Innervation and neuromuscular physiology of intrinsic foregut muscles in the blue crab and spiny lobster

Abstract

1. The innervation pattern of the intrinsic foregut musculature was determined using physiological methods in blue crabs (Callinectes) and spiny lobsters (Panulirus) (Tables 1, 2). The distributions of individual axons were observed in methylene blue stained preparations and their physiological effects were observed by recording excitatory postsynaptic potentials (EPSPs) in various muscles while stimulating the axons directly or while the stomatogastric ganglion was active (Figs. 4–8). 2. InCallinectes the gastric mill muscles were all singly motor innervated, while many of the pyloric muscles were multiply innervated with as many as 3 motor axons to a single muscle (Table 1). InPanulirus the pyloric muscles also showed multiple innervation with a maximum of 5 axons to a single muscle (Table 2). 3. EPSPs varied in shape and amplitude from one muscle to another in both species even when two or more muscles were innervated by the same axon (Fig. 4). InPanulirus variation of EPSPs along individual muscle fibers was observed, indicating non-uniform innervation. 4. InCallinectes the variation in size of the EPSP from one recording site to another was correlated with degree of facilitation and with sarcomere length. Fibers with small EPSPs generally showed more facilitation and relatively shorter sarcomere lengths than fibers with large EPSPs (Table 1). However, instances were found in which the usual correlation did not hold. 5. Also in the blue crab a correlation was found between resting sarcomere length and contractile properties with short sarcomered fibers giving faster contractions than longer sarcomered fibers, as in other crustacean muscles. 6. The complex variety of muscle fiber types and EPSP properties apparently serves to translate the bursts of impulses from the stomatogastric ganglion into the necessary stomach movements. 7. Comparison of the innervation patterns of the gastric mill and pyloric regions suggest that the innervation patterns may be related to the functional roles of these two regions. Multiple innervation of the pyloric region allows rapid depolarization of the muscles to be achieved with a comparatively low frequency of impulses in each axon.