Final answer:
The nerve fibers of the autonomic nervous system end in numerous bulbous swellings on smooth muscle cells known as varicosities or 'boutons' which release neurotransmitters that cause the muscle to contract.
Step-by-step explanation:
The nerve fibers of the autonomic nervous system innervate smooth muscle cells and end in numerous bulbous swellings called varicosities or 'boutons'.
These structures form motor units through the smooth muscle and are responsible for releasing neurotransmitters into the synaptic cleft, which enables smooth muscle contraction.
Smooth muscle tissue, found in the walls of hollow organs like the stomach, intestines, and urinary bladder, is characterized by being involuntary and not under conscious control, has a single nucleus, and lacks striations.