Understanding Action Potentials & Ion Currents in Nerve Axons: Historical & Experimental V, Study notes of Biology

Download Understanding Action Potentials & Ion Currents in Nerve Axons: Historical & Experimental V and more Study notes Biology in PDF only on Docsity! Action Potentials and Conduction • Generation of action potentials • Voltage-clamp and membrane currents underlying action potential • Sodium and potassium currents • Inactivation • Relationship to single-channel currents Intracellular recording of the action potential in a squid axon (from Hodgkin and Katz, 1949) Voltage clamp of the squid giant axon reveals non-linear, voltage- dependent currents (Hodgkin, Huxley and Katz, 1952). The peak early and late currents are typically plotted as a function of voltage following subtraction of the capacitive and leakage currents. The early-inward current is eliminated by replacement of external Na+ with an impermeant cation. Subtraction reveals the late-outward current, which can be shown to be carried by K+ (from Hodgkin and Huxley,1952). As expected, the early and late currents become zero when the axon is clamped at the equilibrium potentials for Na+ and K+ respectively. Membrane current (mA/em2) Two methods for separating currents: use with caution lon replacement Oomv -60 i (Na* — free seawater) A Outward hot (Seawater) Inward Ina (hot = Ik) B Pharmacological blockade (1) Control ms 7 5 40 75mV 60 = 45 10: 30 nA Oo “15 10 45 aly Ee TTX: K* current (4) Za 60 45 <0 15 So “15 (3) TEA: Na* current (dy) vi 60 45 2 “4s 8 -30 -is— Copyright © 2002, Elsevier Science (USA). All rights reserved. Model of the nerve cell axon and currents underlying the action potential (from Hodgkin and Huxley, 1952) The Hodgkin/Huxley model for Na+ channel gating is largely borne out by information about the molecular structure and single-channel currents. “Patch” pipettes can be used to measure single- channel or whole-cell current. The conformation can be varied to allow intracellular and/or extracellular solutions to be controlled. The whole-cell mode may also be used for voltage recording. The voltage- dependence of the macroscopic (whole- cell) Na+ current reflects the voltage- dependent probability of opening for single channels. Propagation occurs as current generated during the action potential depolarizes adjacent axonal membrane to bring it to threshold. The longitudinal spread of current, and hence the speed of action potential propagation, is enhanced by myelination.