Innervation of the muscularis externa in the stomach of a coral fish, Chelmon rostratus Cuvier

Abstract

The muscularis externa of the stomach of the coral fish, Chelmon rostratus Cuvier, consists of four discrete layers - an outermost longitudinal layer followed by a circular, an oblique and an innermost layer. The muscle bundles of the oblique layer appear to be surrounded by thin prolongations of the innermost layer. In longitudinal sections of the innermost layer only rarely were small bundles of unmyelinated axons seen to course between the muscle cells. Vesiculated axon profiles within such nerve bundles occasionally came into apposition with the surface of muscle cells. The number of cross sectioned muscle fibres counted in the oblique, circular and longitudinal layers was 1365, 1408 and 745 respectively. The number of nerve bundles (single and multiple axon profiles) per 1000 muscle fibres was 62 (oblique layer), 30 (circular layer) and 28 (longitudinal layer). Single axon profiles comprised 51% (oblique layer), 36% (circular layer) and 10% (longitudinal layer). The number of axon profiles in the nerve bundles varied between 2 and 43. Nerve bundles containing 2-5 axon profiles formed 43% (oblique layer), 55% (circular layer) and 38% (longitudinal layer) of all nerve bundles. Vesiculated axon profiles within the nerve bundles made up approximately 55% (oblique layer), 41% (circular layer) and 28% (longitudinal layer). Axon profiles containing round agranular vesicles (AGVs) comprised 35% (oblique layer), 30% (circular layer) and 10% (longitudinal layer). Axon profiles containing large granulated vesicles (LGVs) comprised 54% (oblique layer), 60% (circular layer) and 85% (longitudinal layer). Axon profiles containing flattened vesicles comprised 11% (oblique and circular layers) and 5% (longitudinal layer). The neuromuscular junctional gaps between vesiculated axon profiles and the surfaces of muscle cells varied between 11-270 nm (oblique layer), 11-290 nm (circular layer) and 31-110 nm (longitudinal layer). Gaps of 11-30 nm formed 48% and 22% in the oblique and circular layers, respectively. In the oblique layer such gaps were made up of 48% of AGVs, 50% of LGVs and 46% of FVs. The corresponding figures in the circular layer were 18%, 35% and 33%. The majority of vesiculated axon profiles contacted one muscle cell. Vesiculated axon profiles contacting two muscle cells comprised 31% (oblique layer) and 21% (circular layer). Vesiculated profiles contacting three muscle cells comprised 7% (oblique layer) and 21% (circular layer). Rarely, an axon profile contacted four muscle cells. An innervated muscle cell was usually contacted by a single axon profile but there were cases where two or even three axon profiles made contact with a single muscle cell.