Excitatory and Inhibitory Pathways in the Arm of Octopus

1. The nervous system of isolated arms of Octopus vulgaris has been investigated with fine stimulating electrodes and lesions. The reflexes of the intact arm are in accordance with earlier reports. 2. The reflexes are abolished if the nerves from the medulla are cut. These nerves carry both sensory and motor signals, serving discrete areas of muscle and skin. The central mechanisms producing single-sucker reflexes are confined to the ventral ‘axial ganglia’. 3. Stimulation in the ventral medulla influences many suckers simultaneously or progressively, implying both through-pathways and polysynaptic pathways. 4. The nervous supply to the main longitudinal and oblique musculature remains obscure. Motor effects are produced by stimulating the median and central medulla, and require intact peripheral supply. The lateral nerve cords do not affect the main musculature. 5. The dermal musculature is affected by stimulation in the medulla and of its peripheral nerves. Only the subdermal muscle layer is excited by stimulation of the lateral nerve cords. 6. Normal expansion of the chromatophores depends on an intact nervous supply from the medulla, and is not due to autotonus of the chromatophore muscles. The final chromatophore nerves are probably medullar, not cerebral, and each controls only a few patches of skin over short lengths of the arm. 7. Through-pathways, affecting many or all of these chromatophore nerves, run in the dorsal medulla. They include local and general excitatory, general inhibitory and possibly local inhibitory channels. Their significance, and that of excitatory and inhibitory supplies to the sucker reflex mechanisms, is discussed.