title: Contractile activation in frog skeletal muscle.
abstract: Contractile activation was studied in frog single muscle fibers treated with tetrodotoxin to block action potentials. The membrane potential in a short segment of the fiber was controlled with a two-electrode voltage clamp, and the contractile response of superficial myofibrils in this segment was observed microscopically. The strength-duration relation for contractile activation was similar to that reported by Adrian, Chandler, and Hodgkin (1969); at 3.9 degrees C, the contraction threshold was -44 mV for long depolarizing pulses (100-ms) and increased to +64 mV for 2-ms depolarizations. Hyperpolarizing postpulses shifted the threshold for 2-ms pulses to more positive values, and a similar, but smaller, effect was produced by hyperpolarizing prepulses. The decay of excitability following subthreshold pulses showed two apparently distinct components; at 3.9 degrees C, excitability fell to 50% of its initial value within 4 ms, while the subsequent decline of excitability proceeded with a half-time of about 20 ms.
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D001001:Anura; D004558:Electric Stimulation; D066298:In Vitro Techniques; D008564:Membrane Potentials; D009119:Muscle Contraction; D009132:Muscles / Q000502:physiology*; D009210:Myofibrils / Q000502:physiology; D011894:Rana pipiens; D013779:Tetrodotoxin / Q000494:pharmacology; D013997:Time Factors
chemical_list: D013779:Tetrodotoxin