The transmission of an impulse from neuron to neuron or neuron to muscle or gland was reviewed. The fact that neurotransmitters can cause stimulation of inhibition of a response was explained by the type of receptor which binds to it. Dilation or constriction of pupils, dilation or relaxation of bronchioles, increase or decrease in heart rate, dilation or constriction of blood vessels, and contraction or relaxation of muscles were all shown to be dependent on the receptor. That many drugs mimic the action of the endogenous neurotransmitter was defined as an example of agonist activity. We also discussed the botanical history of most of the original medications.