The corpus mamillare is a structure in the diencephalon and forms a component of the limbic system. It is also the origin of the tractus mamillothalamicus and the tractus mamillotegmentalis. Damage to the corpus mamillare can lead to memory impairment.
What is the corpus mamillare?
Located in the diencephalon, the corpus mamillare is part of the hypothalamus. It is also known as the nipple body because of its shape, and is located between the two cerebral crura cerebri. These belong to the midbrain (mesencephalon) and are found there in the anterior region. Due to its anatomical connections and functions, the corpus mamillare belongs to the limbic system. In its entirety, the limbic system is primarily responsible for emotions and memory processes, but its overall tasks are very complex. In addition to the corpus mamillare, it includes the cingulate gyrus, the hippocampus, the parahippocampal gyrus, the amygdala, the septum pellucidum, parts of the thalamus, and the fornix. The corpus mamillare is present only once in the brain of most animals, but in humans and other primates the nipple is present in pairs.
Anatomy and structure
The corpus mamillare has a roundish shape. Inside it are two nuclei composed of a dense mass of nerve cell bodies. Anatomy distinguishes between the lateral nucleus mamillaris lateralis and the nucleus mamillaris medialis, which is oriented toward the center. These nuclei play an important role in processing information in the corpus mamillare. The corpus mamillare is connected to the subiculum. The subiculum is the junction from the hippocampus to the parahippocampal gyrus. It is adjacent to the ammonic horn (cornu ammonis) and is part of the hippocampus. From the subiculum, nerve tracts travel to the fornix, among other places, which they traverse to terminate at the corpus mamillare. Other nerve fibers begin in the two nuclei of the corpus mamillare and run to two structures. One such nerve pathway is the tractus mamillothalamicus, which connects the corpus mamillare to the anterior nuclei of the thalamus. These anterior nuclei are the thalamic nucleus anteroventralis, the nucleus anteromedialis, and the nucleus anterodorsalis. Another nerve pathway that begins at the corpus mamillare is the tractus mamillotegmentalis. It provides a direct connection to the midbrain cap (tegmentum mesencephali).
Function and tasks
The corpus mamillare represents a structure of the limbic system. Phylogenetically, the limbic system represents a relatively ancient part of the brain and is concerned with emotions and memory processes. Researchers today continue to discover new functions and connections within the limbic system. However, the corpus mamillare appears to participate primarily in processes involving memory. The tractus mamillothalamicus, which connects the corpus mamillare to the three anterior nuclei of the thalamus, is part of the Papez neuron circuit. James Papez concluded in 1937 that a network of anatomical structures and neural pathways in the brain is responsible for processing emotions. Papez assumed that the neuronal circuit he discovered caused the development of emotions in the central nervous system. Later, however, this assumption turned out to be a mistake. Today it is known that the activity of the corpus mamillare correlates primarily with memory processes. In the Papez circle, the corpus mamillare is connected to the hippocampus via the fornix. The latter also has a connection to the entorhinal cortex on the other side. This connecting nerve pathway is the tractus perforans. The tractus mamillothalamicus connects the corpus mamillare in the Papez neuron circuit with the thalamus. Subsequently, nerve fibers travel to the cingulate gyrus and on to the entorhinal cortex. The latter is located in the parahippocampal gyrus and plays an important role in Alzheimer’s dementia.
Diseases
In the Papez neuron circuit, the corpus mamillare participates in memory processes. Damage to the mamillary body itself or to structures that also belong to the Papez circuit is therefore typically associated with memory impairment. A well-known example is Alzheimer’s dementia. This is a neurodegenerative disease, the majority of which begins at an advanced age. The disease is characterized by the loss of nerve cells, which progresses and causes a variety of symptoms.The first signs include difficulties in remembering, which initially typically only affect short-term memory. The exact developmental processes of Alzheimer’s dementia are still unknown, so causal treatment is currently not possible. Various therapies aim to slow the progression of the disease and focus on different symptoms. The corpus mamillare can also be affected by malformations, injuries, tumors, hemorrhage and inflammation. In this case, memory impairment is also possible. Damage to the tractus mamillothalamicus is associated with both short-term memory impairment and long-term memory impairment. In the case of long-term memory, the ability to shift new memories from short-term memory to long-term memory suffers in particular. The extent of the impairment is very individual. One possible physical consequence of malnutrition is Wernicke’s encephalopathy, which is also known in medicine as Wernicke-Korsakow syndrome. The disease is based on a deficiency of vitamin B1 (thiamine) and can affect the corpus mamillare, the corpus geniculatum, the nuclei of the thalamus and other brain structures. Wernicke’s encephalopathy often occurs as a consequence of alcohol abuse. Eating disorders such as bulimia, purging-type anorexia, and vomiting in other mental disorders can also lead to vitamin B1 deficiency and thus Wernicke-Korsakow syndrome. Affected individuals are often disoriented and suffer from disorders of consciousness. Disorders of the eye muscles and wide-legged unsteady walking (gait ataxia) form other core symptoms.
