Practice question

Action potential propagation in excitable cells depends on rapid voltage dependent changes in sodium and potassium conductances. Resting potential near minus 70 millivolts maintained by K+ leak and Na+/K+ ATPase. Upon synaptic depolarization reaching threshold about minus 55 millivolts, voltage sensor S4 helices of sodium channels containing repeated Arg-X-X motifs move outward within 100 microseconds, opening activation gate formed by S6 crossing. Sodium electrochemical gradient strong due to 145 millimolar outside, 12 millimolar inside and negative membrane potential drives inward current, depolarizing further to plus 30 millivolts regenerative upstroke. This Hodgkin Huxley sodium current underlies phase 0. Ligand gated nicotinic or glutamate receptors contribute to synaptic depolarization but require chemical transmitter and deactivate slower, not participating in axonal conduction. Aquaporins selectively conduct water via NPA motifs preventing proton conduction, GLUT transporters uniport glucose via rocker switch without ionic current. Subsequent activation of delayed rectifier Kv channels repolarizes. Hence voltage gated Na+ channel confers voltage sensitive excitation for action potential generation.