CurrentlyAtLevel
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I can say that the muscle fiber is a few centimeters long and is multinucleated and that it contains myofibrils. In addition to that, It has a sarcoplasmic reticulum.

That’s all I got, does anybody have a better a level description?

And about slow/fast twitch muscle fibres, I know a lot of structural differences but no physiological differences I don’t think, can someone give me examples of some ?
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CurrentlyAtLevel
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Jpw1097 do you mind answering this question ?
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icecreamisbae
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this AQA?
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Describe the interaction between actin and myosin fibres
(Original post by CurrentlyAtLevel)
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I can say that the muscle fiber is a few centimeters long and is multinucleated and that it contains myofibrils. In addition to that, It has a sarcoplasmic reticulum.

That’s all I got, does anybody have a better a level description?

And about slow/fast twitch muscle fibres, I know a lot of structural differences but no physiological differences I don’t think, can someone give me examples of some ?
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CuriosityYay
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(Original post by icecreamisbae)
this AQA?
edexcel maybe?
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icecreamisbae
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y, do u do AQA?
(Original post by CuriosityYay)
edexcel maybe?
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(Original post by icecreamisbae)
y, do u do AQA?
no
I do edexcel
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Jpw1097
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(Original post by CurrentlyAtLevel)
Jpw1097 do you mind answering this question ?
Okay, slow twitch muscle fibres are less powerful than fast twitch fuvres but are more resistant to fatigue and so these can be used for long periods of time (e.g. postural muscles keeping us upright). Slow twitch fibres have a large supply of triglycerides - these triglycerides are broken down into free fatty acids and then into acetyl CoA (via beta oxidation) which are then fed into the Krebs cycle. Simply, triglycerides can be used as an energy source for aerobic respiration. This is why they are resistant to fatigue. They also have a lot of myoglobin and mitochondria as they need a lot of oxygen since they respire aerobically - the high myoglobin quantity is why these muscle fibres are red.
Fast twitch fibres are more powerful as they are much larger. They are less resistant to fatigue and therefore are only used for short bursts - this is because they respire anaerobically, causing a build up of acid (and lactate). Fast twitch fibres rely on glycolysis - therefore they require a lot of glucose, therefore they have large amounts of glycogen and little triglycerides. They also don't need many mitochondria or much myoglobin as they do not carry out aerobic respiration so they don't need much oxygen - the lack of myoglobin is why these fibres appear white. Also as a side note, there are two sub-types of fast twitch fibres - you have fast glycolytic fibres (which are very fast and powerful as described here) and fast oxidative glycolytic (they are somewhere in between slow and fast twitch fibres, as the name suggests, they carry out both anaerobic (glycolytic) and aerobic (oxidative) respiration).

Hope that helps.
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CurrentlyAtLevel
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(Original post by Jpw1097)
I didn’t know how to reply to sections so I included my replies in this
Okay slow twitch muscle fibres are less powerful than fast twitch fuvres but are more resistant to fatigue and so these can be used for long periods of time (e.g. postural muscles keeping us upright). Slow twitch fibres have a large supply of triglycerides - these triglycerides are broken down into free fatty acids and then into acetyl CoA (via beta oxidation) which are then fed into the Krebs cycle. Simply, triglycerides can be used as an energy source for aerobic respiration. This is why they are resistant to fatigue.

Do you mean that they are less likely to run out of a respiratory substrate so less likely to respire anaerobically so less likely to cause the build up of acid hence resistant to fatigue ?

They also have a lot of myoglobin and mitochondria as they need a lot of oxygen since they respire aerobically - the high myoglobin quantity is why these muscle fibres are red.
Fast twitch fibres are more powerful as they are much larger. They are less resistant to fatigue and therefore are only used for short bursts - this is because they respire anaerobically, causing a build up of acid (and lactate). Fast twitch fibres rely on glycolysis - therefore they require a lot of glucose, therefore they have large amounts of glycogen and little triglycerides.

I don’t think I know the link between triglycerides and the the type of respiration . Why is it that they have little triglycerides ?

They also don't need many mitochondria or much myoglobin as they do not carry out aerobic respiration so they don't need much oxygen - the lack of myoglobin is why these fibres appear white. Also as a side note, there are two sub-types of fast twitch fibres - you have fast glycolytic fibres (which are very fast and powerful as described here) and fast oxidative glycolytic (they are somewhere in between slow and fast twitch fibres, as the name suggests, they carry out both anaerobic (glycolytic) and aerobic (oxidative) respiration).

Hope that helps.
Very much appreciated. What about the description of the structure of a muscle fiber ? Is what I included at the very beginning of the thread enough or is there something else I should say ?
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macpatgh-Sheldon
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CurrentlyAtLevel

Hi apologies that I am putting this reply here - it is for your "very difficult" ECG Q - it seems any Q I touch today "turns to stone" just like King Edward's magician Midas :mad:.

(I have an unfair advantage of having seen cardiac patients and therefore a large number of ECGs - I shall try to put myself in the shoes of an A level student)

3. If you look at the intervals (on x axis = horizontally) between the beats , this interval is steady on the ECG on left [normal], but is shorter between the 1st normal beat and the ectopic beat on ECG on right [abnormal], yeah? So the ectopic beat is premature i.e. it has come too early.

6. ECG on right [abnormal]: For the interval between the 2nd [= ectopic] beat and the 3rd beat [normal one] the gap is much longer, so this tells you that "a beat has been missed" [to put it simply], with me?

7. (v unfair Q for A level): the pulmonary artery pressure trace on right (abnormal) shows no peak at the time of the ectopic beat [looking directly vertically below the abnormal ECG above it], so this tells you that there was effectively no blood pumped out by the heart in timing with the ectopic beat [however, it is actually not ideal to measure pulmonary artery pressure here but aortic pressure, but I suppose at A level, these items cannot be fully correct [the examiners are extremely unlikely to be doctors]].

8. (look up refractory period in your books).

9. (difficult to predict this wording i.e. difficult to read examiner's mind here)

10. (as in 8).

EXTRA INFO: Yes, this is a very difficult Q for A level.

1. An ectopic beat has a different waveform from normal beats (I use this word loosely to mean QRS complexes) (you can see that the shape of "E" is very different from that of the other QRS complexes.

2. "E" has no P wave before it, yeah? Do you remember what generates the P wave?

Yes, correct: atrial depolarization - so the absence of a P wave tells you that this is a VENTRICLAR ectopic beat (although you would get a similar ECG with a junctional ectopic beat (= one originating in the junction between the atria and the ventricles)

3. (NB: ONLY for super-nerds or budding cardiologists): There are several other highly complex ways of distinguishing between ventricular and supraventricular (supra = above so above ventricles = originating in atria or junctional area) ectopic beats BUT MIGHT JUST BE USEFUL TO NOTE ONE ITEM IN THIS Q:- If you measure the time interval between the normal beat (abnormal trace [on right]) i.e. 1st beat, and the next normal beat on the same trace (right one) i.e. 3rd beat, it is twice the distance between individual beats on the normal trace (left one}. Assuming that the traces are from the same patient at different times RATHER THAN from different patients, this tells us that the last beat (3rd one - one after "E") in the abnormal trace (right one) occurs when expected - this is one clue to the ventricular origin of the ectopic beat [because the electrical activity generated cannot travel retrogradely (backwards) to the SA node and so the pause between the ectopic beat and the next normal beat is a "compensatory" one, NOT shorter [due to an early conduction from the SA node of the retrogradely transmitted impulse] OR longer.

Most students: please ignore text in italics.

M
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