The flight engine

As in most insects, flying is achieved by cyclical movements of four wings, borne on the posterior two segments of the thorax. As the wings move up and down, they twist so that they constantly generate lift to keep the insect airborne (Fig. 7.1a). Each wing is moved by ten muscles, which can be divided into three main groups (Fig. 7.1b). The first group contains just one large muscle for each wing, oriented along the animal's long axis. When these dorsal longitudinal muscles contract, they distort the stiff cuticular box structure of the thorax in a manner that causes the wing tips to move downwards, and they are called 'indirect depressor' muscles. The other two groups of muscles lie upright in the thorax and pull directly on the wing base. One group pulls outside the fulcrum of the wing hinge, and so these muscles are direct depressors, and the other group pulls on the inside of the fulcrum and so these muscles are direct elevators (Fig. 7.1c). The three largest direct depressor muscles of each wing control the way the wing twists around its long axis, and are important in altering the pattern of wing beat during steering manoeuvres.

The pattern of innervation of the wing muscles is straightforward because each flight muscle is usually controlled by just one or two motor neurons. A spike in a flight motor neuron mediates a rapid, strong twitch of its muscle. The motor neurons make synapses along the length of each muscle fibre, and these operate in the same way as synapses in the central nervous system. When the presynaptic terminals are depolarised by the arrival of a spike, each releases a tiny squirt of neurotransmitter, in this case the amino acid glutamic acid. The ion channels in the muscle cell membranes that bind glutamic acid cause large EPSPs. Through a series of events, the electrical signal is transduced into the development of tension by individual muscle fibres.

The simple pattern of innervation of locust flight muscles contrasts with that found in most vertebrate skeletal muscles, where each muscle is controlled by several tens or hundreds of motor neurons. It also

Figure 7.1 Movements and muscles involved in locust flight. (a) Movements during one wing-beat cycle. The drawings show three stages during a down-stroke; the hindwings move slightly before the forewings. Below, the path of movement of the tip of the left forewing during one wing beat and the angle of the wing during one wing-beat cycle are shown. The wing is twisted during the upstroke to help maintain lift throughout the cycle. (b) and (c) The main flight muscles of the third thoracic segment (bold outline in the left drawing in a). The right half of the segment is shown in medial view (b) and anterior view (c). Muscles 112, 128 and 129 are wing depressors; muscles 113, 118 and 119 are wing elevators. The wing hinge is indicated by an arrow in (c). (a redrawn after Pringle, 1975; b and c redrawn after Snodgrass, 1935.)

Figure 7.1 Movements and muscles involved in locust flight. (a) Movements during one wing-beat cycle. The drawings show three stages during a down-stroke; the hindwings move slightly before the forewings. Below, the path of movement of the tip of the left forewing during one wing beat and the angle of the wing during one wing-beat cycle are shown. The wing is twisted during the upstroke to help maintain lift throughout the cycle. (b) and (c) The main flight muscles of the third thoracic segment (bold outline in the left drawing in a). The right half of the segment is shown in medial view (b) and anterior view (c). Muscles 112, 128 and 129 are wing depressors; muscles 113, 118 and 119 are wing elevators. The wing hinge is indicated by an arrow in (c). (a redrawn after Pringle, 1975; b and c redrawn after Snodgrass, 1935.)

contrasts with the innervation of other muscles in locusts, such as those which move the legs. Each leg muscle is innervated by only a few motor neurons, but not all of them mediate fast, twitch-like contractions. Some, the slow motor neurons, mediate small, individual twitches that sum together gradually to develop graded, strong, slow contractions when the motor neuron generates a train of spikes. Other motor neurons are inhibitory and employ gamma aminobutyric acid as their neuro-transmitter. The inhibitory motor neurons oppose the action of the slow excitors by causing hyperpolarising postsynaptic potentials in their muscle fibres. For further information about muscle innervation, see Aidley (1998).

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Essentials of Human Physiology

Essentials of Human Physiology

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