Excitation is the process in which action potentials in the nerve fiber lead to action potentials in the muscle fiber. The steps in excitation are shown in figure 11.8.
- A nerve signal arrives at the synaptic knob and stimulates voltage-gated calcium channels to open. Calcium ions enter the synaptic knob.
- Calcium ions stimulate exocytosis of the synaptic vesicles, which release acetylcholine (ACh) into the synaptic cleft. One action potential causes exocytosis of about 60 synaptic vesicles, and each vesicle releases about 10,000 molecules of ACh.
- ACh diffuses across the synaptic cleft and binds to receptor proteins on the sarcolemma.
- These receptors are ligand-gated ion channels. When ACh (the ligand) binds to them, they change shape and open an ion channel through the middle of the receptor protein. Each channel allows Na+ to diffuse quickly into the cell and K+ to diffuse outward. As a result of these ion movements, the sarcolemma reverses polarity—its voltage quickly jumps from the RMP of —90 mV to a peak of +75 mV as Na+ enters, and then falls back to a level close to the RMP as K+ diffuses out. This rapid fluctuation in membrane voltage at the motor end plate is called the end-plate potential (EPP).
- Areas of sarcolemma next to the end plate have voltage-gated ion channels that open in response to the EPP. Some of the voltage-gated channels are specific for Na+ and admit it to the cell, while others are specific for K+ and allow it to leave. These ion movements create an action potential. The muscle fiber is now excited.
_Think About It_
An impulse begins at the middle of a 100-mm-long muscle fiber and travels 5 m/sec. How long would it take to reach the ends of the muscle fiber?
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