Fornix (or Cerebral Trigone): Anatomy And Functions
The limbic system is a group of cortical and subcortical structures that play fundamental roles in processes such as emotion, motivation, and the consolidation of memories.
In this article we will describe The anatomy and functions of the fornix, one of the main components of the limbic system which connects the rest of the regions and fibers of this group of structures, allowing them to carry out key processes for human cognition and emotion.
The fornix or trigone is a set of nerve fibers in the brain that is part of the limbic system, involved in emotion, motivation, and long-term memory It connects different regions of the brain such as the hippocampus, thalamus, hypothalamus, septal nuclei and nucleus accumbens, very relevant in the function of dopamine and GABA.
Since it is composed of neuronal axons covered by myelin sheaths with a role in promoting electrochemical transmission, the fornix is considered to be one of the components of the white matter of the brain. In contrast, what we call gray matter is made up mainly of the bodies of neurons.
The word “fornix” comes from Latin and can be translated as “arch.” This term is used to refer to different parts of the body with an arched shape; In addition to the fornix of the brain, there are also structures with the same name in the conjunctiva of the eyes and in the vagina. It is also known as the trigon, the cul-de-sac and the vault of the four pillars
The fornix has its origin in the hippocampus, a fundamental structure for long-term memory and spatial orientation. From the hippocampus, a series of nerve fibers arise that are called the fimbria and are considered part of the fornix. These neuronal bundles then surround the thalamus and head towards the frontal cortex of the brain.
The fornix projects towards the anterior commissure, which is located between the temporal lobes. In this point fibers from the fornix divide to connect with various subcortical structures : the septal and preoptic nuclei, the ventral striatum, and the mammillary bodies of the hypothalamus, which regulates the release of hormones allowing basic physiological functions.
In this structure we find a nucleus, the body of the fornix, and two anterior and two posterior projections; These are also called pillars, which explains the name “vault of the four pillars”. The anterior ones connect with the mammillary bodies of the hypothalamus, while the posterior ones go to the amygdaloid bodies of the hippocampus.
Functions of the fornix
The fornix plays significant roles in cognition due to its importance as part of the limbic system, which we will discuss in the next section. However, its main functions seem to be related to the consolidation and recovery of memories, as has been found through studies in people with damage to this structure.
Lesions in the fornix, which are frequently caused by tumors, neurodegenerative diseases, and stroke, can cause anterograde amnesia, which consists of a inability to consolidate new information into long-term memory
Damage to the fornix has been found to affect free recall to a greater extent than recognition memory, such that deficits are primarily related to the retrieval of episodic memories.
It is important to note that the fornix allows the connection between the different regions that make up the limbic system as well as the two hemispheres of the brain (thanks to their links with the large bundle of fibers that we know as the corpus callosum) and the anterior and posterior regions of the cortex.
The Papez circuit and the limbic system
In 1937, the neuroanatomist James Papez made a description of the brain circuit that, according to this author, constituted the biological basis of emotions. The fornix was one of the structures that made up this network, currently known as the “Papez circuit.”
According to Papez, the appearance of an emotional stimulus would be detected by the thalamus; From this the information would be transmitted to the hypothalamus and the sensory cortex. The perception and evaluation of emotional experience would take place in the cingulate cortex. The fornix connects the cortex with the thalamus, hypothalamus, and hippocampus so it would be key in this process.
It is currently known that, although Papez’s contributions were relevant and useful for the advancement of research on these structures, the brain circuit on which emotions depend is much more extensive. Nowadays we talk more about the limbic system, which includes these regions and others such as the amygdala, midbrain, and corpus callosum
The structures that make up the limbic system are not only associated with emotions, but are also fundamental for the consolidation and retrieval of memories, attention, motivation, basic physiological needs or sexual responses.
