Use of an Operational Amplifier Signal Differ- Entiator Reveals That Octopamine Increases the Rate of Development of Neurally Evoked Tension in Insect Muscle

Octopamine is a biogenic amine that is widespread in the insect nervous system (Evans, 1978; Dymond & Evans, 1979). In the locust, it potentiates the amplitude and increases the rate of relaxation of tension, in the extensor tibiae muscle of a hindleg, induced by the stimulation of the slow excitatory motoneurone (SETi) (1979) determined relaxation rates by graphical analysis of the data; a process which proved to be very time consuming and subject to error in the measurement at high relaxation rates. The device we describe here allows the continuous measurement of contraction and relaxation rates, and any changes in these parameters, during the course of an experiment. It was developed for use in a comparative pharmacological study of the octopamine receptors producing the above effects (Evans, 1980; P. D. Evans, in preparation). The electrical output of the tension transducer which is attached to the muscle tendon to monitor tension almost isometrically (see O'Shea & Evans, 1979), is first differentiated and then separated into the two components representing the contraction and relaxation rates. These two signals can be displayed separately and the voltage of each is proportional to the maximum rate of increase or decrease of tension respectively. This operation is performed automatically by the circuit shown in Fig. 1. Amplifier 1 provides a low impedance source for the differentiator (Amplifier 2) which is designed to deal with low frequency signals. Amplifiers 3 and 4 are half wave precision rectifiers which separate the positive and negative halves of the differentiated signal. Amplifier 5 inverts the rectified output of Amplifier 4 providing a positive going signal for ease of comparison with the output of Amplifier 3. The performance of the differentiator agrees closely with the theoretical calculated performance shown in Fig. 2 A where f x (0-08 Hz) is the characteristic frequency of the differentiator, f % (140 Hz) is the first pole frequency of the close loop circuit, and / 3 (560 Hz) is the second pole frequency of the closed loop circuit. Between the frequencies of 0-08 Hz (f a) and 140 Hz (f 2) the circuit acts as a differentiator, whilst above 140 Hz the circuit becomes a straightforward amplifier of gain 32 dB, rolling off at 20 dB/decade (6 dB/Octave) above 560 Hz. There is a linear relationship be