Figure 47.8 How do Autonomic Nervous System Neuro-transmitters Control the Firing Rate of Smooth Muscle Cells? (A) When a strip of intestinal smooth muscle is mounted in an incubation bath so that its generation of action potentials and its force of contraction can be recorded, it is possible to test the effects of sympathetic and parasympathetic neurotransmitters on that tissue. (B) Acetylcholine depolarizes (makes less negative) the membranes and increases action potentials; norepinephrine hyperpolarizes (makes more negative) the membranes and decreases action potentials.