BRAIN STEM
The brain stem looks like a stalk supporting the cerebrum. It has three subdivisions: the medulla, pons, and midbrain. The medulla, in the lower portion of the brain stem anterior to the cerebellum, runs uninterrupted with the spinal cord. The pons, a large mass of fibers, is superior to the medulla. Located just above the pons, the midbrain is the hub of visual and auditory reflexes.

The medulla mostly consists of nerves from the spinal cord that connect to the rest of the brain. In complex vertebrates, neuron cell bodies in the medulla control respiration, heartbeat, and blood pressure. Other reflex centers in the medulla control actions such as swallowing, coughing, and vomiting.

The pons functions as a relay station, connecting neuronal pathways from the spinal cord, cerebrum, and cerebellum. It contains nuclei (clusters of cell bodies within the central nervous system) that relay impulses from the cerebrum to the cerebellum. In addition, centers in the pons regulate sleep and respiration. The midbrain receives incoming sensory messages, integrates them, and transmits decisions to the appropriate motor nerves.

In mammals, the midbrain is composed of the superior colliculi (controlling visual reflexes such as pupil constriction and blinking) and the inferior colliculi (controlling certain auditory reflexes). The mammalian midbrain also includes a center (the red nucleus) that assists in the maintenance of muscle tone and posture.