In this video, we will talk about twitch, summation of force, and tetanus in a muscle contraction. Let's get started. A muscle has multiple muscle fibers. The contraction of a single muscle fiber by a single stimulus is called a twitch. It's the smallest unit of muscle contraction.
Being very small, an individual twitch is very weak. To increase the total force of contraction, multiple twitches need to be added together. There are two ways. by which this addition can happen.
One is stimulating the same muscle fiber at a higher frequency, so its individual twitches are added together. This is called frequency summation. And second is stimulating multiple muscle fibers simultaneously.
This is called multiple fiber summation. Now let's talk about each type in more detail. First, frequency summation.
This graph shows time on the x-axis. tension developed by a muscle fiber on the y-axis. And these are individual twitches. Individual twitch lasts for 25 to 200 milliseconds. This duration is long compared to the action potential that stimulates the fiber.
So it's possible to stimulate the fiber again before the first twitch is over. If we do so, the second twitch is added to the first twitch. So the total tension generated by a fiber increases.
As we keep increasing the frequency of stimulation, the tension generated by the fiber increases. Eventually, a state is reached where individual twitches occur so close that they cannot be distinguished from each other. This state is called tetanus. In this state, the tension remains at a steady plateau. The reason behind the tetanus is that at a high frequency of stimulation, there is not enough time to pump the calcium back into the sarcoplasmic reticulum.
So the fiber cannot relax. Above this fusion frequency, very little increase in tension is seen. So this was frequency summation.
Now let's talk about multiple fiber summation. This is a schematic diagram of a muscle. Each block inside represents a muscle fiber. Now in the video on motor unit, we have seen that one motor neuron supplies more than one muscle fiber. The group of muscle fibers supplied by the same neuron is called a motor unit.
In this figure, I have shown six motor units. To generate more force, more motor neurons are stimulated. So multiple fibers contract simultaneously.
So the force generated by individual fiber is added together. This is called multiple fiber summation. There are two interesting things to know regarding multiple fiber summation.
One is the size principle. See, the motor units come in all sizes, from small to large. If less force needs to be generated by the muscle, the brain sends a weak stimulus to the spinal cord.
Such weak stimuli can stimulate small neurons only, not the larger ones. So small motor units are activated first. Because of this, initially, the force increment occurs in small steps. As the stimulus gets stronger, progressively larger motor neurons are activated. So large motor units are activated at very strong stimuli.
Because of this, a progressively larger increment in force is seen as the stimulus gets stronger. This is called the size principle. It allows fine adjustment in force generation at weak contractions. For example, if you are making some art piece, you need very fine control over how much force you exert. This fine control is possible because at this range, Only small motor units are recruited, so you can make very tiny adjustments in force.
On the other hand, if you are doing some heavy work, you want more force. In such case, the recruitment of larger and larger motor units increases force more rapidly. So this was the size principle. The second interesting thing to know about multiple fiber summation is the asynchronous activation of motor units.
In this diagram, I am showing... different motor units of similar size. Now let's say you want to hold a cup of coffee for a minute, and to generate the force needed for that, activation of three motor units is needed.
Here, the same three motor units do not remain contracted for the entire one minute. The motor units keep alternating with each other, so at any given time, some units are generating tension, and others are relaxing, and they keep alternating. This alternation is so smooth, that the whole muscle force is maintained relatively constant throughout the contraction. So this was multiple fiber summation.
Now let's have a quick summary. Twitch is a contraction of a single muscle fiber by a single stimulus. It is the smallest unit of muscle contraction. Frequency summation is adding up multiple twitches of the same muscle fibers by stimulating them at a higher frequency.
In frequency summation, tetanus is seen when individual twitches fuse together, and they are not distinguishable from one another. Multiple fiber summation is stimulating multiple fibers simultaneously. At weak stimulus, smaller motor units are activated, and with increasing strength of the stimulus, large motor units are recruited. Finally, different motor units are activated asynchronously. That's it for this video.
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