Anterior commissure

At a glance

English

Anterior commissure

Latin (TA2)

Commissura anterior

System

Brain and spinal cord

Region

Head

Side

Single, on the midline

About the anterior commissure

The anterior commissure is a small, rounded bundle of white matter that crosses from one cerebral hemisphere to the other at the front of the third ventricle. It runs through the upper part of the lamina terminalis, the thin wall that closes the ventricle in front, and lies just in front of the two columns of the fornix as they curve down towards the hypothalamus. In a midline section it shows as an oval about half a centimetre tall. It is a standard landmark in brain imaging: the line joining it to the posterior commissure, the AC-PC line, is used to align scans and to plan stereotactic surgery.

Traced sideways, the commissure looks like the handlebars of a bicycle. Each half passes laterally beneath the lentiform nucleus and the rest of the corpus striatum, then splits. A small anterior limb turns forwards towards the olfactory tract and the anterior perforated substance, and a much larger posterior limb fans backwards and downwards into the temporal lobe.

These limbs carry two main groups of fibres. The anterior part links the olfactory bulbs, anterior olfactory nuclei and olfactory cortex of the two sides, so that smell information from each nostril can be shared. The posterior part, by far the larger in humans, joins the middle and inferior temporal gyri and the regions around the amygdala and parahippocampal gyrus of the two hemispheres. Through these links it contributes to memory, emotion, hearing and language functions carried out in the temporal lobes. In humans the corpus callosum is many times larger and carries most traffic between the hemispheres, and the hippocampal commissure links the two hippocampi, but the anterior commissure is a distinct pathway in all mammals; in marsupials, which lack a corpus callosum, it carries all fibres between the neocortex of the two sides.

Because it is spared when the corpus callosum fails to develop or is cut, the anterior commissure may let some information, such as colour or emotional tone, still pass between the hemispheres, and imaging studies suggest it can act as a compensating route. Its exact role is still incompletely understood. Its blood comes from small perforating vessels at the base of the brain.

Function and supply

Function

Connects the olfactory regions of the two sides and the middle and inferior temporal gyri and amygdaloid regions of both hemispheres, allowing exchange of smell, memory, emotional and auditory information.

Blood supply

Small perforating branches of the anterior cerebral and anterior communicating arteries, with twigs from the lenticulostriate branches of the middle cerebral artery.