Neuromuscular system of the flexible arm of the octopus: Physiological characterization

The octopus arm is an outstanding example of an efficient boneless and high ly flexible appendage. We have begun characterizing the neuromuscular syste m of the octopus arm in both innervated muscle preparations and dissociated muscle cells. Functionally antagonistic longitudinal and transverse muscle fibers showed no differences in membrane properties and mode of innervatio n. The muscle cells are excitable but have a broad range of linear membrane properties. They are electrotonically very compact so that localized synap tic inputs can control rbe membrane potential of the entire muscle cell. Th ree distinct excitatory neuronal inputs to each arm muscle cell were identi fied; their reversal potentials were extrapolated to be about -10 mV. These appear to be cholinergic as they are blocked by hexamethonium, D-tubocurar ine, and atropine. Two inputs have low quantal amplitude (1-7 mV:) and slow rise times (4-15 ms), whereas the third has a large size (5-25 mV) and fas t rise time (2-4 ms). This large synaptic input is most likely due to excep tionally large quantal events. The probability of release is rather low, su ggesting a stochastic activation of muscle cells. All inputs demonstrated a modest activity-dependent plasticity typical of fast neuromuscular systems . The pre- and postsynaptic properties suggest a rather direct relation bet ween neuronal activity and muscle action. The lack of significant electrica l coupling between muscle fibers and the indications for the small size of the motor units suggest that the neuromuscular system of the octopus arm ha s evolved to ensure a high level of precise localization in the neural cont rol of arm function.