not occur. During experimentation with MEPPs, it was noticed that often spontaneous action potentials would occur, called end plate spikes in normal striated muscle without any stimulus. It was believed that these end plate spikes occurred as a result of injury or irritation of the muscles fibers due to the electrodes. Recent experiments have shown that these end plate spikes are actually caused by muscle spindles and have two distinct patterns: small and large. Small end plate spikes have a negative onset without signal propagation and large end plate spikes resemble motor unit potentials (MUPs). Muscle spindles are sensory receptors that measure muscle elongation or stretch and relay the information to the spinal cord or brain for the appropriate response. Threshold potential ("All or None") When an action potential causes the release of many acetylcholine vesicles, acetylcholine diffuses across the neuromuscular junction and binds to ligand-gated nicotinic receptors (non-selective cation channels) on the muscle fiber. This allows for increased flow of sodium and potassium ions, causing depolarization of the sarcolemma (muscle cell membrane). The small depolarization associated with the release of acetylcholine from an individual synaptic vesicle is called a miniature end-plate potential (MEPP), and has a magnitude of about +0.4mV. MEPPs are additive, eventually increasing the end-plate potential (EPPs) from about -100mV up to the threshold potential of -60mV, at which level the voltage-gated ion channels in the postsynaptic membrane open, allowing a sudden flow of sodium ions from the synapse and a sharp spike in depolarization. This depolarization voltage spike triggers an action potential which propagates down the postsynaptic membrane leading to muscle contraction. It is important to note that EPPs are not action potentials, but that they trigger action potentials. In a normal muscular contraction, approximately 100-200 acetylcholine vesicles are released causing a depolarization that is 100 times greater in magnitude than a MEPP. This causes the membrane potential to depolarize +40mV (100 x 0.4mV = 40mV) from -100mV to -60mV where it reaches threshold. Action potential phases Once the membrane potential reaches threshold, an action potential occurs and causes a sharp spike in membrane polarity. There are five phases of an action potential: threshold, depolarization, peak, repolarization, and hyperpolarization. Threshold is when the summation of MEPPs reaches a certain potential and induces the opening of the voltage-gated ion channels. The rapid influx of sodium ions causes the membrane potential to reach a positive charge. The potassium