gandalf
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11.21.05 (3 years ago)
#3
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I guess chand22sg is right as usual....
GANONG:
Sequence of Events During Transmission
The events occurring during transmission of impulses from the motor nerve to the muscle (see Table 3-2) are somewhat similar to those occurring at other synapses. The impulse arriving in the end of the motor neuron increases the permeability of its endings to Ca2+. Ca2+ enters the endings and triggers a marked increase in exocytosis of the acetylcholine-containing vesicles. The acetylcholine diffuses to the muscle type nicotinic acetylcholine receptors (Figure 4-18), which are concentrated at the tops of the junctional folds of the membrane of the motor endplate. Binding of acetylcholine to these receptors increases the Na+ and K+ conductance of the membrane, and the resultant influx of Na+ produces a depolarizing potential, the endplate potential. The current sink created by this local potential depolarizes the adjacent muscle membrane to its firing level. Acetylcholine is then removed from the synaptic cleft by acetylcholinesterase, which is present in high concentration at the neuromuscular junction. Action potentials are generated on either side of the endplate and are conducted away from the endplate in both directions along the muscle fiber. The muscle action potential, in turn, initiates muscle contraction, as described in Chapter 3.
Endplate Potential
An average human endplate contains about 15-40 million acetylcholine receptors. Each nerve impulse releases about 60 acetylcholine vesicles, and each vesicle contains about 10,000 molecules of the neurotransmitter. This amount is enough to activate about 10 times the number of acetylcholine receptors needed to produce a full endplate potential. Therefore, a propagated response in the muscle is regularly produced, and this large response obscures the endplate potential. However, the endplate potential can be seen if the 10-fold safety factor is overcome and the potential is reduced to a size that is insufficient to fire the adjacent muscle membrane. This can be accomplished by administration of small doses of curare, a drug that competes with acetylcholine for binding to muscle type nicotinic acetylcholine receptors. The response is then recorded only at the endplate region and decreases exponentially away from it. Under these conditions, endplate potentials can be shown to undergo temporal summation.
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