Keskaju

Keskaju is the Estonian term for the midbrain, the central portion of the brainstem that connects the forebrain with the hindbrain. It lies between the diencephalon and the pons and forms the upper part of the brainstem. Anatomic components include the tectum with the superior and inferior colliculi, and the tegmentum, which houses cranial nerve nuclei and reticular formation. The cerebral aqueduct passes through the midbrain, linking the third and fourth ventricles. Notable structures include the red nucleus and the substantia nigra, involved in motor control. The oculomotor (CN III) and trochlear (CN IV) nerve nuclei reside here and control most eye movements.

Developmentally, it forms from the embryonic mesencephalon. Functionally, the midbrain participates in visual and auditory reflexes, motor coordination, and regulation of arousal via parts of the reticular formation. Clinically, lesions can cause oculomotor palsy and vertical gaze abnormalities; stroke, hemorrhage, or tumor can impair pathways. Degeneration of the substantia nigra is associated with Parkinson's disease. Etymology: Keskaju translates to "middle brain" in Estonian. Related terms include mesencephalon and brainstem.