Once an action potential reaches the ends of an axon and allows calcium to enter into the cell, synaptic vesicles release their contents into the synaptic cleft. It is these contents that actually act as the message between the two neurons. The contents of the synaptic vesicles are called neurotransmitters. Neurotransmitters can be of a variety of classes of molecules, but all have a specific receptor on the postsynaptic neuron, and all mediate a specific set of events to occur (they induce a change in the post synaptic membrane resistance). A such neurotransmitters are also known as chemical messengers. It easy to confuse neurotransmitters with another form of chemical messenger known as a hormone. Certain neurotransmitters released by neurons are also released by glands. If a chemical messenger is released by a gland it is called a hormone, even if that same chemical is released by a neuron, and is thus termed a neurotransmitter. There are differences between the two that one must note. Glands generally excrete much larger amounts of a hormone, than a nerve excretes of a neurotransmitter. Hormones are carried in the blood stream, and often times effect very distant tissue. Neurotransmitter on the other hand is released into the synaptic cleft, and it?s signaling distance is very short. Also a neurotransmitters target is generally a single neuron, where as a hormone may effect many different systems. It is important to be able to differentiate between the two especially since the endocrine system is under neurological control. Thus the two systems work side by side, both important communication systems within the body, but both separate.

During studies of the neuromuscular junction (the place where neurons stimulate muscle fibers), it was noted that even when there was no action potential, or true neural stimulation, the muscle fibers showed a slight depolarization event of about 1 mV that lasted for a few milliseconds. This depolarization was measured and showed to be a finite amount that was conserved throughout the muscle. Further research showed that the cause of this was the release of the contents of a single synaptic vesicle from the presynaptic neuron at regular time intervals. This quanta of neurotransmitter release was found to be a common feature of a multitude of neuromuscular junctions. From this bit of information it was deduced that each synaptic vesicle contained approximately the same number of neurotransmitter molecules. It was also concluded that any stimulation of the muscle fiber would be by an integral number of this quanta (since one either releases a whole synaptic vesicle or none, a nerve can not release part of a vesicle, it is like the action potential, an all or none phenomenon). Through this relationship one can now deduce either the magnitude of the muscle stimulation (if the number of vesicles released was known) or determine the number of vesicles released (if the magnitude of the muscle fiber stimulation was known).