"what is the relationship between the motor neuron axons and muscle fibers"

Motor neurons carry impulses to the effectors in the body, which are the muscles or glands that produce a change based on a stimulus sensed by the receptors. The motor neurons "innervate" muscle fibers, which essentially means that they cause them to move. The fewer the muscle fibers associated with a motor neuron, the more precise are the movements of those fibers. Having to control a small number of fibers, the neuron is able to coordinate more dexterous motions. An example of a region where there are not a lot of muscle fibers related to each neuron is in your fingers.

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