Wave summation occurs when additional Ca++ is released from the SR by subsequent stimuli while the Ca++ levels in the sarcoplasm are still elevated from the first stimulus. This buildup of Ca++ causes peak tension generated during fused tetanus to be 5-10x larger than the peak tension produced during a single twitch. This is also partially due to the stretch of elastic components such as tendons and connective tissues around muscle fibres. During wave summation, the elastic components are not given much time to spring back between contractions and thus remain taut. In this state, they do not require as much stretch prior to beginning the next muscular contraction. The combination of the above tautness, and the partially contracted state enables the force of another contraction to be greater than the one before. However, smooth sustained voluntary muscle contractions are achieved mainly by out-of- synchrony unfused tetanus of different motor units.
Muscle Tone
At rest a skeletal muscle exhibits Muscle Tone which is a small amount of tautness or tension in the muscle fibre due to weak, involuntary contraction of its motor units. To sustain tone, small groups of motor units are alternatively active and inactive in a constant shifting pattern. Muscle tone keeps skeletal muscles firm but not enough to produce movement. This is necessary for posture maintenance. Muscle tone is also necessary in smooth muscle such as in blood vessels – an important role in maintaining blood pressure. **Recall that skeletal muscles contraction in response to ACh released by nerve impulses in its motor neurons. Therefore, muscle tone is established by the CNS. **
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