Final answer:
In the NMJ region, swellings of axon terminals are known as synaptic end bulbs, which release neurotransmitters such as ACh to trigger muscle contractions. In the ANS, these swellings are called varicosities, which release neurotransmitters into the synaptic cleft as they pass through smooth muscle.
Step-by-step explanation:
In the neuromuscular junction (NMJ) region, the swellings of axon terminals are referred to as synaptic end bulbs. These structures are at the end of an axon and are responsible for the release of neurotransmitter molecules across the synapse onto a target cell, which is typically a muscle fiber. At the NMJ, the specific neurotransmitter involved is acetylcholine (ACh), which diffuses across the synaptic cleft and binds to ACh receptors on the muscle fiber's sarcolemma, triggering a muscle contraction. In the autonomic nervous system (ANS), similar swellings are known as varicosities or "boutons," and they release neurotransmitters along the length of a postganglionic fiber as it courses through smooth muscle, forming motor units in a less organized manner compared to NMJs between motor neurons and skeletal muscle fibers.
Varicosities are neurotransmitter-filled bulges that release neurotransmitters into the synaptic cleft, whereas synaptic end bulbs are the highly organized structures found at the NMJs with skeletal muscle fibers.