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FUNCTIONAL ANATOMY education Muscle Spindles By Christy Cael In past editions of Functional Anatomy, we have primarily focused on muscles and muscle groups. Several other structures are addressed and affected by bodywork, and I want to highlight some of these in the next several columns, beginning with the muscle spindle. PROPRIOCEPTION Proprioception is an overall awareness of body position. It serves as an important safety mechanism and adaptive strategy for muscles and joints. This aspect of positional awareness is independent of vision and critical in preventing injury and creating efficient movement. The nervous system communicates with muscles, tendons, and joints through different proprioceptors to sense and alter body position. MUSCLE SPINDLE ANATOMY Muscle spindles are proprioceptors distributed throughout skeletal muscle tissue that monitor changes in tissue length. A muscle spindle includes specialized skeletal muscle fibers called intrafusal fibers surrounded by a coil of sensory nerve endings. The sensory nerves, or afferent fibers, monitor the rate and magnitude of stretch within the muscle. If a stretch is strong or fast enough to potentially damage tissue, the alpha efferent fibers prompt the surrounding extrafusal fibers to contract and shorten the muscle, thus protecting it from harmful overstretching. This response is called the myotatic reflex. If you have ever had a physician test your reflexes, you have Anatomy of a muscle spindle.