Final answer:
The incorrect statement about skeletal muscle innervation is that all motor units in a skeletal muscle usually contract together. While the muscle fibers in a single motor unit do contract together, different motor units activate at different times for gradated control of muscle movement.
Step-by-step explanation:
The statement that is NOT true about the innervation of skeletal muscle is that all of the motor units of a skeletal muscle usually contract together. Here's why this statement is incorrect:
While it is true that the cell bodies of somatic motor neurons are within the CNS and axons extend out to skeletal muscle fibers, and that a motor unit is a single somatic motor neuron and all of the muscle fibers it innervates, skeletal muscles do not always contract as a whole with all motor units acting at once. Somatic motor neurons typically activate in a pattern that allows for varied and controlled movement rather than full contraction of all motor units at once. The gradation of muscle force is achieved by recruiting more or fewer motor units depending on the demand placed on the muscle. For delicate and precise movements, fewer motor units are activated, whereas more motor units are recruited for powerful, forceful movements.
The muscle fibers of a single motor unit usually contract together in response to an action potential in that motor neuron. However, different motor units within the same muscle may contract at different times to provide fine control over muscle movement, or collectively for more powerful contractions. This is referred to as motor unit recruitment.