The flight program

Recordings of the electrical signals that cause contractions of a muscle are called electromyograms (EMGs). The technique employed for recording EMGs during locust flight is similar to that used for recording electrocardiograms in humans, except that the electrodes are fine wires, inserted through small holes in the cuticle and secured in place with glue or wax (Fig. 7.2a). It is possible to fit a locust with a tiny radio transmitter to record EMGs from pairs of muscles while it is flying unrestrained (Kutsch et al., 1993). More usually, a locust is tethered, often to a solid bar but sometimes to a harness attached to counterweights so that some of the forces generated by the wing movements can be measured. It is quite easy to induce a suspended, tethered locust to flap its wings in a flight-like manner by blowing a current of air over its head. The air current excites wind-sensitive sensory hairs. During straight, level flight the wing-beat frequency is between 15 and 20/s. The frequency tends to fall gradually during a long flight, perhaps because the locust needs less power after it has burned some of its fuel.

Each flight motor neuron usually produces one or two spikes per wing beat, and excitation of elevator motor neurons alternates with excitation of depressors (Fig. 7.2b). Over a range of different wing-beat frequencies, the delay between excitation of the elevator and depressor motor neurons is fairly constant (Fig. 7.2c), as is the duration of the depressor activity. The most variable event during a wing-beat cycle is the duration of the burst of spikes in elevator motor neurons (Fig. 7.2c). Hindwing depressors spike

Fly Motor Neuron

Figure 7.2 The motor program for locust flight. (a) The locust is tethered to a rod, and induced to fly by wind directed at the head. Fine wire electrodes are inserted into flight muscles, and are attached to amplifiers (triangle symbols). The positions of the leg attachments are shown as circles. (b) Electromyograms from a depressor and an elevator muscle of a forewing. Usually, two motor neuron spikes are registered in each muscle per wingbeat cycle. In midcycle, small spikes are picked up from other muscles as cross talk. (c) During a long flight, the gradual increase in the duration of wing-beat cycles was due to an increase in the delay between spikes in depressor and elevator motor neurons. The delay between spikes in elevator and depressor motor neurons remained constant. (a modified after Horsman, Heinzel & Wendler, 1983; b from Pearson & Wolf, 1987; copyright © 1987 Springer-Verlag; c modified after Hedwig & Pearson, 1984.)

Figure 7.2 The motor program for locust flight. (a) The locust is tethered to a rod, and induced to fly by wind directed at the head. Fine wire electrodes are inserted into flight muscles, and are attached to amplifiers (triangle symbols). The positions of the leg attachments are shown as circles. (b) Electromyograms from a depressor and an elevator muscle of a forewing. Usually, two motor neuron spikes are registered in each muscle per wingbeat cycle. In midcycle, small spikes are picked up from other muscles as cross talk. (c) During a long flight, the gradual increase in the duration of wing-beat cycles was due to an increase in the delay between spikes in depressor and elevator motor neurons. The delay between spikes in elevator and depressor motor neurons remained constant. (a modified after Horsman, Heinzel & Wendler, 1983; b from Pearson & Wolf, 1987; copyright © 1987 Springer-Verlag; c modified after Hedwig & Pearson, 1984.)

5ms before forewing depressors, although elevators of all four wings are active synchronously. The result is that the hindwings are depressed before the forewings in each wing-beat cycle, so that they are not moving into the turbulent air caused by forewing movements.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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