The most widely accepted theory of the restirng potential of muscle is that the electrical potential difference between the inside and outside of a muscle fibre arises from the concentration gradients of the potassium and chloride ions. If we follow Boyle & Conway (1941), the membrane is assumed to be permeable to K and Cl but to be impermeable or sparingly permeable to other ions. Since K is more concentrated inside and Cl is more concentrated outside, the interior of the fibre should be electrically negative to the external solution. If K and Cl are distributed passively, the concentration ratios and the membrane potential under equilibrium conditions ought to conform to the relationwhere [ ]O and [ ]i indicate concentrations outside and inside the fibre, V is the internal potential, F is Faraday's constant, R is the gas constant and T is the absolute temperature. When the external potassium concentration is less than 10 mm agreement with equation (1) is not perfect, but at higher concentrations the relation seems to hold with considerable accuracy (Adrian, 1956;Conway, 1957). However, it is only possible to calculate the membrane potential by equation (1) when the concentrations of K and Cl inside the fibre have come into equilibrium with those in the external solution. In order to deal with other situations one must know the relative conductances or permeabilities of the two ions. Information on this point can be obtained by measuring the effect on membrane potential of sudden changes in the concentrations of K or Cl. Such experiments are difficult to interpret unless the effect of changing the external solution can be determined in a time which is so short that there is no alteration in the internal concentration of Cl or K. The present observations were carried out with single muscle fibres, with an

It has been known for a long time that muscles can be made to contract by raising the potassium concentration in the external medium. In skeletal muscles these contractions are difficult to study, because potassium ions take some time to diffuse through the interspaces and the contractions which they set up are usually transient. The difficulties of interpretation become les acute if single fibres are employed; in that case, as was first shown by Kuffler (1946), a rise in external potassium concentration causes a rapid depolarization and a prompt contraction. The quantitative effects of sudden changes of potassium concentration on membrane potential have been described previously (Hodgkin & Horowicz, 1959, 1960; the present article deals with the effect of similar changes on the tension developed by single muscle fibres. For relevant work on whole muscles the excellent review of Sandow (1955) should be consulted. METHODSAll the experiments were carried out with single fibres from the semitendinosus muscle of Rana temporaria. The apparatus and method of recording have been described in previous articles (Hodgkin & Horowicz, 1959, 1960. The only additional item of equipment was a device for changing solutions rapidly. In the method described previously, solutions were allowed to run into the cell from reservoirs at a height of about 50 cm above the cell; the flow obtained with this method was 1-3 ml./sec and the mean velocity in the channel containing the fibre was 10-30 cm/sec. In some of the present experiments (Fibres D, E, F in Table 1 and Figs. 1 and 2) the reservoirs were replaced by large syringes (internal diameter 2.1 cm) whose plungers were driven by pistons operated by compressed air between stops spaced 0*7 cm apart. This forced 2 ml. of solution through the cell in about 0-3 sec. Tests with dyes showed that the solution in the cell (volume about 0-3 ml.) was fully changed in one flush. The rate of flow (about 60 cm/sec) produced by the piston-operated syringes was too high for the experiments in which the membrane potential was recorded and most of the results described here were obtained with the original method.Solutions were made up on the same lines as those described previously (Hodgkin & Horowicz, 1959,

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In an earlier paper on single muscle fibres (Hodgkin & Horowicz, 1959), we showed that the displacements of membrane potential produced by changes in the external ionic concentration were quantitatively consistent with the idea that the membrane potential is determined by the concentration ratios of the potassium and chloride ions. Without making further measurements it might be supposed that the rate of change of membrane potential in response to a sudden alteration of [K]o or [Cl]o would also be of a relatively simple kind. If ions in the extemal solution could reach the membrane without appreciable delay, the rate of change of membrane potential should be determined by the membrane time constant, which is normally about 30 msec (Fatt & Katz, 1951). If there were a straightforward diffusion delay it should be possible to measure the delay in one record and to predict the shape of all other records. The results described here show that the actual situation is more complicated, the main findings being that [Cl]o affects the membrane potential more rapidly than [K]o and that the depolarization associated with a rise of [K]0 is quicker than the repolarization associated with a fall. A partial explanation is that the sites which are sensitive to potassium ions are less accessible than those which are sensitive to chloride and that appreciable quantities of potassium can be retained in a special region of the fibre. This suggestion accounts for a number of puzzling observations, but something further is needed to explain why the effect of a rise in [K]0 is so much more rapid than that of a fall.In the present paper the words rapid and slow have a different meaning from those used in our previous article (Hodgkin & Horowicz, 1959). In describing the earlier results, any change taking place in less than a minute was regarded as 'rapid' and the word 'slow' was reserved for the drifts in membrane potential, lasting minutes or hours, that are associated

The aim of the experiments described here was to investigate the movements of labelled sodium and potassium ions through the surface membrane of single muscle fibres, and to determine the effect of activity on these movements. The advantages of using a single fibre are that the surface area can be measured fairly accurately and that radioactive ions have rapid access to the surface. The second point is important because frog muscle fatigues in a few minutes if stimulated at more than about 3 c/s. Activity can be maintained for long periods at low frequencies but the changes in ionic flux are then too small to be measured accurately. For this reason it is desirable to collect or apply tracer over well-defined periods lasting only a few minutes. This condition is difficult to satisfy with whole muscle. Other disadvantages of whole muscle are the variation in fibre diameter within the muscle and the difficulty of distinguishing between extracellular and intracellular sodium ions. Complications may also arise from the presence of muscle spindles, slow fibres and blood vessels. The disadvantages of the single fibre are the low counting rate associated with the small quantity of tracer inside the fibre and the difficulty of isolating fibres which will survive for long periods of time. However, since neither difficulty is insuperable it seemed desirable to attempt a quantitative analysis of ionic movements along lines similar to those followed in investigating giant nerve fibres (Keynes, 1951). The results agree with the observations of Fenn & Cobb (1936) on mammalian muscle in showing that activity is associated with an entry of sodium ions and a somewhat smaller loss of potassium ions. They also show that labelled K+ and Na+ are lost exponentially from single muscle fibres and that the movements of these ions are of the kind expected in a system in which exchange is limited by a single barrier such as the surface membrane. METHODS MaterialSingle fibres were isolated from the semitendinosus muscle of Rana temporaria, by methods similar to those used by Ramsey & Street (1940), Sten-Knudsen (1953) A. L. HODGKIN AND P. HOROWICZ After a fibre had been isolated it was tested for excitability and then left for about an hour in Ringer's fluid. At the end of this time it was carefully examined under a microscope; if there was any sign of damage, the fibre was rejected, even though still excitable. Tests of excitability were made at intervals throughout the experiments, some of which lasted 16 hr. In the majority of cases fibres were still giving all-or-nothing twitches at the end of the experiment. Fibres were never taken through an interface and were transferred from the dissecting dish to the tracer apparatus with a small glass spoon. When the fibres were mounted in the influx or efflux apparatus they were stretched to 4/3 of their slack length; measurements on other fibres showed that this stretch corresponded to a sarcomere length of about 3 t.An important factor in obtaining good fibres is the condition of the frogs; th...