The human brain is made up of a set of organs, some of which are so small that they are not easily recognized by the naked eye. Mammillary bodies are one of these.

In this article we will see what the mammillary bodies are, what their function is in the brain and how it relates to various parts of the brain.

What are mammillary bodies?

The mammillary bodies are a pair of small, spherical-shaped brain structures located in the limbic system , the part of the brain that is responsible for generating and regulating emotions.

Specifically, they are located in the fornix, also called the trine, an area that connects the hippocampus (responsible for managing the storage and retrieval of memories) to the diencephalon, the latter being a structure located right in the center of the brain and responsible for many vitally important tasks.

As for the composition of the mammillary bodies themselves, consists of a grouping of neuronal nuclei , that is, units in which different neurons are grouped together according to the tasks in which they participate (which, although they may be very similar, differ in different aspects).

The connections of these areas of the brain

The mammillary bodies, being located in the brain trine, intervene in mental processes associated with emotions and memory.

Specifically, the mammillary bodies receive nerve impulses from the amygdala, related to the regulation of hormone levels and to intense emotional responses , and from the hippocampus, which as we have already seen is a kind of directory of memories that are stored in other parts of the brain.

Specifically, the hippocampus works with memories belonging to the declarative memory, while the amygdala manages the emotional memory, i.e., the emotional carcass of memories.

On the other hand, the mammillary bodies send information to the thalamus , the largest brain structure in the diencephalon, which is responsible for integrating sensory information and generating immediate responses to certain stimuli.

But this should not be interpreted as a sign that the mammillary bodies “bridge the gap between the hippocampus and the amygdala on the one hand and the thalamus on the other.

This would be the case if the mammillary bodies were simply a stretch of white substance, i.e. a part of the brain composed simply of neuronal axons (the long part of these nerve cells, which are responsible for sending nerve impulses to remote areas), but let us remember that the composition of these is based on the neuronal nuclei, i.e. grey matter, areas in which the neuronal somas pile up and are in almost direct contact with each other.

So the mammillary bodies do not play a passive role in the brain, they do not just send nerve signals to other areas, they process that information and transform it into something else, even if it is only slightly different and destined to mix with the tangle of nerve signals that go up to the top of the brain.

What is your function?

At the moment little is known about the exact function of the mammillary bodies, among other things because they are so small that it is complicated to isolate their processes from others that are part of the overall functioning of the trine. We only know the generic role that it has in connecting areas involved with emotion and memory with the thalamus , which in turn is one of the most complex structures of our nervous system (for example, it has been attributed the function of being the “button” that turns on consciousness).

Further research is therefore needed to find out exactly what the function of these tiny sets of neurons is and to bear in mind that it is perhaps more useful to consider them not as something isolated but as part of a larger process carried out by the trine or even by the joint action of the trine and other parts of the brain.

Associated brain injuries, and their effects

Injuries to the mammillary bodies have been found to be often associated with Wernicke-Korsakoff syndrome.

As the connectivity of the mammillary bodies suggests, among the most prominent symptoms of this syndrome are memory problems. Specifically, antegrade amnesia stands out, whereby it is not possible or very difficult to create new memories from what is experienced in the present.

Thus, clinical cases of patients with this type of lesions suggest that mammillary bodies are highly involved in memory. However, this region is also particularly damaged in other diseases, such as Alzheimer’s or schizophrenia .

The fact that each of these neurological or psychiatric conditions has a very wide range of symptoms and that it is not known whether a symptom is specifically due to damage to the mammillary bodies or to injury in other parts of the central nervous system makes it unclear what the specific function of this set of brain structures is.