It apparently does this by inhibiting the release of neurotransmitters, chemicals that carry nerve impulses from one neuron to the next. (10) Like many other agents that affect neuron firing, adenosine must first bind to specific receptors on neuronal membranes. | GMAT READING COMPREHENSION SAMPLE QUESTIONS Instruction This file contains 285 sample questions on GMAT Reading Comprehension and explanations for 24 of them. For answers with complete explanations to other 261 questions please order the Complete GMAT Prep Course which includes Verbal Study Guide Essay Prep Course and Math Review. READING COMPREHENSION Passage 1 Caffeine the stimulant in coffee has been called the most widely used psychoactive substance on Earth . Synder Daly and Bruns have recently proposed that caffeine affects behavior by countering the activity in 5 the human brain of a naturally occurring chemical called adenosine. Adenosine normally depresses neuron firing in many areas of the brain. It apparently does this by inhibiting the release of neurotransmitters chemicals that carry nerve impulses from one neuron to the next. 10 Like many other agents that affect neuron firing adenosine must first bind to specific receptors on neuronal membranes. There are at least two classes of these receptors which have been designated A1 and A2. Snyder et al propose that caffeine which is struc- 15 turally similar to adenosine is able to bind to both types of receptors which prevents adenosine from attaching there and allows the neurons to fire more readily than they otherwise would. For many years caffeine s effects have been attri- 20 buted to its inhibition of the production of phosphodiesterase an enzyme that breaks down the chemical called cyclic number of neurotransmitters exert their effects by first increasing cyclic AMP concentrations in target neurons. Therefore prolonged periods at 25 the elevated concentrations as might be brought about by a phosphodiesterase inhibitor could lead to a greater amount of neuron firing and consequently to behavioral stimulation. But Snyder et al point out that the caffeine concentrations needed to inhibit the production 30 of phosphodiesterase in the brain are much higher than those that produce stimulation. .
