Final answer:
Neurotransmitters can have either excitatory or inhibitory effects on smooth muscle cells, depending on the type of neurotransmitter and the receptors it activates.
Step-by-step explanation:
The statement is true: Neurotransmitters may excite or inhibit smooth muscle cells. Neurotransmitters like acetylcholine or epinephrine can have excitatory effects on muscles, leading to contraction, while others like serotonin or GABA might have inhibitory effects, resulting in relaxation of smooth muscle cells. In the autonomic nervous system (ANS), axons have varicosities that release neurotransmitters into the synaptic cleft with smooth muscle, which can either excite or inhibit the muscle cells.