Final answer:
Twitch summation occurs when successive muscle twitches combine due to a sustained increase in cytosolic Ca2+ levels, leading to greater overall muscle tension. This process, known as wave summation, can intensify to incomplete or complete tetanus with increased stimulation frequency.
Step-by-step explanation:
Twitch summation is a phenomenon observed in muscle physiology where the tension in a skeletal muscle increases due to a sustained elevation in cytosolic Ca2+ levels. When the motor neurons increase their rate of firing action potentials, the successive twitches add up because there is not enough time for the muscle to fully relax between stimulations. This process is known as wave summation and is depicted in muscle response curves called myograms. During wave summation, a second stimulus arrives before the muscle has completely relaxed from the first, resulting in a more potent contraction. This is associated with an increased release of Ca2+ ions, which are essential for initiating muscle contraction by interacting with contractile proteins within muscle fibers. The ions remain elevated in the sarcoplasm, activating additional sarcomeres and enhancing the contractile force.
If the frequency of stimulation continues to rise, it can lead to a state known as incomplete tetanus, where the muscle experiences sustained contraction with only brief relaxation phases. If the stimuli are delivered even more rapidly, the muscle may reach a state of complete tetanus, resulting in continuous contraction without any relaxation phase.