Fornix Brain

There are many illnesses or medical reasons why you have sudden onset headaches or early onset memory loss, but one unknown area of the brain may be a cause. The section of the brain that may be responsible is known as the fornix.

The fornix in the brain is a C-shaped area of myelinated fibers of the telencephalon. They are situated in the mesial aspect of the cerebral hemispheres. It projects to the hypothalamus from the hippocampal formation and is part of the limbic system. It controls memory, cognition, and emotions.

Several studies indicate a connection between tumors or dysfunction of the fornix to diseases such as Alzheimer's Disease, Multiple Sclerosis, and Schizophrenia. If you’ve never heard of the fornix, don’t feel alone, but here is an in-depth look.

What Is The Fornix Of The Brain?

The fornix in the brain is a C-shaped area of myelinated fibers of the telencephalon situated in the mesial aspect of the hemispheres. It projects from the hippocampal formation through to the hypothalamus.

Thought to be a part of the limbic system, the part of the brain responsible for emotional and behavioral responses, the fornix controls memory, cognition, and emotions. Any deformity or injury of the fornix can have immediate and severe effects on the individual.

How Is The Fornix Connected In The Brain?

There are two major pathways in and out of the hippocampus. The hippocampal commissure fibers are interconnected with the contralateral hippocampi of the two hemispheres.

The fornix provides these two main connections are described as follows –

The Postcommissural Fornix –

The postcommissural fornix has fibers that originate in the subiculum of the hippocampus. They project through to the mamillary bodies of the hypothalamus. They form the indirect subiculothalamic pathway.

These fibers are then relayed to the anterior nucleus of the thalamus. This forms the direct subiculothalamic pathway.

The Precommissural Fornix -

The precommissural fornix has fibers that directly connect the septal nuclei, preoptic nuclei, orbital cortex, ventral striatum, and anterior cingulate cortex to the hippocampus. To reach the habenular nuclei, a section of those fibers turns backward and enters the stria medullaris.

The dorsal fornix fibers reach the gyrus cinguli and splenial gyrus.

The Fornix Anatomy

The main parts of the fornix can be identified as follows –

• Fimbria
• Alveus
• Body
• Crura
• Columns of fornix

The fornix connections are as follows –

• Hippocampus  -  Mammillary bodies of the hypothalamus.
• Mammillary bodies – Anterior nuclei of the thalamus
• Hippocampus – Septal nuclei and Nucleus accumbens

As described aforehand – The fornix is a conspicuous white-matter bundle of fibers that are visible on the medial aspects of both the cerebral hemispheres. The alveus fibers start as alveus in the hippocampus and travel posteromedially and cumulate, forming the fimbria of the hippocampus.

The bulk of the fimbria is located on either side of the cerebral hemisphere. The fimbria form the crus of the fornix. The alveus is a group of myelinated fibers that are found in the middle of the temporal horn floor of the lateral ventricle.

There are two crura or posterior pillars. There is one crus for each cerebral hemisphere. They arch anterosuperiorly under the corpus callosum’s splenium. The hippocampal commissure establishes a connection between the crura across the midline. The fibers of the hippocampal commissure are also known as David’s lyre or commissure of psalterium Davidi.

The midline merging of both crura forms the body of the fornix. The fornix body arches under the septum pellucidum and over the thalamus, from where it is connected to the corpus callosum.

The columns of the fornix are made from the anterior, forking extension of the fornix body. These two columns turn down in front of the intraventricular foramen of Monro. From there, it passes through the hypothalamus, reaching the mammillary body.

At the same level of the anterior commissure, the fornix columns divide to form the precommissural fornix and postcommissural fornix. The fibers descending in front of the anterior commissure make up the precommissural fornix.

These fibers relay to the medial olfactory area of septal nuclei, the cingulate gyrus, and the ventral striatum.

The fibers that descend behind the anterior commissure make up the postcommissural fornix. They relay onto the anterior nuclei of the thalamus and the mammillary bodies.

 A few of the fornical fibers reach the structures above the corpus callosum. These fibers are the dorsal fornix and pass over the splenium of the corpus callosum.

What Is The Function Of The Fornix In The Brain?

The fornix is the main output of the hippocampus. Its primary function is transmitting information to the mammillary bodies and the anterior nuclei of the thalamus from the hippocampus.

It is believed the fornix plays a significant role in various aspects of our memory, more specifically in our long-term memory recall or more details of past events. Autobiographical information relies on declarative memories. They are part of our episodic memories and, as a whole, make up the details of an event.

Here is a critical component or pathway of the fornix explained -

The Papez Circuit

The fornix makes up a crucial piece of the Papez circuit – a closed circuit consisting of the following established pathways –

• The Papez circuit originates in the hippocampal formation or subiculum.
• The subiculum extends to the mammillary bodies through the postcommissural fornix via the subiculothalamic tract.
• The mammillary bodies are projected to the anterior thalamic nuclei through the mammillothalamic tract.
• The anterior thalamic nuclei projects to the cingulate gyrus.
• The cingulate gyrus projects to the parahippocampal gyrus.
• The parahippocampal gyrus projects to the hippocampal formation and entorhinal cortex
• The circuit is complete.

What Is The Papez Circuit’s Function?

 The complex Papez circuit is involved in the following cognitive functions –

• Social behavior
• Memory
• Emotion
• Emotional experience

It was originally an idea that the Papez circuit was an anatomical substrate of any emotional experience. Recently the basal forebrain, prefrontal cortex, and amygdala have been recognized as parts of the Papez circuit. They play a more significant role in human emotion than believed.

Is The Fornix Part Of The Hypothalamus?

The C-shaped fornix extends from the hippocampus to the anterior nuclei of the hypothalamus and the mammillary bodies of the hypothalamus. The alveus is made from myelinated fibers and joins the fornix to form the fimbria of the hippocampus.

The hypothalamus forms an important part of the limbic system along with the hippocampus and amygdala. The limbic system in the brain controls and regulates cognitive functions such as emotional response and survival, such as fright or flight, reproduction, protecting the weak, and eating.

Does The Fornix Regulate Memory?

Without the fornix, memory formation would not be possible. It serves as a conduit for acetylcholine and theta rhythms. It also provides mnemonic representations to the deep brain formations responsible for guiding motivated behaviors such as where to sleep and where and when to eat.

There is clinical data and experimental research that suggests the fornix fibers are functionally distributed. The left fornix mainly carries our verbal memory information, and the right fornix mainly carries the visuospatial memory information.

Additionally, fibers from the medial fornix are carried from the caudal hippocampus. These fibers process exteroceptive or exterior signals integrating spatial context with object recognition, for example, when learning a scene for a film or theatre.

The rostral hippocampus processes interoceptive signals for memory and motivational and emotional learning. The lateral aspect of the fornix carries these projections.

Damage to the fornix in humans will manifest clinically as anterograde amnesia. Research points to the fornix's role in Alzheimer’s Disease and mild cognitive impairment. The fornix can possibly predict the conversion from mild impairment to Alzheimer’s Disease.

Some studies link disturbances in the fornix to mood disorders, eating disorders, and schizophrenia. Deep brain stimulation, such as electric impulses, can improve mood and significantly reduce memory loss and certain seizures.

A healthy fornix is essential for a memory system that is fully functioning. The fornix is implicated in all the neurological functions reliant on the hippocampus.  

What Are Clinical Aspects Of The Fornix?

Clinical aspects refer to the medical observations made in terms of a diagnosis. When an embryo develops without issue, the commissural fibers are actively guided across the midline to their targets – the contralateral hemisphere.

Sometimes the mechanisms that regulate the guidance of the commissural fibers fail, and a pathological abnormality will form. Sometimes, the fornix is damaged in forming these abnormalities because it forms part of the limbic system responsible for the major connective pathways between structures.

Anomalies of the fornix –

Glioblastoma Multiforme

The fornix is rather susceptible to tumors due to the fact that they favor the route of the midline white matter tract. The fornix can spread tumors like hypothalamic glioblastoma multiforme because of the white matter tract. These patients will present with a memory deficit.

Glioblastoma multiforme is the most common adult intracranial neoplasm, accounting for less than 50% of astrocytomas and 15% of intracranial neoplasms. They carry the worst prognosis because they have the worst prognosis of the way they spread over the white matter, such as the corpus callosum, corticospinal tracts, and the fornix.

The glioblastoma multiforme often gives rise to the butterfly glioma involving the contralateral hemisphere.

Multiple Sclerosis

Demyelination of the hypothalamus and fornix fiber bundles during embryogenesis is considered a functional consequence of multiple sclerosis. Symptoms of cognitive dysfunctions such as dementia, learning impairments, and short-term memory loss are also observed.

Holoprosencephaly

While in the third and fourth week of gestation, a failed forebrain division will result in a forebrain malformation. Although a congenital absence of the fornix is rare, it is a finding in the holoprosencephaly spectrum.

The prosencephalon or forebrain will partly cleave into the left and right hemispheres, the olfactory bulb, deep brain structures, optic bulbs, and tracts. In holoprosencephaly, the most severe malformations of the brain include –

• Middle interhemispheric variants
• Semi-lobar
• Lobar
• A-lobar

Herpes Simplex Encephalitis

Herpes simplex encephalitis is an infection of the limbic system and the fornix. This infection typically leads to atrophy of the gliosis of the hippocampus and the fornix. It predominantly affects gray matter areas or parts of the limbic system.

Herpes simplex encephalitis can result in abnormalities of the fornix fimbria and presents with significant swelling of the fornix body. Failure to rapidly diagnose and treat this infection can result in permanent brain damage or death.

Asymmetry Of The Fornix

Asymmetry of the fornix means that there is more fornical volume on one side of the cerebral hemispheres. The asymmetry will be noticeable in the position of the fornix columns as it relates to the septum pellucidum.

If there is an abnormality such as asymmetry of the fornix or hippocampal sclerosis, it will be detected on the ipsilateral side of the hippocampus that is abnormal. Neuronal loss in the hippocampus can cause Wellerian degeneration and ultimately atrophy of the ipsilateral fornix.

The following neurodegenerative syndromes and diseases are linked to abnormalities of the fornix -

• Schizophrenia
• Mesial temporal sclerosis
• Alzheimer’s Disease
• Wernicke Encephalopathy

What Happens If The Fornix Is Damaged?

Damage or lesions to the fornix will result in irreversible damage in the form of anterograde amnesia, and there is a link between cognitive aging, epilepsy, Alzheimer's Disease, psychosis, and fornix pathology.

In a cited paper, a young woman in her mid-twenties was admitted to the ER with progressive headaches over the course of 60 days. An MRI scan was done on her brain, and an unusual lesion on the anterior right lateral ventricle was seen.

This lesion was causing an obstruction in the foramen of Monro and was displacing and impinging the columns of the fornices. The displacement and impingement were more pronounced on the right than on the left.

The advice from radiology to the neurosurgeons was that the close proximity to the medial margin of the tumor and the fornices causes a huge risk of memory loss to the patient. She elected for surgery and was scheduled for endoscopic subtotal resection.

The lesion or tumor was identified as a glioneuronal neoplasm. It was tightly attached to the fornical columns. Postoperatively, the young woman displayed memory loss and deficits. She could not remember why she had surgery and even that she had surgery at all.

Several months later, she displayed delayed recall of visuospatial and verbal materials at a neurological and neuropsychological checkup. Although it does not indicate if it was malignant or benign, the very rare tumor permanently affected parts of her cognition and life.

Conclusion

A well-functioning, healthy fornix is critical to the overall cognitive well-being of the human brain and body. Any impairment of the fornix seems to greatly impact the surrounding bodies, such as the corpus callosum, hippocampus, hypothalamus, and amygdala.

Having your doctor check any sudden onset signs such as memory loss, progressive headaches, and neuropathy in the extremities is never a bad idea. Rapid action can extend good quality of life even with progressive diseases like Alzheimer's.