Electrical Excitability of Squid Giant Axons

Amir GolnabiENGS166

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

outk

Clin

k

Nain

ink

Clout

k

Naout

m

ClPPNaP

PK

ClPPNaP

PKlog

F

RTV

• 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

Squid Neuron

Cytoplasm concentration

Extracellular concentration

Na+ 50 mM 440 mM

K+ 400 mM 20 mM

Cl- 51 mM 560 mM

F

RT

mV.

..

..logVm 12860

56045050040400

504504400402058

Propagation or Conduction:

• Nerve impulses must travel from the trigger zone to the axon terminals

• Two types: Continuous and Saltatory (figure1)

A.Continuous: muscle fibers and unmyelinated axons →

Relatively short distance propagation and a steady flow along the

membrane

B.Saltatory: discontinuous myelin sheaths that act as an electrical insulation. Action potential can be generated only at nodes of Ranvier → Faster propagation http://om

lc.ogi.ed

u/spectra/

hemoglo

bin/hemestru

ct/heme-

struct.gif

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

RC

dKV

4

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:

secm.sec

cm.

.V 18272718

104354

10470106

secm.sec

cm..

V 21616121104354

1047022

Hodgkin and Huxley:

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

RC

dKV

4

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.