Final answer:
The force produced by muscles is affected by the frequency of action potentials. Increased frequency leads to wave summation and higher calcium ion concentration, resulting in stronger and more sustained muscle contractions until the muscle reaches its peak force output.
Step-by-step explanation:
The frequency of action potentials influences the force production in muscles. When a motor neuron fires rapidly, this leads to an increased release of calcium ions (Ca++) into the muscle fibers, which binds to troponin, causing tropomyosin to uncover binding sites on actin filaments. This process, known as wave summation, results in greater muscle contraction because more cross-bridges are formed between actin and myosin filaments, thus producing more tension. As the frequency of action potentials increases, muscles are able to generate a stronger contraction up to a point.
If a muscle fiber receives a new action potential before it has relaxed from a previous twitch, the new contraction will be stronger due to the presence of residual calcium ions in the sarcoplasm. This summation of twitches can lead to a sustained muscle contraction called tetanus, which yields a peak force output for the muscle. This mechanism provides the nervous system with a means of finely tuning muscle contraction force by simply adjusting the frequency of action potentials sent to muscle fibers.