Chemical messengers known as neurotransmitters are housed in small sac-like structures known as prior to being released into the synapse.

The answer is vesicles.

Vesicles locates at the end of a nerve cell and they contain neurotransmitters.

Vesicles would start releasing the neurotransmitters into the synapse once the electric Impulses arrives the end of the neuron so that vesicles fuses with the cell membrane.

In addition, neurotransmitters would diffuse through the synapse and bind into receptors, located at the end of the synapse, which is the start of the next neuron. Note that neurotransmitters are complementary to the receptors, and electrical impulse would be transmitted in the next neuron once neurotransmitters binds into receptors.