17.11: Brainstem
The brainstem, located inferior to the brain and superior to the spinal cord, serves as a bridge between the cerebrum and the spinal cord. It plays a vital role in relaying information and controlling critical life functions. It comprises three primary regions: the midbrain, pons, and medulla oblongata.
The Midbrain
The midbrain is located beneath the diencephalon and connects the cerebrum with the lower parts of the brain. The cerebral peduncles are prominent midbrain structures that house the crus cerebri. The crus cerebri contain the corticospinal tract — a bundle of nerve fibers originating from the cerebral cortex. The corticospinal tract carries signals from the brain to the spinal cord, allowing voluntary movements such as walking, reaching, or grabbing objects. Damage to the crus cerebri can result in motor deficits and muscle weakness on the opposite side of the body due to how these nerve fibers cross over in the brainstem.
On its posterior side, the midbrain houses the corpora quadrigemina, consisting of four large nuclei — two superior colliculi involved in visual processing and two inferior colliculi involved in auditory processing. The red nuclei of the midbrain have a rich blood supply and high iron content and are involved in motor coordination, especially the fine motor skills of the limbs. The midbrain also contains the substantia nigra, known for its dopamine-releasing neurons crucial for movement control. When these neurons degenerate, such as with Parkinson’s disease, it results in motor symptoms such as tremors, rigidity, bradykinesia, and postural instability.
The Pons
The pons is a major communication and coordination center between the cerebrum and cerebellum, mainly through the middle cerebellar peduncles. It houses numerous pontine nuclei that transmit signals from the motor cortex to the cerebellum, essential for motor control, coordination, and autonomic regulation. It regulates breathing, contributes to the sleep-wake cycle, and contains cranial nerve nuclei responsible for facial movement and hearing. It also plays a role in coordinating swallowing reflexes and processing taste sensations.
The Medulla Oblongata
The medulla oblongata forms the lower portion of the brainstem and is directly connected to the spinal cord. It features two longitudinal ridges known as the pyramids. They comprise large motor tracts descending from the brain to the spinal cord and play a critical role in controlling voluntary movements. The medulla is a hub for several nuclei that manage both motor and sensory information. It includes the nuclei for the hypoglossal nerve, which controls tongue movements, and vagus nerves, which provide parasympathetic control. The inferior olivary nuclei are involved in motor learning and aid in refining motor skills and coordination. The vestibular nuclei are essential for maintaining balance and gaze stability by processing signals from the inner ear, while the cochlear nuclei process auditory information, enabling hearing and interpretation of sounds from the environment.
