Re: What is the chemical composition of neurotransmitters?

In each case you mention (acetylcholine, serotonin, and norepinephrine), the chemical elements that make up these molecules are carbon, hydrogen, nitrogen, and oxygen. The chemical formulas are:

Acetylcholine : C₇H₁₆NO₂
Serotonin : C₁₀H₁₂N₂O
Norepinephrin : C₈H₁₁NO₃

Neurotransmitters are used to transmit a signal. Electrical signals cannot travel directly between two cells (such as neurons in the brain). The electrical impulse can transmit from one end of a cell to another, but cannot cross a gap between two cells. So chemical signals, called neurotransmitters are used. When an electrical signal arrives at the end of a cell, neurotransmitters are released into the gap. The second neuron has receptors (a certain type of protein) that recognize the presence of those neurotransmitters and then convert the signal back into an electrical impulse to continue through that cell. So there is a chemical interaction between the neurotransmitter and the receptor. Then when the neurotransmitter is no longer needed in the gap, an enzyme (such as acetylcholinesterase) catalyzes a reaction between the acetylcholine and water to produce choline and acetic acid. This destroys the chemical messenger, allowing the system to return to a state in which another signal can be transmitted.