Sliding Filaments

1 Sliding Filaments

On the left of this slide, there are two pictures. One shows a bicep muscle at rest. It has a myofibrilCylindrical structures within muscle fibers that contain myofilaments. (NOT a muscle fiber) also at rest.  On the bottom left we have the biceps brachiiFlexor / Supinator Front of upper arm; bends elbow and turns palm upward. muscle contracting and we can see that one myofibril is shortening.  Recall from another minilecture that these Z lines are the boundaries of a sarcomere. It is the smallest subunit of a muscle that still shortens in a contraction.  Let’s also highlight the Z lines on these pictures here with actin and myosin.  Compare the top picture with the bottom. 

The Z lines the contraction of a sarcomere seem to get closer together.  The H zoneThe middle of the sarcomere where only thick filaments are present. seems to get smaller, as do these I or light bands.  The only thin that gets bigger is the A or dark bands – those areas of overlap.  This is because as a contraction occurs, actin and myosin slide past each other.  As they do this, the sarcomere shortens. All the other sarcomeres on a myofibril also shorten. Then, all the muscle fibers that contain those myofibrils shorten as well.  This starts a chain reaction.  The muscle fibers shorten. They pull on the endomysium, which then pulls on the perimysium. This sequentially pulls on the epimysium and then on the tendon, aponeuroses, or the bone itself to move the bone.  Just a reminder here that we are looking at skeletal muscle, which is defined by its attachment to a bone.  

2 Cross Bridges

Let’s do some labeling here before we even get started.  Right?  Always establish the anatomyThe study of the structure of the human body. first.  The big blue thingies at the bottom of the slide form the thick filament. They are made of lots of myosin. These myosins have their tails and heads.  The reddish spheres are the actin subunitsthe small spheres of actin in the thin filament of the thin filament.  This purple line that seems to twist around the actin subunits is a protein called tropomyosin.  Take a really really close look at the thin filaments on the bottom pictures.  On the left, which is a relaxed muscle, tropomyosin seems to cover those active sites on the actin subunits.  By covering them, the myosin heads can’t connect to make a contraction. 

Tropomyosin seems to have twisted on the picture on the right. It represents a contracted muscle and reveals those actin active sites.  In factA statement based on direct observation that is repeatedly confirmed., you can see this myosin headRounded proximal end that fits into the acetabulum of the hip bone. here. It is attaching to the active siteThe specific region of an enzyme where a substrate binds and a reaction occurs.. This forms something we call a cross bridgeThe connection formed between myosin heads and actin filaments..  There is one more protein called troponin.  This is the three part structure that you can see more clearly in the top pictures.  On the left picture, troponin is holding tropomyosin in place, covering the actin active sites.  In the picture on the right, there is a green calcium cation attached to the troponin. The tropomyosin is not covering those active sites. This allows a cross bridge to form again. 

Now, let’s put it all in motion.  If a muscle fiber becomes flooded with calcium from the sarcoplasmic reticulum, then calcium cations attach to all these troponins.  Calcium is like a key and troponin is the lock.  Putting the key into the lock opens the door, or moves tropomyosin to reveal those active sites.  These two pictures illustrate what a cross bridge is. It is the attachment of a myosin head to an actin active site. The pictures show us a relaxed muscle on the left. They also show a contracted muscle on the right.

3 Making & Moving the Cross Bridge

OK, so, cross bridges are analogous with contraction?  Yes.  In fact, a million little cross bridges are forming, breaking, and reforming when you contract a muscle.  They do this continuously. People sometimes call this the cross bridge cycle (as if you needed me to explain that one).  This picture here shows that cycle, breaking it down into a couple of steps.  This picture starts at 12 o’clock.  This picture at the top is the first picture you should look at.

 Before I explain these, you should do something.  At this point in the minilectures, we have defined every anatomical structure that you see on this picture.  We just haven’t yet put it all together.  I challenge you to do that before listening to the rest of this slide.  Go to each picture and just describe what you see.  You don’t have to interpret the picture, just describe it.  For example, let’s do the first picture together.  I see that the myosin head of the thick filament is in a resting position.  The myosin head is not attached to the actin active site on the thin filament.  Tropomyosin is covering those active sites.  Troponin does not have calcium attached.  OK, now you go and do at least the next two pictures.

Did you use the word cross bridge for picture 2 here?  If not, go back and do so.  Picture 3 has an added arrow to indicate the movementA fundamental property of life involving motion of the body or its parts. of the actin filament past the myosin filament.  This is what is referred to as the power stroke.  This is when the two filaments, actin and myosin, slide past each other.  This is a theoryA well-tested and widely accepted explanation., not a fact.  Sliding filaments is a theory with a lot of support.  Although no one has directly observed it with all the fancy microscopes we have. We have microscopes like electron microscopes. We have seen shadows. These shadows tell us that this theory is probably pretty close to the truth. 

A cross bridge and the power stroke have all happened without the use of ATPThe energy currency of cells used for muscle contraction.. I want to emphasize this point.  Up until this point, we’ve used no energyThe capacity to do work or cause change..  YOU USE NO ENERGY TO MAKE A CONTRACTION!  Not even the power stroke uses energy, despite the misleading name.  So, these first three pictures illustrate the creation of movement in a contraction. They show the shortening of the sarcomere. This process makes the disarticulated arm on the left side of the screen contract its biceps brachii.

4 Detaching the Cross Bridge

While making a crossbridge and allowing filaments to slide past each other requires no ATP. However, ATP is needed to reset the filaments for another contraction. ATP is essential to detach the cross bridge. It also resets the filaments for a second or more cross bridge. That is exactly what you see in these two pictures right here or pictures 4 and 5 on this diagram.  In picture 4, the cross bridge is still attached and ATP is present.  In picture 5, the cross bridge is detached and ATP is present.  If we cycle back again to our beginning picture, the cross bridge is detached. The myosin head is also cocked back into its original position.  ATP is not present on this beginning picture. 

ADPA molecule produced when ATP releases energy. is present because we have used the ATP molecule to detach the cross bridge and cock the myosin head.  This is what allows the cycle to start again.  Writer’s cramp is the result of not enough ATP in the muscles of your hand.  It’s because these muscles are designed for sprinting through a quick quiz, not marathon testing.  These muscles naturally fatigue quickly. You don’t actually bring in enough oxygen to detach the cross bridges of these muscles. Breathing heavily while writing would allow them to detach.   The “cramp” or inability to move comes from too many cross bridges attached and no ATP to reset the cycle.  It’s like they all did their power stroke, but then couldn’t finish the cycle.

Rigor mortis occurs in the same way.  Rigor mortis is the last contraction you will ever have.  It is the last cross bridges you will ever make.  You don’t need ATP to make the cross bridges. About 24 hours after death, your body stiffens up. This happens as your muscles make as many cross bridges as they can.  Hours later, the actin and myosin proteinsLarge molecules made of amino acids with various functions in the body. will start to break down. They will degrade, releasing you from the stiffness of rigor mortis.