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. It is front part of the brainstem, and any disruption to this area can cause irreversible damage and impairment. Illnesses most commonly associated with this region of the brain are stroke, schizophrenia, and Parkinson's disease.
Structures that Relay Visual and Auditory Input
The tectum is the dorsal, or roof, part of the midbrain and controls visual and auditory reflexes. It's divided into the corpora quadrigemina, which consists of two superior and two inferior colliculi. In anatomy, the terms superior and inferior refer to the positions of the structures, with superior structures being located above inferior ones.
The colliculi are control centers for visual and auditory reflexes. The superior ones are located below the thalamus and receive visual sensory input from the oculomotor nucleus, a bundle of nerves that connect to the eyes. The inferior colliculi are involved with processing auditory stimuli that come from the ears through the trochlear nerve bundle.
These control centers relay the information they receive to the thalamus, which in turn sends it to the cerebral cortex. There, the neural connections are made that enable the brain to decide how to act in response to the sensory information it receives. The tectum and the four colliculi within it are, therefore, the first step of the neural pathway that determines how people react to what they see and hear.
Structures that Control Movement
Located beneath the colliculi, the tegmentum is the lowest region of the midbrain. It regulates autonomic functions, those that the body carries out without conscious thought, such as digestion, heart rate, and breathing rate. In addition, motor skills and basic awareness are dependent on this part of the brain.
Within this region is the red nucleus, which is involved in motor coordination, and the substantia nigra, the brain’s largest dopamine-producing center. Dopamine is a neurotransmitter, a type of chemical that is essential for the movement of electrical signals between brain cells. This chemical has many roles in the brain, and can affect behavior, sleep, mood, and memory.
The substantia nigra plays an important role in movement, learning, and addiction. This area of the brain consists of the pars compacta and the pars reticulate. These two regions work together as a kind of circuit. The pars compacta is the input portion of the circuit and supplies the dopamine to the basal ganglia, the area that controls movement. The pars reticulate serves as the output portion of the circuit, and transmits signals from the basal ganglia to the rest of the brain.
Diseases of the Midbrain
Parkinson’s disease is one of the most common disorders of the midbrain. This progressive illness develops when dopamine-producing nerve cells in the pars compacta die off in large numbers. These nerve cells are important in regulating motor function and emotion, and cell death leads to symptoms such as tremors, physical instability, and emotional changes. This disease is typically treated with medications that provide the brain with additional dopamine, but the drugs involved have side effects and are not always effective. In some cases, a device can be placed in the brain to stimulate the areas related to movement and help control symptoms, but this is usually only recommended for patients who don't respond to medications.
A stroke in the midbrain, also called a posterior cerebral artery stroke, is less common than those which affect the anterior or middle cerebral arteries. Midbrain strokes typically affect an individual's motor and sensory functions including speech, vision, body movement, and sensation. They are usually the result of a cardioembolism, a blood vessel obstruction within or around the heart muscle. The damage is irreversible, and treatment is centered on rehabilitation and preventing another stroke.
The midbrain may also be linked to some forms of mental illness. The dopamine hypothesis of psychosis developed as researchers noted that dopamine production is often abnormally high in people with certain mental illnesses, like schizophrenia. There are several pieces of evidence to support this hypothesis, including the fact that some the most effective medications for the treatment of psychosis are those that reduce dopamine activity. Another strong piece of evidence is that the substantia nigra, where most dopamine is produced, has been seen to undergo structural and cellular changes in a person with schizophrenia.