The Control of Walking in Orthoptera Ii. Motor Neurone Activity in Normal Free-walking Animals

A brief description is given of the anatomy, innervation and mechanical properties of the extensor tibiae muscles of the locust. Each is innervated by a 'fast' (FETi) and 'slow' (SETi) excitatory axon, one branch of a common inhibitor (CI) and a fourth small axon (DUMETi). The prothoracic and mesothoracic extensors contract more rapidly than the metathoracic muscle but exhibit a stronger 'catch', which can be relaxed by CI or FETi activity. Records were made of electrical activity in the extensor motor nerves in all the legs of locusts and lubber grasshoppers. During standing only the SETi axons were active. During straight line walking in the locust all three motor axons were active, except in the metathoracic leg. A detailed description of the activity pattern of each axon is given. The activity in the grasshopper was similar, but rather more variable. Measurements were made of the mechanical responses of the extensor muscles to these patterns of activity. As walking speeds increased the response to SETi activity approached a constant tension or muscle length which could be strongly modulated by the phasic contractions due to FETi activity. It is suggested that the timing of SETi activity is only important at low walking speeds, and that at high speeds it simply provides a return force for the flexor muscle. The CI produces a slow effect and fires at the wrong time in the step to phasically relax the prothoracic or mesothoracic extensors, so it is suggested that its main role is in the relaxation of coxal muscles. The sources of motor neurone activity are discussed.