A muscle spindle is a sensory receptor located within skeletal muscles. It detects changes in muscle length and the rate of that change, helping the body maintain balance and posture. Muscle spindles are essential for the proprioceptive system, which provides information about body position and movement. These spindles consist of specialized muscle fibers called intrafusal fibers, surrounded by a capsule. They are innervated by sensory neurons that send signals to the central nervous system when the muscle is stretched. This feedback allows for coordinated movements and adjustments, playing a crucial role in activities like walking and running.