midbrain

n. Brain area situated between the diencepha- lons (rostrally) and the pons (caudally). In the human brain it is a relatively small area approximately 2 cm long. The midbrain (also known as mesencephalon) forms with the pons and the medulla oblongata the brainstem. The dorsal surface of the midbrain forms the tectum. The floor of the midbrain is known as the tegmentum. The tegmentum is situated around the cerebral aqueduct (connection between the third and fourth ventricles). Two large neural pathways connecting the cerebral cortex and the brainstem and spinal cord are found in the ventral surface of the midbrain, known as the cerebral peduncles. The midbrain also contains the substantia nigra, a major component of the basal ganglia. Neurons of the substantia nigra use dopamine as a neurotransmitter and are involved in controlling movements. Parkinson's disease is associated with a degeneration of the dopaminergic neurons of the substantia nigra. The midbrain also contains the inferior and superior colliculi, both forming the corpora quadrigemina. Superior colliculi are involved in eye movements, and the inferior colliculi are the principal midbrain nucleus of the auditory pathway.

- AA

▶ See also COLLICULUS