The motor nervous system of the starfish, Astropecten irregularis (Pennant), with special reference to the innervation of the tube feet and ampullae

The motor nervous system of Astropecten iregularis, lodged within the mesothelial linings of the coelomic body cavities, has a bilateral and metameric arrangement in conformity with the longitudinal seriation of the tube feet and of the ossicles and muscles of the ventral wall of the arm. Bilateral and metameric groups of central distributory motor neurons, lodged above the radial nerve cord in the floor of the radial perihaemal sinus, have long axon processes which extend outwards on either side of each body segment through the lateral perihaemal canals to circumscribe the base of the foot. The central distributory axons may be regarded as pre-ganglionic fibres since they terminate in one or other of two ganglionic complexes: (a) the lateral motor complex on the outer side of the foot in the perihaemal cavity and (b) the foot complex within the foot cavity. The former is interposed into the tracts which supply the arm musculature, the latter serves as a relay to the several muscle systems of the foot and ampulla. The post-ganglionic innervation of the arm musculature is effected through chains of distributory internuncial neurons linked in series. They form clearly defined tracts, each of which terminates in ultimate motor neurons. The axons of these neurons, initially of small diameter (c. 0.5μ and circular in section, expand distally into broad ribbons (20 to 30 x 0.5μ) which invest the muscle fibres and have a continuous surface of synapsis with them. Some of the fibres of each muscle system are supplied by collateral branches of the ribbon axons. The neurons of the foot complex are lodged in two more or less bilaterally disposed bulbs of supporting tissue within the base of the foot. One is lateral, the other (bilobed) medial. There are three component series of neurons in each bulb complex, α neurons synapse with and are in receipt of excitation from the central distributory axons external to the foot in the adjacent perihaemal canals. They transmit excitation to a β neuron complex which functions as a relay to the foot and ampulla musculature. Chain neurons conduct excitation from the β neurons to the ampulla. They terminate in ultimate motor (ribbon axon) neurons whose ribbon axons and axon collaterals innervate the ampulla musculature. The ultimate neurons serving the retractor muscles are in direct contact with the relay neurons of the β complex. Those supplying the postural muscles connect with the β complex through radially arranged fibres. The latter provide, in all probability, for the sectorial contraction of the musculature by which a 360° range of pointing and stepping can be effected. Circular fibres connecting the ultimate neurons of the various muscle sectors are thought to play a role in the display of autonomous and individual patterns of pointing. The fine structure of the unique ribbon axons is described. Of particular interest is the presence of an axiolemma sheath. Comparison is made of the echinoderm nervous system with that of other invertebrates.