Electrical Excitability of Squid Giant Axons

1
Electrical Excitability of Squid Giant Axons

• Amir Golnabi
• ENGS166
• Spring 2008

2

• Outlines
• Resting membrane potential of squid giant axon
• Propagation of nerve impulses
• Velocity of nerve conduction in squid axon
• Conclusion
• References

3

• 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

4

• Squid Giant Axon 500-1000 µm in diameter
• Human axons merely 2 µm in diameter
• Relative permeabilities for K, Na, and Cl-
  1.00.040.45
• 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

Squid Neuron Cytoplasm concentration Extracellular concentration
Na 50 mM 440 mM
K 400 mM 20 mM
Cl- 51 mM 560 mM
5

• 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
6

• 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

Hodgkin and Huxley
d 0.1 cm R 35.4 O in temperature 18.5 ºC C
1.0 µF/cm2 K 10.47 1/msec If d20 µm like
the largest axon in our body
7

• 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

8

• 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
  lthttp//en.wikipedia.org/wiki/Squid_giant_axongt.
• Tortora, Gerard, and Bryan Derrickson. Principles
  of Anatomy and Physiology. Wiley, 2006.