The corpus callosum is a large white matter tract that connects the two hemispheres of the brain. It is an incredibly important structural and functional part of the brain. It allows us to perceive depth and enables the two sides of our brain to communicate. In this article we will discuss the gross and functional anatomy of the corpus callosum. We will also discuss the clinical relevance of the structure, and provide a summary of key points at the end of the article. We will finally conclude with some review questions to test the reader’s understanding of the article content.
The corpus callosum gets its name from the Latin for “tough body”. It is the largest white matter structure in the brain both in terms of size (700 square millimeters for the midsagittal cross-section) and number of axonal projections (200 million) between the two hemispheres. This contrasts with the much smaller number of fibres in the optic nerve (1.5 million) and the cochlear nerve (32000).
The corpus callosum gives rise to white matter fibres that project to a number of regions within the cerebral cortex. The corpus callosum is divided into four parts: rostrum, genu, body/trunk and splenium. The rostrum is continuous with the lamina terminalis and connects the orbital surfaces of the frontal lobes. The genu is the bend of the anterior corpus callosum and the forceps minor is a tract that projects fibres from the genu to connect the medial and lateral surfaces of the frontal lobes. The body forms the long central section and its fibres pass through the corona radiata to reach the surface of the hemispheres. The splenium tapers away at the posterior section, with the forceps major projecting fibres from the splenium to connect the two occipital lobes. White matter fibres projecting from the body and fibres from the splenium not included in the forceps major are known as the tapetum, and run along the entire lateral occipital and temporal horns of the lateral ventricle.
Broadly speaking there are two types of connections: homotopic and heterotopic. Homotopic connections link similar regions from the left and right sides of the brain while heterotopic connections connect dissimilar areas of the left and right sides of the brain. Superior to the body of the corpus callosum lies the interhemispheric fissure which the falx cerebri runs within, as well as the anterior cerebral vessels. The superior aspect of the corpus callosum is covered with a thin layer of glial tissue under the pia mater, known as the indusium griseum, over which run on either side of the midline one or two strips of grey matter, i.e. the longitudinal striae.
The cingulate gyrus is a component of the limbic system and lies superolateral to the corpus callosum. The callosal sulcus separates the two structures. The anterior part of the corpus callosum (rostrum, genu, body) is attached inferiorly to the fornix by the septum pellucidum while the splenium is attached inferiorly to the crura and the commissure of the fornix.
The blood supply to the corpus callosum is rich and strengthened by anastomoses, which means the area is rarely affected by infarcts. The anterior cerebral artery can be divided along its course as segment A1 between the internal carotid artery and the anterior communicating artery and as the pericallosal artery distal to the anterior communicating artery. The vast majority of the blood supply to the corpus callosum comes from callosal branches of the pericallosal artery, whose course follows the outer border of the corpus callosum, first in the subcallosal area then in the callosal sulcus or in the cingulate sulcus.
The posterior pericallosal (splenial) artery usually is a branch of the posterior cerebral artery but its origin is variable. It bifurcates into an inferior and a superior branch, which supply the inferior and superior part of the splenium respectively. The superior branch usually anastomoses with the pericallosal artery.
In the majority of patients, the corpus callosum also gets blood supply from the anterior communicating artery via either the median callosal or the subcallosal artery. The subcallosal artery supplies the hypothalamus in part, but also supplies the rostrum and genu of the corpus callosum. The median callosal artery is essentially the same as a subcallosal artery, but it runs further in order to reach the body of the corpus callosum.
Agenesis of the corpus callosum- This is a rare congenital disorder, and is defined as the partial or complete absence of the corpus callosum. The disease usually occurs between the 3rd and 12th week of gestational life, the period when neurological development is at its peak. Symptoms are highly variable but include hypotonia, swallowing and chewing difficulties, low pain perception, delays in motor milestones such as walking and sitting, poor motor coordination is also common. Seizures occur in up to two thirds of patients and symptoms also depend on the presence of associated brain malformations.
Stereopsis/binocular vision- This is our ability to perceive depth, and is dependent on our corpus callosum, allowing us to interpret visual information from one eye in conjunction with the visual information from our other eye.
Corpus callosum impingement syndrome- This is impingement of the corpus callosum fibres on the inferior free margin of the falx cerebri. It usually results from longstanding hydrocephalus. Ischaemia results and eventually causes atrophy. It does not usually produce symptoms.
Disconnection syndrome- This is a syndrome that occurs when the connection between the two hemispheres is disrupted, either as a result of brain surgery, stroke or trauma. The patient may be completely normal to their family and friends, but specific tests elicit the abnormalities. The left side of the brain is important in for understanding and producing speech. Hence when these patients are presented with a stimulus on the left side of their vision (and hence perceived by the right hemisphere), they are unable to name the stimulus/object. This occurs as the language centre in the left hemispheres are not receiving any stimulus. Over time the right hemisphere develops some ability to produce speech or the left hemisphere gains ipsilateral control. Right ear advantage (when different words are presented to each ear simultaneously) and left hand apraxia (right hemisphere has poor language comprehension) are also manifestations of the disease.
- The corpus callosum is a large white matter tract that connects the two hemispheres of the brain.
- It lies deep to the cingulate gyrus.
- It lies superficial to the fornix, to which it is connected anteriorly by the septum pellucidum.
- The corpus callosum gets its blood supply from a branch of the anterior communicating artery (median callosal or subcallosal artery), from the pericallosal artery (distal part of the anterior cerebral artery) and from the posterior pericallosal artery (usually a branch of the posterior cerebral artery).
- Agenesis of the corpus callosum is a rare congenital disorder, which gives rise to a range of symptoms.
- Contrastingly, corpus callosum impingement syndrome (which usually results from long term hydrocephalus) does not usually cause symptoms.