which is why it earned the nickname "reptilian brain." Evolutionary studies show that in complex animals, the brain stem was one of the first parts of the brain to develop.
From the spinal cord upward, the brain stem includes the medulla, the pons, and the midbrain. The medulla looks a bit like a thicker continuation of the spinal cord and contains the nuclei of the ninth, 10th, 11th, and 12 th cranial nerves. It is responsible for relaying taste sensations from the tongue and signals to muscles controlling speech, tongue, and neck movements. The medulla also houses groups of nerve cells that regulate heartbeat, breathing, blood pressure, and digestion and relaying information about these activities via the
VAGUS (TENTH CRANIAL) NERVE.
The pons is much wider than the medulla and contains bundles of nerves connecting with the cerebellum lying behind the brain stem. The pons receives information from ear, face, and teeth via its nuclei for the fifth through eighth cranial nerves and also receives signals controlling the jaw, facial expression, and eye movement. The pons is also important for breathing regulation.
The midbrain is the smallest part of the brain stem, located above the pons and containing the nuclei of the third and fourth cranial nerves, which control eye movement and pupil size.
Scattered throughout the brain stem are groups of nerve cells known collectively as the reticular formation, which is believed to control incoming sensory information and governs such basic activities as awareness, attention, sleep, and waking, breathing, and heart rate.
The cerebellum is a separate brain organ that is attached directly to the back of the brain stem and is concerned mostly with balance and coordinated movement.
Running through the middle of the brain stem is a canal that widens into the fourth ventricle of the brain, home of the circulating cerebrospinal fluid.
The brain stem is susceptible to the same type of problems that can beset the rest of the central nervous system; damage to different parts of the brain stem will result in different problems. Any sort of damage to the vitally important centers in the medulla can be quickly fatal; damage to the reticular formation can result in coma. Likewise, damage to a specific cranial nerve can have particular effects; for example, damage to the seventh (facial) cranial nerve will result in facial palsy. Degeneration of the substantia nigra in the midbrain is linked to the development of Parkinson's disease.
brain syndrome, organic See organic brain syndrome.
brain tissue transplants Brain damage can be repaired by implanting brain tissue from aborted fetuses into a patient's impaired brain. Scientists agree that one of the greatest problems in treating brain damage and a range of degenerative brain diseases is that up to now, brain tissue has not been able to regenerate itself. When the brain is damaged, other regions do not always take over.
Rat studies have shown that grafted brain tissue can become effective parts of the animal's brain and can even improve age-related learning impairments. Scientists have also shown improvement in some Parkinson's disease patients after transplanting fetal brain cells to produce dopamine, a key brain chemical lacking in the disease.
In 1992, a Swedish team from the University of Lund performed a fetal tissue transplant on two Americans who had destroyed their substantia nigra after injecting themselves with tainted synthetic heroin. For seven years the two had not been able to use their voluntary muscles; after receiving brain tissue from more than one fetus, both recovered enough to be able to live independently.
In the United States, scientists have implanted fetal tissue into the brains of patients with Parkinson's disease and reported that patients showed improvement in movement examinations. Scientists hope that someday they will be able to treat a range of other brain diseases with this technique.
However, the moral and philosophical issues surrounding the use of tissue implants presents problems; the fact that successful transplants require the use of fetal tissue adds to the ethical and legal dilemma.
Both the Ronald Reagan and George H. W. Bush administrations banned the use of federal funds for
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