The brain stem is an extension of the spinal cord into the cranial cavity consisting of the medulla, pons, and mesencephalon. The brain stem contains the motor and sensory nuclei that control sensorial and motor functions of the face and the head, whereas the spinal cord controls the movements of the rest of the body below the neck. The brain stem also accounts for many other involuntary and autonomic functions, such as the control of respiration, eye movement, equilibrium, gastrointestinal function, cardiovascular system, and several stereotyped body movements as well.

Higher brain neural centers (i.e., motor cortical areas, cerebellum, hypothalamus, and limbic system) send signals to the brain stem that trigger or change specific control functions throughout the body.

The brain stem's reticular nuclei and vestibular nuclei support the body against gravity by controlling the degree of muscle contraction...