Innervation Patterns of Fast and Slow Muscle in the Uropods of Crayfish

In previous publications from this laboratory (Kennedy & Takeda, 1965a, b; Evoy & Kennedy, 1967; Kennedy, Evoy & Hanawalt, 1966; Evoy, Kennedy & Wilson, 1967) we have analysed the innervation of abdominal flexor and extensor muscles in the crayfish, and have specified the role of central interneurones in evoking co-ordinated, multisegmental postural movements. Movements involving the axial muscles of the abdomen occur in only a single plane, and are based upon relatively simple reciprocal relationships between excitatory and inhibitory motor outflows to the antagonistic slow flexor and extensor muscles. In the investigations described in this and the following paper we have extended this method of analysis to a more complex system controlling the appendages of the terminal abdominal segment. The uropods of crayfish perform a variety of movements concerned with righting responses, steering during locomotion and the like; and their motor control is more complex in several respects than that of the axial musculature. First, they are movable about three axes (rotation, extension/flexion and promotion/remotion) instead of just one. Secondly, their movements are often bilaterally asymmetric, as in steering reactions. Thirdly, phasic and tonic elements are more intermixed than in the abdominal musculature, and various types of movements can be produced by either category. Finally, some of the muscles operating the uropods show homologies primarily with the abdominal series, while others belong more clearly to the appendages themselves. The situation is thus intermediate between the neuromuscular systems of the abdomen and those of the well-studied thoracic limbs (for reviews see Kennedy, 1967; Atwood, 1967). The first paper in this series describes the innervation and electrical characteristics of 12 of the more accessible uropod muscles, and provides some information on nearby muscles that flex or extend the telson. It will be shown that some muscles are purely phasic; these exhibit electrically excitable membrane responses and are innervated by phasic motoneurones lacking spontaneous activity. Others are tonic; they show only graded junctional potentials (j.p.s) generated by tonic, often 'spontaneous* motor activity. Still others have tonic and phasic portions, either with or without overlap of innervation from motoneurones of the two classes. We have gathered data on the number of motoneurones innervating the different muscles, the number innervating