14.12: Motor Unit Stimulation
When the neuron of a motor unit fires an action potential, it triggers a series of events, leading to a twitch contraction in the muscle fibers. The process of excitation-contraction coupling is crucial in relaying the action potential to the muscle fibers.
The latent period of contraction marks the onset of excitation-contraction coupling, when the action potential propagates across the sarcolemma, preparing the muscle fibers for contraction. As the fibers enter the contraction phase, the tension rapidly peaks, resulting in a forceful contraction. Although a single twitch contraction is detectable in an experimental setup, it is generally insufficient for most functional activities. Instead, an active muscle undergoes a continuous series of smooth contractions, known as graded muscle contractions, which vary in strength. These contractions are crucial in allowing us to perform a wide range of precise movements.
Temporal Summation
One way muscle contractions can be graded is through temporal summation by increasing the stimulation frequency. In this phenomenon, successive stimuli arrive before the complete relaxation of the muscle fiber from the previous twitch. Each subsequent impulse cumulatively adds to the residual force from the prior contraction, causing the muscle tension to build up with each successive stimulus. The result is a smoother and stronger muscle contraction compared to the response elicited by a single stimulus, allowing more precise and forceful movements.
Motor Unit Recruitment
Another way muscle contractions can be graded is through variable recruitment of motor units. The strength of a muscle contraction is determined by how many motor units are activated and the frequency of their activation. When a light force is needed, the nervous system recruits smaller motor units, which consist of fewer muscle fibers and are more easily excited. These small motor units are often sufficient for tasks requiring precision rather than strength, such as writing or gently lifting an object. As the demand for force increases, larger motor units, with more and larger fibers, are progressively recruited to increase the muscle's power output.
This ability to vary the stimulation frequency and recruit different motor units allows muscles to contract with a range of forces, providing the precise control necessary for complex tasks and the strength required for more demanding activities.
