Electrical Excitability of Squid Giant Axons Amir Golnabi ENGS166 Spring 2008
Outlines: Resting membrane potential of squid giant axon Propagation of nerve impulses Velocity of nerve conduction in squid axon Conclusion References
Membrane potential: difference of electrical charges across a plasma membrane Electrochemical gradients The rate of ions flow through the membrane – Permeability of ions Excess of negative charge inside and an excess of positive charge outside the cell membrane at rest: resting membrane potential Goldman Equation: R: gas constant T: absolute temperature in kelvins F: Faraday constant [ion]: concentration of the ion P: permeability
Squid Giant Axon: 500-1000 μm in diameter Human axons: merely 2 μm in diameter Relative permeabilities for K+, Na+, and Cl-: 1.0:0.04:0.45 = 58 mV (at 20ºC) inside and outside concentration of ions Resting membrane potential of large mammalian nerve fibers: -90 mV: ~ 50% more than in squid axons
Propagation or Conduction: Nerve impulses must travel from the trigger zone to the axon terminals Two types: Continuous and Saltatory (figure1) Continuous: muscle fibers and unmyelinated axons → Relatively short distance propagation and a steady flow along the membrane Saltatory: discontinuous myelin sheaths that act as an electrical insulation. Action potential can be generated only at nodes of Ranvier→ Faster propagation http://omlc.ogi.edu/spectra/hemoglobin/hemestruct/heme-struct.gif
Hodgkin and Huxley: • d = 0.1 cm • R = 35.4 Ω in temperature 18.5 ºC • C = 1.0 μF/cm^2 • K = 10.47 1/msec • If d=20 μm like the largest axon in our body: Velocity of nerve conduction based on the cable properties of the nerve fibers: V: conduction velocity d: diameter of the axon R: resistance of axoplasm C: capacity per unit area of the membrane. K: constant which depends only on properties of the membrane
Velocity of nerve conduction based on the cable properties of the nerve fibers (cont.) Other important factors in conduction velocity: Temperature Resistance of axoplasm Capacity per unit area of the membrane Squid: Life in the ocean, prayed on by fish and whales Giant axons: ability to make very fast movements through the water
References: Becker, Wayne, Lewis Kleinsmith, and Jeff Hardin. The World of the Cell. San Francisco: Benjamin Cummings, 2002. Freeman, Scott. Biological Science. Upper Saddle River, NJ: Pearson Prentice Hall, 2005. Guyton, Arthur, and John Hall. Textbook of Medical Physiology. PA: Elsevier, 2006. HODGKIN, A. L., and A. F.HUXLEY. "A Quantitative Description of Membrane Current and Its Application and Excitation In Nerve." 117(1952): 500-544. Matsumoto, Gen, and Ichiji Tasaki. "A study of conduction velocity in nonmyelinated nerve fibers." 20(1977). "Squid giant axon". Wikipedia. April 2008 <http://en.wikipedia.org/wiki/Squid_giant_axon>. Tortora, Gerard, and Bryan Derrickson. Principles of Anatomy and Physiology. Wiley, 2006.