The midbrain, also called the mesencephalon, is a small region of the brain that serves as a relay center for visual, auditory, and motor system information. As the rostral or front part of the brainstem, the midbrain is directly involved with motor and sensory functions, and any disruptions can cause irreversible damage. Disorders most commonly associated with the midbrain are stroke and Parkinson's disease.

The tectum is considered the dorsal, or roof, part of the midbrain and controls visual and auditory reflexes. The tectum is divided into what is called the corpora quadrigemina, which consists of the two superior and two inferior colliculi. The colliculi are essentially the individual control centers for visual and auditory reflexes respectively. The superior colliculi are located below the thalamus and receive visual sensory input. The inferior colliculi are located below the superior colliculi and are involved with processing auditory stimuli.

Forming the floor of the midbrain is the tegmentum, which regulates autonomic functions and awareness and controls motor skills. As the ventral part of the midbrain, the tegmentum extends from the substantia to the cerebral aqueduct. The red nucleus is a prominent part of the tegmentum that is involved in motor coordination.

The substantia nigra is part of the basal ganglia and consists of the pars compacta and pars reticulate, which work together as a kind of circuit. The pars compacta is the input portion of the circuit and supplies dopamine to the major input system of the basal ganglia. The pars reticulate serves as the output portion of the circuit that transmits signals from the basal ganglia to the rest of the brain.

The mesencephalon can be affected by disorders such as Parkinson's disease and stroke. Both disorders not only affect an individual's motor system, but can severely impact mental functioning. Symptoms of either condition are indicative of midbrain disruption and their consequences are usually permanent.

Parkinson's disease is a progressive condition that results from the impairment, or death, of dopamine producing nerve cells in the substantia nigra. These nerve cells serve as messengers that relay signals to the corpus striatum that allow for movement coordination. When these cells become unbalanced, other movement control centers are affected and the resulting disruption manifests as the onset of symptoms including tremors and instability. Symptoms associated with Parkinson's may be treated with drug therapies and deep brain stimulation. In some cases, surgery is employed when symptoms prove to be resistant to traditional treatment.

A midbrain stroke, referred to as a posterior cerebral artery (PCA) stroke, is less common than strokes that affect the anterior or middle cerebral arteries. When a midbrain stroke does occur, however, it can affect an individual's motor and sensory functions including speech, vision, and body movement and sensation. Midbrain strokes are generally the result of a blood vessel obstruction within or around the heart muscle, known as a cardioembolism. Damage caused by stroke is irreversible and, as a result, focus is centered on rehabilitation and prevention of another stroke.