In humans, the medulla, the pons, and the cerebellum represent the hindbrain. In both structure and function, the medulla is very much like an enlarge extension of the spinal cord, the medulla has neuron cell bodies at its center, surrounded by a layer of myelin-covered axons. The medulla controls several automatic functions, such as breathing, heart rate, blood pressure, and swallowing. Certain neurons in the pons, located above the medulla, appear to influence transitions between sleep and wakefulness and between stages of sleep. Others influence the rate and pattern of breathing. The cerebellum is crucially important in coordinating movements of the body. It receives information from command centers in the higher, conscious areas of the brain that control movement and also from position sensors in muscles and joints. By comparing what the command centers ordered with information from the position sensors, the cerebellum guides smooth accurate motions and body position. Not surprisingly, the cerebellum is largest in animals whose activities require fine coordination. It is best developed in birds, who engage in the complex activity of flight.