Motor Unit Distribution of the Dorsal Longitudinal Flight Muscles In Locusts

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Abstract
1. The peripheral pathways of the ‘fast’ motor fibres to the locust dorsal longitudinal flight muscles are described at the single unit level, from electrophysiological and histological studies. This is summarized in a diagram on Pl. 1. 2. Both pterothoracic dorsal longitudinal muscles consist of five anatomically distinct motor units, arranged in layers from dorsal to ventral. Each of the four more ventral units of both muscles receives a motor axon from the segment in front via the recurrent nerve, whereas the uppermost motor unit is innervated in each case from the segment containing the muscle. The motor units are nearly equal both in size and capability for work. 3. Each of the five ‘fast’ motor axons innervates one topographically distinct bundle of muscle fibres. There is no overlap between muscle motor units. 4. Even within a single muscle, motor units are capable of vibrating at independent frequencies. This indicates that the coupling of units which occurs during flight is neither structurally nor functionally rigid. 5. With respect to peripheral features, the motor units within each dorsal longitudinal muscle are designed for fast response which improves the synchronization when the relevant neurons fire simultaneously (large motor axons, 15-25 µ in average diameter with high propagation velocity, 8 m./sec. at flight temperature). 6. It is suggested that a tonic motor output, containing at least three units, which was recorded from mesothoracic nerve IBa, travels to the small lateral dorsal muscles.