Logan Steele
Muscle summation occurs when a muscle contraction is stimulated before a previous muscle twitch has completely relaxed, increasing the total amount of tension produced in the muscle. Summation can be achieved by increasing the frequency of stimulation or by recruiting additional muscle fibres within a muscle. It is the additive effect of several electrical impulses on a neuromuscular junction, the junction between a nerve cell and a muscle cell.
| Characteristics | Values |
|---|---|
| Definition | Summation is the additive effect of several electrical impulses on a neuromuscular junction, the junction between a nerve cell and a muscle cell |
| Cause | A 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 |
| Result | Greater contraction of the motor unit |
| Stimulus | Individually the stimuli cannot evoke a response, but collectively they can generate a response |
| Muscle tension | 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 |
| Twitch contraction | The period of contraction and relaxation of a muscle after a single stimulation |
| Summation | The occurrence of additional twitch contractions before the previous twitch has completely relaxed |
| Muscle fibres | Summation can be achieved by recruiting additional muscle fibres within a muscle |
| Contraction frequency | Summation can also be achieved by increasing the frequency of contraction |
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What You'll Learn
Summation occurs at the molecular level
Summation is the occurrence of additional twitch contractions before the previous twitch has completely relaxed. This can be achieved by increasing the frequency of stimulation, or by recruiting additional muscle fibres within a muscle. A twitch contraction is the period of contraction and relaxation of a muscle after a single stimulation.
In terms of the neuromuscular junction, summation is the additive effect of several electrical impulses. Individually, the stimuli cannot evoke a response, but collectively they can generate a response.
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Summation increases muscle tension
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. 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. This is distinct from tetanus, which occurs when the frequency of muscle contraction is such that the maximal force of tension is generated without any relaxation of the muscle.
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Summation occurs when a muscle is stimulated before it has completely relaxed
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. 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. When a weak signal is sent by the central nervous system to contract a muscle, the smaller motor units, being more excitable than the larger ones, are stimulated first. As the strength of the signal increases, more (and larger) motor units are excited.
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Summation can be achieved by increasing the frequency of stimulation
Summation is the occurrence of additional twitch contractions before the previous twitch has completely relaxed. It can be achieved by increasing the frequency of stimulation, or by recruiting additional muscle fibres within a muscle.
The force of the second twitch is typically higher than the force of the first twitch if the second twitch is initiated before the muscle has completely relaxed from the first twitch. This is because the additional action potential stimulates a muscle contraction before a previous muscle twitch has completely relaxed, increasing the total amount of tension produced in the muscle.
Summation is influenced by the time between successive stimuli (the inter-pulse interval, or IPI), twitch force and twitch duration. It is a non-linear process.
Within a muscle, summation can occur across motor units to recruit more muscle fibres, and also within motor units by increasing the frequency of contraction. When a weak signal is sent by the central nervous system to contract a muscle, the smaller motor units, being more excitable than the larger ones, are stimulated first. As the strength of the signal increases, more (and larger) motor units are excited.
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Summation can be achieved by recruiting additional muscle fibres
Summation occurs when additional twitch contractions happen before the previous twitch has completely relaxed, increasing the total amount of tension produced in the muscle. This can be achieved by recruiting additional muscle fibres within a muscle.
When a weak signal is sent by the central nervous system to contract a muscle, the smaller motor units are stimulated first, as they are more excitable than the larger ones. As the strength of the signal increases, more and larger motor units are excited. A single motor neuron can innervate multiple muscle fibres, causing them to contract at the same time.
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. 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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Frequently asked questions
Summation is the occurrence of additional twitch contractions before the previous twitch has completely relaxed.
Summation occurs because a 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. 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.
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