midthjerne

The midthjerne, known in English as the midbrain, is the upper part of the brainstem, extending from the diencephalon above to the pons below. It houses several nuclei and tracts essential for movement, sensation, and reflexes.

Anatomy: It is divided into tectum (roof) with the superior and inferior colliculi, tegmentum (floor of the aqueduct) containing the red nucleus, substantia nigra, and oculomotor and trochlear nuclei, and the crus cerebri (cerebral peduncles) containing descending motor fibers. The cerebral aqueduct runs through it, connecting the third and fourth ventricles.

Functions: The superior colliculus contributes to visual reflexes; inferior colliculus to auditory processing. The tegmentum and peduncles arbitrate voluntary motor control via corticospinal tracts; the oculomotor nerve nuclei regulate most eye movements and pupil constriction; the red nucleus and substantia nigra participate in movement coordination and dopaminergic signaling. The periaqueductal gray modulates pain, and the midbrain participates in arousal and autonomic responses.

Blood supply and development: It is supplied mainly by branches of the basilar artery, including perforators, the superior cerebellar artery, and posterior cerebral artery; embryologically, it arises from the mesencephalon.

Clinical relevance: Lesions here can produce oculomotor palsy (often with fixed dilated pupil), ataxia, and contralateral motor deficits (as seen in Weber and Benedikt syndromes). Midbrain strokes, tumors, demyelinating lesions, and raised intracranial pressure affecting the aqueduct can lead to hydrocephalus and gaze abnormalities. Diagnostic imaging (MRI/CT) delineates pathology.