Final answer:
The axon hillock is the most common place for the initiation of action potentials in efferent neurons, which are motor neurons that transmit signals to muscles.
Step-by-step explanation:
The most common location where action potentials start in efferent neurons is the axon hillock. This is the initial segment of the axon, where summation of graded potentials takes place, leading to the initiation of an action potential. This region has a high concentration of voltage-gated Na+ channels needed for the depolarizing phase of the action potential. The axon of efferent, or motor neurons, extends from the spinal cord to the muscles, transmitting the action potential to the muscle fibers, which results in muscle contraction.