Neuronal control of locomotion in the lobster,Homarus americanus

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1. Lobsters that are tethered in place on a treadmill (Fig. 3) walk against the direction of belt movement (Table 2). Forward and backward locomotion over the full range of step frequencies can be controlled by this method, even in the absence of visual input. The passive traction provided by a moving substrate is therefore an effective stimulus for walking and presumably operates in parallel with previously described optomotor pathways to provide positive feedback reinforcement of locomotory behavior. 2. The movements (Figs. 1, 6) and muscular anatomy (Fig. 2) of a lobster walking leg are described. On the basis of simultaneous extracellular recording from several leg muscles (Fig. 5), and motion picture analysis, the overall patterns of joint movement and muscular coordination underlying forward and backward walking are described (Figs. 5, 6, 7). 3. Some muscles that are synergic for forward walking are antagonistic for backward walking (Figs. 6, 7). Similarly movements that are synergic for lateral walking on the leading side are antagonistic for lateral walking on the trailing side (Fig. 6). 4. Quantitative analysis of leg movements (Fig. 9) and electromyograms (Fig. 10) have shown that the walking muscles can be subdivided into three different functional classes: return stroke muscles, which exhibit bursts of relatively constant duration irrespective of step frequency (Fig. 10A); power stroke muscles in which burst duration varies linearly with step frequency (Fig. 10B); and bifunctional muscles, which exhibit the discharge characteristics of either return or power stroke muscles, depending on the direction of walking (Fig. 10C). 5. Several lines of evidence (Table 3, Figs. 6, 7, 9, 10, 12) suggest that the limb elevator motoneurones (or their central antecedents) function as the central pacemaker of the walking system, and that other cyclic leg movements are appended to the basic elevation/depression cycle as appropriate to the direction of walking. Evidence is presented that proprioceptive inputs provided by passive traction are capable of controlling the direction of locomotion (Table 2), and determining the periodicity of stepping (Fig. 4), by altering the duration of powerstroke bursts (Figs. 9, 10, 15).