The medulla oblongata together with the the inferior olive, pyramids and pons make up the region of the brain known as the brainstem. This region is responsible for the autonomic nervous system processes that control heart rate, respiration, pain sensitivity, vomiting and vasomotor reflexes. As the portion of the brainstem that connects the pons to the spinal cord, the medulla oblongata would be easy to mistake for spinal tissue were it not for the distinct pyramidal structures on its sides. The medulla oblongata is predominantly made up of white and gray matter; it also houses the communication tracts that carry messages between hemispheres as well as between the brain and the spinal cord. Considering that the entire structure measures less than 1.2 inches (3 cm) in length, the individual functions of most of its microscopic elements are not well understood.
Traversing the entirety of the upper half of the medulla oblongata are the arciform fibers. These are found both deep within the medulla and on its surface. The deepest fibers are the most common, connecting the nuclei of the funiculus gracilis, the funiculus cuneatus and the restiform body to the raphe.
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The raphe is found just above the pyramids in the center of the medulla oblongata. It is comprised of a dense mixture of neurons and nerve fibers. Its fibers extend in numerous directions, some of which connect to the floor of the fourth ventricle.
Sometimes considered the lowermost portion of the pons, the fourth ventricle also can be considered the uppermost portion of the medulla oblongata. The hypoglossal nucleus and the nucleus ambiguous are found within it, as well as the point of origin of both the hypoglossal and glossopharyngeal accessory nerves. This diamond-shaped structure has both a floor and a roof. The roof is formed by the medullary vela, and the floor is made up of the rhomboid fossa and formatio reticularis.
The formatio reticularis is an unusual structure that is found behind the olive and the pyramids. It is made up of nerve fiber bundles arranged at 90-degree angles to each other. They connect to the olive, the spinal cord, the funiculus cuneatus and the funiculus gracilis.
Gray matter is found throughout the lateral part of the medulla. Within it lie a large number of vital nuclei that are used for transmitting sensory information from the peripheral nervous system. The gray matter also contains many cranial nerves.