The motor system makes use of this phenomenon in the following way: When commands from the brain are issued to move a limb, both the alpha motor neurons and the gamma motor neurons are activated. The alpha motor neurons start the muscle contracting. If there is little resistance, both the extrafusal and intrafusal muscle fibers will contract at approximately the same rate, and little activity will be seen from the afferent axons of the muscle spindle. However, if the limb meets with resistance, the intrafusal muscle fibers will shorten more than the extrafusal muscle fibers, and hence sensory axons will begin to fire and cause the monosynaptic stretch reflex to strengthen the contraction. Thus, the brain makes use of the gamma motor system in moving the limbs. By establishing a rate of firing in the gamma motor system, the brain controls the length of the muscle spindles and, indirectly, the length of the entire muscle