0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 Amplitude of End-plate Potentials (mV)
FIGURE 17 Quantal nature of transmitter release. (A) In a low-Ca2+ solution, the motor axon is repeatedly stimulated, and the amplitude of the EPP is monitored. In such a solution, the amplitude of the EPP is small and variable. Asterisks indicate spontaneous MEPPs. (Modified from Liley AW. J Physiol 1956; 133:571-587.) (B) A histogram showing the number of times EPPs of various amplitudes are recorded in the low-Ca2+ solution. There are many EPPs that have an amplitude identical to the amplitude of the MEPP (0.4 mV). Other peaks are multiples of the first. (Modified from Boyd IA, Martin AR. J Physiol 1956; 132:74-91.)
Ca2+ concentration in the extracellular medium is reduced to a point at which the evoked EPPs are quite small (approximately the same size as the MEPPs). The left side of the traces in Fig. 17A illustrate some of the spontaneous MEPPs (approximately 0.4 mV in amplitude [asterisks]). The small vertical lines on the traces illustrate the point in time at which the stimulus is applied to the motor axon. When the motor axon is stimulated in the low-Ca2+ solution, very small EPPs are produced, but there is considerable variation in their size. For example, sometimes the EPP is about the same size as the MEPP (spontaneously occurring) (trace 3, Fig. 17A). Such a response is called a unit EPP. Sometimes no EPP is produced by the stimulus (traces 2 and 6, Fig. 17A) and at other times an EPP is produced that is about twice the size of the unit (double) (traces 4, 7, and 8, Fig. 17A). In other cases, EPPs are produced that are about three times the size of the unit or the spontaneous EPP. Sometimes an EPP is produced that is about four times the size of the unit EPP and is referred to as a quadruple (trace 1, Fig. 17A).
By measuring the amplitude of all the evoked EPPs, a plot can be drawn of the number of times the EPPs of a given amplitude are observed. Similarly, an amplitude distribution of the spontaneous MEPPs can be made (e.g., Fig. 16B). As we have already learned, the spontaneous MEPPs have an average value of approximately 0.4 mV. The plot of the evoked EPPs produced in low Ca2+ reveals a remarkable finding; namely, there is a multimodal distribution of sizes (Fig. 17B). There is a large proportion of EPPs that have an average value of approximately 0.4 mV, another large percentage that have an average value of approximately 0.8 mV, and some that have an average value of approximately 1.2 mV. The critical finding is that the size of the first peak of the evoked EPP (0.4 mV) is nearly identical to the size of the spontaneous MEPPs and that subsequent peaks are multiples of the first.
Based on these observations, Katz and his colleagues proposed the quantal nature of transmitter release. The hypothesis was developed at the neuromuscular junction, but the quantal nature of release appears to be widely applicable to chemical synapses. Transmitter release at some synapses may be nonvesicular, however. According to the quantal hypothesis, the size of the EPP fluctuates because different numbers of packages (quanta) of ACh are released with each stimulus. The smallest EPP is due to the release of a single package of ACh containing 104 molecules. An EPP twice that size is due to the release of two packages, and so forth. The EPP of about 50 mV produced in normal concentrations of Ca2+ is a compound of the individual contributions of many packages of ACh. Because each package of ACh produces an amplitude of approximately 0.4 mV, g 14-
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