50 mV 10 ms 1 ms 50 mV 2 ms 50 mV depol n hyperpol n repol n

extracellular intracellular [Na] 145 mM 10 mM [K] 4.5 mM 140 mM A - Nernst equation for a permeable ion (e.g.K + ) E m (mV) = 61 log 10 [K + ] o [K + ] i Assume K + is permeable E K = 61 log 10 E K = -92 mV Assume Na + is permeable E Na = 61 log 10 E Na = +71 mV

action potential g Na gKgK E m mV Na + influx g, mS.cm EKEK E Na EKEK 0.5 ms B K + efflux

1 ms 0 mV 50 mV relative stimulus strength Absolute refractory period Relative refractory period

action potential cytoplasm action potential cytoplasm action potential A cytoplasm action potential cytoplasm cytoplasm 50 mV 1 ms i) ii) iii) iv) v)

direction of propagation myelin cyto- plasm Action Potential B axon myelin sheath axon node of Ranvier

A axon 2 mV 10 ms AP subthreshold 50 mV Axon hillock/ Initial segment AP axon APs + B temporal summation spatial summation