Coronal section - Midbrain

The midbrain (mesencephalon) is the uppermost part of the brainstem, situated between the pons and diencephalon. It surrounds the cerebral aqueduct and is broadly divided into the tectum, tegmentum and cerebral peduncles. Important structures within the midbrain include the substantia nigra, red nucleus, oculomotor and trochlear nerve nuclei, along with major ascending and descending neural pathways. The superior and inferior colliculi are involved in visual and auditory reflexes respectively, while the cerebral peduncles carry important descending motor fibres. Midbrain lesions may produce combinations of oculomotor abnormalities, contralateral motor weakness, tremor and movement disorders. Degeneration of dopaminergic neurons in the substantia nigra pars compacta is a hallmark of Parkinson disease.