Neuromuscular control involves understanding how muscle structures create movement through coordination. Messages from the brain initiate muscle contractions by motor neurons stimulating muscle fibers in motor units. The number and size of motor units varies depending on a muscle's function, with large muscles for gross movements having more motor units than smaller muscles. Muscles contract according to the all-or-nothing principle, where motor units either contract fully or not at all depending on whether nerve impulses reach the threshold level.