The mammillary bodies are a pair of small, rounded structures in the brain, named for their resemblance to breasts. Located on the undersurface of the brain, they are part of the diencephalon. These structures are involved in several neurological processes.
Anatomy and Location of the Mammillary Bodies
The mammillary bodies are situated at the posterior end of the hypothalamus, forming part of the limbic system. Anatomically, they are positioned at the ends of the anterior arches of the fornix, a C-shaped bundle of nerve fibers. This placement situates them between the midbrain and the main portion of the hypothalamus.
Each mammillary body is composed of two main groups of nuclei: the medial mammillary nuclei and the lateral mammillary nuclei. The medial nucleus is the larger of the two. These nuclei serve as connection points within a larger network, linking different brain regions. Their blood is supplied by branches of the posterior communicating artery.
The mammillary bodies have connections to other brain structures. They receive incoming nerve fibers, known as afferent connections, primarily from the hippocampus through the fornix. In turn, they send outgoing fibers, or efferent connections, to the anterior nuclei of the thalamus. This specific pathway is called the mammillothalamic tract.
Primary Functions of the Mammillary Bodies
The most well-documented function of the mammillary bodies relates to recollective memory. Studies involving rats with lesions on their mammillary bodies have shown that damage, particularly to the medial mammillary nucleus, results in spatial memory deficits. Other functions may include involvement in emotion and goal-directed behaviors.
These structures act as a relay station for impulses traveling from the hippocampus and amygdala to the thalamus, placing them within a neural network known as the Papez circuit. This circuit is involved in memory formation. Within this network, the mammillary bodies process information from the hippocampus and transmit these signals to the anterior thalamus. This transmission is a component of the brain’s system for consolidating and retrieving memories.
Impact of Mammillary Body Damage
Damage to the mammillary bodies can lead to significant memory impairments. The most prominent condition associated with this damage is Korsakoff syndrome. This neurological disorder is most often caused by a deficiency in thiamine, which is frequently linked to chronic alcoholism, but can also arise from severe malnutrition or other medical conditions.
A lack of sufficient thiamine leads to cellular damage and atrophy, or shrinkage, of the mammillary bodies. This damage disrupts the Papez circuit, interrupting the flow of information necessary for memory. Individuals with Korsakoff syndrome typically exhibit a profound inability to form new memories, a condition known as anterograde amnesia.
They may also experience a loss of past memories, which is called retrograde amnesia. Another characteristic symptom is confabulation, where an individual creates stories to fill in gaps in their memory. This is not intentional deception but rather an unconscious process of the brain attempting to make sense of fragmented information. Other causes of damage to the mammillary bodies, though less common, include localized strokes, tumors, or injury during brain surgery.