The innervation of the ampulla of the tube foot in the starfish Astropecten irregularis

Abstract

In the ampullae of the starfish Astropecten irregularis extensions stretch from the main soma of the muscle cells down the seams of the ampullae into bulbs of tissue in the tube feet. Their terminations range from 0.2 to 4.0 μm in diameter. Here axons in tracts from the radial cord form synapses upon the extensions of the muscle cells. In sections examined with the electron microscope the extensions are distinguished from nerves by their central core of myofilaments, surrounded by clear cytoplasm. The junctions between the extensions of the muscle cells and the nerve fibres show no peculiarities of the membrane and are recognized as synapses only by their vesicles. No nerve fibres or neuromuscular junctions can be identified in the ampulla, and structures described from material vitally stained with methylene blue must be reinterpreted. The fibres which run along the seam of the ampullae, formerly described as axons, are extensions of muscle cells, to which they can be traced. The muscle fibres of the ampullae have large fluid-filled spaces around the myofilaments and it is proposed that some stain selectively and give rise to the structures originally described as ribbon axons.