Synaptic boutons:

Synaptic boutons indicated by arrows.

aka axon terminals, synaptic boutons are small swellings that are found at the terminal ends of axons. Synaptic boutons are typically the sites where synapses with other neurons are found, and neurotransmitters are stored here to communicate with other neurons via these synapses.

Watch this 2-Minute Neuroscience video to learn more about the structures of a neuron.