muscle spindle
/ˈmʌsl ˌspɪndl/Anatomy and Physiology
noun
A fusiform bundle of specialized muscle fibres with sensory and motor innervation enclosed in a capsule of connective tissue, present in large numbers in skeletal muscle and sensitive especially to stretch.
Origin
Late 19th century; earliest use found in Journal of Physiology. From muscle + spindle, after German Muskelspindel.