Neuromuscular physiology of the longitudinal muscle of the earthworm, Lumbricus terrestris. 3. Mapping of motor fields.

The functional properties of the neuromuscular systems of animals are frequently studied by using isolated nerve-muscle preparations and a suitable physiological saline solution. In general suitable salines are obtained by determining the concentrations of major ions in an animal's body fluids and duplicating these concentrations in the form of a saline solution. Any alteration in the concentrations of major ions often leads to malfunctioning or rapid deterioration of the preparation (Lockwood, 1961). In the case of the earthworm, analyses of the ionic composition of body fluids have been carried out only recently. From these analyses it is obvious that the ionic composition of earthworm body fluids does not coincide with the ionic composition of commonly used earthworm salines. For example, in the saline developed by Pantin (1946) the concentrations of Na+, Ca, and Cl~ are all considerably different from the concentrations of these ions in either blood or coelomic fluid (Kamemoto, Spalding & Keister, 1962; Dietz & Alvarado, 1970). It is these differences in ionic composition which may account for some of the difficulties encountered in previous investigations of earthworm neuromuscular physiology. For example, the results of Hidaka et al. (1969 a-c) and Chang (1969) indicate the presence of low muscle resting potentials, spontaneous spiking, and erratic junction potentials in earthworm longitudinal muscle. Further difficulties were encountered by Drewes & Pax (1971), who reported prolonged firing in longitudinal muscle following single-pulse stimulation of individual segmental nerves. The nature of such difficulties suggests abnormal excitability of nerve and muscle brought about perhaps by improper ion concentrations in the bathing medium. For these reasons we have developed a new saline which is much nearer earthworm coelomic fluid and blood with respect to ion composition than the earthworm salines used by previous investigators. In this paper we present a comparison of the activity of earthworm longitudinal muscle when bathed in this new saline and when bathed in the saline most commonly used by previous investigators, the saline recommended by Pantin (1946).