anterior commissure

**Anatomy** - Compact bundle of myelinated fibers crossing the midline in the rostral wall of the third ventricle (lamina terminalis), immediately anterior to the columns of the fornix. - Two parts: (1) the smaller anterior (olfactory) part connects the anterior olfactory nuclei and olfactory tracts of both sides. (2) The larger posterior (temporal) part connects the amygdalae, the inferior and middle temporal gyri, and the temporal pole of both hemispheres. It is the main interhemispheric link for the paleocortex and the temporal lobe. - It is phylogenetically older than the corpus callosum. The corpus callosum connects mostly neocortex.

**Function** - Interhemispheric transfer of olfactory information. Because most olfactory input is ipsilateral, the anterior commissure allows some bilateral integration. - Transfers emotional and memory-related information between the amygdalae and temporal lobes, and some auditory and visual information from the temporal lobes. - Carries some pain-related and sexually dimorphic signals. It is reported to be larger in females, though this is of limited board relevance.

**Clinical significance** - Agenesis of the corpus callosum: the anterior commissure is often intact or enlarged and serves as a compensatory interhemispheric pathway. Patients may have relatively mild deficits. Disconnection signs are usually subtle, such as impaired bimanual coordination and slow interhemispheric transfer. - Split-brain surgery (callosotomy) for refractory epilepsy: sparing the anterior commissure leaves some interhemispheric transfer. Complete commissurotomy produces classic disconnection syndromes, such as the left hand being unable to name an object. - Midline malformations: it may be absent or abnormal in holoprosencephaly and septo-optic dysplasia. - Seizure spread: it can carry epileptic activity between the temporal lobes. - Imaging: it is seen as a small bright dot or band on midsagittal MRI just anterior to the fornix columns. It is a landmark for stereotactic neurosurgery, such as the AC-PC line used in deep brain stimulation targeting.

**High-yield board points** - Location: lamina terminalis, anterior to the fornix columns. - Connects the amygdalae and temporal lobes plus olfactory structures. - Other major commissures: corpus callosum (largest), posterior commissure (pupillary light reflex and vertical gaze, at the junction of the third ventricle and cerebral aqueduct), hippocampal (fornical) commissure, habenular commissure. - The posterior commissure is involved in the bilateral consensual pupillary light reflex and vertical gaze. Compression by a pineal tumor causes Parinaud syndrome. - Commissural fibers connect corresponding areas of the two hemispheres. Association fibers connect areas within one hemisphere. Projection fibers connect the cortex with subcortical structures.