Fornix of the brain
The fornix is a C-shaped bundle of white matter that is found in the mesial aspect of the cerebral hemispheres, below the corpus callosum. The fornix is an important part of the limbic system, and represents the largest single pathway of the hippocampus, connecting it to various subcortical structures.
The fornix originates from the hippocampus in the mesial temporal lobe and stretches longitudinally to the diencephalon and basal forebrain, forming an arch over the thalamus. Being a part of the limbic system, the fornix is associated with the processes of cognition, memory consolidation, emotions and sexual responses.
This article will discuss the anatomy and function of the fornix of the brain.
|Parts||Alveus, fimbria, crura, body, columns of fornix|
|Connections||Hippocampus to the mammillary bodies of the hypothalamus
Mammillary bodies to the anterior nuclei of thalamus
Hippocampus to the septal nuclei and nucleus accumbens
|Functions||Cognition, memory, emotions|
- Structure and parts
- Clinical aspects
Structure and parts
The fornix is a prominent bundle of white matter fibers seen on the medial aspects of the cerebral hemispheres. It begins in the hippocampus as the alveus, which is a collection of myelinated fibers found medially to the floor of the temporal horn of the lateral ventricle. The fibers of the alveus travel posteromedially and aggregate to form the fimbria of the hippocampus. The main bulk of fimbriae continues on each side of the cerebral hemisphere to form the crus of the fornix.
The two crura (posterior pillars), one for each hemisphere, arch anterosuperiorly under the splenium of the corpus callosum. The crura establish a connection with each other across the midline by the partially decussating fibers called the hippocampal commissure (commissure of psalterium Davidi or David’s lyre). Both crura continue anteriorly and merge in the midline to form the body of the fornix. The body of the fornix arches over the thalamus and under the septum pellucidum, which connects it to the corpus callosum.
The body of the fornix extends anteriorly and bifurcates into the right and left halves called the columns of the 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 columns of the fornix divide at the level of the anterior commissure and form the precommissural and postcommissural fornix.
- The precommissural fornix is composed of the fibers that descend in front of the anterior commissure. They relay onto the septal nuclei (medial olfactory area), the ventral striatum, and the cingulate gyrus.
- The postcommissural fornix are the fibers that pass behind the anterior commissure and relay onto the mammillary bodies and the anterior nuclei of the thalamus. A small amount of fornical fibers pass above the splenium of corpus callosum to reach the structures above the corpus callosum. Those fibers constitute the dorsal fornix.
The fornix provides the major pathway into and out of the hippocampus. The main connections of the fornix are the following:
- The fibers of the hippocampal commissure interconnect the contralateral hippocampi of the two cerebral hemispheres.
- Fibers from the postcommissural fornix originate from the subiculum of the hippocampus and project to the mammillary bodies of the hypothalamus, forming the indirect subiculothalamic pathway. Those fibers are then relayed to the anterior nucleus of the thalamus, forming the direct subiculothalamic pathway.
- Fibers from the precommissural fornix connect the hippocampus to the septal nuclei, preoptic nuclei, ventral striatum, orbital cortex and anterior cingulate cortex. Some of those fibers turn backward to enter the stria medullaris thalami and reach the habenular nuclei.
- Fibers from the dorsal fornix reach the splenial gyrus and the gyrus cinguli
Learn everything about the fornix anatomy and function with our articles, video tutorials, quizzes and labeled diagrams.
Being the main output tract of the hippocampus, the primary role of the fornix is to transmit the information from the hippocampus to the mammillary bodies and to the anterior nuclei of thalamus.
Although the specific function of the fornix is not entirely defined, it is believed that it plays a key part in some aspects of memory, specifically in recall of long-term memory such as details from past events. The fornix is especially involved in the regulation of episodic memories, which are a type of declarative memories that encompass autobiographical information.
It’s difficult to understand the fornix without mastering the basics of brain anatomy. To be in the clear, check out our quizzes and diagrams on the parts of the brain!
The fornix is also a critical component of the Papez circuit. The Papez circuit is a closed circuit that consists of the following pathways:
- The Papez circuit starts in the subiculum (hippocampal formation). The subiculum projects to the mammillary bodies via the postcommissural fornix through the previously described subiculothalamic tract.
- The mammillary bodies then project to the anterior thalamic nuclei via the mammillothalamic tract.
- The anterior thalamic nuclei next project to the cingulate gyrus, which in turn projects to the parahippocampal gyrus.
- Finally, the parahippocampal gyrus projects to the entorhinal cortex and the hippocampal formation, completing the circuit.
The Papez circuit is involved in learning and memory, emotion, and social behavior, and was originally thought to be the anatomical substrate of emotional experience. However, the amygdala, basal forebrain and prefrontal cortex have recently been recognized as additional parts of the Papez circuit that play a more profound role in emotion.
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