Printed in Great Britain AXON/SCHWANN7CELL RELATIONSHIPS IN THE GIANT NERVE FIBRE OF THE SQUID

This communication summarizes the experimental evidence obtained in the giant nerve fibre of the tropical squid Sepioteuthis sepioidea, on the nature of the mechanism responsible for the long-lasting effects of axonal excitation on the membrane potential of the Schwann cell. In these nerve fibres the propagation of a train of nerve impulses by the axon is followed by a prolonged hyperpolarization of the Schwann cell which can be reproduced, or modified,.by the external application of cholinergic compounds. The pre-sence and exact localization of the different components of the acetylcholine system directly involved in such Schwann-cell responses was detected by means of pharmacological, histochemical and chemical procedures. Thus, the results of the experiments herein discussed revealed that, under physio-logical conditions: the Schwann cell is able to synthesize, store and release acetylcholine; and that it has acetylcholine receptors of the nicotinic type, and acetylcholinesterase enzyme activity in its plasma membrane. On the other hand, the axon has low acetyltransferase activity and acetylcholine con-centration in its axoplasm, and a high acetylcholinesterase activity in its axolemma. It was also found that acetylcholine hyperpolarizes the Schwann cell by increasing its relative permeability to the potassium ion. The distri-bution pattern of the acetylcholine system indicates that it operates as a feedback mechanism for the regulation of the Schwann-cell membrane potential and ionic permeability following axonal excitation. Since their description by Young (1936) the giant nerve fibres of the squid have been extensively used for exploring fundamental mechanisms of nerve function. The giant fibres and synapses constitute a fast conducting system that synchronously drives the muscle fibres of the mantle during the fast swimming behaviour of the squid. The ultrastructural studies of these nerve fibres, carried out by Villegas & Villega