The brainstem is the stalklike portion of the brain that joins higher brain centers to the spinal cord. It contains several nuclei that are surrounded by white matter. Ascending (sensory) and descending (motor) axons between higher brain centers and the spinal cord pass through the brainstem. The components of the brainstem include the midbrain, pons, and medulla oblongata (see figure 8.13).
The midbrain is the most superior portion of the brainstem. It is located posterior to the hypothalamus and superior to the pons. It contains reflex centers for head, eye, and body movements in response to visual and auditory stimuli. For example, reflexively turning the head to enable better vision or better hearing is activated by the midbrain.
The pons lies between the midbrain and the medulla oblongata and is recognizable by its bulblike anterior portion. It consists primarily of axons. Longitudinal axons connect lower and higher brain centers, and transverse axons connect with the cerebellum. The pons also works with the medulla oblongata by controlling the rate and depth of breathing.
The medulla oblongata is the most inferior portion of the brain, and it is the connecting link with the spinal cord. Descending (motor) axons extending between the brain and the spinal cord cross over to the opposite side of the brain within the medulla oblongata. The medulla oblongata contains three integration centers that are vital for homeostasis:
- The respiratory rhythmicity center controls the basic rhythm of breathing by triggering each cycle of inhale and exhale. It is also involved in associated reflexes such as coughing and sneezing.
- The cardiac control center regulates the rate and force of heart contractions.
- The vasomotor center regulates blood pressure and blood flow by controlling the diameter of blood vessels.