What is the Midbrain?
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.
What sort of psychological damage could occur if the mid-brain was damaged? Suppose someone had a brick fall on the top of their head.
What does the midbrain consist of?
I have a tumor in my midbrain, found when I was 52, and it has grown a little. At first it was thought to be a hematoma, but since I had a lumbar punch and my protein was 124 so it was determined to be a tumor. The doctor says it's a pilocytic astrocytoma, which is considered a low grade pediatric tumor which is only dealt with in a child and as an adult. I am now 60, and they are not going to do anything.
I recently had to have a defibrillator implanted and now I cannot have MRIs. I hear ringing in my ears, and it is not in my ears, as my actual hearing is not affected, but the ringing is louder at times, and I feel pressure also. The tumor is next to the hydrocephalus drain. I am just so confused. No one entertains the idea that the ringing is due to this growth, and it started not long after the tumor. I also am out of work due to cognitive issues, and frankly, I have no idea how to find a really good brain surgeon or diagnostician who would look at tying all these things together. At one point I was told I had MS and I do not have it. I am told now that I have another autoimmune disease, more rare, called CIDP. Any comments?
The midbrain can also be affected by brain lesions which affect its function. Common results or symptoms of a midbrain lesion include facial palsy, weakening of nerve and motor functions and functioning of limbs, most particularly the arms.
Did you know that the midbrain plays a big part in addiction? It's the ventral midbrain which holds neurons releasing dopamine. The ventral midbrain neurons also hold nicotinic acetylcholine receptors (nAChR), a key player in neuronal communication. A significant aspect of this part of the midbrain is that it is associated with the feeling of pleasure. Addictive substances such as nicotine bind to and activate the dopamine-releasing neurons in the ventral midbrain, which is precisely what makes these substances addictive.
We're really lucky to be living in a time when there are so many treatments available for midbrain problems. For example, in the past dopamine was the only treatment for diseases such as Parkinson's disease and Dopa-Responsive Dystonia.
However, dopamine cannot cross the blood-brain barrier and affect the central nervous system. Instead, dopa, the synthetic precursor of dopamine can be given. When dopa crosses from the bloodstream into the brain, a carboxyl group is removed (decarboxylation) forming dopamine. Dopa was approved by the FDA in 1970 as a drug and it has revolutionized the treatment of Parkinson’s disease.
I know a lot about this because my grandfather was recently put on dopa -- and I have to say I'm just so glad that we have that option to help him.
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