Electrical properties of toad sartorius muscle fibres in summer and winter

The area and circumference of surface fibres of sartorius muscles were measured from photomicrographs of frozen sections of whole muscles, and compared with the values obtained assuming a circular cross‐section. The latter assumption gave an over‐estimate of the mean area of 28%, but only a 2% over‐estimate of the circumference. In isolated, single fibres, the assumption gave over‐estimates of 25 and 6%, of area and circumference respectively. 2. The passive electrical properties of fibres were different in summer and winter. The mean internal resistivity, membrane resistance and membrane capacitance were 147 Ω.cm, 7·6 kΩ.cm 2 and 4 μF/cm 2 in summer, and 194 Ω.cm, 3·9 kΩ.cm 2 and 6·7 μF/cm 2 in winter, in fibres of comparable diameters in situ . In single fibres in summer, the mean values were 120 Ω.cm, 8·6 kΩ.cm 2 and 3·6 μF/cm 2 . 3. In glycerol‐treated fibres the mean specific membrane capacitance was 1·0 μF/cm 2 in summer and 2·0 μF/cm 2 in winter. The internal resistivity and specific membrane resistance were 167 Ω.cm and 8·9 kΩ.cm 2 in summer, and 232 Ω.cm and 3·9 kΩ.cm 2 in winter. 4. Early after‐depolarizations were recorded in glycerol‐treated fibres which had a low membrane capacitance, did not twitch and showed little ‘creep’. Electron micrographs of glycerol‐treated fibres showed disruption of the transverse tubular system and sarcoplasmic reticulum. 5. After exposure of muscles to 400 m M urea or acetamide for 1 hr, muscle fibres did not twitch and had a reduced membrane capacitance in Ringer solution.