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SAT阅读模拟题之brain下面为大家整理的是一篇关于brain SAT阅读模拟题，后面附有相关题目和正确答案。SAT阅读考试中的文章阅读在题目的设置上非常有特点，大家一起来看看详细内容吧，看看有哪些题目是特别的。
It was once believed that the brain was independent
of metabolic processes occurring elsewhere in the body.
In recent studies, however, we have discovered that the
production and release in brain neurons of the neuro-
(5) transmitter serotonin (neurotransmitters are compounds
that neurons use to transmit signals to other cells)
depend directly on the food that the body processes.
Our first studies sought to determine whether the
increase in serotonin observed in rats given a large injec-
(10)tion of the amino acid tryptophan might also occur after
rats ate meals that change tryptophan levels in the
blood. We found that, immediately after the rats began
to eat, parallel elevations occurred in blood tryptophan,
brain tryptophan, and brain serotonin levels. These find-
(15)ings suggested that the production and release of sero-
tonin in brain neurons were normally coupled with
blood-tryptophan increases. In later studies we found
that injecting insulin into a rat’s bloodstream also caused
parallel elevations in blood and brain tryptophan levels
(20) and in serotonin levels. We then decided to see whether
the secretion of the animal’s own insulin similarly affected
serotonin production. We gave the rats a carbohydrate-
containing meal that we knew would elicit insulin secre-
tion. As we had hypothesized, the blood tryptophan
(25) level and the concentrations of tryptophan
serotonin in the brain increased after the meal.
Surprisingly, however, when we added a large
amount of protein to the meal, brain tryptophan and
serotonin levels fell. Since protein contains tryptophan,
(30)why should it depress brain tryptophan levels? The
answer lies in the mechanism that provides blood tryp-
tophan to the brain cells. This same mechanism also
provides the brain cells with other amino acids found in
protein, such as tyrosine and leucine. The consumption
(35)of protein increases blood concentration of the other
amino acids much more, proportionately, than it does
that of tryptophan. The more protein in the meal, the
lower is the ratio of the resulting blood-tryptophan
concentration to the concentration of competing amino
(40) acids, and the more slowly is tryptophan provided to
the brain. Thus the more protein in a meal, the less
serotonin subsequently produced and released.
