Cellular organization of the frog muscle spindle as revealed by serial sections for electron microscopy

Abstract

By means of extensive group serial sectioning it was possible to obtain a detailed and topographical picture of the frog muscle spindle through motor and sensory regions. The distribution of different cell types was analyzed. On the basis of the topographical morphology of the intrafusal muscle fiber and its innervation, a descriptive nomenclature of parts of the spindle was adopted. The spindle envelope is divided into outer and inner capsules. The outer capsule encapsulated groups of intrafusal muscle fibers at their sensory regions. Inner capsule cells were found in the sensory zone, dominating and surrounding each intrafusal muscle fiber in the reticular zone. Extracellular filamentous material was found to follow the distribution of inner capsule cells. The intrafusals were found to alter their overall diameter within a wide range outside of the reticular zone, but only slightly within that zone. In this zone the diameter of all muscle fibers in a specific bundle was about equal. In the sensory zone nucleated satellite cells, e.g., intrafusal satellite cells, were found in intimate contact with the muscle fibers. They commonly occurred in the sensory compact zones but were sparse in reticular or motor compact zones. Each intrafusal muscle fiber was surrounded by sensory chains of nerve bulbs that were joined together with thin links. A general bimodal distribution of these sensory structures was found with maxima at the borders between the reticular and the sensory compact zones. Motor regions were observed situated inside the capsular envelope of the muscle spindle. The results with respect to the cellular topography of the frog muscle spindle are discussed.