|MadSci Network: Anatomy|
Dear Pradu, Thank you very much for your interesting question. I hope you are able to understand my anwer. You seem to be familiar with the sliding filament model of muscle contraction. This is now accepted as the definite answer to the question of how muscles shorten, with the exception of one or two individuals who are considered to be madder than the rest of us. This is summarised well in a previous Mad Sci network answer, which includes links that I have included here for speed of access, first to an overview of the mechanisms of muscle contraction and secondly to a movie of the sliding filament model. I shall now get into answering your question, which appears to be in several parts so it is probably best if I deal with it in parts. Muscle shortens because the head of myosin (thick filaments) binds to actin (thin filaments) undergoes a change in shape (isomerization), which pulls on the actin past the myosin. The thing that determines how fast a muscle can shorten is how fast the myosin can undergo this process and then return to its starting position to get hold of actin and pull it along again. I strongly recommend looking at the movie of the sliding filament model to help you understand this if you haven't done so already. For a more detailed look at what a single molecule of myosin is doing to a part of an actin filament look here. The human body contains different skeletal muscle fibre types, which are termed type I and type II. Type I are much slower compared with type II. The reason for this is they contain different isoforms of myosin. The isoforms of myosin are termed alpha and beta myosin. alpha myosin is the faster isoform of myosin relative to beta myosin and is therefore the predominant isoform found in the fast type II fibres. alpha myosin attaches to actin and very quickly undergoes the isomerization pulling on actin and then rapidly detaches from actin returning to its original shape so that it can reattach to actin and pull on it again. Therefore it is capable of making the muscle shorten very quickly. beta myosin, on the other hand, attaches to actin and then the time taken to undergo isomerization and detachment from actin is much longer. Therefore the muscle containing this myosin type will shorten much more slowly. However the myosin remains attached to the actin for a longer period of time. Whilst myosin is attached to actin after it has gone through its isomerization step then it is generating force, so beta myosin generates more force than alpha myosin. So you can see that muscle that we want to shorten quickly, like the ones we use for running would be fast muscles and contain mostly alpha myosin, whilst muscles that we use, for example to keep us standing upright that need to generate a lot of force but not to shorten very quickly, will contain mostly beta myosin. I will now provide brief answers to your questions in light of this information. What determines the speed of reaction to the enzymes? The enzyme in this reaction is myosin and as you can now probably work out it is the myosin itself that is responsible for the rate limiting step under ideal conditions where there is free availability of all reaction products (I shall come back to this in relation to your question about oxygen consumption). Is there a limit to a speed that a muscle can contract? The simple answer is yes. This is called the unloaded shortening velocity of a muscle, ie how quickly the muscle can shorten when there is no load applied to it. When this is investigated in a piece of muscle, when the conditions are set so that there are no other reaction products limiting the rate of reaction, then it is as fast as the myosin is able to interact with the actin. Because of the arrangement of muscle sarcomeres in series then the rate of contraction is dependent on its length. The maximum unloaded shortening velocity is around 6 muscle lengths per second for a fast muscle fibre, eg a muscle 1 cm long will contract at 6cm per second. However, this can only occur for a very short time because as the muscle shortens the sarcomeres shorten and before very long they will not be able to shorten any more. How is the rate of Oxygen input determined? You may have noticed when looking at what a single molecule of myosin is doing to a part of an actin filament that whilst the actin attaches and detaches from actin a single molecule of ATP (adenosine triphosphate) is broken down to form ADP (adenosinediphosphate)and Pi(phosphate). This is the energy source for the reaction. So for muscle contraction to continue the muscle needs a ready supply of ATP. This is produced from the breakdown of glucose to carbon dioxide and water, which produces 38 molecules of ATP. At this point I would like to refer you to a previous ans wer that I have given on this website, which should provide all the background information that you need about the production of ATP. The fast type II muscle fibres can be divided into different categories, type IIa that do not fatigue quickly because they fully metabolise the ATP by oxidative phosphorylation and type IIb fibres do fatigue quickly because they primarily utilise glycolysis only. This is because they are specifically adapted to their particular function. The type IIb (fatigable) fast fibres are designed for very short bursts of high activity that can be performed using stored ATP and maybe some glycolysis, but it would be inefficient to maintain all the requirements within the cell to perform oxidative phosphorylation in these types of fibres so they have very low oxygen demands. Type IIa (non fatigable) fast fibres are utilised with slightly less intensity for a longer time and therefore it is important for them to utilise oxygen to work efficiently. So the short answer to your question about how the input of oxygen rate is determined is by specialisation of the muscle fibres. Those fibres that have a high oxygen demand have all the components necessary to fully metabolise glucose and consume oxygen, whilst those that only need to work for short periods without oxygen do not contain these specialised structures. If a fibre runs short of ATP then it will no longer be able to contract as quickly. If this happens then it is no longer the myosin that determines how quickly the muscle is able to contract but the amount of ATP becomes the limiting factor in the equation. I hope I have managed to answer your questions. I have tried to be as comprehensive as possible, which has resulted in a very long answer for which I apologise. If you have further questions on this then my email address is firstname.lastname@example.org and I will be happy to explain my answer to you further if necessary. Good luck and thank you for your interest Dave Burton
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