The Hippocampus in 400 Words

I have to apologize for the paucity of posts on this blog. The grind of my semester got to me. After a few week break it’s time for a neuroanatomy post.

WIthin the temporal lobe of the brain is an elongated structure called the hippocampus. Some people have compared its shape to that of a seahorse (the word hippocampus comes from the Greek {hippos + campos}, which roughly means “seahorse”). This structure is special for a number of reasons. One is its role in memory encoding and consolidation.

From cytoarchitectonic standpoint, the hippocampus is special because unlike the surrounding cortex, it consists of only three layers instead of six. The hippocampus is phylogenetically an old part of the cortex, which means that it is an older branch on the evolutionary tree, whereas the rest of the cortex (more accurately called the neocortex), especially cortex of the frontal lobes, is a much newer development.

The hippocampus (to be more accurate, there are two hippocampi – one in each cerebral hemisphere) resides within the medial portion of the temporal lobe. It is continuous with the parahippocampal cortex, entorhinal cortex (the hippocampus receives its main input from this cortex), and perirhinal cortex. I’ll not write about the internal structure of the hippocampus, which becomes fairly complex, due to the brief nature of this post.

The hippocampus sends white matter tracts off its dorsal and posterior portions (the hippocampus also communicates through other tracts and pathways – this circuit is not the only output of the hippocampus). These white matter tracts are the fimbria of the hippocampus (technically, the fimbria are the “offshoots” of the alveus of the hippocampus). The fimbria proceeds upwards from the posterior portion of the hippocampus, at which point it ceases to be the fimbria and is called the fornix.

The fornices (plural of fornix) are prominent white matter tracts passing above the thalamus and medially in the brain. The fibers travel forward, then turn downward just posterior to the anterior commissure (a white matter tract that connects both hemispheres) to terminate in the mammillary bodies, two bumps on the ventral side of the brain. They are part of the hypothalamus of the brain. From there, the pathway courses upward through the mammilothalamic tract (MTT) to the anterior nucleus of the thalamus. From there axons course to the cingulate gyrus, then to the underlying cingulum (large white matter tract), and back to the hippocampus (via the parahippocampal and entorhinal cortices). This circuit is part of the limbic system and is called the Papez (pronounced “papes” – rhymes with capes) circuit. This circuit is important for emotion (and memory).