Final answer:
The effects of excitatory input on motor neurons depend on the size and excitation threshold of the motor neurons. Small motor neurons, associated with low threshold motor units, generate a relatively small degree of contractile strength when excited. In contrast, larger motor neurons, associated with higher threshold motor units, generate a greater degree of contractile strength.
Step-by-step explanation:
The different effects of excitatory input on low threshold (small diameter) motor neurons compared with higher threshold (larger diameter) motor neurons is due to the recruitment of different motor units. Motor units are composed of motor neurons and the muscle fibers they control.
Small motor units, associated with low threshold motor neurons, have smaller, more excitable motor neurons and smaller muscle fibers. When excitatory input is received by these motor neurons, a relatively small degree of contractile strength (tension) is generated.
In contrast, larger motor units, associated with higher threshold motor neurons, have bigger, less excitable motor neurons and larger muscle fibers. Excitatory input to these motor neurons results in a greater degree of contractile strength.