Fornix

Fornix is a C-shaped bundle of nerve fibers located deep in the cerebral hemispheres. It is an important output tract of the hippocampus. It is considered as the main connecting tract of the limbic system. Being a part of the limbic system is associated with the processes of memory, emotions and sexual responses.

In this article, we will talk about the fornix and other nerve tracts of the limbic system. We will discuss the structure, histology, and connections of these nerve tracts. We will also talk about their physiological role in the limbic system and the clinical conditions resulting from the destruction of these nerve tracts.

Structure

In this section, we will talk about the structure,
histology, connections, and relations of fornix.

The word fornix is a Latin word meaning “arch”. This
name is used because of the arch-shaped structure. The fornix consists of three
parts; crus, body, commissure, and anterior columns. We will discuss the details
of all these parts below.

Crus of Fornix

The crus is the first part of the fornix. It is made
by fibers continuing from the fimbriae. The crus of the fornix begins at the
posterior end of the hippocampus. One crus is present on each side. The crus
from each side curves posteriorly and superiorly under the splenium of corpus
callosum. The two crura curve around the posterior surface of the thalamus. The
two crura then join together, in the midline, to form the body of the fornix.

Body of the Fornix

The body of fornix begins by joining together of the two crura. They join in the midline, just under the corpus callosum, to form the body of fornix. The body of the fornix is very closely applied to the inferior surface of the corpus callosum.

Septum pellucidum is a very thin nervous structure that connects the body of fornix to the undersurface of corpus callosum. The tela choroidea and the ependymal roof of the third ventricle are located just below the body of fornix.

Commissure

The commissure of fornix is the collection of transverse fibers that connect the two crura of the fornix. The commissure is present just before the beginning of the body of fornix. The function of the commissure is to connect the outputs of hippocampal formations present on the two sides. The transverse fibers in commissure also decussate.

Columns of the Fornix

The body of the fornix anteriorly divides into two columns. The two columns diverge slightly lateral to the midline. These columns then curve downwards. They arch just in front of the interventricular foramen.

These columns descend behind the anterior commissure. They disappear in the lateral wall of the third ventricle, pass through the grey matter and reach the base of the brain. Here, they end in the mammillary bodies.

Other Connections of Hippocampus

It is important to understand other connections of the
Limbic system along with the fornix.  The
connections associated with the fornix include the following.

Alveus

The output tract of the hippocampus or the limbic system begins with the alveus. It is a thin layer of white matter located just above the ventricular surface of the hippocampus. It, in fact, covers the superior surface of the hippocampus.

It consists of the nerve fibers originating from the cortex of the hippocampus. The fibers of the alveus converge on the medial side of the hippocampus to form the fimbria.  

Fimbria

Fimbria are the main output tract of the hippocampus.
They leave the posterior end of the hippocampus and continue as the crura of
the fornix.

Mammillothalamic Tract

It is the terminal part of the output tract of the
limbic system. The mammillothalamic tract begins in the mammillary bodies and
terminates in the anterior nucleus of the thalamus. In this way, the output
tract of the limbic system reaches its final destination.

Physiology

In this section, we will talk about the functions of the fornix. It should be kept in mind that no specific function is associated with the fornix. The functions of the fornix are the same as that of the hippocampus and the limbic system.

It assists the limbic system to carry out its functions properly. So, we will also include the functions of the limbic system in this section.

Serving  as a
Connecting Tract

Fornix is the main output tract of the hippocampus. Its main function is to transmit the information from the hippocampus to the mammillary bodies and the anterior nucleus of thalamus.

The commissure of the fornix also serves to connect the two hippocampal formations. In this way, the information arising in the two hippocampal formations can be exchanged freely.

Role in Recall Memory

Fornix is considered to play an important role in memory. It is important for the recall memory. People who are deprived of the fornix are unable to perform the delayed recall tasks. The cognitive memory is not associated with the fornix.

Role in Emotional Memory

The fornix, along with the mammillothalamic tract is associated with the emotional memory. It is a general experience that the events that have some emotional importance remain buried in our minds for quite a long time.

They are very hard to forget and can be recalled easily, even after months or years. This fast recall of the emotional memory is due to the combined activation of the fornix and the mammillothalamic tract.

Functions of the Limbic System

The functions of the limbic system that are assisted
by the fornix include the following:

• Spatial memory
• Learning, especially deep learning
• Emotional processing
• Emotional response and behavior
• Control  of sexual behavior
• Social processing
• Eating behaviors of a person

All these functions are mainly performed by the
hippocampus and amygdala. The fornix just assists them by connecting the limbic
system to other parts of the brain.

Clinical Conditions

Tumors

Fornix may be affected by the tumors that spread along the route of fornix. The most common is glioblastoma. The tumors that involve the fornix can cause loss of memory and behavior of a person. Profound changes can be seen in the behavior of patients with the fornix involvement. The involvement of fornix also causes memory dysfunction.  It results in anterograde amnesia and retrograde amnesia.

The important tumors that involve the fornix include
the following;

• Glioblastomas
• Astrocytomas
• Lymphomas

Infections

Infections of the brain and meninges can also involve the fornix and the limbic system. The involvement of fornix is most commonly seen in patients with herpes simplex encephalitis.

A research study proved the involvement of fornix in nine out of ten herpes simplex encephalitis patients. The brain of these patients was studied after 6 months of acute infection.

Multiple Sclerosis

A research study found the demyelination of the nerve
fibers in fornix of a patient who had died of multiple sclerosis. However, any
other suitable evidence is not present in literature about the involvement of
fornix in multiple sclerosis.

Wernicke’s Encephalopathy

This syndrome of the brain is characterized by ataxia, altered consciousness, and ophthalmoplegia along with anterograde amnesia. The anterograde amnesia seen in Wernicke’s encephalopathy is suggestive of the involvement of fornix in this disease.

The imaging studies have confirmed the involvement of fornix in two cases of Wernicke’s encephalopathy studied so far.

Conclusion/Summary

The fornix is the main output tract of the limbic system connecting mainly the hippocampal formations, present on the two sides, with the thalamus.

The structure of fornix is divided into 4 parts; the crura,
body, commissure and the columns.

The crura of fornix begin on each side as a continuation of the fimbria.  The curve over the hypothalamus and join in the midline to form the body of the fornix.

Just before the union of crura, the commissure of the
fornix connects them. The commissure comprises of transverse fibers and serves
to connect the two hippocampal outputs together.

The body of the fornix continues in midline, just beneath the corpus callosum, separated from it via septum pellucidum.

The body then divides into two anterior columns that
curve downwards, pass through the lateral wall of the third ventricle and end
in the mammillary bodies.

The fornix is a continuation of fimbria which
originate from the alveus of the hippocampus. The fornix anteriorly continues
as the mammillothalamic tract that originates from the mammillary bodies and
ends in the anterior nucleus of thalamus.

Regarding the physiology of the fornix,  no specific function is associated with with this structure. However, it assists in the functions of the limbic system such as memory, behavior, and learning.

The clinical conditions that can involve the fornix
include;

• Tumors
• Multiple sclerosis
• Wernicke’s encephalopathy
• Infections

References

1. Adam G. Thomas;  Panos Koumellis; Robert A. Dineen. “The Fornix in
   Health and Disease: An Imaging Review” Published Online:Jul 8 2011 https://doi.org/10.1148/rg.314105729.
2. Gaffan, D. (1994). "Scene-specific
   memory for objects: a model of episodic memory impairment in monkeys with
   fornix transection". Journal of
   Cognitive Neuroscience. 6 (4): 305–320. doi:10.1162/jocn.1994.6.4.305. ISSN 0898-929X. PMID 23961727.
3. Aggleton, J.
   P.; McMackin, D.; Carpenter, K.; Hornak, J.; Kapur, N.; Halpin, S.; Wiles,
   C. M.; Kamel, H.; Brennan, P.; Carton, S.; Gaffan, D. (2000-04-01). "Differential
   cognitive effects of colloid cysts in the third ventricle that spare or
   compromise the fornix". Brain. 123 (4):
   800–815. doi:10.1093/brain/123.4.800. ISSN 0006-8950.