The mammillary bodies are two structures, the medial mammillary nucleus and the lateral mammillary nucleus, found on the underside of the brain and considered parts of the limbic system. Both are linked to the hypothalamus via a nerve path called the fornix and feature a series of neural projections that connect to other parts of the brain. The mammillary bodies are also a relay for signals that travel from the hippocampus and amygdale to the thalamus, a system called the Papez circuit. They are important to memory processing, so damage caused by either physical destruction or nutritional deficiencies can lead to amnesia.
Both the lateral and medial mammillary nuclei have connections to both the hippocampus and thalamus, carried by nerve fibers through the fornix. The nuclei connect with the same parts of the brain, although each is linked to differing sections of the same structures. They also have different functions, as the medial mammilliary nuclei has cells that fire according to the direction someone is facing and how fast the head moves. There are also cells that do this in the thalamus and the hippocampal area.
The medial nuclei of the mammillary bodies are comprised of one to five subnuclei depending on the type of animal, while the lateral mammillary nucleus contains the largest neurons in the system. Cellular signals for head motion and angle in the lateral nucleus are relayed to the hippocampus, which generates a theta rhythm that is important for spatial memory. Cells in the medial mammillary nucleus fire in the same pattern as those in the hippocampus, and the mammillary bodies seem to relay this rhythm to other areas.
Damage to the mammillary bodies leads to amnesia. The study of memory loss due to such damage or from a deficiency in thiamine was studied in the late 1800s and is known as Wernicke-Korsakov syndrome. Amnesia characterized by a loss of memories following a traumatic event is common with this condition, but long-term memories might be unaffected. Lesions in this area and other parts of the limbic system are often found in people that are chronically amnesic, and hypoxia due to sleep apnea can cause damage as well.
The mammillary bodies form a crucial component of the brain’s circuitry. Integrally connected to other areas of the brain, biologists thought they were another section of the hypothalamus. Memory, recognition, spatial awareness, and interpreting the context of events are affected when the area is damaged or dysfunctional.