The muscle spindle has intrigued physiologists since it was first described anatomically more than 100 years ago. Its in-parallel response during twitch contractions of muscle was defined in 1933 by B. H. C. Matthews. The fascination has been heightened by the fact that the muscle spindle is a sensory organ that receives a motor innervation, an unusual but not unique property. The attention paid to the muscle spindle in the control of movement has often been at the expense of the Golgi tendon organ (cf. Chapter 6), intramuscular free nerve endings and, in particular, cutaneous mechanoreceptors (cf. Chapter 9), all of which play important roles in modulating motor output and (probably) in generating appropriate movement or contraction-related sensations. The attention paid to the muscle spindle/fusimotor system may be disproportionate for its role in the control of normal and pathological movement. However, despite this attention, its role is still the subject of debate, possibly because no unitary hypothesis satisfactorily explains the findings in all animal species. Human muscle spindles are innervated, as in the cat, by γ and β motoneurones, the former exclusively motor to the ‘intrafusal’ muscle fibres within the muscle spindle, the latter motor to both the contractile (‘extrafusal’) muscle as well as the spindle. The term ‘fusimotor’ (i.e. motor to the fusiform structure) was coined by Hunt and Paintal (1958) to refer to the γ efferent innervation of the muscle spindle, and ‘skeleto-fusimotor’ by Emonet-Dénand, Jami & Laporte (1975) for β innervation.