The mesencephalon, or midbrain, is located between the dien-cephalon and the pons. The corpora quadrigemina are four rounded elevations on the dorsal surface of the midbrain (see fig. 8.16). The two upper mounds, the superior colliculi, are involved in visual reflexes; the inferior colliculi, immediately below, are relay centers for auditory information.
The midbrain also contains the cerebral peduncles, red nucleus, substantia nigra, and other nuclei. The cerebral peduncles are a pair of structures composed of ascending and descending fiber tracts. The red nucleus, an area of gray matter deep in the midbrain, maintains connections with the cerebrum and cerebellum and is involved in motor coordination.
As discussed in chapter 7, the midbrain has two systems of dopaminergic (dopamine-releasing) neurons that project to other areas of the brain. The nigrostriatal system projects from the substantia nigra to the corpus striatum of the basal nuclei; this system is required for motor coordination, and it is the degeneration of these fibers that produces Parkinson's disease. Other dopaminergic neurons that are part of the mesolimbic system project from nuclei adjacent to the substantia nigra to the limbic system of the forebrain (fig. 8.18). This system is in-
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.