Muscle Contraction and (Microscopic) Anatomy. Some copyright issues but I won t tell if you

Transcription

1 Muscle Contraction and (Microscopic) Anatomy Some copyright issues but I won t tell if you don t

2 What is a contraction? It is so important to visualise the fact that muscles work their magic by simply getting shorter. I mentioned a couple of times that our muscles perform their feats of strength, either to move, support or stabilize, in a similar way that hydraulic systems provide the force for a piece of heavy construction equipment. I managed to get a picture of one of the smaller machines this morning, which you can see below. The hydraulic cylinders are clearly visible and you don t need to be expert to figure out which joint each of the cylinders actuates. But you can also see that there is only one hydraulic cylinder per joint, which can either push or pull and move the joint in both directions. We know that muscle can only move in one direction (pull), so we need to have two muscles (minimum) in opposition in order to move a joint in two directions. We ll be discussing muscle antagonistic pairs next week. But for now it s important to understand that muscles contract - get shorter - and that the mechanical energy from this shortening is used to pull on a bone and provide the incentive to move said bone that s at the macro level We need to appreciate the micro level of the muscle - picturing and understanding what and how the contractile unit of the muscle cell can provide this overall shortening of the muscle. This means picturing and understanding the interactions and energy requirements of the sliding filament theory - i.e. how the thin filaments (actin) and thick filaments (myosin) are arranged and move within the visible banding patterns known as the sarcomere (the contractile unit) In it s simplest form the two filaments interact much like the in the diagram on the next page. the guy in the middle is representing the myosin filaments and he s holding on to the actin. during a contraction he moves his had up the actin filament and grabs and pulls, which brings the z-band closer. Compare that to the standard filament diagram above and you can see the heads of the myosin filaments grab on the actin and pull in towards the centre. the steps of this process are shown a few pages in. As I mentioned in class you should know the for stages of this process s indicated on that page.

3 Sliding Filament Model The link above will take you to a Nature paper that describes all of the details of the sliding filament theory and is a good resource to keep for the future.. What follows from here are a couple of pictures that I borrowed from one of my ipad apps that has a similar blow-up of the muscle structure from the molecular to the gross anatomical. It just happens to look more natural then the one in your textbook so I provide it here for labelling practise. After those diagrams I have summarised the sliding filament model (which I got from the nature paper) and also explains how the whole thing starts when Ca ions are released from the sarcoplasmic reticulum, where it is stored after the muscle has received a signal from the nervous system

4 Nice muscle structure pictures from DK: The Human Body ipad App with labels

5 Nice muscle structure pictures from DK: The Human Body ipad App without labels

6 In this simple diagram, the cycle of Myosin Filament (MF) head attachment to Actin Filament (AF) is shown along with the ATP hydrolysis cycle. which represents the contraction process of a muscle fibre The Sliding ffment Model. What is not shown here are the changes to the AFs necessary to allow this process to go on in the first place. In the relaxed state there are proteins on the AF that prevent the MF heads from forming cross bridges (step 2 in this diagram). On the next page I try to show diagrammatically in keeping with style of this simple presentation - the necessary beginning step of Calcium release, as well as how the contraction process It s important to know all of the steps below even though they only demonstrate the cycle once a contraction starts. 1. AF ADP! P i! 2. ADP! P i! MF Weakly Bound ATP hydrolysis to ADP and P i MF Head returns to cocked position 1. Loss of P i bond between MF Head and AF binding site stengthens 2. Loss of ADP - Powerstoke ATP! ATP bonds to MF Head, releases from AF binding site

7 Calcium, troponin and tropomyosin and actin too The Actin Filament (AF) is a complex of proteins made up mostly of the same actin filament (the oranges) found in the cytoskeleton of most cells. In the myofibril, the actin is surrounded by another fibre called tropomyosin (the blue line), that has troponin molecules (yellow spheres) at various spots. The Tropomyosin covers up areas on the actin (the black spots) that the heads of the Myosin filament (MF) could bind too. When a contraction is called for, the muscle cell secretes Calcium ions from its store house the sarcoplasmic reticulum and the Ca 2+ binds to the Troponin molecules, causing them to change shape, which drags the tropomyosin with them moving the tropomyosin away from the MF binding sites which means the cycle shown in the previous slide can take place 1. Calcium ion release and binding to troponin 2. Troponin changes shape and moves the tropomyosin, exposing the binding sites blah, blah, blah sliding filament 3. Calcium ions are pumped back into the sarcoplasmic reticulum from whence they came and the troponin/ tropomyosin go back to where they were yada, yada, yada no more cross bridging with the Myosin no more contractions