There is evidence that Na+ ions carry an important part of the charge through the active membrane of the sensory endings of the Pacinian corpuscle of the cat, since Diamond, Gray & Inman (1958) have shown that perfusion of the corpuscle with saline solutions in which choline or sucrose have been substituted for Na+ results in a reduction of the receptor potential to 10 % of its initial value. However, soaking the Pacinian corpuscle in Na+-free solutions does not affect the receptor potential, possibly because the movements of Na+ ions through the Pacinian capsule are slow (Gray & Sato, 1953, 1955. Similarly, in the muscle spindle of the frog the receptor potential persists even after prolonged soaking of the muscle in Na+-free Ringer's solution (Katz, 1950). This apparent lack of sensitivity of the muscle spindle nerve endings to Na+ deprivation has suggested experiments to determine whether, and if so how, ionic changes affect the behaviour of the spindle. The results show that a major part of the charge is carried through the spindle nerve endings membrane by Na+ ions, and that Ca2+ and Mg2+ ions also have some effect on the receptor potential of the spindle.While the manuscript of this paper was being prepared, Ottoson's work (1964) was published; it deals with some of the problems of these experiments and there is agreement in the results obtained.
METHODSThe spindles of the ext. long. dig. IV of Rana temporaria were used in these experiments. The muscle was set up in a bath with the proximal end fixed and the distal end tied to an electro-magnetic puller energized by a 20-50 msec pulse synchronized with the time base of the oscilloscope. The solutions in the bath could be withdrawn and replaced with others by the use of two syringes and a two-way tap on the bath outlet. Recording electrodes were of Ag/AgCl; one was placed on the nerve as near as possible to the muscle, and the other on the proximal end of the muscle or in the solution in the bath. A condenser-coupled amplifier was used (RC = 5 sec). In Na+-deficient solutions the normal osmotic relations were maintained by adding equimolecular concentrations of choline; the sodium phosphate buffer system was not used, but the pH of the solutions was adjusted to that of normal Ringer's solution by addition of small and physiologically insignificant amounts of K2HPO4.