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Analyzing the axon, myelin

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PH 122
Brian Jones

21 February The electric field inside the cell membrane A typical cell membrane has a thickness of 7.0 nm. What is the strength of the electric field inside the cell membrane? ∆V 70x10 V3 E = = −9 ∆d 7.0x10 m 7 E = 1.0 x 10 V/m Uninsulated axon signal propagation A wave of potential travels down the axon. Sodium channels open → potassium channels open → membrane recovery Analyzing the axon, Part I: Resistance What is the resistance along the length of a typical axon of length 1.0 mm and radius 5.0 μm? Ignore the resistance of the membrane, and assume that the resistivity of the fluid inside the axon is 2.0 Ωm. ρL R = A −3 2.0Ωm (1x10 m) R = −6 2 π(5x10 m) R = 25 MΩ Analyzing the axon, Part II: Capacitance What is the capacitance of a typical axon of length 1.0 mm and radius 5.0 μm? The dielectric constant of the cell membrane is 9.0, and the thickness is a typical 7.0 nm. κε0A C = d ( ) −3 9 (ε0)(2π7x10 m) C = 7x10 m3 C = 360 pf Analyzing the axon, Part III: Signal speed For the typical axon of the previous two slides, how fast will an action potential propagate from one end to the other? L L .001m
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