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In 1951, during the period from February to May, Huxley and I made a series of observations, using glass micro-electrodes (Ling and Gerard, 1949 Nastuk and Hogkin, 1950), on the action potentials recorded from inside the frog motor nerve fiber. A large motor fiber was isolated from a sciatic-gastrocnemius preparation, and the operated region of the preparation, immersed in Ringer's fluid, was brought under a polarizing microscope with an objective lens of water-immersion type. A micro-electrode of below 0.5 micron in diameter, held by means of a micro-manipulator, was pushed into the nerve fiber, and the potential difference between the intra-cellular electrode and the outside fluid medium was recorded with a cathode-ray oscillograph used in conjunction with a DC amplifier. Action potentials were elicited from the fiber by induction shocks applied to the proximal nerve trunk. Muscular contractions were taken as an index of the propagation of impulses across the impaled region. The main features of the results obtained are as follows: 1. When the tip of the micro-electrode was just under the layer of myelin or when a dimpling of myelin into the axis-cylinder was seen at the tip of the micro-electrode, there was practically no measurable resting potential . But a propagated nerve impulse could very often evoke sizable monophasic action potentials (generally not exceeding 30 mV in peak value) . At this stage, propagation of impulses across the impaled region was not impaired. 2. When the tip of the micro-electrode was introduced deep inside the axis-cylinder, a resting potential of 10 to 40 mV was observed. The action potentials observed at this stage were large at the beginning (20 to 40 mV), but soon a notch appeared at the peak of the action potential . As time elapsed, this notch became more pronounced, and generally within 2 to 5 minutes after introduction of the micro-electrode, a stage was reached at which the part of action potential following the notch suddenly disappeared . Propagation across the impaled region was blocked at this moment . The first part of the present investigation is an extension of the abovestated experiments done in England. In order to correlate the action potentials observed with an internal electrode with the action currents recorded with the

PROPERTIES OF MYELINATED FIBERS IN FROG SCIATIC NERVE AND IN SPINAL CORD AS EXAMINED WITH MICRO-ELECTRODES