Muscle spindle
（重定向自Neuromuscular spindles）
![Mammalian muscle spindle showing typical position in a muscle (left), neuronal connections in spinal cord (middle) and expanded schematic (right). The spindle is a stretch receptor with its own motor supply consisting of several intrafusal muscle fibres. The sensory endings of a primary (group Ia) afferent and a secondary (group II) afferent coil around the non-contractile central portions of the intrafusal fibres. Gamma motoneurons activate the intrafusal muscle fibres, changing the resting firing rate and stretch-sensitivity of the afferents. Animated version: Arthur Prochazka's Lab, University of Alberta](/Images/godic/202501/29/Muscle_spindle_model0950.jpg")
![Light microscope photograph of a muscle spindle. HE stain.](/Images/godic/202501/29/Muscle_Spindle_LM_HE_stain0950.jpg")
Muscle spindles are sensory receptors within the belly of a muscle that primarily detect changes in the length of this muscle. They convey length information to the central nervous system via sensory neurons. This information can be processed by the brain to determine the position of body parts. The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles, by activating motor neurons via the stretch reflex to resist muscle stretch.