Nervous system control
The strength of muscle contraction can be controlled by the nervous system. One of the ways is to vary the strength of action potential. Different motor units are innervated by different motor neurons. By applying different voltages to the muscle, different motor units will be activated depending on whether its neuronal connections are depolarised or not, sending action potentials down their way. Each individual motor unit obeys the all or nothing law. However, the strength of skeletal muscle contraction varies, depending on the number of activated motor units. This process is called recruitment.
The greater the voltage, the more motor neurons become depolarised and the more motor units are recruited. These cause a greater muscular contraction as a whole. However, the strength of contraction is capped at a maximum because there is only a finite number of motor units in the muscle. No more motor units can be recruited once all are activated. These principles are illustrated in Graph 1 presented below.
Graph 1. Recruitment of muscles, demonstrating the relationship between the intensity of stimuli and muscle tension.
The nervous system can also control the frequency of action potentials and alter muscular contraction. The additive effect of stronger muscle contraction following more frequent depolarisations is called summation.
Figure 2. Graphs showing muscle summation, demonstrating the relationship between stimulus frequency and muscle tension. Full explanation given below.
At 1 on Figure 2, a low frequency of action potential arrivals (approximately 5 Hz). Individual twitches can be distinguished. This occurs as the resting interval in between action potentials is long enough for intracellular Ca2+ concentrations to be restored to baseline levels.
By quickening the stimuli (up to 20 Hz), successive twitches become stronger and show a treppe phenomenon, as shown in 2. The graph looks like a flight of stairs. There is very little time between action potentials for intracellular Ca2+ to be fully reabsorbed back into sarcoplasmic reticulum before the next action potential arrives. Muscle relaxation is hence limited. The following muscular contraction also increases in strength as the existing intracellular Ca2+ supplements the incoming Ca2+ from the store. Heat released from actively respiring muscle cells also causes muscle enzymes, e.g. myosin ATPase, to work more efficiently (enzymes catalyse reactions faster at higher temperatures up until the body temperature). Twitches become stronger as muscles warm up.
At a greater frequency of stimulation (up to 40 Hz), summation becomes more effective (3). Tension of muscle builds up with each twitch and muscles have little time to relax in between action potentials before having to contract again. This phenomenon is called temporal succession. The build-up of twitches by the muscle is called incomplete tetanus.
Complete tetanus is achieved when at even higher frequency (40 Hz and greater), twitches occur so closely that they fuse into a continuous contraction like that in 4. Ca2+ levels cannot drop below the level that induces relaxation. Muscle tension is also maintained. This is attributed to the asynchronous contraction of different muscle units at different times, akin to taking shifts in jobs. The contracting muscle will soon be fatigued and relax, thus lessening its tension.
- Graph 1 and Figure 2 adapted and modified from Saladin (2003, p. 424-425)