the presynaptic terminals. Finalized neurotransmitter vesicles are bound to the presynaptic membrane. When an action potential propagates down the motor neuron axon and arrives at the axon terminal, it causes a depolarization of the axon terminal and opens calcium channels. This causes the release of the neurotransmitters via vesicle exocytosis. After exocytosis, vesicles are recycled during a process known as the synaptic vesicle cycle. The retrieved vesicular membranes are passed through several intracellular compartments where they are modified to make new synaptic vesicles. They are then stored in a reserve pool until they are needed again for transport and release of neurotransmitters. Unlike the reserve pool, the readily releasable pool of synaptic vesicles is ready to be activated. Vesicle depletion from the readily releasable pool occurs during high frequency stimulation of long duration and the size of the evoked EPP reduces. This neuromuscular depression is due to less neurotransmitter release during stimulation. In order for depletion not to occur, there must be a balance between repletion and depletion which can happen at low stimulation frequencies of less than 30 Hz. When a vesicle releases its neurotransmitters via exocytosis, it empties its entire contents into the synaptic cleft. Neurotransmitter release from vesicles is therefore stated to be quantal because only whole numbers of vesicles can be released. In 1970, Bernard Katz from the University of London won the Nobel Prize for Physiology or Medicine for statistically determining the quantal size of acetylcholine vesicles based on noise analysis in the neuromuscular junction. Using a book on mechanical statistics, he was able to infer the size of individual events going on at the same time. The synaptic vesicles of acetylcholine are clear core synaptic vesicles with a diameter of 30 nm. Each acetylcholine vesicle contains approximately 5000 acetylcholine molecules. The vesicles release their entire quantity of acetylcholine and this causes miniature end plate potentials (MEPPs) to occur which are less than 1mV in amplitude and not enough to reach threshold. Miniature end plate potentials (MEPPs) Miniature end plate potentials are the small (~0.4mV) depolarizations of the postsynaptic terminal caused by the release of a single vesicle into the synaptic cleft. Neurotransmitter vesicles containing acetylcholine collide spontaneously with the nerve terminal and release acetylcholine into the neuromuscular junction even without a signal from the axon. These small depolarizations are not enough to reach threshold and so an action potential in the postsynaptic membrane does