When which neurons fire, they transmit muscle action potentials to contract all extrafusal muscle fibers?

Alpha motor neurons are the primary neurons responsible for transmitting muscle action potentials that initiate the contraction of extrafusal muscle fibers. These neurons are located in the anterior horn of the spinal cord and directly innervate skeletal muscle fibers, leading to voluntary muscle movement.

When alpha motor neurons fire, they release neurotransmitters at the neuromuscular junction, triggering muscle fibers to contract. This mechanism is crucial for motor control and is fundamental to the execution of voluntary movements.

In contrast, other types of motor neurons have different functions. For example, beta motor neurons do not exist in the conventional classification of motor neurons; their designation might confuse individuals as beta fibers relate more to sensory pathways than motor control. Gamma motor neurons primarily innervate intrafusal muscle fibers in the muscle spindle, involved in proprioception and the regulation of muscle tone, instead of contracting extrafusal fibers. Delta motor neurons do not have a specific classification in standard neuromuscular physiology or clinical practice related to muscle action.

Therefore, the function of alpha motor neurons in muscle contraction makes them the correct choice in this question.