 through mossy fibers, which end in large synapses close to each cell's body. That makes them powerful, even though in rats each CA3 cell gets only about 46 of them.
- CA1 is also called "Sommer's sector". Sommer was one of the doctors who first described the scarring of Ammon's horn seen in epilepsy, in the late 1800s.
By the numbers
Where the name comes from
Its name means "Ammon's horn". Ammon was an Egyptian god shown with a ram's head, and a slice across this tissue shows a C shape that looks like a ram's horn. The "CA" in CA1 to CA4 simply stands for Cornu Ammonis.
Quick quiz
3 questions. Your answers are saved and come back in your daily review.
Which of these is a job of the Cornu Ammonis (CA fields)?
Show the answer
Recalling from a clue. CA3 cells are richly connected to one another. According to one influential theory, this lets a small part of a memory call back the whole thing.
Recalling from a clue. CA3 cells are richly connected to one another. According to one influential theory, this lets a small part of a memory call back the whole thing.
The Cornu Ammonis (CA fields) is part of which larger structure?
Show the answer
Hippocampus. It belongs to the hippocampus.
Hippocampus. It belongs to the hippocampus.
Which part fits this description? "Like an assembly line with four stations, where the raw details of an experience are linked, checked and packaged before being sent out as a memory."
Show the answer
Cornu Ammonis (CA fields). It is the Cornu Ammonis (CA fields). Ammon's horn: the curled band of the hippocampus where memories take shape.
Cornu Ammonis (CA fields). It is the Cornu Ammonis (CA fields). Ammon's horn: the curled band of the hippocampus where memories take shape.
These parts now come back in your daily review, just before you would forget them.
Practice it and its neighbors in 3DSources
- The Anatomy of the Hippocampus, Cerebral Ischemia (Exon Publications), NCBI Bookshelf
- Neuroanatomy, Hippocampus, StatPearls, NCBI Bookshelf
- Temporal Seizure, StatPearls, NCBI Bookshelf
- The mechanisms for pattern completion and pattern separation in the hippocampus, Frontiers in Systems Neuroscience
- The hippocampal CA2 region is essential for social memory, Nature
- Replay and time compression of recurring spike sequences in the hippocampus, Journal of Neuroscience
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