Effect of ouabain on electrical activity of the isolated frog muscle spindle

Changes in electrical activity of the isolated frog muscle spindle were studied in Ringer's solution containing ouabain. The presence of ouabain in the solution increased the spontaneous firing rate of the receptors up to a maximum and then reduced it quickly to zero. The amplitude of the action potentials was reduced on the average to 40% of normal. Ouabain causes initial disappearance of the hyperpolarization phase of the receptor potential and a subsequent decrease in amplitude of its dynamic phase to zero. The decrease in amplitude of the receptor potential and action potential and also the changes in firing rate in the solution with ouabain depend on the frequency of their spontaneous activity. The changes observed can be explained by depolarization of the membrane of the nerve endings and the first node of Ranvier, developing as a result of blocking of the sodium pump by ouabain.Translated from Neirofiziologiya, Vol. 5, No. 6, pp. 576–582, November–December, 1973.     

Effect of hypertonic solutions on electrical activity of the isolated frog muscle spindle

Changes in electrical activity of the isolated frog muscle spindle were investigated in hypertonic solutions obtained by adding 400 mM sucrose, glucose, or glycerol to Ringer's solution. The spontaneous firing rate in hypertonic sucrose and glucose solutions increased at first (for 3–5 min) and then fell rapidly to zero; the receptor potential and evoked spike activity diminished under these conditions and disappeared. In the hypertonic solution with glycerol a similar effect was observed but, unlike in the first two media, in this case spike activity returned after its initial increase to the normal level; a second rise in the firing rate was then observed up to a steady value which was higher than normal. After rinsing out the hypertonic sucrose and glucose solutions with ordinary Ringer's solution the spontaneous and evoked activity gradually returned to normal with a small overshoot. During the rinsing out of the hypertonic glycerol solution a sharp and considerable rise in spontaneous activity was first observed, while the changes in frequency of the evoked activity were negligible. The spike activity then returned to normal. The observed changes in electrical activity of the muscle spindle in hypertonic media are attributed to deformation of the sensory terminals and intrafusal muscle fibers (in the glycerol medium), leading to depolarization of the receptor membrane.P. K. Anokhin Institute of Normal Physiology, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 8, No. 3, pp. 291–299, May–June, 1976.     

Effect of tetraethylammonium ions on electrical activity of the isolated frog muscle spindle

Changes in electrical activity of the isolated frog muscle spindle in Ringer's solution containing tetraethylammonium (TEA) ions were studied. An increase in the frequency of spontaneour activity was observed, but with continued perfusion with TEA solution both spontaneous afferent impulses and action potentials generated during stretching of the muscle receptor were blocked. The dynamic component of the depolarization phase of the receptor potential was reduced in amplitude and increased in duration. Rinsing the receptor in normal physiological saline did not restore its responses completely.Institute of Physiology, Leningrad State University. Translated from Neirofiziologiya, Vol. 4, No. 2, pp. 208–215, March–April, 1972.     

Effect of a polarizing current on receptor potential of the isolated frog muscle spindle

The effect of a polarizing current on electrical activity of the isolated frog muscle spindle was studied. A depolarizing current increased the frequency and reduced the amplitude of afferent spike activity, both spontaneous and evoked by mechanical stimulation. A hyperpolarizing current produced the opposite effect. The amplitude of the receptor potential in response to a mechanical stimulus varied as a linear function of the intensity of the polarizing currentA. A. Ukhtomskii Physiological Institute, Leningrad State University. Translated from Neirofiziologiya, Vol. 5, No. 1, pp. 95–101, January–February, 1973.     

Effect of adequate stimulation on the discharge pattern of an isolated frog muscle spindle

Discharges from an isolated frog muscle spindle during mechanical stimulation of varied amplitude, velocity, and shape were investigated. The firing rate during a linear increase in strength of the stimulus is determined by its amplitude, whereas the change in firing rate is determined by the rate of increase of amplitude. With sinusoidal stimulation the firing rate apparently reproduces stimulus shape, i.e., the muscle spindle is sensitive not only to amplitude and velocity, but also to acceleration of the stimulus. Sensitivity to acceleration is most probably due to the change in threshold of appearance of action potentials observed during variation of the speed of stretching.P. K. Anokhin Institute of Normal Physiology, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 8, No. 4, pp. 426–433, July–August, 1976.     

Role of active ion transport in reception of the generator potential of the isolated frog muscle spindle

The effect of complete replacement of sodium ions by lithium ions in Ringer's solution and of 10⁻⁴ M ouabain on the receptor potential of the isolated frog muscle spindle was investigated. Initially, under the influence of lithium ions and ouabain, the hyperpolarization phase of this potential diminished and disappeared, and later the same fate befell the static and dynamic part of its depolarization phase. The rise time of the receptor potential was increased in a solution containing lithium ions but in the solution with ouabain it remained the same as initially. No appreciable changes were found in the rate of fall of the dynamic part of the depolarization phase. On rinsing the muscle spindle in normal Ringer's solution in the experiment with lithium ions recovery was incomplete, and in the experiments with ouabain the receptor responses were not restored.     

Effects of acetylcholine and succinylcholine on isolated frog neuromuscular spindle]

The mode of action of acetylcholine (ACh) and succinylcholine (SCh) on the isolated frog's muscle spindle has been studied. Receptor afferent nervous supply was maintained; the appropriate spinal roots were dissected for stimulating motor axons and recording from sensory fibres. Excitatory effects on the afferent activity, when the receptor was held still and during stretching, were found with ACh or SCh concentrations of 10(-8) to 10(-3); 10(-6) g/ml being usually effective. These effects are similar to those obtained by stimulating fusimotor nerve fibres. The contractile activity of intrafusal muscle fibres which occurred during these effects was observed. Seldom, and only for high concentrations of ACh and SCh, a decrease in afferent activity following the excitatory effects was found. Tubocurarine chloride (10(-5)-10(04) g/ml) in the bath prevented both motor fibres and drugs effects. Sometimes slight transient excitation occurred at very high concentrations of the two tested substances; however, this effect was prevented by stronger curarization. The observed blocking effects were always reversed by removing tubocurarine from the bath. No more excitatory effects by motor fibres stimulation and by ACh and SCh action could be found after destruction of intrafusal muscle fibres, by pinching them as close as possible to the ends of the spindle. It is suggested that ACh and SCh act indirectly by causing mechanical changes in intrafusal muscle fibres, and that a direct action on sensory nerve endings, if any, cannot, by itself, increase the afferent activity of the receptor.