Attwell D, Iles J F
Proc R Soc Lond B Biol Sci. 1979 Nov 30;206(1162):115-31. doi: 10.1098/rspb.1979.0095.
Currents flowing through the postsynaptic membrane of an active synapse will tend to change the concentrations of ions in the synaptic cleft. Published experimental data are used to predict (a) the sodium and potassium concentration changes in the cleft at the frog neuromuscular junction, and (b) the sodium depletion in the cleft under a Ia synaptic bouton on a cat motoneuron. Significant concentration changes are predicted at both synapses. These changes will contribute to the time dependence of the observed current and will cause the reversal potential of the current to be time dependent. At the frog neuromuscular junction, the time course of the endplate current has been shown previously to depend on the magnitude of the current flowing (at a given potential). We attribute this to changes of the cleft ion concentration. The time dependent changes of the endplate current reversal potential that we predict for the neuromuscular junction are probably too small to be detected. This is because the effects of sodium depletion and potassium accumulation on the reversal potential almost cancel. We predict that near the reversal potential small currents of complex time course will remain, i.e. no true reversl potential exists. Such currents have previously been experimentally. At the cat Ia synapse, the synaptic current is predicted to deplete a significant fraction of the available extracellular sodium ions. Consequently, the magnitude of the synaptic current should be relatively independent of the number of postsynaptic channels activated, and of the membrane potental, as has previously been found experimentally.
流经活跃突触后膜的电流往往会改变突触间隙中离子的浓度。已发表的实验数据用于预测：（a）青蛙神经肌肉接头处突触间隙中钠和钾浓度的变化，以及（b）猫运动神经元Ia突触终扣下突触间隙中的钠耗竭情况。预计在这两种突触处都会发生显著的浓度变化。这些变化将导致所观察到的电流具有时间依赖性，并使电流的反转电位也具有时间依赖性。在青蛙神经肌肉接头处，终板电流的时间进程先前已表明取决于（在给定电位下）流动电流的大小。我们将此归因于突触间隙离子浓度的变化。我们预测的神经肌肉接头处终板电流反转电位的时间依赖性变化可能太小而无法检测到。这是因为钠耗竭和钾积累对反转电位的影响几乎相互抵消。我们预测在反转电位附近会存在复杂时间进程的小电流，即不存在真正的反转电位。此前已通过实验观察到此类电流。在猫的Ia突触处，预计突触电流会消耗相当一部分细胞外可用的钠离子。因此，突触电流的大小应该相对独立于激活的突触后通道数量以及膜电位，这与先前的实验发现一致。
