Final answer:
Secondary endings of nerve fibers on intrafusal muscle fibers are known as gamma motor neurons, which maintain the sensitivity of muscle spindles during muscle contractions.
Step-by-step explanation:
The secondary endings of nerve fibers on intrafusal muscle fibers are referred to as gamma motor neurons. These neurons play a role in adjusting the sensitivity of the muscle spindle, a sensory receptor within the muscle that detects changes in muscle length. The gamma motor neurons do this by causing the intrafusal muscle fibers to contract, thereby keeping the muscle spindle taut and sensitive to stretch even when the muscle itself is contracted. This is in contrast to alpha motor neurons, which innervate the extrafusal muscle fibers and are responsible for the contraction of the main body of the muscle. The question refers to anatomical components of the neuromuscular system, which dictate how the body interprets sensory information and controls muscle movement.