Effect of pH and calcium of the surrounding medium on electrical activity of the isolated muscle spindle

The effect of pH and the calcium ions of the surrounding medium on the receptor potential and unit activity of the isolated frog muscle spindle was investigated. With a decrease in pH of the medium the amplitude and duration of the hyperpolarization phase of the receptor potential increased and the amplitude of its depolarization phase decreased. The frequencies of the spontaneous and evoked activity were reversibly reduced under these conditions. A change in pH of the surrounding medium toward the alkaline side led to a decrease in the amplitude and duration of the hyperpolarization phase of the receptor potential. The firing rate initially increased and then decreased to 30–50% of normal. An increase in the calcium concentration in an alkaline medium led to an increase, but in an acid and normal medium to a decrease, in firing rate. The observed changes in electrical activity of the muscle spindle in solutions can be explained by changes in the velocity of active (electrogenic) and passive transport of sodium and calcium ions.