¹ From the Department of Neurology, College of Physicians and Surgeons, Columbia University, New York, and the Marine Biological Laboratory, Woods Hole. Dr. Cerf is a "Chercheur qualifié" of the Belgian National Foundation for Scientific Research. Fulbright Research Scholar, United States Educational Foundation in Belgium.
² From the Department of Neurology, College of Physicians and Surgeons, Columbia University, New York, and the Marine Biological Laboratory, Woods Hole.
³ From the Department of Neurology, College of Physicians and Surgeons, Columbia University, New York, and the Marine Biological Laboratory, Woods Hole. Dr. Hoyle was a Fellow of The Rockefeller Foundation.

Present address: Départment d'Electrophysiologie, Faculté des Sciences, University of Brussels. Present address: Department of Zoology, University of Glasgow, Scotland. Dr. McCann's present address is Department of Physiology, Dartmouth Medical School, Hanover.

Dually innervated Romalea muscle fibers which respond differently to stimulation of their fast and slow axons are excited by intracellularly applied depolarizing stimuli. The responses, though spike-like in appearance, are graded in amplitude depending upon the strength of the stimuli and do not exceed about 30 mv. in height. In other respects, however, these graded responses possess properties that are characteristic of electrically excitable activity: vanishingly brief latency; refractoriness; a post-spike undershoot. They are blocked by hyperpolarizing the fiber membrane; respond repetitively to prolonged depolarization, and are subject to depolarizing inactivation. As graded activity, these responses propagate decrementally.

The fast and slow axons of the dually responsive muscle fibers initiate respectively large and small postsynaptic potentials (p.s.p.'s) in the muscle fiber. These responses possess properties that characterize electrically inexcitable depolarizing activity. They are augmented by hyperpolarization and diminished by depolarization. Their latency is independent of the membrane potential. They have no refractory period, thus being capable of summation. The fast p.s.p. evokes a considerable or maximal electrically excitable response. The combination, which resembles a spike, leads to a twitch-like contraction of the muscle fiber. The individual slow p.s.p.'s elicit no or only little electrically excitable responses, and they evoke slower smaller contractile responses. The functional aspects of dual responsiveness and the several aspects of the theoretical importance of the gradedly responsive, electrically excitable component are discussed.

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