MEDULLA OBLONGATA

also called Medulla, the lowest part of the brain and the lowest portion of the brain stem. The medulla oblongata is connected by the pons to the midbrain and is continuous posteriorly with the spinal cord, with which it merges at the opening (foramen magnum) at the base of the skull. The medulla encloses the fourth ventricle and contains nuclei associated with most of the cranial nerves, along with major fibre tracts that link the spinal with higher nerve centres. The medulla also contains various nerve centres mediating the control of involuntary vital functions, such as respiration. Like the cerebrum and cerebellum, the medulla oblongata consists of both myelinated (white matter) and unmyelinated (gray matter) nerve fibres. In the medulla, however, the normal anatomic relationship of the two is reversed, with the white matter on the outside and the gray matter lying on the inside, around the fourth ventricle (a fluid-filled cavity formed by the expansion of the central canal of the spinal cord upon entering the brain). A complex network of medullary nerve cells and processes from elsewhere in the central nervous system enables the medulla to carry on complex integrative functions. The medulla contains several functional centres that govern autonomic nervous activity, regulating respiration, heart rate, and digestive processes. Other activities of the medulla include control of movement (along with the cerebellum), relaying of somatic sensory information from internal organs, and control of arousal and sleep (along with the thalamus). The last seven cranial nerves emerge from the medulla, which influences their functional activities. Injuries or disease affecting the middle portion of the medulla produce paralysis of the opposite side of the body, loss of the senses of touch and position, and partial paralysis of the tongue. Injuries or disease of the lateral medulla cause loss of pain and temperature sensations, loss of the gag reflex, difficulty in swallowing, vertigo, vomiting, and loss of coordination.