Final answer:
The longest part of a neuron carrying messages to a leg muscle is the axon. These can be exceptionally long in motor neurons found in the ventral horn of the spinal cord, with some axons reaching up to a meter to innervate leg muscles.
Step-by-step explanation:
The longest part of a neuron carrying messages to a leg muscle is the axon. In the case of the motor neurons that control leg muscles, these axons can be exceptionally long, spanning from the spinal cord to the muscles they innervate. Specifically, neurons such as the large multipolar motor neurons found in the ventral horn of the spinal cord have extensive axons that travel considerable distances through the peripheral nervous system to reach the muscle fibers, with some reaching lengths of up to a meter.
For example, the motor neurons that cause contraction in muscles of the leg are anchored in the spinal cord and send their axons through the ventral nerve root to merge with the spinal nerves and finally extend to the muscles, such as those controlling the movement of the toes. These axons are bundled together with those of other neurons to form the nerves, like the sciatic nerve, which is known to be the longest and thickest nerve in the body.