ALL-OR-NONE CONTRACTION AND SODIUM-CHANNELS IN A SUBSET OF CIRCULAR MUSCLE-FIBERS OF SQUID MANTLE

Motor function in squid (Loligo) mantle reflects the highly coordinate d activity of two motor pathways associated with giant and non-giant m otor axons that respectively produce all-or-none and graded contractio ns in mantle muscle. Whereas both types of axons innervate circular ma ntle muscle fibers, precise nerve-muscle relationships remain unclear. Are squid like most invertebrates, in which single muscle fibers rece ive dual innervation from giant and non-giant motor axons, or is squid mantle configured more like vertebrates, in which parallel motor axon systems innervate distinct fast and slow muscle fibers? In this repor t, we describe giant and non-giant motor pathways that appear to contr ol different pools of circular muscle fibers in squid A subset of circ ular muscle fibers possesses large Na currents, and these fibers are p roposed to employ Na-dependent action potentials to produce fast, all- or-none muscle twitches associated with giant axon stimulation.