Coxal depressor muscles of the cockroach and the role of peripheral inhibition.

In a previous paper (Pearson & lies, 1971) the pattern of innervation of the coxal depressor muscles of the cockroach Periplaneta americana was described. On the basis of microelectrode recordings these muscles could be classified into three types: (i) those receiving one 'fast' axon, (ii) those with one 'slow' and branches of three different common inhibitory neurones, and (iii) those whose fibres received both the' fast' and ' slow' axons but no inhibitory innervation. The mechanical properties of the mesothoracic muscles of the first type (136, 137, notation of Carbonell, 1947) have been studied with both isotonic (Becht, 1959; Becht & Dresden, 1956) and isometric (Becht, Hoyle & Usherwood, i960; Usherwood, 1962) techniques. Nerve stimulation was found to result in a single twitch response of the muscle which antifacilitated at frequencies above 10/s. This is consistent with innervation by a single fast axon, and we have found the same result in the metathoracic homologues (178, 179, Pearson & lies, 1971). The position with regard to the mechanical properties of the second and third types is more confused. Becht (1959) discovered that the mesothoracic muscles 135D, ,E were each anatomically divisible into two parts of differing colour. Two of these, i3Sd', e' gave twitch responses to nerve stimulation, and the others, 133d, e were darker in colour and showed slow responses. Inhibition was found in 135c and possibly in i35d. We have shown that i35d', e' (and also the metathoracic homologues i77d', e') receive one fast and one slow axon; and i35d, e (i77d, e) receive the same slow axon and three inhibitory axons (Pearson & lies, 1971). This is consistent with Becht's mechanical results. These findings are not entirely supported by the observations of Usherwood (1962). From records of the isometric tension generated by the whole muscle complex 135D, E it was concluded that innervation was by two fast and possibly two slow axons, but not by any inhibitory axon. The experiments described in the present paper were in part designed to reinvestigate the mechanical properties of muscles 135(177) D, E and to find a correlation with the results from microelectrode studies (Pearson & lies, 1971). The coxal depressor muscles are active during walking in the cockroach and the discharge of the excitatory and inhibitory axons has been studied (Pearson & lies,