Brainstem

The brainstem comprises three regions: the midbrain, pons, and medulla oblongata. The midbrain sits below the diencephalon and connects the cerebrum to lower brain structures. The pons lies inferior to the midbrain and connects to the cerebellum. The medulla oblongata forms the lowest portion and connects directly to the spinal cord, serving as a bridge between the brain and spinal cord.

The midbrain contains the cerebral peduncles with crus cerebri, which house the corticospinal tract carrying voluntary movement signals to the spinal cord. The corpora quadrigemina consist of superior colliculi for visual processing and inferior colliculi for auditory processing. The substantia nigra contains dopamine-releasing neurons crucial for movement control, and the red nuclei coordinate fine motor skills of the limbs.

Damage to the crus cerebri, which contains the corticospinal tract, results in motor deficits and muscle weakness on the opposite side of the body. This occurs because corticospinal tract fibers cross over in the brainstem before descending to the spinal cord. The severity of weakness depends on the extent and location of the damage within these critical motor pathways.

The pons serves as a major communication center between the cerebrum and cerebellum through the middle cerebellar peduncles. Pontine nuclei transmit signals from the motor cortex to the cerebellum for motor control and coordination. The pons also regulates breathing, contributes to the sleep-wake cycle, coordinates swallowing reflexes, and contains cranial nerve nuclei for facial movement and hearing.

The pyramids are two longitudinal ridges in the medulla oblongata composed of large motor tracts descending from the brain to the spinal cord. These tracts play a critical role in controlling voluntary movements. The pyramids represent the major pathway through which motor commands from the cerebral cortex reach the spinal cord to execute purposeful actions.

The medulla contains several nuclei managing both motor and sensory functions. The hypoglossal nerve nuclei control tongue movements, while vagus nerve nuclei provide parasympathetic control. The inferior olivary nuclei aid motor learning and coordination refinement. Vestibular nuclei maintain balance and gaze stability, and cochlear nuclei process auditory information for hearing and sound interpretation.

Degeneration of substantia nigra dopamine-releasing neurons, as occurs in Parkinson's disease, results in motor symptoms including tremors, rigidity, bradykinesia (slow movement), and postural instability. Since the substantia nigra is crucial for movement control, loss of dopamine-producing neurons disrupts the brain's ability to regulate smooth, coordinated motor function and voluntary movement initiation.