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  3. When the heart breaks down, it beats irregularly or not at all. A bone can chip or snap. But when the complex network of neurons

When the heart breaks down, it beats irregularly or not at all. A bone can chip or snap. But when the complex network of neurons

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问题     When the heart breaks down, it beats irregularly or not at all. A bone can chip or snap. But when the complex network of neurons in our brain fails to work, the result can be a near-endless variety and combinations of mental illnesses.
    It is normal to sometimes be sad, happy, anxious, confused, forgetful or fearful, but when a person’s emotions, thoughts or behavior frequently trouble them, or disrupt their lives, they may be subject to mental illness. According to the World Health Organization(WHO), about 450 million people worldwide are affected by mental, neurological or behavioral problems at any time.
However, determining that someone has a mental illness, and which one, is one of the challenges psychiatrists face.
    Historically, some symptoms of mental illness, such as eccentric behavior and hearing voices, have been taken as evidence of heavenly communication or demonic(恶魔的)possession.
    More recently, brain scans have directly linked these conditions with changes in levels of neurotransmitters — chemicals that convey messages across neurons — or alterations in the number or structure of neurons in different brain areas. For instance, people suffering from depression often display lowered levels of the neurotransmitter serotonin(血清素).
    In a few cases, the immediate cause of the malfunction has been identified. Alzheimer’s disease, a major source of dementia and memory loss in the elderly, is caused by the accumulation of protein plaques which choke neurons in the brain.
    Some infectious diseases can also develop into a mental illness. Untreated HIV infection can cause dementia, as can the uncontrolled replication of the germ that causes syphilis(梅毒).
    In many cases the precise cause is unclear and experts suspect that many different factors are involved. The chance that identical twins both develop schizophrenia(精神分裂症)is much higher than that for fraternal twins or siblings, proving the strong role of inherited genes. But scientists are accumulating a growing list of other risk factors that predispose people to this condition, including prenatal exposure to famine conditions, certain infections or exposure to lead. The season of their birth also seems important — birth in winter or early spring increases the risk, as does an older father and, controversially, child abuse.
    Genes are also thought to influence many other mental health problems, including: anorexia, autism, Alzheimer’s disease and bipolar disorder.
    Some researchers believe that smoking cigarettes and taking recreational drugs may elevate a user’s risk of mental illnesses.
What are neurotransmitters responsible for according to the passage?
选项
答案Conveying messages across neurons．
解析 由题干中的neurotransmitters将本题出处定位到第五段首句。该句提到…neurotransmitters—chemicals thatconvey messages across neurons。破折号起解释说明的作用，其后的chemicals指的就是前面的neurotransmitters，that引导的定语从句修饰chemicals并说明它的作用。由此可知，that后的内容为本题答案出处。由于问题是整句，所以需要把动宾结构的convey messages across neurons转化为名词短语，并大写首字母，故答案为Conveying messages across neurons．。
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