Final answer:
The strength of muscle contraction is determined by the firing rate of alpha motor neurons, which, when firing action potentials rapidly, release ACh at the neuromuscular junction, causing a stronger muscular contraction through wave summation.
Step-by-step explanation:
The firing rate of the alpha motor neurons determines the strength of contraction of a muscle. Alpha motor neurons are part of a motor unit, which also includes the muscle fibers innervated by these neurons. When an alpha motor neuron fires action potentials, it releases the neurotransmitter acetylcholine (ACh) at the neuromuscular junction, leading to muscle fiber contraction. The frequency of the motor neuron's action potentials affects the tension the muscle can produce. If the action potentials are fired more rapidly, this results in wave summation, where each successive twitch builds upon the previous one, ultimately leading to a stronger muscular contraction. This process depends on the release of more Ca++ ions that contribute to sarcomere activation for greater muscle tension during the contractions.