The fornix is one of the three brain commissures, i.e. an arch-shaped bundle of white matter neuronal fibers. It connects two identical parts across the left and right hemispheres of the brain, as well as different structures with the same function.
It’s located under the corpus callosum, in the middle of the cerebral cortex. The triangular shape of the fornix resembles two symmetrical and arched columns, each stretching toward one of the brain’s hemispheres.
The structures located in the diencephalon and the basal forebrain supply the fornix with the primary information received from the sensory stimuli, which play a big role in the consolidation of memory and emotions.
What Is the Function of the Fornix?
The fornix is a part of the limbic system of the brain. By function, it’s correlated with some particular aspects of memory, as well as with the navigational ability of the brain.
One of its functions is to serve as the primary output of information to the hippocampus. Namely, the sensory signals which the brain receives from the sensory organs form chemical or electrical response in the hippocampus (the part of the brain in charge of the memory processes).
From here, the fornix aids the transmission of information to the septal nuclei (responsible for the formation of feelings of pleasure) where they are processed and translated into the corresponding behavior, reaction, or emotion.
What Regions and Structures of the Brain Does the Fornix Connect?
The fornix connects the hippocampus, the hypothalamus, and the thalamus by creating tracts with its fibers.
The fornix fibers can be divided into:
- Anterior fibers (also called: precommissural fibers)
- Posterior fibers (also called: postcommisural fibers)
These fibers create the main output tracts of the fornix to the other brain structures – the mammillary bodies, septal nuclei, nucleus accumbens, and anterior nuclei of the thalamus.
Some white matter fibers of the fornix also create attachments between the hippocampus and the septal nuclei, the preoptic nuclei, the ventral striatum, the orbital cortex, as well as with the anterior cingulate cortex.
What Are the Elements That Comprise the Fornix?
The body of the fornix (corpus fornicis) has a triangular shape. Its constitutional elements connect at one point in the front and split behind, forming the shape of a triangle.
The fornical structure consists of 4 elements:
-
- Body
- Crura
- Commissure
- Columns
The back parts of this part of the fornix are called columns of the fornix and they make the body of the fornix, while its back parts are labeled as the crura, or ‘legs’.
The side parts of the body of the fornix are linked by a thin triangular lamina behind, which is labeled as psalterium (Lyra). It consists of transverse fibers that join the two lobes of the hippocampus together, thus creating the hippocampal commissure (known as the Commissure of psalterium Davidi or David’s lyre).
Also, there is a horizontal cleft, referred to as the ventricle of the fornix.
The elements which comprise the fornix are located on both sides of the mid-sagittal plane and create connections among themselves across that plane.
What Is the function of the Columns of the Fornix?
The columns of the fornix are, in fact, pillars created by the neural axons which aid the transmission of sensory signals. The two columns of the fornix part ways in the front and behind, while the central parts are joined together in the middle.
There are three types of columns of the fornix:
- Columna fornicis
- Anterior pillars
- Fornicolumns
As a part of the limbic system, they support the brain structures involved with the formation of memory (the hippocampus and the mammillary bodies).
The Indirect Role of the Fornix in the Limbic System
The fornix supports many of the functions performed by the limbic system by helping it connect with the other brain centers. They include:
- Emotional behavior
- Learning processes
- Formation of spatial memory
- Control of the urges
- Social processing
- Compulsive behavior
The Indirect Role of Fornix in Retrieving Memory
The formation of memory mostly occurs in the limbic system of the human brain. As a part of the limbic system and by its axons’ connection to the hippocampus, the fornix is an important part of the processes that integrate the received sensory information with the memory in the human brain.
At the beginning of this process, short term memories are temporarily stored in the cerebral cortex, where the brain adds meaning and context to the stimuli received from the five senses (smell, sight, touch, taste, and hearing).
After that, the new information is transmitted from this area of the brain to the hippocampus. This is the place in the cerebral cortex where the information is joined together and consolidated with the previously stored similar memories.
Since the fornix spreads its neuron fibers through the hippocampus, it participates in the process of episodic memory formation by creating synapses with the neurons of this part of the brain.
The Indirect Role of the Fornix in Episodic Memory
The fornix plays a substantial role in the formation of episodic memory. This type of memory refers to the capacity to re-experience specific personal experiences and events by recalling long-term memory.
Since memory is, in fact, an electric impulse, it carries the signal of the information received from the sensory stimuli to the hippocampus. This part of the brain serves as the primary brain area where the memory is formed.
Due to the fact that the neural fibers of the fornix pass through the hippocampus, the fornix supports all the processes activated in this part of the cerebral cortex, including the formation of episodic memory.
The Indirect Role of the Fornix in Emotional and Spatial Memory
The fornix is also associated with emotional memory by its connections to the mammillothalamic tract.
The mammillothalamic tract is a bundle of nerve fibers that stem from the fornix and the mammillary body and branch into the gray matter of the cerebral cortex. From here, they connect to the thalamic nucleus.
By direct connection of the fibers of the fornix to major structures of the limbic system – the amygdala, thalami, and the hippocampus – the fornix supports the transmission of the information in order to integrate both emotional and spatial memory.
What Happens If There Is Damage to the Fornix?
The damage to the fornix is primarily related to memory loss. Anterograde amnesia is one of the most common diseases which occur due to the dysfunctionality of the fornix. It’s an inability of the affected individual to create new memories.
Regarding the specific types of memory, both episodic and recognition memory are strongly affected by any damage to the brain in the fornix area.
Furthermore, neurodegeneration of the fornix can lead to Alzheimer’s disease – a brain disease that is characterized by a decline in memory, thinking, and reasoning.
Other clinical reports point to the following medical conditions and diseases, also caused by damage to the fornix:
- Holoprosencephaly – This is a rare medical condition when the fornix is not formed during the fetal development of the brain. It’s a disorder caused by a congenital failure of the prosencephalon.
- Glioblastoma multiforme – This is the most aggressive and invasive type of cancer that occurs in the brain structures. It develops in the glial astrocyte oligodendrocyte cells of the front and temporal lobes of the brain. Since it also severely inhibits the blood supply to the brain, the delivery of medicaments is limited, which further contributes to the negative outcome of this disease.
- Herpes Simplex encephalitis – It’s an infection that affects the gray matter elements of the limbic system, including the fornix. The infection causes swelling of the fornical body.
- Multiple sclerosis – An unpredictable disease of the central nervous system which curtails the transmission of both chemical and electrical signals within the brain, which results in dementia, and difficulties in learning and storing long-term memories.
- Asymmetry of the fornix – Asymmetry of the fornix occurs as a result of an abnormality of the hippocampus. This is noted as a loss of volume of the fornix due to the degeneration of hippocampal neurons.
- Wernicke’s Encephalopathy – This is a neurological and life-threatening condition that affects both the peripheral and the central nervous system. It stems from a lack of vitamin B-1 and manifests with loss of consciousness and abnormal movement of the eyes.
To Sum Up
The fornix is a brain commissure responsible for the transmission of the cognitive information between the left and right sides of the brain. It has no specific function, but it supports several of the vital cognitive processes that happen in the brain.
From a physiological point of view, the fornix connects the hippocampus with the hypothalamus, links the mammillary bodies of the hypothalamus with the anterior nuclei of the thalamus, and annexes the hippocampus to both the septal nuclei and the nucleus accumbens.
Our ability to remember some past events or feelings from our life involves two parts of the cerebral cortex: the hippocampus and the middle part of the prefrontal cortex.
Since the fornix, as a commissure made of white matter, probes the hippocampus with its neural fibers, it is also a part of the memory storage and the integrational memory processes that develop in the human brain.
Fun Facts
Did you know?
- Forgetting recently learned information and confusion could be some of the first warning signs of Alzheimer’s disease.
- Apart from disease-related memory loss, there is also age-related memory loss which is part of the normal processes of the brain that function as prevention from information overload. Such everyday examples are when we forget to record our favorite TV show or get confused about what day of the week it is.
- As part of the diencephalon, the mamillary bodies play a vital role in the formation of recollective memory. They are a paired structure, found on both of the brain’s hemispheres. Apart from their role in memory, they have also been credited with the support of maintaining a sense of direction.
- The blood supply system of the fornix consists of the perforating branches of the hypothalamic arteries, the anterior cerebral artery, the posterior pericallosal artery, and the lateral posterior choroidal artery.
- Lack of sufficient accumulation of Vitamin B in the brain can trigger the Wernicke-Korsakoff syndrome, and lead to memory loss.
- The retrieval of episodic memory requires consciousness. It is not possible to unconsciously recollect any ‘episode’ of our personal life.
- According to the decaying theory, memories can be forgotten if we don’t try to recollect them often.
- The plural form of fornix is fornices.
- The Latin meaning of the term fornix is arch, denoting the white matter fibers positioned like an arch around the thalamus.
- The fornix does not have a specific function in the brain but is nonetheless one of the most active ‘assistants’ to the other parts of the brain.