The limbic system is a grouping of cortical and subcortical structures that play fundamental roles in processes such as emotion, motivation and memory consolidation.

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 other regions and fibres of this group of structures allowing them to carry out key processes for human cognition and emotion.

What is the fornix?

The phornix or trine 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, the thalamus, the hypothalamus, the septal nuclei and the nucleus accumbens, which is very relevant to 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 phornix is considered to be one of the components of the white matter of the brain. In contrast, what we call the grey matter is mainly made up of the bodies of the neurons.

The word “fornix” comes from Latin and can be translated as “bow”. 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 in the conjunctiva of the eyes and in the vagina with the same name. It is also known as the trine, sack and vault of the four pillars .

Location and anatomy

The fornix has its origin in the hippocampus, a fundamental structure for long-term memory and spatial orientation. These neural beams then surround the thalamus and are directed towards the frontal cortex of the brain.

The fornix projects towards the anterior commissure, which is located between the temporal lobes. At this point the fibers of the fornix divide to connect to various subcortical structures : the septal and preoptic nuclei, the ventral striatum, and the mammillary bodies of the hypothalamus, which regulate the release of hormones allowing for 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 to the mammillary bodies of the hypothalamus, while the posterior ones go to the tonsil bodies of the hippocampus.

Functions of the Fornix

The fornix plays significant roles in cognition because of 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 consolidation and recovery of memories, as found through studies in people with damage to this structure.

Lesions in the fornix, which are often caused by tumours, neurodegenerative diseases and strokes, can lead to antegrade amnesia, which consists of an inability to consolidate new information in long-term memory .

Damage to the fornix has been found to affect free memory more than recognition memory, so the deficits are mainly related to the recovery of episodic memories.

It is important to point out 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 its links with the large bundle of fibres 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 which, according to this author, was the biological basis of emotions. The fornix was one of the structures that made up this network, known today as the “Papez circuit”.

According to Papez, the appearance of an emotional stimulus would be detected by the thalamus; from there the information would be transmitted to the hypothalamus and the sensory cortex. The perception and evaluation of the 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 now known that, although Papez’s contributions were relevant and useful in advancing research on these structures, the brain circuitry 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, the midbrain and the corpus callosum .

The structures that make up the limbic system are not only associated with emotions, but are also fundamental to the consolidation and recovery of memories, attention, motivation, basic physiological needs or sexual responses.