- The brainstem is a critical, evolutionarily ancient structure located at the base of the brain, connecting the cerebrum and cerebellum to the spinal cord.
- Despite its relatively small size, the brainstem plays an indispensable role in maintaining vital bodily functions, including breathing, heart rate, blood pressure, consciousness, and reflexes. It acts as a conduit for all sensory and motor information traveling between the brain and body and serves as the control center for many autonomic and involuntary processes essential to life.
- The brainstem is composed of three main parts: the midbrain (mesencephalon), pons, and medulla oblongata, each with specialized functions but working together as an integrated unit. The midbrain, the uppermost portion, is involved in visual and auditory processing, eye movement, and motor control, and contains structures such as the superior and inferior colliculi and the substantia nigra, the latter of which plays a role in movement and is notably affected in Parkinson’s disease. The pons, located below the midbrain, serves as a bridge between the cerebrum and cerebellum, coordinating movement and playing a major role in sleep regulation, respiration, and facial sensations and expressions.
- The medulla oblongata, the lowest portion of the brainstem, is directly continuous with the spinal cord and contains centers that regulate vital autonomic functions, such as heartbeat, blood vessel diameter, breathing, swallowing, vomiting, and coughing. Damage to the medulla can be life-threatening because of its role in controlling basic life-support systems. It also houses the decussation of pyramids, where most motor fibers from the cerebral cortex cross over to the opposite side of the body, explaining why brain damage on one side of the brain often results in motor impairment on the opposite side.
- The brainstem contains cranial nerve nuclei for 10 of the 12 cranial nerves (III through XII), making it central to the control of many sensory and motor functions of the face and head, including hearing, taste, eye movement, balance, facial expression, and swallowing. These cranial nerves arise from various levels of the brainstem and relay information to and from the peripheral nervous system.
- In addition, the brainstem contains the reticular formation, a network of interconnected neurons that spans all three regions and plays a fundamental role in arousal, attention, sleep-wake cycles, and consciousness. The ascending reticular activating system (ARAS), part of this network, is essential for maintaining wakefulness; damage to it can result in coma or impaired levels of consciousness.
- Clinically, brainstem dysfunction can arise from stroke, tumors, trauma, demyelinating diseases (such as multiple sclerosis), or congenital malformations. Because so many vital functions are controlled in this small region, even minor lesions can have severe and sometimes fatal consequences. For example, brainstem strokes can lead to conditions such as locked-in syndrome, where a person is fully conscious but unable to move or speak due to paralysis of nearly all voluntary muscles.
