Fornix of the Brain
A brief structural organization of the cerebrum was outlined under the topic “Cerebral Cortex”, in which the human brain commissures (the commissural fibers) were defined as a category of white matter of the cerebral hemisphere which connect identical areas of the cortices (singular: cortex) of right and left cerebral hemispheres (that is, they interconnect identical in the two hemispheres). Other categories of white matter tract include the Projection fibers and Association fibers. It was also mentioned that the corpus callosum is the largest of the commissural fibers. Other commissures of the brain includes the following:
- Anterior commissure
- Posterior commissure
However, the fornix does not only connect identical areas across the two cerebral hemispheres, as indicated in the definition above, but also connects different structures belonging to and that have common functions. The fornix primarily function to connect the:
- Hippocampus to the mammillary bodies of the hypothalamus
- Mammillary bodies to the anterior nuclei of thalamus
- Hippocampus to the septal nuclei and nucleus accumbens, etc
All of these brain structures – hippocampus, hypothalamus, thalamus, septal nuclei, as well as the amygdala, cingulate cortex, entorhinal cortex, perirhinal cortex, parahippocampal cortex, reticular formation of the midbrain, and the olfactory areas constitutes a system known as the Limbic System. The limbic system is involved in motivation, emotion, learning, memory, as well as other cognitive functions. It also forms the area of the brain where the subcortical structures meet the cerebral cortex.
Some authors describe the “Limbic”, which is a Latin word for “border”, as an intermediate or transition state, which is a border. Hence in the case of the limbic system of the brain, the system forms a border between the neocortex and the subcortical structures (the diencephalon). Therefore, the fornix is a commissural fiber of the brain that forms an integral part of the limbic system, and it is important for the transfer of complex cognitive information between the cerebral hemispheres.
Embryogenesis of the fornix begins as two fiber bundles arising in the area of precommissuralis and passes dorsally into the hippocampal primordium of the lamina reunions (lamina terminalis). Those fibers (future fornices – pleural for fornix) pass towards the medial wall of their ipsilateral hemispheric vesicle as early as week 8 of intra-uterine life, and diverge as they do so.
By weeks 10 and 11, some of the fornical fibers cross the midline and form the early hippocampal commissure. During the 14 – 15 weeks of development, glial cells (expressing acidic proteins) appear around the fornix to support and nourish the development of the fornix. Myelination of fornical fibers is only evident at about some few weeks to term, that is, towards the end of the 9th month of pregnancy, with the presence of myelin basic proteins. Myelination in the hippocampus proper, continues even after embryonic life and increases in childhood until adolescence. The early fornix is a short and slightly curved bundle containing more hippocampal-septal or septohippocampal fibers that connect the hippocampus to the septal area.
Characteristics & Location
The fornix is a C-shaped bundle of fibrous commissural fibers (axons) that extends from the hippocampus to the mammillary bodies of the hypothalamus, the anterior nuclei of the thalamus, and forms an arch over the thalamus. It is located on the medial aspects of the cerebral hemispheres. Components of the fornix lie to both sides of the mid-sagittal plane and connect with each other across that plane.
The fornix is comprised of a body, a posterior part called the crura or “legs” (singular: crus), which further extend posteriorly as the alveus- and finally the fimbria of the hippocampus. The fimbria is prolonged into the uncus of the parahippocampal gyrus. The fornix is also made up of an anterior part, which like the posterior part, is an anterior extension of the body of fornix. This anterior part is called the column of the fornix.
The fornix is a prominent bundle of fibers seen on the medial aspects of the cerebral hemispheres, and it is predominantly made up of fibers arising in the hippocampus as fimbria. These fibers (fimbria) extend anteriorly, giving off some lateral strands of fibers which forms the alveus of the head of the hippocampus.
The main bulk of fimbria continues on each side of the cerebral hemisphere further anteriorly to form the crus of the fornix. Both crura (in the left and right hemispheres) continues anteriorly and merge to form a midline part of the fornix called the body of the fornix (which belong to both the right and left hemispheres). The body of the fornix is suspended from the corpus callosum by the septal pellucidum and comes into close relationship with the tela choroidae in the roof of the 3rd ventricle.
The body of the fornix extends anteriorly and divides at the level of the anterior commissure into the right and left halves called the columns of the fornix (which are the precommissural fornix and the postcommissural fornix). Each column turns downwards just in front of the interventricular foramen of Monro and passes through the hypothalamus to reach the mammillary body.
The precommissural fornix are the fibers that descend in front of the anterior commissure. The postcommissural fornix lie behind the anterior commissure. Some other fibers of the fornix pass above the splenium of the corpus callosum to reach the structures above the corpus callosum. Those fibers constitutes the dorsal fornix.
In addition, the left and right crura of the fornix, communicates with each other across the midline by the partially decussating fibers called commissures of the fornix or hippocampal commissure (commissure of psalterium Davidi or David’s lyre). The commissure of psalterium Davidi provides one of the major communicating pathway between the hippocampi of the left and right hemispheres.
Connections of the Fornix
The following are the main connections of the fornix:
- Hippocampal commissure: These are fibers of the fornix interconnecting the hippocampi of the two cerebral hemispheres.
- Fibers from the postcommissural fornix connects the hippocampus to the mammillary body of the hypothalamus. Those fibers are then relayed to the anterior nucleus of the thalamus.
- Some fibers of the fornix connects, directly, the hypothalamus, while some others also directly connects the anterior nucleus of the thalamus.
- Fibers from the precommissural fornix connects the hippocampus to the septal nuclei, preoptic nuclei, ventral striatum, orbital cortex and anterior cingulate cortex. Some of those fibers from the precommissural fornix turn backwards to enter the stria medullaris thalami and reach the habenular nuclei.
- Fibers from the dorsal fornix reach the splenial gyrus and the gyrus cinguli.
The column of the fornix is supplied by:
- Perforating branches of the hypothalamic arteries
- Anterior cerebral artery
The crus of the fornix is supplied by:
- Inferior branch of the posterior pericallosal artery
- Lateral posterior choroidal artery
In normal embryonic development, commissural fibers must be actively guided across the midline to reach their targets in the contralateral hemisphere. When the underlying mechanisms regulating the guidance of the commissural fibers fail, pathological dysgenesis of one or more commissures ensues. The fornix is sometimes implicated in anomalies of the limbic system, since it is a part of the limbic system forming major connecting pathways between structures of this system.
The following are some anomalies of the fornix:
Congenital absence of the fornix is very rare, but has been described as a finding in the holoprosencephaly spectrum, in which there is malformation of the forebrain in humans, resulting from failed or incomplete forebrain division in the third to fourth weeks of gestation. The forebrain (prosencephalon) incompletely cleaves into right and left hemispheres, deep brain structures, and the olfactory and optic bulbs and tracts. The most severe types of brain malformations in holoprosencephaly includes alobar, semilobar, lobar, and middle interhemispheric variant (in an order of decreasing severity).
As a midline white matter tract, the fornix is particularly susceptible to involvement with tumors that favor this route of spread. The fornix is well involved in the spread of tumors of the white matter tract like hypothalamic glioblastoma multiforme in which patients present with memory deficit. Glioblastoma, also known as glioblastoma multiforme, is the most common adult primary intracranial neoplasm. It accounts for 12-15% of all intracranial neoplasms and ~50% of astrocytomas. Unfortunately, it also carries the worst prognosis. They have a preferential spreading along the condensed white matter tracts such as corticospinal tracts, corpus callosum and the fornix. They often spread across white matter commisural tracts such as the corpus collosum and fornix, and can give rise to the so called butterfly glioma, to involve the contralateral hemisphere. Glioblastomas rarely involve the meninges.
Herpes Simplex Encephalitis
This is an infection that involves the limbic system and thus the fornix, and in most cases leads to atrophy of the fornix and gliosis of the hippocampus. Herpes simplex encephalitis predominantly affects gray matter components of the limbic system but also results in abnormalities of the fimbria of fornix which is adjacent to the acutely involved hippocampus, as well as the forniceal crus. There is also marked swellings of the forniceal body.
During embryogenesis, demyelination of fiber bundles of the fornix and hypothalamus has being identified as a functional consequence of multiple sclerosis with symptoms of cognitive dysfunctions like dementia - short term memory impairment and learning impairments.
Asymmetry of the Fornix
This is a difference in the fornical volume between the right and left cerebral hemispheres. This asymmetry is present in the position of the two columns of the fornix in relation to the septum pellucidum.
In abnormality of the hippocampus, for example, hippocampal sclerosis, asymmetric volume loss in the fornix is detected on the ipsilateral side of the abnormal hippocampus. Hippocampal neuronal loss may result in Wallerian degeneration and subsequently, atrophy of the ipsilateral fornix.
A growing number of current studies also suggest the involvement of the fornix in neurodegenerative diseases and syndromes like:
- Wernicke Encephalopathy
- Alzheimer Disease
- Mesial Temporal Sclerosis