The human brain is divided into two halves, or the left and right hemispheres. Each hemisphere controls the contralateral, or opposite, side of the body. The anterior commissure is a nerve bundle that connects these two hemispheres. Also called the precommissure, it can be found in the brains of all mammals.
The brain is made up of two different types of tissue: white matter and gray matter. Gray matter contains the cell bodies of neurons and the dendrites that branch out from there. White matter contains the axons of neurons, which are covered in a white fatty substance called myelin. The anterior commissure is made up of white matter.
The anterior commissure connects the two temporal lobes, which are located on the lower part of each side of the brain, behind the ear. The temporal lobes are involved with hearing and language. It also connects the two amygdalae, nuclei that regulate emotion and play a role in memory. It also holds the olfactory tract fibers, which cross over to opposite sides of the body in the anterior commissure.
The neospinothalamic tract is one of the pathways pain stimuli takes through the nervous system. The neurons carrying fast pain signals decussate, or cross over to the contralateral side of the brain, at the anterior commissure. These fibers terminate in the thalamus, where they communicate with the somatosensory cortex.
There are several pathways that connect the left and right hemispheres of the brain. The largest one is the corpus callosum, which is more than ten times larger than the anterior commissure. It is also made of white matter and is in fact the largest white matter structure in the brain.
In a sagittal cut of the brain, which divides the brain down the middle into left and right sections, the anterior commissure can be clearly seen. It is oval in shape and from top to bottom it measures about 5 mm. It is located below the corpus callosum and in front of the columns of the fornix, a brain structure that carries signals from the hippocampus. It is slightly larger in women than in men.
Some studies suggest that there may be neurological differences between homosexual and heterosexual men. One such study found that the anterior commissure was larger in the brains of homosexual men than in heterosexual men. This finding has not been replicated, however, so further study is necessary before drawing conclusions about neurological indicators of homosexuality.