Final answer:
When a motor unit is stimulated, every muscle fiber within that unit contracts fully due to the all-or-none principle. This is essential for muscle function and allows for a graded muscle response through the recruitment of more motor units for increased tension.
Step-by-step explanation:
When a motor unit is stimulated, every muscle fiber it innervates fully contracts due to the all-or-none principle. This means that once the motor neuron's action potential is initiated, all muscle fibers within that particular motor unit will contract completely or not contract at all. It is an important aspect of the nervous system's control over muscle functions.
The size of a motor unit is variable, and it plays a role in the graded muscle response. The muscles that require fine control, like the muscles controlling eye movements, have very small motor units to allow for precise movements. In contrast, muscles that require more force and power, like the quadriceps, contain larger motor units.
During muscle contraction, recruitment ensures that as the need for force increases, more and larger motor units are activated, which increases the overall muscle tension. This concept is known as recruitment or the principle of multiple motor unit summation and allows for graded responses in muscle tension.