Muscle spindle is a fusiform sensory receptor within skeletal muscle, made up of intrafusal fibers, sensitive to change in muscle length and the velocity of lengthening.
About 100 µm in diameter, 5 to 10 mm long, in a connective tissue capsule, with 2 to 12 intrafusal fibers.
Structure and Intrafusal Fibers
Two intrafusal fiber types: nuclear bag fibers (usually 2, longer, central bag with nuclei) and nuclear chain fibers (usually 4 or more, thinner, shorter).
The central noncontractile part of the spindle carries the sensory receptors; the peripheral parts are contractile, so stretch of the central part activates the spindle.
Nuclear chain fiber nuclei are arranged in a row, unlike the clustered central "bag" of nuclei in bag fibers.