Studies on Myo-Neural Mechanisms in Arthropoda

Citation

Ripley, Sherman Harvey (1953) Studies on Myo-Neural Mechanisms in Arthropoda. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/S8TQ-7214. https://resolver.caltech.edu/CaltechETD:etd-05122003-120648

Abstract

PART I. Innervation Patterns of Crustacean Limbs. In contrast to the vertebrates where the skeletal muscle is built up of large numbers of motor units, the limb muscles of decapod Crustacea are innervated by a small number of motor and inhibitory axons. It is possible to determine the details of the innervation and of the muscular response elicited by each of the several axons by isolating them in the nerve trunk. By such techniques, the previously determined innervation patterns of the walking legs in the four tribes of the Decapoda Reptantia have been confirmed and extended. Incomplete innervation patterns for the Decapoda Natantia and Stomatopoda have also been determined. Many characteristic features of innervation and associated motor and inhibitory effects were found to be common to all the Decapoda and the Stomatopoda. These physiologically derived anatomical data support the classification of the Decapoda into Reptantia and Natantia with division of the former into four tribes. A tentative evolutionary hypothesis is presented to explain the divergence of the different innervation patterns. PART II. The Effect of Spaced Stimulation of Excitatory and Inhibitory Axons of the Cray-fish. If a single motor axon to one of the limb muscles of a crustacean is stimulated with pairs of shocks repeated at regular intervals, the resulting contraction is greater than that elicited by the same number of regularly repeated single shocks. This effect was previously studied in the fast and slow contractions of muscles in various decapod Crustacea. The authors concluded that the effect was much more pronounced in the fast than in the slow system. In order to extend these observations and to determine whether or not it is a true myo-neural junctional phenomenon, the effect of unequally spaced stimulation of both the excitatory and inhibitory axons in the abductor of the dactylopodite was studied. The contractions resulting from such stimulation of the excitatory axon were more difficult to inhibit than were the normal contractions. Stimulation of the inhibitory axon with unequally spaced shocks resulted in a much greater reduction of the contraction than when it was stimulated with the same number of regularly recurring single shocks. It is concluded therefore, that the spacing effect occurs at the myo-neural junction and that it is not a mechanical effect at the level of the contraction. PART III. Neuromuscular Mechanisms in the Grasshopper, Romalea microptera (Beauv.). Insect myo-neural physiology has not been extensively studied but recently the existence of single and double innervation and of fast and slow contractions has been reported. The occurrence of peripheral inhibition has been claimed but the observations cannot be considered conclusive. In view of the phylogenetic relationship between the Crustacea and Insecta it might be expected that the two sub-phyla would have many characteristics of the myo-neural system in common. Both histological and physiological observations of the limb muscles of Romalea have confirmed this expectation. With one exception, these muscles are innervated by about six axons, each of which gives rise to a characteristic contraction. The majority of these contractions are of the fast type. True slow contractions have not been observed. There is no marked facilitation of either the contractions or the action potentials. The large 'jump' muscles of the third limb are specialized and exhibit only a single fast contraction type. No evidence for peripheral inhibition has been obtained.

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