Corpus Callosum

The two cerebral hemispheres of the human brain are connected via a band of fibers called corpus callosum that allows the two sides to communicate and stay in sync. In this patient, damage to these fibers due to a stroke did unleash a more labile and emotionally volatile right hemisphere controlling her left hand, resulting in such an abnormality.

1.0Corpus Callosum Anatomy

• Commissural fibers connect wide areas of the cerebral cortex of two hemispheres. These fibers are mainly developed from axons of pyramidal cells across the midline, and the corpus callosum is the most prominent band of the neocortex. 
• It forms an arched band with convexity upward and is a quintessential white matter structure composed of dense myelinated fibers connecting the homologous areas of two hemispheres. 
• The primary visual cortex (area 17) and somesthetic areas (areas 3, 4, and 5) do not have fibres connected by the corpus callosum. 
• Parts of the corpus callosum (collosal commissure) are splenium, trunk/body, genus and rostrum from behind forward. 
• The corpus callosum, a bridge between the brain's hemispheres, has distinct regions: the posterior section is the splenium, the anterior is the genu, and between them lies the trunk or body. 
• The isthmus, often notably thin, connects the body to the splenium. The rostrum extends from the genu in a posterior and downward direction, named for its resemblance to a bird's beak.

2.0Structure of Corpus Callosum

Corpus Callosum Anatomical Relation:

• The upper convex surface of the corpus callosum is covered by a thin sheet of grey matter called indusium griseum. Here, the medial and lateral longitudinal striae are embedded, representing the remnants of the upper part of the hippocampal formation. 
• On each side, the anterior and superior aspects of the corpus callosum are related to anterior cerebral vessels. 
• The lower concave surface in the posterior part comes in direct contact with the crura of the fornix. Still, anteriorly, it is separated by a bilaminar fold of neural tissue named septum pellucidum. 
• The trunk of the corpus callosum forms the roof of the central part, and the anterior horn of lateral ventricles is lined by ependymal cells; the genu forms the anterior wall, while the rostrum forms a part of the floor of the anterior horn of the lateral ventricle. 
• The great cerebral vein traverses below the splenium in the transverse fissure to drain backwards into the straight sinus.

Arrangements of Fibers:

• Both hemispheres' orbital surfaces are connected via the fibers passing through the rostrum. 
• The lateral and medial surfaces of the frontal lobes are connected by the fibers of the genu, which proceed forward and laterally as forceps minor. 
• Projection fibers of corona radiata intersect the fibers passing via the trunk. Some truncal fibers, named tapetum, form the roof and lateral wall of the posterior horn and the lateral part of the roof of the inferior horn of the lateral ventricle. 
• The forceps major is formed by the extension of splenial fibers backwards, connecting the occipital lobes of two hemispheres.

Corpus Callosum Blood Supply:

• Branches from anterior cerebral arteries supply the corpus callosum's rostrum, genu, and body, while the splenium receives branches from posterior cerebral arteries. 
• The anterior and posterior cerebral arteries provide the major blood supply of the corpus callosum via the pericallosal artery and small penetrating vessels that run perpendicular to the parent artery.

Corpus Callosum Composition:

• The corpus callosum comprises about 300 million densely packed myelinated white fibers, hence susceptible to shear injury/trauma and demyelination. 
• Due to the density of fibers, only glioblastoma multiforme or lymphoma may cross or affect the corpus callosum.

3.0Functions of Corpus Callosum

• Memory, which is developed through learning from sensory and motor experiences, is initially confined to one hemisphere of the brain.  However, both hemispheres communicate and share information via the corpus callosum. For instance, the left cerebral hemisphere processes input from and controls movements on the right side of the body, and vice versa. 
• This means that the corpus callosum facilitates the transfer of memory traces from the hemisphere that has been trained to the hemisphere that has not.
• The dominant brain is concerned with speech and consciousness for self, while the nondominant brain is mute.
• Four cortical areas of the left hemisphere (Wernicke's area, angular gyrus, supramarginal gyrus and Broca's area) are integral in the expression of speech. 
• These regions are linked to their counterparts in the right hemisphere through the corpus callosum: The genu connects the prefrontal cortex between the two brain hemispheres. The body and splenium interconnect areas of the premotor, supplementary motor regions, and motor cortex.The posterior body of the corpus callosum facilitates communication of somatosensory information between the two halves of the parietal lobe and the visual center of the occipital lobe.

4.0Dysfunction of the Corpus Callosum

1. Agenesis of Corpus Callosum:

• It is a congenital condition characterized by either complete or partial absence of the corpus callosum. 
• Callosal agenesis is related to the absence of a commissural plate, which acts as a bridge for axonal passage. 
• The development of fibers that would otherwise form the corpus callosum becomes longitudinally oriented within each hemisphere and forms Probst bundle structures. Some fibers may pass into the anterior commissure, and occasionally, some may descend within the internal capsule with the corticospinal tract as far as the spinal cord.

2. Split Brain Syndrome/Callosal Syndrome:

• Partial/complete destruction of the corpus callosum may lead to a disconnection syndrome due to the isolation of the left hemisphere's language and perception areas from those of the right hemisphere.
• The patients may present with: 

Hemialexia: Unable to read matter presented to the left visual field, as the symbols seen in the left visual field are projected to the right hemisphere, which has no access to the left hemisphere language centers. 

Left ideomotor apraxia: Unable to work with left hand on verbal command. 

Left agraphia: Unable to write from dictation with left hand.

Left tactile anomia: Unable to name an object held in the left hand with closed eyes.

3. Tumors

• Tumors of the corpus callosum, especially those involving the anterior portion, may cause behavioral symptoms like catatonia, depression, psychotic symptoms, and personality changes.

Dysfunction in Psychiatric Illness

• Attention deficit hyperactivity disorder, dyslexia— anterior part of the corpus callosum 
• Autism—posterior part of the corpus callosum  
• Obsessive-compulsive disorders—rostrum and orbitofrontal cortex 
• Post-traumatic stress disorder—smaller intracranial volume and corpus callosum area 
• Schizophrenia—callosal pruning and myelination, as well as interhemispheric coherence, continue to develop into early adulthood, a factor that may be relevant to the age of onset in schizophrenia. Impairments in callosal transfer have been reported in patients, implicating alterations in callosal connectivity. Structural modifications in asymmetric perisylvian regions linked by the callosum have been reported. The corpus callosum spans across the superior horns of the lateral ventricles, which are often enlarged in individuals with schizophrenia.