brainstem

The brainstem is the stem-like region that connects the cerebrum and cerebellum to the spinal cord. It consists of three major parts: the midbrain (mesencephalon) at the top, the pons in the middle, and the medulla oblongata at the base. The brainstem contains pathways that carry motor and sensory information between the brain and the body, as well as cranial nerve nuclei that control most of the head and neck. It also houses the reticular formation, a network involved in arousal and autonomic regulation.

Anatomy and landmarks: The midbrain contains the cerebral peduncles, superior and inferior colliculi, and nuclei of the oculomotor and trochlear nerves. The pons contains the pontine nuclei and origins of several cranial nerves and the corticospinal tract fibers that traverse the ventral surface. The medulla houses the nuclei of cranial nerves IX through XII, and structures such as the pyramids where motor fibers decussate. The brainstem also connects to the cerebellum via the peduncles and interfaces with the thalamus and spinal cord through ascending and descending tracts.

Functions: The brainstem regulates essential life-sustaining functions, including breathing, heart rate, and blood pressure, largely through the medullary and pontine autonomic centers. It mediates reflexes such as swallowing, vomiting, coughing, and gagging and relays somatosensory and motor information between the brain and body. The reticular activating system within the brainstem contributes to wakefulness and attention.

Clinical relevance: Brainstem lesions can produce cranial nerve deficits, long tract signs, and disorders of autonomic control or consciousness. Stroke, demyelination, or trauma involving the brainstem can be life-threatening due to disruption of vital functions.