Brain's Acetylcholine, Dopamine, Serotonin, Histamine and GABA (GHB) systems are considered to be essential for one's physical and psychological health.
Acetylcholine powers the parasympathetic, sympathetic and sensory nervous functions, and assists the dopamine function. The erectile mechanism is mainly driven by the Acetylcholine-Parasympathetic action, which produces Nitric Oxide (NO) (erectile neurotransmitter) and cGMP (erectile dilator). On the other hand, the dopamine-norepinephrine axis boosts libido and promotes the burning of testosterone, which expands the penile tissues to an extreme state. Then, a large amount of norepinephrine becomes epinephrine (adrenaline) by conversion, this triggers the sympathetic "Fight or Flight" response, where used in this particular context, "Fight" means ejaculation and orgasm (if acetylcholine action is sufficient) and "Flight" the erection or engorgement withdrawal (if the acetylcholine action is insufficient).
Serotonin can block or reduce the norepinephrine-epinephrine conversion. During foreplay and numerous erectile states, excessive epinephrine can block the incoming parasympathetic motoring communication from the brain to the targeted organ and is considered one of the main enemies of erection and/or engorgement.