Dominic Stone
The rate at which a motor neuron fires action potentials affects the tension produced in the skeletal muscle. This is called wave summation, and it occurs when the fibres are stimulated while a previous twitch is still occurring, resulting in a stronger second twitch. Summation and subsequent muscle tension in the motor unit continue to rise until they reach a peak point, which is referred to as incomplete tetanus. During this state, the muscle goes through quick cycles of contraction with a short relaxation phase.
| Characteristics | Values |
|---|---|
| Motor neuron signalling frequency increases | Muscle tension rises |
| Tension at peak point | Three to four times greater than the tension of a single twitch |
| State at peak point | Incomplete tetanus |
| Muscle state during incomplete tetanus | Quick cycles of contraction with a short relaxation phase |
| Stimulus frequency during complete tetanus | High enough that the relaxation phase disappears |
| Muscle state during complete tetanus | Continuous contractions |
| Ca++ ions during tetanus | Allow virtually all of the sarcomeres to form cross-bridges and shorten |
| Muscle state during tetanus | Contraction continues uninterrupted until muscle fatigues |
| Muscle fibres during summation | Activated by the next action potential before they have had time to completely relax |
| Forces generated by temporally overlapping contractions | Summed |
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What You'll Learn
Motor neuron signalling increases muscle tension
The frequency of the action potentials generated by motor neurons also contributes to the regulation of muscle tension. The increase in force that occurs with increased firing rate reflects the summation of successive muscle contractions. The muscle fibres are activated by the next action potential before they have had time to completely relax, and the forces generated by the temporally overlapping contractions are summed. As the firing rate of individual units rises, the amount of force produced increases.
If the frequency of motor neuron signalling increases, summation and subsequent muscle tension in the motor unit continues to rise until it reaches a peak point. The tension at this point is about three to four times greater than the tension of a single twitch, a state referred to as incomplete tetanus. During incomplete tetanus, the muscle goes through quick cycles of contraction with a short relaxation phase for each.
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Muscle tension reaches a peak point
At the molecular level, summation occurs because the second stimulus triggers the release of more Ca++ ions, which become available to activate additional sarcomeres while the muscle is still contracting from the first stimulus. The tension at this point is about three to four times greater than the tension of a single twitch, a state referred to as incomplete tetanus. During incomplete tetanus, the muscle goes through quick cycles of contraction with a short relaxation phase for each.
If the stimulus frequency is so high that the relaxation phase disappears completely, contractions become continuous in a process called complete tetanus. During tetanus, the concentration of Ca++ ions in the sarcoplasm allows virtually all of the sarcomeres to form cross-bridges and shorten, so that a contraction can continue uninterrupted (until the muscle fatigues and can no longer produce tension).
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Incomplete tetanus
Summation increases muscle tension by increasing the frequency of motor neuron signalling. This causes the muscle fibres to be activated by the next action potential before they have had time to completely relax, resulting in greater contraction of the motor unit. This response is called wave summation, because the excitation-contraction coupling effects of successive motor neuron signalling are added together. At the molecular level, summation occurs because the second stimulus triggers the release of more Ca++ ions, which become available to activate additional sarcomeres while the muscle is still contracting from the first stimulus.
The rate at which a motor neuron fires action potentials affects the tension produced in skeletal muscle. If the fibres are stimulated while a previous twitch is still occurring, the second twitch will be stronger. This is because the second stimulus triggers the release of more Ca++ ions, which become available to activate additional sarcomeres. As the firing rate of individual units rises, the amount of force produced increases.
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Wave summation
The rate at which a motor neuron fires action potentials affects the tension produced in the skeletal muscle. If the fibres are stimulated while a previous twitch is still occurring, the second twitch will be stronger. This response is called wave summation, because the excitation-contraction coupling effects of successive motor neuron signalling are summed, or added together.
At the molecular level, summation occurs because the second stimulus triggers the release of more Ca++ ions, which become available to activate additional sarcomeres while the muscle is still contracting from the first stimulus. Summation results in greater contraction of the motor unit. The tension at this point is about three to four times greater than the tension of a single twitch, a state referred to as incomplete tetanus. During incomplete tetanus, the muscle goes through quick cycles of contraction with a short relaxation phase for each.
The frequency of the action potentials generated by motor neurons also contributes to the regulation of muscle tension. The increase in force that occurs with increased firing rate reflects the summation of successive muscle contractions. The muscle fibres are activated by the next action potential before they have had time to completely relax, and the forces generated by the temporally overlapping contractions are summed. As the firing rate of individual units rises to a maximum of about 20-25 per second, the amount of force produced increases.
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The release of Ca++ ions
Summation increases muscle tension by increasing the frequency of motor neuron signalling. This causes the muscle fibres to be activated by the next action potential before they have had time to completely relax, resulting in greater contraction of the motor unit. This response is called wave summation.
At the molecular level, summation occurs because the second stimulus triggers the release of more Ca++ ions. These ions become available to activate additional sarcomeres while the muscle is still contracting from the first stimulus.
During incomplete tetanus, the muscle goes through quick cycles of contraction with a short relaxation phase for each. The concentration of Ca++ ions in the sarcoplasm allows virtually all of the sarcomeres to form cross-bridges and shorten, so that a contraction can continue uninterrupted until the muscle fatigues and can no longer produce tension.
As the firing rate of individual units rises, the amount of force produced increases. This is because the muscle fibres are activated by the next action potential before they have had time to completely relax, and the forces generated by the temporally overlapping contractions are summed. The release of Ca++ ions is a key part of this process, as they are responsible for activating the additional sarcomeres that lead to greater contraction.
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Frequently asked questions
Summation increases muscle tension by causing the excitation-contraction coupling effects of successive motor neuron signalling to be added together. This is called wave summation.
Wave summation is the response that occurs when the fibres of a skeletal muscle are stimulated while a previous twitch is still occurring, resulting in a stronger twitch.
Ca++ ions are released during wave summation, activating additional sarcomeres while the muscle is still contracting from the first stimulus.
Wave summation results in greater contraction of the motor unit, with tension about three to four times greater than the tension of a single twitch.
This state of increased tension is called incomplete tetanus. During incomplete tetanus, the muscle goes through quick cycles of contraction with a short relaxation phase for each.
