Muscle spindles: stretch receptors (sensor) in skeletal muscles. Normal muscle cells extrafusal muscle fibers (regular muscle fibers) innervated by alpha motor neurons. Muscle spindle cells intrafusal fibers innervated by motor neurons. Muscle spindles monitor muscle length and prevent overstretching. When muscles stretch and lengthen, muscle spindle sensory afferent neurons fire more. The reflex response is mucle contraction to prevent damage from over stretching: muscle stretch. Increased afferent signals to spinal cord: spinal cord. Increased efferent output through alpha motor neurons: muscle contracts. More frequent action potentials=more of a stretch of the muscle. Alpha (extrafusal) and gamma (intrafusal) motor neurons fire together (normally happens) As the muscle shortens, so does the muscle spindles this keeps the muscle spindle ready or recalibrated to respond to a stretch. Alpha-gamma coactivation maintains spindle function when muscle contracts and shortens! This keeps the muscle spindle ready or recalibrated to respond to a stretch. Gamma motor neurons cause intrafusal fibers (of spindle organ) to contrac.