In 1952, Fatt and Katz, using extra- and intracellular recording from the subsynaptic membrane of resting muscle, observed what they called spontaneous miniature endplate potentials (SMEPPs) (Katz, 1966; Eccles, 1964). These SMEPPs were transient depolarizations of the membrane under the motor end plate having the same time course as a muscle twitch potential caused by the arrival of a single motoneuron spike at the motor end plate. The SMEPPs occurred randomly at a mean rate of about one per second. The most frequently observed amplitude was 0.4 mV peak. Long-term recording of SMEPPs from resting muscle subsynaptic membrane showed that the SMEPPs were quantized, i.e., most were about 0.4 mV, and with decreasing frequency of occurrence, some were 0.8. 1.2, 1.6, 2.0, and 2.4 mV peak. In each SMEPP height class, the amplitudes were distributed around the mean with an approximate normal distribution, with the standard deviations being proportional to the means.
As neurophysiologists began to inquire about the source of SMEPPs, it became evident from the electron micrographs of motor end plates and chemical synapses, that chemical neurotransmission was due to the action potential-stimulated release of neurotransmitter from vesicles found in close proximity to the inside of the presynaptic membrane. Since all of the vesicles are of a fairly uniform diameter, and they contain neurotransmitter molecules, an arriving spike causes the release of the contents of about 150 vesicles, producing a normal (full-sized) motor end plate potential of about +70 mV (Kandel et al., 1991). It was estimated that the spontaneous release of the contents of one vesicle (one quantum) into the subsynaptic cleft would cause the basic amplitude (0.4 mV) SMEPP. Based on a vesicle diameter of 50 nm and an internal ACh concentration of 0.15 M (the isotonic concentration), about 6000 NT molecules are in a typical motor end plate vesicle. Of much lower probability is the (nearly) simultaneous, spontaneous release of the contents of two vesicles (two quanta), producing a SMEPP of 0.8 mV peak.
The mean rate of SMEPPs was shown to be proportional to a low level of dc depolarization imposed on the presynaptic terminal by either electrical or external ion substitution methods. For example, when ammonium ions are substituted for extracellular Na+, the resultant presynaptic membrane depolarization causes a marked increase in the rate of SMEPPs (Eccles, 1964).
Needless to say, spontaneous postsynaptic potentials are present in neuron-neuron chemical synapses as well as in muscle motor end plates (Kandel et al., 1991). Quantal noise on motoneron epsps was observed by Eccles (1964). Such noise is potentially more disturbing, because spike excitation of neural-neural synapses normally causes the release of far fewer quanta of neurotransmitter (1 to 10) than at a motor end plate (~150). So it would appear that convergent resting chemical synapses have the potential to inject large amounts of noise into the membrane potential of a postsynaptic neuron. However, the convergence of many synapses from the same presynaptic neuron on the dendrites of the target postsynaptic neuron offers a clue to how this noise is mitigated. What occurs is spatiotemporal averaging; conduction down the passive core-conductor of a den-drite low-pass filters and smoothes the spontaneous essps (SEPSPs) from a given synapse. Presumably, the SEPSPs from the other N - 1 synapses are all generated with the same statistics, but are uncorrelated. They, too are low-pass-filtered by the dendrites, and sum together on the soma to give a slightly raised dc resting potential without appreciable noise by what can be viewed as a spatial ensemble averaging process. In neuro-sensory systems, the spatiotemporal averaging process inherent in multisynaptic transmission can produce an improved signal-to-noise ratio, and permits detection of lower threshold stimuli than would be possible with fewer synapses (Northrop, 1975).
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