Corpus Callosum and Commissures

The commissures are white-matter pathways whose fibres cross the midline to connect the two cerebral hemispheres. The largest by far is the corpus callosum, an arched band of some hundreds of millions of fibres linking corresponding and non-corresponding cortical regions; the anterior commissure and the smaller posterior and hippocampal commissures connect more restricted territories. Together they coordinate the activity of the two hemispheres and allow information to be shared between them.

Definition

The cerebral commissures are commissural white-matter tracts whose axons cross the midline to interconnect the two hemispheres; the corpus callosum is the principal commissure, with the anterior, posterior, and hippocampal commissures connecting smaller regions.

Scope

This entry covers the commissural fibre systems of the cerebrum: the gross subdivisions of the corpus callosum (rostrum, genu, body, splenium), its fibre composition and topography, and the anterior and other commissures, treated as reference anatomy. It does not cover developmental disorders or surgical division of the callosum as clinical topics, beyond noting their anatomical basis.

Key concepts

• Corpus callosum (rostrum, genu, body, splenium)
• Commissural versus association and projection fibres
• Anterior commissure
• Topographic organisation of callosal fibres
• Fibre calibre and conduction
• Interhemispheric transfer
• Forceps minor and forceps major

Mechanisms

The corpus callosum is organised topographically, so that anterior cortical regions are connected through its genu and anterior body and posterior regions through the splenium, with frontal fibres looping forward as the forceps minor and occipital fibres looping back as the forceps major. Aboitiz and colleagues showed that the human callosum contains a mix of fibre calibres, with many small thin axons and a smaller number of large fast-conducting fibres, and that this composition varies along its length in register with the cortical regions it links. The anterior commissure connects parts of the temporal lobes and olfactory regions. By transferring sensory, motor, and cognitive information across the midline, the commissures integrate the two hemispheres; Gazzaniga reviews how studies of patients whose callosum had been divided revealed the extent of hemispheric specialisation and the role of the callosum in unifying experience.

Clinical relevance

The topography of the corpus callosum means that focal lesions produce deficits referable to the cortical regions whose connecting fibres are interrupted, and that division of the callosum can impair the transfer of information between hemispheres. The entry presents this anatomy for reference and is not clinical guidance.

Evidence & guidelines

Callosal anatomy and fibre composition are established from gross dissection, histological fibre counts, and studies of patients with callosal lesions or surgical section, together with in vivo diffusion MRI tractography that reconstructs its subdivisions. These imaging reconstructions are descriptive and share the general limitations of tractography.

History

The corpus callosum was described by classical anatomists, and its functional importance became clear in the twentieth century through split-brain research on patients whose callosum had been surgically divided, work associated with Roger Sperry and Michael Gazzaniga. Quantitative histology by Aboitiz and colleagues later characterised the fibre composition of the human callosum, and diffusion MRI subsequently mapped its subdivisions in living brains.

Key figures

• Michael Gazzaniga
• Francisco Aboitiz
• Roger Sperry

Seminal works

• aboitiz1992
• gazzaniga2000
• wakana2004

Frequently asked questions

What are the main parts of the corpus callosum?

From front to back it is divided into the rostrum, genu, body (trunk), and splenium; frontal fibres curving through the genu form the forceps minor and occipital fibres through the splenium form the forceps major.

What is the difference between a commissure and an association tract?

Commissural fibres cross the midline to connect the two hemispheres, whereas association fibres connect regions within the same hemisphere; both are distinct from projection fibres that link the cortex with lower centres.