Further studies of rapid mechanical changes in squid giant axon associated with action potential production

Mechanical changes in the squid giant axon associated with the production of an action potential are examined further by using piezoelectric and optical methods. The peak of swelling of the axon coincides with the peak of the action potential recorded internally at the site of mechanical recording. Mechanical changes produced by a train of action potentials do not summate. Repetitively fired action potentials induced by lowering the external Ca-ion concentration are preceded by a gradual swelling of the axon. An inward current through the membrane causes shrinkage and an outward current produced swelling of the axon. An inward current enhances and an outward current depresses the mechanical changes associated with the action potential. There is a transient shortening followed by an elongation of the axon when an action potential travels along the axon. It is argued that the experimental results obtained are consistent with the colloid chemical, or macromolecular, theory of excitation.