Medulla Oblongata

The medulla oblongata is an enlarged continuation of the spinal cord, extending from the level of the foramen magnum to the pons (see fig. 11.20). Its dorsal surface flattens to form the floor of the fourth ventricle, and its ventral surface is marked by the corticospinal tracts, most of whose fibers cross over at this level. On each side of the medulla oblongata is an oval swelling called the olive, from which a large bundle of nerve fibers arises and passes to the cerebellum.

Because of the medulla oblongata's location, all the ascending and descending nerve fibers connecting the brain and spinal cord must pass through it. As in the spinal cord, the white matter of the medulla surrounds a central mass of gray matter. Here, however, the gray matter breaks up into nuclei that are separated by nerve fibers. Some of these nuclei relay ascending impulses to the other side of the brain stem and then on to higher brain centers. The nucleus gracilis and the nucleus cuneatus, for example, receive sensory impulses from fibers of the fasciculus gracilis and the fasciculus cuneatus and pass them on to the thalamus or the cerebellum.

Other nuclei within the medulla oblongata control vital visceral activities. These centers include the following:

1. Cardiac center. Peripheral nerves transmit impulses originating in the cardiac center to the heart, where they increase or decrease heart rate.

2. Vasomotor center. Certain cells of the vasomotor center initiate impulses that travel to smooth muscles in the walls of blood vessels and stimulate them to contract, constricting the vessels (vasoconstriction) and thereby increasing blood pressure. A decrease in the activity of these cells can produce the opposite effect—dilation of the blood vessels (vasodilation) and a consequent drop in blood pressure.

3. Respiratory center. The respiratory center acts with centers in the pons to regulate the rate, rhythm, and depth of breathing.

Some nuclei within the medulla oblongata are centers for certain nonvital reflexes, such as those associated with coughing, sneezing, swallowing, and vomiting.

However, since the medulla also contains vital reflex centers, injuries to this part of the brain stem are often fatal.

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