Final answer:
Swellings containing vesicles filled with neurotransmitters are known as synaptic end bulbs or varicosities, which are essential for neural signal transmission at synapses or neuroeffector junctions.
Step-by-step explanation:
Swellings that contain vesicles filled with neurotransmitter are known as synaptic end bulbs or varicosities. The synaptic end bulb is a swelling at the end of an axon where neurotransmitter molecules are released onto a target cell across a synapse. Additionally, there are a series of axon-like swellings called varicosities that occur in autonomic neurons. These structures form motor units as they course through smooth muscle, releasing neurotransmitters into the synaptic cleft without forming the highly organized neuromuscular junctions (NMJs) seen in skeletal muscles. Varicosities and synaptic end bulbs are crucial components for the transmission of neural signals across synapses or neuroeffector junctions.