End plate potentials (
EPPs) are the depolarizations of
skeletal muscle fibers caused by
neurotransmitters binding to the postsynaptic membrane in the
neuromuscular junction. They are called "end plates" because the postsynaptic terminals of muscle fibers have a large, saucer-like appearance. When an
action potential reaches the
axon terminal of a
motor neuron, vesicles carrying neurotransmitters (mostly
acetylcholine) are
exocytosed and the contents are released into the neuromuscular junction. These neurotransmitters bind to receptors on the postsynaptic membrane and lead to its depolarization. In the absence of an action potential, acetylcholine vesicles spontaneously leak into the neuromuscular junction and cause very small depolarizations in the postsynaptic membrane. This small response (~0.5mV) is called a
miniature end plate potential (
MEPP) and is generated by one acetylcholine-containing vesicle. It represents the smallest possible depolarization which can be induced in a muscle.