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( Brainstem)
The brain stem is the lower part of the brain, adjoining and structurally continuous with the spinal cord. Some taxonomies describe the brain stem as the medulla and mesencephalon, where as others include diencephalic regions. The most medial part of the medulla is the anterior median fissure. Moving laterally on each side are the pyramids. The pyramids contain the fibers of the corticospinal tract, or the upper motor neuronal axons as they head inferiorly to synapse on lower motor neuronal cell bodies within the ventral horn of the spinal cord.
The anterolateral sulcus is lateral to the pyramids. Emerging from the anterolateral sulci are the hypoglossal nerve (CN XII) rootlets. Lateral to these rootlets and the anterolateral sulci are the olives. The olives are swellings in the medulla containing underlying inferior olivary nuclei (containing various nuclei and afferent fibers). Lateral (and dorsal) to the olives are the rootlets for cranial nerves IX and X (glossopharyngeal and vagus, respectively). The pyramids end at the pontomedullary junction, noted most obviously by the large basal pons. Between the basal pons, cranial nerve 6, 7 and 8 emerge (medial to lateral). These cranial nerves are the abducens nerve, facial nerve and the vestibulocochlear nerve, respectively. At the level of the midpons, the large trigeminal nerve, CN V, emerges. At the rostral pons, the occulomotor nerve emerges at the midline. Laterally, the trochlear nerve has emerged after emerging out of the dorsal rostral pons and wrapping around to the anterior. The most medial part of the medulla is the posterior median fissure. Moving laterally on each side is the fasciculus gracilis, and lateral to that is the fasciculus cuneatus. Superior to each of these, and directly inferior to the obex, are the gracile tubercles and cuteanus tubercles, respectively. Underlying these are their respective nuclei. The obex marks the end of the 4th ventricle and the beginning of the central canal. The posterior intermediate sulci separates the fasciculi gracilis from the fasciculi cuneatus. Lateral to the fasciculi cuneatus is the lateral funiculus.
Superior to the obex is the floor of the 4th ventricle. In the floor of the 4th ventricle, various nuclei can be visualized by the small bumps that they make in the overlying tissue. In the midline and directly superior to the obex is the vagal trigone and superior to that it the hypoglossal trigone. Underlying each of these are motor nuclei for the respective cranial nerves. Superior to these trigones are fibers running laterally in both directions. These fibers are known collectively as the striae medullares. Continuing in a rostral direction, the large bumps are called the facial colliculi. Each facial colliculus, contrary to their names, do not contain the facial nerve nuclei. Instead, they have facial nerve axons traversing superficial to underlying abducens (CN VI) nuclei. Lateral to all these bumps previously discussed is an indented line, or sulcus that runs rostrally, and is known as the sulcus limitans. This separates the medial motor neurons from the lateral sensory neurons. Lateral to the sulcus limitans is the area collectively known as the vestibular area, which is involved in special sensation. Moving rostrally, the inferior, middle, and superior cerebellar peduncles are found connecting the midbrain to the cerebellum. Directly rostral to the superior cerebellar peduncle, there is the superior medullary velum and then the two trochlear nerves. This marks the end of the pons as the inferior colliculus is directly rostral and marks the caudal midbrain. Spinal Cord to Medulla Transitional Landmark From a ventral view, there can be seen a decussation of fibers between the two pyramids. This decussation marks the transition from medulla to spinal cord. Superior to the decussation is the medulla and inferior to it is the spinal cord.
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