Ionic basis of the receptor potential in primary endings of mammalian muscle spindles.

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Abstract
The effect of changing the ionic composition of bathing fluid on the receptor potential of primary endings was examined in isolated cat spindles whose capsule was removed in the sensory region. After impulse activity was blocked by tetrodotoxin, ramp-and-hold stretch evoked a characteristic pattern of potential change consisting of a greater dynamic depolarization during the ramp phase and a smaller static depolarization during the hold phase. After a high-velocity ramp there was a transient post-dynamic undershoot to below the static level. On release from hold stretch, the potential showed a postrelease undershoot relative to base line. The depolarization produced by stretch was rapidly decreased by the removal of Na+ and Ca2+. Addition of normal Ca2+ partly restored the response. Stretch appeared to increase the conductance to Na+ and Ca2+ in the sensory terminals. The postdynamic understood was diminished by raising external K+ and blocked by tetraethylammonium (TEA). It apparently resulted from a voltage-dependent K conductance. The postrelease undershoot was decreased by raising external K+, but was not blocked by TEA. It may be caused by a relative increase in K conductance on release. Substitution of isethionate for Cl- or the addition of ouabain did not alter the postdynamic and postrelease undershoots.