Primary and secondary sensory nerve fibers spiral around and terminate on the central portions of the intrafusal muscle fibers, providing the sensory component of the structure via stretch-sensitive ion-channels of the axons. In mammals including humans, the motor component is provided by up to a dozen gamma motoneurons and to a lesser extent by one or two beta motoneurons. Gamma and beta motoneurons are called fusimotor neurons, because they activate the intrafusal muscle fibers.