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The midbrain, most rostral segment of the brainstem, consists of a tectum, a tegmentum, and a large important fiber bundle, the crus cerebri. The cavity of the midbrain, the cerebral aqueduct, is surrounded by cells, the periaqueductal gray (PAG), a structure important in the modulation of pain pathways. Prominent structures of the tectum are the eminences of the superior (related to visual function) and inferior (related to auditory function) colliculi, the nuclei of these elevations, and upper portions of the PAG. The central region of the midbrain, the tegmentum, contains the red nucleus (motor function), important ascending (sensory) and descending (motor) tracts, and a number of smaller tracts that interconnect the various segments of the brainstem with each other and/or with the cerebellum. The substantia nigra and the crus cerebri collectively form the basis pedunculi; the former is the most important source of dopaminergic fibers to the basal nuclei of the telencephalon, and the latter is made up of important descending fibers to the basilar pontine nuclei, the motor nuclei of cranial nerves, and the spinal cord motor neurons. The cranial nerve nuclei associated with the midbrain are the oculomotor (and Edinger-Westphal, level of the superior colliculus), the trochlear (level of the inferior colliculus), and portions of the trigeminal complex. At its position within the tentorial notch, the midbrain is susceptible to a variety of clinical insults, especially those related to increases in intracranial pressure. Caudally the cerebral aqueduct opens into the fourth ventricle. |
