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medulla oblongata[mə′dəl·ə ‚äb‚lȯŋ′gäd·ə]
the most posterior (inferior) part of the brain, extending from the spinal cord below to the pons varolii above (toward the front). The posterior surface of the medulla oblongata forms the lower part of the floor of the fourth ventricle.
The medulla oblongata transmits (often after modification) signals from the spinal cord to the brain by centripetal conduction paths and from the brain to the spinal cord by centrifugal paths. The neurons of the medulla oblongata (the nuclei of the reticular formation and craniocerebral nerves) help regulate blood circulation, respiration, digestion, and the functioning of the higher portions of the brain and of the segmental apparatus of the spinal cord, including sleep. Motor impulses are transmitted at the level of the medulla oblongata to spinal cord neurons through the pyramidal system of conduction paths (the corticospinal tract), which forms a chiasma, and through the extrapyramidal system.
The medial portions of the reticular formation of the medulla oblongata contain accumulations of nerve cells that form the descending reticular spinal system, which inhibits the motor apparatus of the spinal cord and mediates the coordinating influences from the cerebral cortex, subcortical nuclei, cerebellum, and other portions of the brain that control movement and position. The raphe nuclei contain neurons that have processes in almost all the higher-situated portions of the brain and synchronize the electrical activity of the cerebral cortex with the onset of the slow sleep phase. The mediator exciting these neurons is serotonin. Destruction of the neurons in experimental animals causes persistent insomnia and behavioral disturbances; pharmacological block of the neuron’s development and release of serotinin has the same effect. On the floor of the fourth ventricle in the medulla oblongata are neurons (the region of the blue spot), which together with the mediator norepinephrine influence other cells of the reticular formation and cause inclusion of the inhibitory reticular spinal system in the rapid sleep phase and inhibition of muscle tone and cerebrospinal reflexes. Thus, the medulla oblongata, the phylogeneti-cally oldest portion of the brain, plays an important role in sleep.
Nerve pathways in the posterosuperior portions of the medulla oblongata transmit from the spinal cord sensory signals from receptors of the skin, muscles and joints, and internal organs. Some of these pathways are intercepted in the medulla oblongata’s nuclei, where second neurons of the sensory pathway are situated; the nerve pathways also proceed to the opposite side, forming a chiasma. The medulla oblongata’s neuronal mechanisms automatically regulate respiration, cardiac rhythm, blood pressure, the secretion of saliva, the secretion and peristalsis of the stomach and small intestine, chewing, swallowing, vomiting, and sneezing; these neuronal mechanisms also issue commands to the speech apparatus (the tongue and the muscles of the soft palate and larynx). This regulation occurs by means of signals proceeding through the sensory fibers of the somatic and autonomic craniocerebral nerves (from the skin, mucous membranes, and muscles of the head, taste receptors, heart, major blood vessels, respiratory tract, lungs, and alimentary canal) and by commands sent through the efferent nerve fibers to the muscular and glandular portions of these organs and to the corresponding skeletal muscles. Impairment of the above-mentioned functions as a result of bilateral injury to the medulla oblongata causes the severe syndrome called bulbar paralysis.
L. P. LATASH