Final answer:
Skeletal muscle fibers produce a brief twitch in response to a single stimulus, but require a series of action potentials for a sustained contraction. Graded muscle responses allow force variation, with key factors including action potential frequency and motor neuron activity. Wave summation and the sliding filament model are essential to understanding muscle contraction dynamics.
Step-by-step explanation:
Skeletal muscle fibers produce a twitch in response to a single stimulus to generate an action potential (AP). However, a twitch on its own is brief and does not result in significant muscle activity. For a muscle contraction that can perform work, a series of action potentials is necessary, leading to a graded muscle response. This response varies in force depending on input from the nervous system.
Several factors influence the force a muscle can produce, most notably the frequency of action potentials and the number of motor neurons that are firing. When a second action potential arrives before a muscle fiber has relaxed from a twitch, this results in wave summation, which makes the contraction stronger because more calcium ions are available to activate sarcomeres.
The sliding filament model explains how muscle contraction works at the molecular level, with neurotransmitters like acetylcholine (ACh) initiating the process, resulting in the movement of muscle fibers. Frequency of stimulation, the proportions of myofibers within the muscle, and the recruitment of multiple motor neurons contribute to the overall tension and strength of the muscle contraction.