Final answer:
The swellings of postganglionic autonomic axons which release neurotransmitters are called varicosities, representing a less organized form of synaptic transmission compared to typical neuromuscular junctions.
Step-by-step explanation:
The swelling of postganglionic autonomic axons from which neurotransmitters are released is called varicosities. These are a series of neurotransmitter-filled bulges along the axon as it courses through smooth muscle. Unlike the typical synaptic end bulb found at the neuromuscular junctions, varicosities are chains of swellings along the length of a postganglionic fiber that release neurotransmitters into the synaptic cleft and can influence multiple cells within a target area, which makes the neurotransmission in the autonomic nervous system somewhat diffuse.