Insect visceral muscle. Responses of the proctodeal muscles to mechanical stretch

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The longitudinal muscle fibres of the cockroach proctodeum were found to be sensitive to stretch despite its striated structure. Mechanical and electrical responses triggered by stretch were recorded. Quick stretches applied to the muscle generated electrical responses which caused contractions. Resting membrane potentials decreased during maintained stretch and in proportion to the amount of stretch. A train of rhythmic action potentials was evoked at a critical depolarization level of the resting membrane potential by stretch. Total number and frequency of the responses increased with increasing strength of the maintained stretch. Tension-length curves of active and resting states were measured. Actively developed tension increased up to the state twice the rest length of the muscle. An action potential was generated by directly applied depolarizing current, and stronger depolarization evoked repetitive action potentials. Hyperpolarization blocked the generation of action potentials. It is concluded that in the longitudinal muscle fibres the action potential may be generated by a simple shift of the resting membrane potential towards a critical depolarization level which is achieved not only by PSP's but also by stretch.