Studies On Certain Sensory and Motor Systems of Decapod Crustaceans

Citation

Furshpan, Edwin Jean (1955) Studies On Certain Sensory and Motor Systems of Decapod Crustaceans. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/JM6N-EC44. https://resolver.caltech.edu/CaltechETD:etd-11212003-113407

Abstract

Part I. Muscle Receptor Organs of the Crayfish.

Some interesting structures were discovered in the abdomens of certain Crustacea by J. S. Alexandrowicz (Quart. J. Micr. Sci. (1951) 92: 163) and in the present study their function was investigated. From his histological observations, Alexandrowicz deduced that these structures would serve as stretch receptors and his assumption was confirmed by the present work. It was found possible to isolate the organs and remove them from the animal. When the organs were stretched, using a specially constructed apparatus, trains of impulses were recorded from the nerve supplying them. There are two types of receptors in each half of each abdominal segment and the responses of each were found to differ from those of the other. One type (RM1) exhibited a prolonged slowly adapting discharge in response to a constant stretch; while the other type (RM2) adapted rapidly. The organs each consist of a muscular strand on which end the dendrites of the nearby sensory cell. It was shown that the muscular parts are unnecessary for the evocation of the sensory discharge by stretch; but that the former, by their contraction, can initiate impulses. The reactions of the receptors to several drugs were tested. Acetylcholine (ACh) in concentrations above a certain low value (10^-6 g/ml.) augmented or initiated discharge in RM1. The effect was abolished by previous administration of atropine and enhanced by eserine. The discharge accompanying stretch was not abolished by atropine and it was concluded that ACh probably did not serve as a mediator of the normal stretch-discharge.

Part II. Excitation in Decapod Crustacean Muscles.

Several muscles in a number of species of decapods were studied using a combination of two techniques. On the one hand was the procedure for isolating and stimulating single motor axons; and on the other, the microelectrode method for recording intracellularly from single muscle fibers. The existence of two classes of responses was corroborated. The first type, termed "junctional potentials," constituted the first muscular response to nerve stimulation. These potentials were found to be distributed to all parts of the muscle fiber by the motor nerve, in corroboration of previous evidence. The junctional potentials could summate when repeated and upon exceeding a certain threshold of depolarization evoked the second type of response: the spike potential. The latter was found to have the unusual properties of being graded and local. The spike was, however, followed by refractoriness. The distribution of motor axons to the individual muscle fibers was also studied. Two types of muscles were studied in this regard: those receiving two motor axons and those receiving four. In the muscles innervated by two axons, most muscle fibers were found to receive each of the axons, although several exceptions were noted. Each axon evoked a response of different character in any particular fiber; and, in addition, the size of the response elicited by either one of the axons varied markedly from muscle fiber to muscle fiber. The situation was even more complicated in the muscles innervated by four motor axons. Fibers were found which received only one axon, some received two, others three, and a small number were innervated by all four. In this muscle, then, aside from heterogeneity of response size, a marked variation in the axon complement of individual fibers was found. The responses evoked by different axons in a muscle fiber were found to summate and could thus act in concert to evoke a spike. The crustacean neuromuscular apparatus is thus a very complex reaction system and has some of the attributes of central nervous systems.

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