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

Abstract

ABSTRACT The innervation of 18 uropod and telson muscles in the crayfish has been studied in varying detail. Some muscles display purely phasic properties (short sarcomeres, electrically excitable membrane responses); others are strictly tonic (long sarcomeres, junctional potentials only, spontaneously active motor neurones); a few are mixed, but with anatomical segregation of fibre types. At least 55 efferent neurones are employed to operate these muscles. The tonic muscles all receive peripheral inhibition, and a given inhibitory axon serves only a single tonic muscle. Inhibitory and excitatory axons serving a given muscle discharge reciprocally and often respond to different reflex inputs. The average number of excitatory axons innervating tonic muscles is 2-3. The phasic muscles differ in the richness of excitatory innervation. Some receive as many as five axons per muscle, and as many as four per fibre; others are innervated by only a single motor axon. Many lack inhibitory innervation; where present, peripheral inhibitors may be shared between different phasic muscles. In no case were excitatory axons found to innervate anatomically separate muscles. In ‘mixed’ muscles there is predominantly separate innervation of phasic and tonic fibres; but in at least one case axons were found to serve fibres of both types. The shared axon was phasic in character. Some of the uropod and telson muscles are clearly homologous with axial muscles in more anterior abdominal segments. In a few of the former it was possible to make direct comparisons with the efferent neurones innervating their serial homologues. Differences were found in the size ratios of homologous axons, in their central location, and in their presence or absence.