Muscle Contraction and Tetanus

Study Notes

Muscle Twitch

A twitch is the smallest unit of muscle contraction, caused by a single stimulus in a single muscle fiber
A single twitch is weak, but multiple twitches can be added together to increase overall force

Frequency Summation

One method for adding up multiple twitches is frequency summation.
This involves stimulating the same muscle fiber at a higher frequency, so that the individual twitches add up
It takes 25 to 200 milliseconds for an individual muscle fiber twitch to occur.
This means the muscle fiber can be stimulated again before the first twitch is over.
The second twitch then adds to the first, increasing the total tension generated by the fiber as a result.
This creates a sustained contraction when repeated enough times
Eventually, the frequency of stimulation will be so high that the individual twitches won't be noticeable.
This is called tetanus, and this state results in a smooth, steady plateau of tension.
Tetanus occurs because the calcium isn't being pumped back into the sarcoplasmic reticulum fast enough, meaning the muscle can't relax.
Above the fusion frequency of tetanus, there is very little increase in tension.

Multiple Fiber Summation

This method involves stimulating multiple muscle fibers simultaneously to increase force production.
Different motor neurons are stimulated to generate different levels of force.
The group of muscle fibers that a motor neuron supplies is called a motor unit.
Increasing stimulus strength can increase force by activating more motor units
The size principle describes how motor neurons are recruited in size from small to large based on stimulus strength.
Small motor units are recruited first, resulting in small steps of force increase.
Larger motor units are recruited only when the stimulus is very strong, resulting in larger increases of force.
This allows for precise control of force production during fine movements, such as performing detailed art work.
Large motor units are recruited in heavy contractions, where more force is required, for example when lifting something heavy.
Asynchronous activation is when different motor units alternate their contraction and relaxation.
This is how sustained force gets generated over time from different motor units.
Motor units will alternate which ones are activated during sustained contraction, keeping muscle force relatively constant.

Muscle Twitch

A muscle twitch is the smallest contractile unit of a muscle fiber
It is triggered by a single stimulus
A single twitch is weak, but multiple twitches can be added together to increase overall force

Frequency Summation

One method to increase force is by increasing the frequency of the stimulus, also known as frequency summation
A muscle fiber can be stimulated again before the first twitch is over, because each twitch takes 25 to 200 milliseconds to occur
Each subsequent twitch adds to the previous one, resulting in increased tension
Repeated stimulation creates a sustained contraction
The frequency of stimulation can reach a point where individual twitches are no longer visible, leading to a smooth and steady plateau of tension - this state is called tetanus
Tetanus occurs because calcium is not pumped back into the sarcoplasmic reticulum fast enough for the muscle to relax

Multiple Fiber Summation

This method involves simultaneously stimulating multiple muscle fibers to increase force production
Different motor neurons are stimulated to generate different levels of force
A motor unit is a group of muscle fibers controlled by a single motor neuron
Increasing stimulus strength activates more motor units, enhancing force production
The "Size Principle" applies to the recruitment of motor neurons, based on their size and stimulus strength
Small motor units are recruited first, resulting in small increments of force
Larger motor units are only recruited when the stimulus is strong, leading to large increases in force
This allows for precise force control during fine movements (e.g., detailed artwork)
Large motor units are needed for heavy contractions (e.g., lifting a heavy object)
Asynchronous activation occurs when different motor units alternate their contraction and relaxation, contributing to sustained force generation over time
Different motor units take turns being activated during sustained contraction, keeping muscle force relatively constant.

Description

This quiz explores the concepts of muscle twitch and frequency summation. Understand how individual muscle fibers respond to stimuli and how sustained contractions occur through tetanus. Test your knowledge on the mechanics of muscle contraction and the physiological principles involved.