单项选择题

Some scientists have concluded that narcolepsy, a disease which disrupts sleep patterns, arises when unknown agents in the environment spur an autoimmune reaction winding up damaging neurons in the brain circuits that Line control arousal and muscle tone. Honda found that all members of a group of narcoleptics had one aspect of their tissue type in common, one of the human leukocyte antigens (HLAs). Whereas the HLA type was clearly not sufficient to induce narcolepsy, Honda’s finding indicated that HLA type greatly affected susceptibility to the disease. HLA molecules are pitchfork-shaped structures that cells of the body use to show pieces of the proteins they contain to the immune system, which ordinarily attacks foreign substances and cells infected by viruses. Evidence for an autoimmune cause of narcolepsy will most likely come from studies of the brains of people who had the disorder. Ever since Gineau correctly named narcolepsy as a distinct syndrome in 1880, researchers have examined the brains of patients in a futile search for neurological damage that could explain the symptoms of the disease. Mignot observed that the brains of narcoleptic dogs had more than the usual number of receptors for the neurotransmitters acetylcholine, dopamine and norepinephrine and higher levels of some of the neurotransmitters themselves, changes identical to those Aldrich later noted human narcoleptic brains.Given that narcolepsy is not progressive but chronic—i.e.after the establishment of symptoms, narcoleptics do not deteriorate or improve-pathologists may have been focusing their efforts on the wrong stage of the disease’s progression. This suggested that the damage might occur during a short time, roughly during the period in which a patient first develops signs of the illness. Debris left over from degenerative processes that occurred at the age of 20 would be removed by the brain’s support cells long before most patients died, and any remaining clues would be obscured by the normal degeneration of aging. As a loss of neurons would be undetectable unless the dead cells were concentrated in a particular area, as in Parkinson’s disease, or unless many neurons died, as in Alzheimer’s disease, I therefore examined the brains of narcoleptic dogs shortly after their symptoms had begun, and found clear evidence that neurons in certain areas of the dogs’ brains were degenerating between one and two months of age, just before and during the onset of symptoms. Evidence of the degeneration largely disappeared by the time the dogs were six months old. At this moment, all we can give patients are drugs to treat narcolepsy’s symptoms, even though this research promises the definite potential of a cure.
 

The passage implies that research into the cause of human narcolepsy has been complicated by what aspect of the human brain（）

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