Midbrain

The midbrain, or mesencephalon, is the upper portion of the brainstem, located between the diencephalon above and the pons below. It connects the forebrain with the hindbrain and houses structures important for vision, hearing, motor control, arousal, and autonomic functions. The midbrain is commonly described as having a dorsal tectum and a ventral tegmentum, with the anterior continuation formed by the cerebral peduncles (crus cerebri).

Anatomically, the midbrain consists of several key regions. The tectum contains the superior colliculi, which are involved in visual reflexes, and the inferior colliculi, which play a role in auditory processing. The tegmentum includes the red nucleus, the substantia nigra, and the periaqueductal gray matter, as well as the cranial nerve nuclei for oculomotor (CN III) and trochlear (CN IV) nerves. The crus cerebri carry descending motor tracts from the cerebral cortex to the brainstem and spinal cord. The cerebral aqueduct runs through the midbrain, connecting the third and fourth ventricles.

Functionally, the midbrain participates in visual and auditory processing, initiation and coordination of movement, regulation of eye movements and pupil responses, pain modulation, and aspects of consciousness and arousal through its reticular formation. The substantia nigra provides dopaminergic input to the basal ganglia, which is critical for movement control.

Clinical significance stems from midbrain lesions, which can produce oculomotor palsies, ataxia, or contralateral motor deficits. Classic midbrain syndromes include Weber syndrome and Benedikt syndrome, reflecting involvement of the cerebral peduncles, nigrostriatal pathways, and brainstem nuclei. The midbrain develops from the embryonic mesencephalon.