Electrical Properties of Fibres from Stridulatory and Flight Muscles of a Tettigoniid

Abstract

ABSTRACT The mesothoracic dorsal longitudinal muscle (DLM) of the katydid Neoconocephahts robustas is used in stridulation and flight; the metathoracic DLM is used in flight only. The DLM’s in the two segments have radically different maximum operating frequencies, 200 Hz for the mesothoracic muscle during stridulation and 20 Hz for the metathoracic muscle during flight. Cable analysis was used to determine the passive electrical properties of mesothoracic and metathoracic DLM fibres. Fibres in the two segments are of similar diameter and have similar sarcoplasmic resistivity. The apparent membrane resistance is lower, the apparent membrane capacitance higher, and the time constant shorter in mesothoracic fibres than in the metathoracic homologues. The depolarization evoked by neural stimulation in both mesothoracic and metathoracic fibres is principally an excitatory junctional potential (e.j.p.) with little or no contribution from voltage-dependent, inward current channels. At shqrt interstimulus intervals the second e.j.p. of a pair is reduced in amplitude relative to the first e.j.p. The period of e.j.p. depression is shorter in mesothoracic than in metathoracic fibres. It is suggested that the faster recovery of e.j.p.’s in mesothoracic fibres is due to more rapid recovery of the transmitter release mechanism in their motorneurones. In mesothoracic but not metathoracic fibres the voltage response to large depolarizing currents is usually oscillatory, and the recovery of e.j.p. amplitude as a function of time in paired shock experiments is sometimes oscillatory. The oscillation frequency is 250–300 Hz (35 °C) which is higher than the natural operating frequency of the muscle.