Contracted vs Relaxed Sarcomere (H zone, A Band, I Band)

Why are sarcomeres called contractile units? How does the appearance of a sarcomere change when it contracts? Where are sarcomeres located? Why are skeletal and cardiac muscle striated?
Covered topics/terms: myofibrils, sarcomeres, z dics, m line, h zone, a band, i band, actin and myosin (thick and thin filaments), elastic filaments (titin)
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[@AnatomyHero]

Anatomy of the neuromuscular junction: kzbin.info/www/bejne/e2XaZYx-Zspnj5o For more of my muscle physiology videos check out this playlist: kzbin.info/aero/PLBM7jL93Kc2T8d_Y8T0r4-8zWJuUi7Mze

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[@AnatomyHero]

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[@AnatomyHero]

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[@peterm92]

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[@alexanderleahy2097]

Hello! Quick question, reading about the " All or Nothing Principle" how can that be true to state that muscle fibers (in this case the sarcomere as a whole) either contract maximally or not at all? I ask because for example, if my arm were to be at rest, and I only curl my arm about halfway, wouldn't the muscle not be at its maximal contraction and only half of what it has the potential to contract to? Or is there something I'm missing/misunderstanding? Thank you for your time! :)

[@alexanderleahy2097]

Just had a thought, even though a singular motor unit cannot vary it's force based off that principle, can the CNS essentially choose to only activate certain motor units based off of necessity? Sorry, I think I may have just wasted your time and answered my own question haha.

[@dr.scientimental2700]

As far as the partial contraction of an entire muscle is concerned, there are different motor units that are stimulated during a muscle contraction. All muscle fibers almost never contract all at once or else muscle contraction would be quick, uncontrolled and would lead to the full approximation of the bones forming the joint. But the nervous system never functions that way. It provides signals to a few units to contract so that the other muscle fibers remain in relative rest and then it alternates the pattern. This way, the limb wouldn't fully contract but just as much as is needed for the activity. Yes sarcomeric contraction in each myofibril would occur completely but not every muscle fiber in a muscle is contracting and so the body can regulate the power generated. Hope I made sense.

[@FutureDrsmith02]

too goooddd

[@dr.scientimental2700]

Can you explain, if each sarcomeric unit shortens due to contraction, why do the Z discs not rip apart. Because adjacent sarcomeres are contracting away from these discs.

[@AnatomyHero]

I have never thought about it in those terms, but that's a really good question... I have never seen an explanation or a model that shows how a myofibril contracts as a results of all of the sarcomeres contracting... But I'm guessing the sarcomeres pulling on the sarcomere next to them (via the z discs) causes everything to bunch in the middle... Because the sarcomeres on the very ends wouldn't have any tension on their open sides... So they would get dragged away from the open side and towards the sarcomere next to them... And so on and so forth until it all ended up in the middle. Thats my best guess!

[@dr.scientimental2700]

@@AnatomyHero yes, even I haven't come across any such thorough model of the contraction of multiple sarcomeric units as one. But I guess your explanation is fairly decent to go by with. Thanks 👍

[@montheral-falojey6995]

Skyler White?

[@shivaswarnkar190]

effective

[@gamagarcia3037]

Good evening. I have a question. Is this knowledge part of the nursing education program?

[@AnatomyHero]

This is something you would typically be taught in a general a&p class before a nursing program...but schools and curriculums differ, some schools have nursing specific A&P and I don't know of this would or wouldnt be included.

[@darknight5467]

😊

[@faithelias2798]

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[@j0425]

Ma'am is this Physiology part?

[@AnatomyHero]

Yes, it's microanatomy with a little bit of physiology!

[@j0425]

@@AnatomyHero Okay thanks! 🥰

[@tysken0251]

how can a single z line move closer to two adjacent m lines simultaneously

[@AnatomyHero]

That's a really good question. I have never seen an explanation or a model that explains how a myofibril contracts as the result of the sarcomeres contracting (which I think would address your question)... But I think that because the z discs on the very ends wouldn't have any tension on their open sides they would get dragged away from the open side and towards the sarcomere next to them... Which would eliminate the tension on the z disc next door...And so on and so forth until it all bunches up in the middle. Thats my best guess! I hope what I'm trying to say makes sense.

[@daddyfrhuh]

@@AnatomyHero can we assume that contraction happen in a particular direction, as the heavy meromysion is projected outwards at regular distance and angle from each other? which could possibly explain the net contraction?

[@AnatomyHero]

@@daddyfrhuh from my understanding..no. The myosin is just pulling the actin towards the m line of each sarcomere and there's no directionality to it beyond that.

[@AnatomyHero]

@@daddyfrhuh I'm not terribly certain what you mean when you say net contraction... But if you're at all thinking of action of the muscle... the body part that ultimately ends up moving just depends on what body part has less resistance.

[@daddyfrhuh]

@@AnatomyHero thank you so much , Im a high schooler you just cleared all my doubts❤

[@jakeavilla2740]

Clutchhh

[@arhamahmed4459]

ur beautiful

[@Ehuuuuu2559]

Why is the H zone is lighter? Even though the are Thick ? I didn’t get that even when the prof was explaining 🫣🫣 please help 🥲

[@AnatomyHero]

The I band is the lightest because it's just thin filaments The A band is where there are thick filaments and is therefore darker than the I band In some parts of the A band there are thick AND thin filaments, so that area is the darkest The H zone is a PART of the A band is lighter than the rest of the A band because it is ONLY thick filaments (not overlapping thick AND thin) but the H zone is still darker than the I band (it's just lighter than the other parts of the A band) Sorry if my video wasn't clear enough, hope that clears it up for you

[@Ehuuuuu2559]

@@AnatomyHero thank you so much I understand now , I love your videos please continue 🙏💜