Characteristics of pigeon gastrocnemius and its muscle spindle supply

The occurrence of spindles was compared in the three heads of the pigeon gastrocnemius, each having distinctive histochemical and contractile characteristics. Spindle densities in pars interna and pars externa were not significantly different from each other, whether expressed per cubic millimeter muscle volume (0.098 receptors and 0.113 receptors) or per alpha motoneuron (0.58 receptors and 0.69 receptors), even though times-to-peak tension (41.6 vs. 31.2 ms) and fatigue indices (59 to 91% vs. 33 to 78%) differed significantly (P <- 0.01), and oxidative indices were dissimilar. The partes interna and media had similar times-to-peak tension (41.6 and 39.8 ms) and oxidative indices (100 and 97%). Their spindle densities were significantly different from each other when based on muscle volume (0.098 vs. 0.213; P <- 0.01), but not if related to alpha motoneurons (0.58 and 0.64; P <- 0.01). These findings indicate that within a muscle complex characteristics of oxidative index or contractile properties may bear no relation to the incidence of muscle spindles. Should muscle tissues of differing histochemical compositions require different sensory inputs to their related motoneuronal pools, this must be accomplished by other means than variations in the number of spindles.

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