The diencephalon (di"en-sef'ah-lon) develops from the posterior forebrain and is located between the cerebral hemispheres and above the brain stem (see figs. 11.15 and 11.20). It surrounds the third ventricle and is largely composed of gray matter. Within the diencephalon, a dense mass, called the thalamus (thal'ah-mus), bulges into the third ventricle from each side. Another region of the diencephalon that includes many nuclei is the hypothalamus (hi"po-thal'ah-mus). It lies below the thalamic nuclei and forms the lower walls and floor of the third ventricle (see reference plates 49 and 53).
The specific symptoms associated with a cerebral injury or abnormality depend upon the areas and extent of damage. A person with damage to the association areas of the frontal lobes may have difficulty concentrating on complex mental tasks, appearing disorganized and easily distracted.
If the general interpretative area of the dominant hemisphere is injured, the person may be unable to interpret sounds as words or to understand written ideas. However, the dominance of one hemisphere usually does not become established until after five or six years of age. Consequently, if the general interpretative area is destroyed in a child, the corresponding region of the other side of the brain may be able to take over the functions, and the child's language abilities may develop normally. If such an injury occurs in an adult, the nondominant hemisphere may develop only limited interpretative functions, producing a severe intellectual disability. Following are three common cerebral abnormalities.
• In a concussion, the brain is jarred against the cranium, usually as a result of a blow to the head, causing loss of consciousness. Short-term memory loss, mental cloudiness, difficulty concentrating and remembering, and a fierce headache may occur in the days after a concussion, but recovery is usually complete. Cerebral palsy (CP) is motor impairment at birth, often stemming from a brain anomaly occurring during prenatal development. Until recently, most cases of CP were blamed on "birth trauma," but recently researchers determined that the most common cause is a blocked cerebral blood vessel, which leads to atrophy of the brain region deprived of its blood supply. Birth trauma and brain infection cause some cases.
CP affects about 1 in every 1,000 births and is especially prevalent among premature babies. One-half to two-thirds of affected babies improve and can even outgrow the condition by age seven. Sometimes seizures or learning disabilities are present. Clinicians classify CP by the number of limbs and the types of neurons affected. In a "stroke," or cerebrovascular accident (CVA), a sudden interruption in blood flow in a vessel supplying brain tissues damages the cerebrum. The affected blood vessel may rupture, bleeding into the brain, or be blocked by a clot. In either case, brain tissues downstream from the vascular accident die or permanently lose function. Temporary interruption in cerebral blood flow, perhaps by a clot that quickly breaks apart, produces a much less serious transient ischemic attack (TIA). ■
Other parts of the diencephalon include (1) the optic tracts and the optic chiasma that is formed by the optic nerve fibers crossing over; (2) the infundibulum, a conical process behind the optic chiasma to which the pituitary gland is attached; (3) the posterior pituitary gland, which hangs from the floor of the hypothalamus; (4) the mam-millary (mam'i-ler"e) bodies, which are two rounded structures behind the infundibulum; and (5) the pineal gland, which forms as a cone-shaped evagination from the roof of the diencephalon (see chapter 13, p. 533).
The thalamus is a selective gateway for sensory impulses ascending from other parts of the nervous system to the cerebral cortex. It receives all sensory impulses (except those associated with the sense of smell) and channels them to appropriate regions of the cortex for interpretation. In addition, all regions of the cerebral cortex can communicate with the thalamus by means of descending fibers.
The thalamus seems to transmit sensory information by synchronizing action potentials. Consider vision. An image on the retina stimulates the lateral geniculate nucleus (LGN) region of the thalamus, which then sends action potentials to a part of the visual cortex. Researchers have observed that those action potentials are synchro-nized—that is, fired simultaneously—by the LGN's neurons only if the stimuli come from a single object, such as a bar. If the stimulus is two black dots, the resulting thala-mic action potentials are not synchronized. The
Parkinson disease affects millions of people worldwide. In this disorder, neurons in the basal nuclei that synthesize the neurotransmitter dopamine degenerate. The resulting decline in dopamine levels in the striatum causes slowed movements, difficulty in initiating voluntary muscular actions, rigidity, loss of balance, and tremors.
The standard treatment for many years has been to give levo-dopa, which is a precursor to dopamine that can cross the blood-brain barrier. Once in the brain, levo-dopa is converted to dopamine, which cannot cross the barrier. However, with prolonged use, the brain becomes dependent on the external supply and decreases its own production of dopamine even further. Eventually levo-dopa only alleviates symptoms intermittently. A new drug, pramipexole, seems to even out the supply of dopamine by mimicking the action of dopamine. If given with levo-dopa, the new drug improves symptoms and even helps lift depression associated with advanced Parkinson disease. Prolonged use of levo-dopa can also cause tardive dyskinesia, a condition that produces uncontrollable facial tics, perhaps by abnormally increasing the dopamine level in brain areas not implicated in Parkinson disease.
A controversial and experimental treatment for Parkinson disease is to transplant fetal brain tissue into patients' brains, where the tissue produces dopamine. These procedures have been done experimentally since the 1980s. At first, results were mixed, because often the patient's immune system destroyed the foreign tissue. By 1995, giving immune-suppressant drugs to transplant recipients greatly improved the success of the procedure. One patient, treated in 1995, experienced a return of function. He could once again feed and dress himself and could lower his daily dosage of levo-dopa. After his death from other causes eighteen months after the transplant, his brain was found to contain dense groupings of neurons in the striatum that had actively secreted dopamine—proof that the transplant had at least partially corrected the disease.
The cause of Parkinson disease isn't known. It has been attributed to exposure to certain chemicals in pesticides and designer drugs and to severe and frequent injury such as occurred in
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