The fornix cerebri is part of the limbic system and forms a curved projection pathway between the mamillary bodies (corpora mamillara) and the hippocampus. The fornix cerebri can be divided into four areas and consists of fibers of the olfactory pathway. It is associated with memory retrieval, which is why damage to the fornix cerebri leads to corresponding memory impairment.
What is the fornix cerebri?
The fornix cerebri consists of nerve fibers that run as a pathway through the cerebrum (telencephalon). It is also known as the cerebral vault and is shaped like a crescent. Neurology counts the fornix cerebri as part of the limbic system, which is responsible for emotions, memory, drive, and some autonomic tasks, among others. The limbic system includes various brain structures that do not form a self-contained anatomical unit but are interconnected in a functional network. Two of these structures are the corpus mamillare and the hippocampus. The white matter of the fornix cerebri provides a connection between these two brain regions. In this process, the hippocampus acts as the starting point. The fornix cerebri is a neighbor of the septum pellucidum, which is located at the anterior horn of the lateral ventricle. In addition, the fornix cerebri is adjacent to the superior edge of the third ventricle of the brain (ventriculus tertius cerebri). This cavity is located in the diencephalon and is filled with fluid originating in the choroid plexus.
Anatomy and structure
Within the fornix cerebri, four areas can be divided: the corpus, the columnae, the crura, and the commissura. The corpus fornicis – its full name – forms the body of the fornix cerebri. Viewed in longitudinal section, the corpus lies centrally in the brain, below the bar (corpus callosum). It forms the highest point of the fornix cerebri. In the front (anterior) area, the corpus divides into the two columnae, which are symmetrically located on the right and left sides. Anatomy also knows them as fornix pillars. In the right and left hemispheres of the brain, they have the same structure and can be further divided into two parts. The pars tecta lies under a layer of tissue originating from the hypothalamus, whereas the pars libera is adjacent to the foramen interventriculare. This connects the lateral ventricle to the 3rd cerebral ventricle, which the pars libera also traverses. In the posterior (posterior) part, the corpus of the fornix cerebri also divides into right and left parts, namely the crura fornicis or fornix limb. Their curvature bends them simultaneously downward and outward, away from the longitudinal axis of the brain. The crura represent the longest section of the fornix cerebri. Spanning between the crura are the filaments of the commissura fornicis, which corresponds to the hippocampal commissure. The ligament is also known as the taenia.
Function and tasks
Little data are available on the precise function of the fornix cerebri. Most knowledge to date has come from clinical observations. From these, it can be concluded that the fornix cerebri is associated with memory functions. Its functions seem to relate primarily to long-term memory, as individuals with severed fornix cerebri have difficulty remembering events in detail. The anatomical connection of the curved brain structure also fits this observation. The fornix cerebri originates at the hippocampus, which is important for both long-term memory and short-term memory. In addition, it participates in linking various memory contents as well as spatial orientation. The fibers of the fornix cerebri arise from the part of the corticofugal olfactory pathway formed by the uncus and the pes hippocampi. As a projection pathway, the fornix cerebri transmits nerve signals to the corpus mamillare. Like the hippocampus and the fornix cerebri, the corpus mamillare is part of the limbic system, whose functions include not only memory functions, but also emotional processes, drive, and autonomic tasks. The fornix fibers that draw to the corpus mamillare are the postcommissural fibers (after the commissura). In contrast, other fibers, the precommissural fibers, extend to the nucleus septales and nucleus accumbens.
Diseases
Damage affecting only the fornix cerebri but no other brain structures is rare.However, in the course of a neurosurgical operation, a lesion of the projection pathway may occur. For example, the corpus is cut transversely, which medicine calls transection. Such lesions on the fornix cerebri can occur, for example, during the removal of a tumor. As a result, memory problems typically occur that are related to long-term memory. Affected individuals find it difficult to recall memories of events that took place before the damage. Detailed memories in particular prove to be a problem. However, the memory impairment often does not represent complete amnesia, and other memory functions are not necessarily impaired. In a single-case study, Calabrese and colleagues also noted in 1995 the consequences of a lesion on both columnae fornicis in an adolescent patient. In this case, they were able to observe anterograde amnesia but not retrograde amnesia. Anterograde amnesia is characterized by the inability or limitation of affected individuals to recall new memories. At the same time, other cognitive abilities (intelligence, attention, and concentration) showed no abnormalities. Short-term memory performance was also not impaired in this case. It can be deduced that the nature of the complaints associated with fornix cerebri depends, among other things, on the location of the damage.
