Posterior commissure in the context of "Cerebral hemispheres"

The cerebrum, or the largest part of the vertebrate brain, is made up of two cerebral hemispheres. The deep groove known as the longitudinal fissure divides the cerebrum into the left and right hemispheres, but the hemispheres remain united by the corpus callosum, a large bundle of nerve fibers in the middle of the brain whose primary function is to integrate sensory and motor signals between the hemispheres. In eutherian (placental) mammals, other bundles of nerve fibers like the corpus callosum exist, including the anterior commissure, the posterior commissure, and the fornix, but compared with the corpus callosum, they are much smaller in size.

Broadly, the hemispheres are made up of two types of tissues. The thin outer layer of the cerebral hemispheres is made up of gray matter, composed of neuronal cell bodies, dendrites, and synapses; this outer layer constitutes the cerebral cortex (cortex is Latin for "bark of a tree"). Below that is the larger inner layer of white matter, composed of axons and myelin.

The anterior commissure (also known as the precommissure) is a white matter tract (a bundle of axons) connecting the two temporal lobes of the cerebral hemispheres across the midline, and placed in front of the columns of the fornix. In all but five species of mammal the great majority of fibers connecting the two hemispheres travel through the corpus callosum, which in humans and all non-monotremes is more than 10 times larger than the anterior commissure. Other routes of communication pass through the hippocampal commissure or, indirectly, via subcortical connections. Nevertheless, the anterior commissure is a significant pathway that can be clearly distinguished in the brains of all mammals.

The anterior commissure plays a key role in pain sensation, more specifically sharp, acute pain. It also contains decussating fibers from the olfactory tracts, vital for the sense of smell and chemoreception. The anterior commissure works with the posterior commissure to link the two cerebral hemispheres of the brain and also interconnects the amygdalae and temporal lobes, contributing to the role of memory, emotion, speech and hearing. It also is involved in olfaction, instinct, and sexual behavior.

A commissure (/ˈkɒməʃər/) is the location at which two objects abut or are joined. The term is used especially in the fields of anatomy and biology.

• The most common usage of the term refers to the brain's commissures, of which there are at least nine. Such a commissure is a bundle of commissural fibers as a tract that crosses the midline at its level of origin or entry (as opposed to a decussation of fibers that cross obliquely). The nine are the anterior commissure, posterior commissure, corpus callosum, commissure of fornix (hippocampal commissure), habenular commissure, ventral supraoptic decussation, Meynert's commissure, anterior hypothalamic commissure of Gasner, and the interthalamic adhesion. They consist of fibre tracts that connect the two cerebral hemispheres and span the longitudinal fissure. In the spinal cord, there are the anterior white commissure, and the gray commissure. Commissural neurons refer to neuronal cells that grow their axons across the midline of the nervous system within the brain and the spinal cord.
• Commissure also often refers to cardiac anatomy of heart valves. In the heart, a commissure is the area where the valve leaflets abut. When such an abutment is abnormally stiffened or even fused, valvular stenosis results, sometimes requiring commissurotomy.
• The term may also refer to the junction of the upper and lower lips (see labial commissure of mouth).
• It may refer to the junction of the upper and lower mandibles of a bird's beak, or alternately, to the full-length apposition of the closed mandibles, from the corners of the mouth to the tip of the beak.
• It may refer to the nasal and temporal meeting points of the upper and lower eyelids (the medial and lateral canthi).
• In the vulva, the joining points of the two folds of the labia majora create two commissures—the anterior commissure just anterior to the prepuce of the clitoris, and the posterior commissure of the labia majora, directly posterior to the frenulum of the labia minora and anterior to the perineal raphe.

In biology, the meeting of the two valves of a brachiopod or clam is a commissure; in botany, the term is used to denote the place where a fern's laterally expanded vein endings come together in a continuous marginal sorus.

The epithalamus (pl.: epithalami) is a posterior (dorsal) segment of the diencephalon. The epithalamus includes the habenular nuclei, the stria medullaris, the anterior and posterior paraventricular nuclei, the posterior commissure, and the pineal gland.

A corpus callosotomy (/kəˈlɔːs(ə)təmiː/) is a palliative surgical procedure for the treatment of medically refractory epilepsy. The procedure was first performed in 1940 by William P. van Wagenen. In this procedure, the corpus callosum is cut through, in an effort to limit the spread of epileptic activity between the two halves of the brain.

Although the corpus callosum is the largest white matter tract connecting the hemispheres, some limited interhemispheric communication is still possible via the anterior and posterior commissures. After the operation, however, the brain often struggles to send messages between hemispheres, which can lead to side effects such as speech irregularities, disconnection syndrome, and alien hand syndrome.