There and back again: 50 years of wandering through terra incognita fusorum.

This paper is in two parts: 'There', which is a review of some of the major advances in the study of spindle structure and function during the past 50 years, serving as an introduction to the symposium entitled 'Mechanotransduction, Muscle Spindles and Proprioception' held in Munich in July 2022; and 'And Back Again', presenting new quantitative morphological results on the equatorial nuclei of intrafusal muscle fibres and of the primary sensory ending in relationship to passive stretch of the spindle. What is the topic of this paper?This paper concerns the relationship between structure and function of the mammalian muscle spindle, its intrafusal muscle fibres and their sensory and motor innervation.What advances does it highlight?The paper summarizes the development of knowledge of the number of types of intrafusal muscle fibres and the pattern and functional significance of their fusimotor innervation. New observations are presented on the quantitative functional morphology of equatorial nuclei and primary sensory endings. [ABSTRACT FROM AUTHOR]