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The kinematics and neural control of high-speed kicking movements in the locust

Malcolm Burrows^* and Genevieve Morris

Department of Zoology, University of Cambridge, Cambridge CB2 3EJ, UK

View larger version (81K): [in a new window]   Fig. 1. Anatomy of the femoro-tibial joint of a left hind leg of a mature locust. (A) Photograph of the anterior (=lateral) surface. The semi-lunar processes and the proximal part of the tibia are black and heavily sclerotised. The curved horns of the tibia are engaged in the domes of the more lightly sclerotised ventral parts of the semi-lunar processes. (B) End-on photograph with the tibia held at an angle of approximately 90° relative to the femur. The hinge apposition of the flat surfaces of the two semi-lunar processes and the horns of the tibia are shown. (C,D) Drawings of the femoro-tibial joint. (C) Anterior surface with the tibia flexed about the femur. The anterior cover plate is drawn as though transparent to show a tibial horn engaged in the ventral dome of a semi-lunar process. The extent of the inwardly projecting process from the ventral wall of the femur, Heitler’s lump, and the plane of weakness about which the tibia bends are indicated by dashed lines. (D) Ventral view of the distal femur and proximal tibia with the joint membrane and muscles removed to show the invaginations of the semi-lunar processes. The tendon of the extensor tibiae inserts on the dorsal wall of the tibia and that of the flexor, after splitting into two, on the ventral wall. The opposing surfaces of the femur and tibia that form the hinge joint are indicated by the curved arrows.

View larger version (77K): [in a new window]   Fig. 2. Distortions of the femur and movements of the tibia during a kick. (A) Selected images taken at 1000 s–1. At the start of co-contraction (–525 ms), the semi-lunar process is not distorted, but at the end of co-contraction (–6 ms) it is bent. The proximal movement of the tibia, seen by reference to the tibial spine in the first two images, results in a widening of the gap between the femur and tibia. As the tibia begins to extend, the semi-lunar process remains fully bent (–3 ms) and begins to unfurl only when the tibia has already extended by 58.5° (–2 ms). As the tibia extends further, the semi-lunar process unfurls to its original state (–1, 0 ms). The cross hairs here and in Fig. 5 and Fig. 6 are aligned on the distal tip of the semi-lunar process. (B,C) Superimposed tracings of individual images from a kick taken at 500 s–1 and with an exposure time of 1/2000 s. (B) Co-contraction. The anterior semi-lunar process is progressively bent from –22 to –10 ms and the dorsal and distal part of the femur is compressed. The tibia also moves progressively ventrally and proximally in its fully flexed position. (C) The rapid extension movement. From –8 to –4 ms, the tibia begins to extend gradually, but the semi-lunar process only begins to unfurl at –4 ms when the tibia is moving at its maximum velocity and appears as a blur (the outlines of the tibia show the extreme positions of its leading and trailing edges during this image). (D) Selected images of an end-on view of the femoro-tibial joint from a kick captured at 1000 s–1. At the start of co-contraction (–800 ms), the tibia is fully flexed but the femur is not distorted. At the end of co-contraction (–5 ms), the femur is compressed dorso-ventrally and expanded laterally. As the tibia extends rapidly (–2 ms), the femur returns to its original shape.

View larger version (27K): [in a new window]   Fig. 3. Graphs of the movements of the tibia and of the distortions of the semi-lunar process during a kick captured at 1000 images s–1. (A) The distortions of the semi-lunar process and the movements of the tibia during the complete sequence are measured from every tenth image (10 ms intervals). (B) An expanded portion during the rapid tibial extension in which measurements from each image (1 ms intervals) are plotted. The semi-lunar process unfurls only when the tibia has already begun to extend. Circles, femoro-tibial angle; squares, proximal movement of the tibia; diamonds, movement of the semi-lunar process.

View larger version (22K): [in a new window]   Fig. 4. Correlations between distortions of the semi-lunar processes and the velocity of tibial movements. Three tibial extensions of different velocities are plotted. Extension 1, open circles; there was no distortion of the semi-lunar process and the tibial extension was slow. Kick 1, open triangles; there was a small distortion of the semi-lunar process and the resulting tibial extension was more rapid. Kick 2, filled squares; there was a large distortion of the semi-lunar process and the resulting tibial extension was even faster. The curves are aligned at the time (0 ms) when maximal tibial extension is achieved.

View larger version (44K): [in a new window]   Fig. 5. The activity of leg motor neurones during a kick and the consequent distortions of the semi-lunar processes and tibial movements. Intracellular recordings are from the fast extensor tibiae motor neurone (FETi) and from a fast flexor tibiae motor neurone. The first spikes in the flexor motor neurone move the tibia into the fully flexed position. When the FETi motor neurone starts to spike, so that the extensor and flexor muscles are now co-contracting, the semi-lunar process begins to bend. The bending continues until the flexor motor neurones are inhibited and the semi-lunar processes unfurl during the rapid extension. The apparently stepwise bending of the semi-lunar process is an artefact of the limited resolution of the measurements of each image. The cross hairs in the images show the progressive bending during FETi spiking and the rapid unfurling once tibial extension has started.

View larger version (71K): [in a new window]   Fig. 6. Tibial bending during a kick. Selected images from a kick captured at 2000 images s–1 and with an exposure time of 1/4000 s. Before the extension movement (–5 ms), the semi-lunar process is fully bent. The semi-lunar process remains fully bent during the initial part of tibial extension (–3 ms) and only unfurls when the tibia is partially extended and moving rapidly (–1 to –0.5 ms). At full extension (0 ms), inertial forces cause the tibia to bend at its proximal plane of weakness. As the tibia rebounds from full extension (+0.5 ms), it is still bent.

View larger version (14K): [in a new window]   Fig. 7. A pulse of sound is generated during a kick. (A) Recordings to show the level of sound that is generated in different kicks by a single locust if the tibia is allowed to extend only to the angle indicated. No sound is produced if extension is prevented at 20°, and only a faint sound occurs when extension is prevented at 50°. (B) The timing of the sound pulse relative to movements of the tibia (circles) and distortions of a semi-lunar process (squares) in an individual kick. The sound begins only when the semi-lunar process is unfurling and when the tibia is already extending. Measurements were made from images captured at 1000 s–1.

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