ABSTRACT
The muscle spindle is the complex sensory structure of the muscle that conveys information about muscle length. Spindles lie in parallel with extrafusal fibers (large muscle fibers that effect gross movement), so when the muscle is length ened, the spindle is stretched, thereby causing la afferent fibers to send impulses to the spinal cord. Shortening of the extrafusal muscle results in shortening of the spindle and silencing of the la afferents. The intrafusal muscle fibers of the spindle complex are innervated by y-motor neurons in the anterior horns of the spinal cord, which act to increase the tension of the intrafusal fiber when it is shortened. This resets the spindle after shortening so it is again sensitive to changes in muscle length.
