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Journal of Experimental Biology 127,173-189 (1987)
Published by Company of Biologists 1987

The Electrical Constants of the Fibres from Two Leg Muscles of the Locust Schistocerca Gregaria

M. E. KORNHUBER ¹ and C. WALTHER ¹

¹ Department of Anatomy, University of Ulm, FRG

1. The passive electrical properties of the fibres from the M. extensor tibiae and the M. retractor unguis in the hindleg of the locust Schistocerca gregaria were investigated using short cable theory. The dependence on various physicochemical parameters was determined.

2. The sarcoplasmic resistivity (R₁) was the same in the extensor and in the retractor muscle. R₁ was 175 cm at 20°C.

3. The specific membrane resistance (R_m) was considerably lower in the retractor muscle (5100 cm²) than in the extensor muscle (13 000 cm²; [K⁺]_o =; 10 mmoll^-1; temperature = 20°C). R_m increased by more than 100 % if the external potassium concentration was lowered from 10 to 5 mmoll^-1 and it decreased by approximately 75 % if the calcium concentration was lowered from 2 to 0.2 mmoll^-1.

4. The specific membrane capacity (C_m) increased with fibre diameter. The different mean values for C_m in the extensor (8.5 µF cm^-2) and retractor muscle (6.3 µF cm^-2) can be accounted for by the different mean fibre diameters.

5. The temperature coefficients (Q₁₀) of the electrical constants were 0.74 for R_i, 0.48 for R_m, 1.01 for C_m and 1.21 for the resting membrane potential (temperature, 16-27°C).

6. There was close agreement between the membrane time constant (_m) derived from the decay of the excitatory junction potential (EJP) and that derived from injection of current pulses. Thus R_m and the length constant () can be derived from the EJP and the fibre diameter if the sarcoplasmic resistivity and the specific membrane capacity are known.

7. The temporospatial dependence of miniature EJPs in a fibre can be predicted satisfactorily from the electrical constants as is demonstrated by an example given in the Appendix.

Note:
Dedicated to the late Graham Hoyle

Key words: locust, cable constants, muscle, synaptic potential

Accepted on September 1, 1986

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