Skeletal Muscle Response to Stimuli

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Questions and Answers

Which of the following best describes the summation effect in skeletal muscle?

Separate curves with identical amplitude.

Total inhibition of muscle response.

A decrease in contraction force over time.

A single curve with increased amplitude and a broader base. (correct)

What occurs when a second stimulus is applied during the first half of the latent period?

The muscle contracts with greater force.

The muscle exhibits superposition of contractions.

No response is obtained to the second stimulus. (correct)

The second stimulus triggers a summation effect.

What is the term used to describe the advantageous effects produced by the first stimulus that assist the second stimulus in generating a contraction?

Treppe

Incomplete tetanus

Beneficial effect (correct)

Superposition

When does superposition occur in relation to muscle contraction?

When a second stimulus is applied during the relaxation phase of the first contraction. Signup and view all the answers

What is the effect when a second stimulus is applied after the relaxation phase of the initial muscle contraction?

A complete contraction curve is produced with increased force. Signup and view all the answers

What phenomenon occurs when the next successive stimulus is applied right after the relaxation phase of the previous twitch, resulting in individual twitches with increasing force?

Treppe Signup and view all the answers

Which effect is characterized by the next stimulus falling on the relaxation phase of the previous twitch, resulting in superimposed contractions?

Incomplete tetanus Signup and view all the answers

Which statement is true regarding maximal versus supramaximal stimuli?

Supramaximal stimuli do not increase contraction force further. Signup and view all the answers

What type of muscle contraction occurs when stimuli are applied at such a frequency that the muscle does not have time to relax between contractions, resulting in a smooth, sustained contraction?

Complete tetanus Signup and view all the answers

What is the defining characteristic of threshold stimuli in muscle contraction?

They always produce a contraction. Signup and view all the answers

How does frequency of stimuli impact muscle contraction?

Increasing frequencies can produce summation effects. Signup and view all the answers

What happens when a subthreshold stimulus is applied to a single muscle fiber?

It evokes no response. Signup and view all the answers

Which of the following factors does NOT affect the contractile response of skeletal muscle?

Color of muscle fibers Signup and view all the answers

What describes the force of contraction at a suprathreshold stimulus level?

It increases with an increase in the strength of the stimulus. Signup and view all the answers

What is 'quantal summation' in muscle physiology?

The phenomenon of multiple muscle fibers being activated. Signup and view all the answers

Under what conditions can discrete responses with a staircase effect occur?

With frequencies less than 10/s Signup and view all the answers

What is the primary factor that decreases muscle viscosity and contributes to the beneficial effect of the first contraction?

Higher muscle temperature Signup and view all the answers

What is indicated by the term 'maximal stimulus'?

A stimulus that excites all motor units. Signup and view all the answers

What does quantal summation in muscles refer to?

The cumulative effect of multiple muscle fibers contracting. Signup and view all the answers

What type of muscle response occurs when no contraction happens due to the subsequent stimulus being applied during the absolute refractory period?

No response Signup and view all the answers

Which condition describes the application of a stimulus that exceeds the threshold necessary for muscle contraction?

Suprathreshold stimulus Signup and view all the answers

When is the refractory period of the muscle twitch primarily observed?

In the first half of the latent period. Signup and view all the answers

How does frequency of stimulation affect muscle contraction?

Higher frequencies can lead to summation of contractions. Signup and view all the answers

What is the effect of a supramaximal stimulus on a muscle's contraction?

It does not increase contraction beyond the maximal level. Signup and view all the answers

Which condition primarily initiates the twitch contraction in a muscle fiber?

Action potential generation Signup and view all the answers

Study Notes

Frequency of stimulus

The effect of repeated stimuli on a skeletal muscle depends on the number of stimuli (contraction frequency), not the strength of the stimulus
The effect of the number of stimuli can be observed by varying the time interval between stimuli while keeping the strength of stimuli constant.

Effect of two successive stimuli

If the second stimulus is applied during the first half of the latent period, no response is obtained. This is because this period corresponds with the absolute refractory period (ARP) of the muscle fibers.
If the second stimulus is applied from the second half of the latent period to the contraction phase, the two stimuli are summed, creating a single curve with greater amplitude and broader base. This phenomenon is called complete summation.
If the second stimulus is applied during the relaxation phase, the relaxation phase is cut short, and a new contraction occurs. This is called superposition or incomplete summation of waves. The second curve is superimposed over the first curve, and has a greater amplitude than the first.
If the second stimulus is applied soon after the relaxation phase of the first stimulus, a second complete curve is obtained. However, the force of the second contraction is greater than that of the first.
The increased force of contraction in the second stimulus, whether it's summation, superposition or a separate contraction, is due to the beneficial effect.

Beneficial effect and its causes

The beneficial effect occurs when the second stimulus is applied after the absolute refractory period
The contraction produced by the first stimulus proves beneficial to the second one, resulting in a stronger contraction for the second stimulus.
The beneficial effect is attributed to:
- Calcium ions released during the first contraction, adding to the calcium released upon the second stimulus
- Viscosity of the muscle decreasing due to the first contraction
- Increase in hydrogen ions concentration due to the first contraction
- Increased muscle temperature due to the first contraction, contributing to the beneficial effect by decreasing viscosity

Effect of multiple stimuli

The response obtained will vary depending on when the next stimulus arrives: after the first twitch, during the relaxation phase, during the contraction phase, or during the second half of the latent period.

Discrete responses

If the successive stimulus falls after the completion of the relaxation phase of the preceding twitch, discrete responses are produced with brief intervals between them.
Each successive twitch has an increased force of contraction due to the beneficial effect of the previous twitch. This phenomenon is called the staircase effect or treppe.

Incomplete tetanus or clonus

If the next stimulus falls on the relaxation phase of the preceding twitch, the incomplete summation of waves occurs creating a superimposed contraction.

Complete tetanus

When the stimulus frequency is high enough to allow no relaxation between contractions, the muscle appears to be in sustained, maximal contraction. This is called complete tetanus and is a state of maximal tension.

Relation between Electrical and Mechanical Events

Action potential and mechanical response (simple muscle twitch curve) can be plotted on the same time scale.
The twitch starts about 2 ms after the start of depolarization but always before the repolarization is completed.
The refractory period (absolute refractory period) is very short and lies in the first half of the latent period of the single muscle twitch.

Factors Affecting Contractile Response

Strength of stimulus:
- A single muscle fiber obeys the all or none law - a subthreshold stimulus evokes no response, while threshold and supramaximal stimuli produce the same contraction.
- Graded response is obtained when stimuli of different intensities are applied through the nerve in a nerve-muscle preparation:
  - Subthreshold stimuli do not evoke any response.
  - Threshold stimulus produces minimal contraction.
  - Suprathreshold stimuli produce a graded response, meaning the force of contraction increases with the increase in strength of the stimulus, until a maximal limit is reached. This is due to quantal or multifibre summation, where more muscle fibers are being recruited into activity.
  - Maximal stimulus produces the maximal response (excites all motor units).
  - Supramaximal stimulus does not increase the response beyond the maximal response, because even at the maximal stimulus, all motor units are already contracting maximally.
Frequency of stimulus: This was discussed above
Load on the muscle (preload and after-load): The greater the load, the slower the contraction and shorter the shortening, and the greater the load, the lower the velocity of shortening.
Initial length of muscle: The length-tension relationship of the muscle determines the force of contraction. Maximal tension is achieved at an optimal length.
Temperature: Increased temperature increases the speed of contraction and relaxation.

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Description

Explore the fascinating dynamics of skeletal muscle contractions in response to repeated stimuli. This quiz examines how contraction frequency and timing of stimuli affect muscle response, including concepts such as absolute refractory period, complete summation, and superposition. Test your understanding of these critical physiological principles!