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10 msec

10 msec

Amplitude of MEPP (mV)

FIGURE 16 The miniature end-plate potential (MEPP). (A) In the absence of nerve stimulation, small spontaneously occurring potentials are recorded. The successive traces are continuous recordings from a neuromuscular junction. The MEPPs are approximately 0.4 mV in amplitude and occur about once every 50 msec. (Modified from Liley AW. J Physiol 1956; 133:571-587.) (B) Histogram representing the number of times a MEPP of a given amplitude is recorded. There is a variation in amplitude, but MEPPs appear to be derived from a single population and have an average amplitude of 0.4 mV. (Modified from Boyd IA, Martin AR. J Physiol 1956; 132:74-91.)

Amplitude of MEPP (mV)

FIGURE 16 The miniature end-plate potential (MEPP). (A) In the absence of nerve stimulation, small spontaneously occurring potentials are recorded. The successive traces are continuous recordings from a neuromuscular junction. The MEPPs are approximately 0.4 mV in amplitude and occur about once every 50 msec. (Modified from Liley AW. J Physiol 1956; 133:571-587.) (B) Histogram representing the number of times a MEPP of a given amplitude is recorded. There is a variation in amplitude, but MEPPs appear to be derived from a single population and have an average amplitude of 0.4 mV. (Modified from Boyd IA, Martin AR. J Physiol 1956; 132:74-91.)

from left to right and down the illustration. (It is important to keep in mind that in this experiment the motor axon is not stimulated and that the recordings are made from the unstimulated muscle cell.) It is evident that without stimulation there is no absence of activity. Indeed, numerous small voltage deflections (on the average only about 0.5 mV in amplitude; Fig. 16B) are observed. They are small compared with the relatively large potential changes observed in response to a presynaptic action potential. The potentials occur in a random fashion at an average rate of about once every 50 msec.

The small potential changes have some interesting features. When the recording electrode is placed in regions distant from the motor end plate, the potentials disappear. For this reason, these potential changes are called miniature EPPs, or MEPPs, because they are EPPs that have occurred in the vicinity of the motor end plate and are much smaller than the EPP normally produced by stimulating the motor axon. If curare is added to the extracellular medium, the size of the MEPPs decreases. If neostigmine is added to the extracellular medium, the size of the MEPPs increases. Based on these considerations, it appears that MEPPs are due to the spontaneous and random release of ACh from the presynaptic terminal. Acetylcholine presumably is in relatively high concentration in the presynaptic terminal. So, by chance, some of that ACh might diffuse out of the presynaptic terminal, bind with postsynaptic receptors, and produce small permeability changes.

How much ACh is necessary to produce a MEPP? The simplest answer is that a MEPP is produced by one ACh molecule binding to a receptor and opening a single ACh-sensitive channel. When small amounts of ACh are ejected into the vicinity of the neuromuscular junction, however, potentials much smaller than MEPPs are produced. Indeed, the single-channel recording techniques (e.g., Fig. 11) indicate that the opening of a single ACh-sensitive channel produces a potential change of approximately 0.4 mV. This indicates that the MEPPs are not due to the release of a single molecule of ACh. Because the opening of a single AChsensitive channel produces a potential change of approximately 0.4 mV, it would take at least 1000 molecules of ACh to produce a potential change the size of one MEPP (0.4 mV). Indeed, because of loss due to diffusion in the synaptic cleft and the fact that two ACh molecules are necessary to open a channel, approximately 104 ACh molecules are required to produce a MEPP. Therefore, it appears that it is not a single molecule of ACh that is spontaneously and randomly released from the presynaptic terminal, but a package of 104 molecules of ACh. It is now well established that the morphologic locus of this package of 104 ACh molecules

6. Neuromuscular and Synaptic Transmission is the synaptic vesicle found in high concentration in a presynaptic terminal. Chemical analyses have revealed that the synaptic vesicles in the motor axon contain ACh.

Is the normal EPP due to the release of these packages of ACh as well? One observation consistent with this hypothesis is that when Ca2+ is injected into the presynaptic terminal, the frequency of occurrence of the MEPPs is increased. This indicates that the release of the vesicles is Ca2+ dependent. As a result, it is attractive to think that, when a larger amount of Ca2+ enters the presynaptic terminal during an action potential, a large number of vesicles is released synchronously.

Figure 17 illustrates a test of this hypothesis. To make the interpretation of the experiment easier, the

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