What does the fornix function as in brain?

The fornix is a C-shaped white matter band that forms part of the limbic system, a group of interconnected brain structures that controls emotional responses, learning, and memory. The medial and lateral fornices come together just above the posterior commissure to form a white matter structure shaped like a pillar; the crus or pillar of the fornix.

The fornix then fans out to form the body of the fornix. The crus and the body of the fornix are located just inferior and anterior to the corpus callosum. At the most superior and posterior aspect of the fornix, the body gives rise to the terminal striae, which travels to the hippocampus. The fornix is thus an important pathway that connects the hippocampus to the hypothalamus, basal forebrain, and septal area.

The fornix functions as the major output tract of the hippocampus, carrying signals to structures in the diencephalon, most importantly to the mammillary bodies of the hypothalamus. Signals arising in the CA1 region of the hippocampus are carried via Schaffer collaterals that terminate in the subiculum. From there, signals are carried via the alvear pathway, which continues as the fimbria that ultimately becomes the body of the fornix. The fornix thus connects both the hippocampus and the entorhinal cortex, which provides sensory input to the hippocampus, to the hypothalamus, thalamus, basal forebrain, and septal area.