The brainstem connects the brain to the spinal cord and includes the medulla oblongata, pons, and midbrain, playing a vital role in basic bodily functions like respiration, circulation, and digestion. The medulla regulates vital functions such as breathing and heart rate, the pons facilitates communication between brain parts and regulates breathing and sleep, while the midbrain is involved in sensory and motor regulation. Key structures like the substantia nigra within the midbrain are crucial for movement control and processing sensory information.