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Even though the brainstem is divided in three sections, sometimes the reticular formation is described as the fourth. I have included the description of the reticular formation in the pons and some deficit syndromes at the end.
The brain stem is continuous with the cerebrum above and with the spinal cord below. It is usually divided into three sections: midbrain (mesencephalon), pons (metencephalon), and medulla oblongata (myelencephalon) (Krebs, 2006).
The mid-brain or mesencephalon connects the pons and cerebellum with the thalamencephalon and cerebral hemispheres. Its components are a) the cerebral peduncles (a pair of cylindrical bodies), b) the corpora quadrigemina (four rounded eminences at the back) and c) the cerebral aqueduct (a tunnel that connects the 3rd and 4th ventricle) (Gray, 2000).
The cerebral peduncles are two cylindrical masses situated at the base of the brain. Each peduncle has a dorsal part called tegmentum and ventral part called the base or crusta separated by the substantia nigra (Gray, 2000). The substantia nigra was named because of the color of the area given by the deep pigmented gray substance it contains. Lesions in the peduncles involving the third craneal nerve can give Weber's syndromeresulting in hemiparesias of the contralateral half of the body, and same side ptosis and inability to move up, down or medially the eyes (University of Florida, 2009).
The base consists almost entirely of three longitudinal bundles of efferent fibers, cerebrospinal, frontopontine, and temporopontine (Gray, 2000).This fibers are concerned with motor control (Tantorsky, 2007)
The tegmentum consists of longitudinal and transverse fibers, together with a considerable amount of gray substance containing the red nucleus and the interpeduncular ganglion (Gray, 2000).The red nucleus was named for its pinkish color due to its high vascularity (D'Alessandro, 2010). A midbrain stroke that involves the third nerve near this nucleus can give Benedikt's syndrome resulting in unilateral third nerve palsy with contralateral ataxia (Uninversity of Florida, 2009). In the tegmentum lies also the rubrospinal tract, an important part of the motor pathway from the cerebellum to the lower motor centers (Tantorsky, 2007). The interpeduncular ganglion if a median collection of nerve cells (Gray, 2000). It contains fibers of the fasciculus retroflexus of Meynert, involved in mechanisms of emotion and behavior (D'Alessandro, 2010).
Besides the two nuclei mentioned, there are small collections of cells which form the dorsal and ventral nuclei and the central nucleus or nucleus of the raphé (Gray, 2000).Their main function is to release serotonin to the rest of the brain (Wikipedia, 2010). You can find roots of the third cranial nerve through the tegmentum that supplies " the levator palpebrae superioris, the superior rectus, inferior rectus, medial rectus, inferior oblique, and constrictor pupillae muscles" (D'Alessandro, 2010, Section Plate 17.340 Mesencephalon: ...
The solution involves a detailed description of the brain stem components - midbrain or mesencephalon, pons or metencephalon, medulla oblongada o myelencephalon and the reticular formation- and their functions in brain physiology. Deficit syndromes are also included.