As the spinal cord enters the skull, it enlarges into the bottommost structure of the brain, the medulla (or medulla oblongata). The medulla controls many of the most basic physiological functions for survival, particularly breathing and the beating of the heart. Reflexes such as vomiting, coughing, sneezing, and salivating are also controlled by the medulla. The medulla is sensitive to opiate and amphetamine drugs, and overdoses of these drugs can impair its normal functioning. Severe impairment can lead to a fatal shutdown of the respiratory and cardiovascular systems.
Just above the medulla lie the pons, parts of the reticular formation, the raphe system, and the locus coeruleus. All these structures play a role in arousal and sleep. The pons plays a major role in initiating rapid eye move-
Anatomy of the Brain
ment (REM) sleep. REM sleep is characterized by repeated horizontal eye movements, increased brain activity, and frequent dreaming. The reticular system (sometimes called the reticular activating system, or RAS) stretches from the pons through the midbrain to projections into the cerebral cortex. Activation of the reticular system, by sensory stimulation or thinking, causes increases in arousal and alertness in diverse areas of the brain. For the brain to pay attention to something, there must be activation from the reticular formation. The raphe system, like the reticular system, can increase the brain's readiness to respond to stimuli. However, unlike the reticular formation, the raphe system can decrease alertness to stimulation, decrease sensitivity to pain, and initiate sleep. Raphe system activity is modulated somewhat by an adjacent structure called the locus coeruleus. Abnormal functioning of this structure has been linked with depression and anxiety.
The largest structure in the metencephalon is the cerebellum, which branches off from the base of the brain and occupies a considerable space in the back of the head. The cerebellum's primary function is the learning and control of coordinated perceptual-motor activities. Learning to walk, run, jump, throw a ball, ride a bike, or perform any other complex motor activity causes chemical changes to occur in the cerebellum that result in the construction of a sort of program for controlling the muscles involved in the particular motor skills. Activation of specific programs enables the performance of particular motor activities. The cerebellum is also involved in other types of learning and performance. Learning language, reading, shifting attention from auditory to visual stimuli, and timing (such as in music or the tapping of fingers) are just a few tasks for which normal cerebellar functioning is essential. People diagnosed with learning disabilities often are found to have abnormalities in the cerebellum.
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