Properties of Muscle Twtich

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owyang

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I just did a lab where we dissected the gastrocnemius muscle of a frog and then stimulated the sciatic nerve and measured the force of response (the resulting muscle twitch). As we increased the voltage of the stimuli, the corresponding twitch response increased in force until it reached a max value and then proceeded to fall as we continued to increase the stim voltage.

My question is why the did the response correspondingly increase with the stimulus voltage. Is it because the sciatic nerve is a bunch of nerves and it started to "recruit" more muscle fibers in the muscle as voltage increased? Or, does it have to do with the amount of neurotransmitter at the NMJ?

thanks.
 

ReachingTruth

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Going with the OP, I had a really interesting lab. A car is traveling at 5 km/h for 2 hours before a bus overtakes it. The bus has only been traveling for 30 minutes. What is the avg. velocity of the bus when it over takes the car?

I also have orgo 2 homework if people want to do that, as well.
 
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nubbey24

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I just did a lab where we dissected the gastrocnemius muscle of a frog and then stimulated the sciatic nerve and measured the force of response (the resulting muscle twitch). As we increased the voltage of the stimuli, the corresponding twitch response increased in force until it reached a max value and then proceeded to fall as we continued to increase the stim voltage.

My question is why the did the response correspondingly increase with the stimulus voltage. Is it because the sciatic nerve is a bunch of nerves and it started to "recruit" more muscle fibers in the muscle as voltage increased? Or, does it have to do with the amount of neurotransmitter at the NMJ?

thanks.

It is because the sciatic nerve contains a bunch of alpha somatic motor neurons (nerves are bundles of neurons), each of which make up a single motor unit and thereby innervate a fixed number of muscle fibers. As you increase the stimulus intensity you are reaching the threshold of more and more of these motor units (i.e. neurons and said muscle fibers they innervate) and are thereby causing an increase in muscle force production. This is motor unit recruitment and is independent of the amount of ACh released at the NMJ. Once a neuron is fired it is an all or nothing phenomenon. You can also play with the frequency though and cause summation (and thus increase muscle force), but that is another phenomenon in it of itself. Hope this helps.

Nubs
 

punkindrublic

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Nubs-

We recently covered this in gen bio 2, and while reading the post I was trying to figure it out, and decided that it must be because you're causing action potentials in more neurons by increasing the voltage and causing more muscle cells to react. Is that what you're saying? Seems intuitive enough.

Why would the force lower after a certain threshold is reached? It seems like there should be some sort of logrhythmic correlation b/w stimulus and response. Does it have something to do with creating too large a potential and screwing up voltage gates?
 

nubbey24

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Nubs-

We recently covered this in gen bio 2, and while reading the post I was trying to figure it out, and decided that it must be because you're causing action potentials in more neurons by increasing the voltage and causing more muscle cells to react. Is that what you're saying? Seems intuitive enough.

Why would the force lower after a certain threshold is reached? It seems like there should be some sort of logrhythmic correlation b/w stimulus and response. Does it have something to do with creating too large a potential and screwing up voltage gates?

Force should not decrease after you keep increasing voltage. Theoretically, once you have reached the threshold of all Neurons within the neuron, you will have achieved maximal motor unit recruitment.

You are correct in your assumption. As you increase voltage you firing more neurons, and thus causing more muscle fibers to contract. The way it is supposed to work is that lower threshold motor units are recruited first and then higher threshold, which usually corresponds to smaller to larger motor units respectively.

Hope this helps,

nubs
 

MonkeyNuts!

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I just did a lab where we dissected the gastrocnemius muscle of a frog and then stimulated the sciatic nerve and measured the force of response (the resulting muscle twitch). As we increased the voltage of the stimuli, the corresponding twitch response increased in force until it reached a max value and then proceeded to fall as we continued to increase the stim voltage.

My question is why the did the response correspondingly increase with the stimulus voltage. Is it because the sciatic nerve is a bunch of nerves and it started to "recruit" more muscle fibers in the muscle as voltage increased? Or, does it have to do with the amount of neurotransmitter at the NMJ?

thanks.

You wouldn't happen to go to JHU?

Hey people, for future reference, don't answer homework questions, I think it's against TOS and it makes braluk thread-moving/closing-trigger happy. This guy only has 1 post, he made this account because he was lazy.
 

DocYuki

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I just did a lab where we dissected the gastrocnemius muscle of a frog and then stimulated the sciatic nerve and measured the force of response (the resulting muscle twitch). As we increased the voltage of the stimuli, the corresponding twitch response increased in force until it reached a max value and then proceeded to fall as we continued to increase the stim voltage.

My question is why the did the response correspondingly increase with the stimulus voltage. Is it because the sciatic nerve is a bunch of nerves and it started to "recruit" more muscle fibers in the muscle as voltage increased? Or, does it have to do with the amount of neurotransmitter at the NMJ?

thanks.
I did my undergraduate research project (~3 years) on neuromuscular junction in frogs and facilitation.

My understanding is that at a certain voltage, within a population of muscle fibers, many of them are not reaching their action potentials yet. Once you hit a higher voltage, yes, you recruit more and more of the fibers until they are all responding. As you keep hitting the muscle with impulses, they will fatigue as a result of intra-cellular dynamics (calcium concentration, etc).

This of course affects the release of calcium at the NMJ (quantal content). Stimulate more and more at high frequency and you'll get a lowered quantal content over time. This gets complex when frequency is increased - you get facilitation which is what I've been working on for a few years. Anyway, it seems you know what's going on so keep rockin!
 
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