Monoamine oxidase (MAO) is an enzyme that breaks down the neurotransmitters dopamine, serotonin, and norepinephrine. Individuals who are involved in extreme sports, such as rock climbing, generally have low levels of MAO and, therefore, higher-than-normal levels of these neurotransmitters.

Dopamine and serotonin are linked to pleasurable feelings. Norepinephrine is released in the fight-or-flight response. One hypothesis for why individuals participate in extreme sports is that in order for individuals with high resting levels of these neurotransmitters to achieve a pleasurable sensation, they require a greater surge of these chemicals than do other people.
The site in the neural pathway where MAO is active is the

A. axon
B. synaptic cleft
C. cell body
D. Schwann cell