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The synaptic sarcolemma (= end-plate) at rest is polarized, meaning the charge inside is relatively negative to the charge outside. This condition is called the resting membrane potential and in muscle fibers measures about -90 mV.
The process of depolarizing the sar-colemma begins when thousands of acetyl-choline (= ACh) are released by the motor neuron. The ACh molecules diffuse across the synaptic cleft and bind to thousands of receptors embedded in the sarcolemma. Chem-ical gates open in response to the ACh, and the receptors become water-filled channels through which Na+ and K+ ions can pass.
Both the diffusion (= chemical) and electrical gradients favor the movement of Na+ into the cell. Thus, more positive ions (Na+) enter the muscle fiber than leave (K+). A gain in positive ions inside the muscle fiber causes the sarcolemma to depolarize, moving from -90 mV toward 0 mV. Around -50 mv, a threshold point is reached and an action potential is produced in the muscle fiber, which causes it to contract. If enough ACh is released during neurotransmis-sion, the sarcolemma may actually reverse polarize (= +20 mV). These changes in sarcolemma polarity are often referred to as the end-plate potential or EPP.
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