Muscling in

I know I haven’t posted anything for a while but I’ll get back into the swing of things very soon, I promise. Now that writing a dissertation is out of the way, I thought I’d write a little something about muscles, namely how they work and how they can be specialised for a particular function.

I gave you all a crash course in muscle anatomy a few posts ago, so if you haven’t read it yet, it’s a great place to start. And here’s a picture to jog your memory:

Figure 1. A schematic diagram of a muscle's microstructure. Credit: medicalpicturesinfo.com
Figure 1. A schematic diagram of a muscle’s microstructure. Credit: medicalpicturesinfo.com

As a quick recap, the actin filaments and myosin filaments (bottom right) overlap with each other between discs of proteins called z-discs. It is the overlapping nature of the filaments which causes the banding you can see in the myofibrils (top right). Two z-discs and the overlapping filaments which are anchored to them form a sarcomere (fig. 2).

A diagram of a sarcomere where actina and myson fibres overlap.
Figure 2. Diagram of a sarcomere with actin and myosin filaments between two z-discs. Dark bands in a myofibril are where actin and myosin overlap. © J. R. Lumbard, 2014.

When we contract a muscle, the myosin filaments pull on the actin filaments through molecular interactions, drawing the z-discs closer to one another and shortening the sarcomere. The combined action of millions of sarcomeres stacked end to end in a myofibril produces a visible contraction of the muscle.

As strong as a feather

Okay, so that’s not a common saying, but let me explain. Notice that I didn’t say above that sarcomeres stacked end to end produce useful muscle force. If you think in terms of string, this concept becomes clear. Five strings working together could theoretically support five times more weight than a single piece of string, while one string five times longer than the other but the same thickness wouldn’t be any stronger. If you’ve ever seen a strong person you’ll have noticed their muscles bulge: many sarcomeres are bundled together within each muscle fibre for greater strength.
This concept is exploited in the strongest muscles of our bodies including our jaw closing muscles and our quadriceps at the front of our thigh. In such muscles, myofibres are arranged arround central tendon at an angle, resembling the barbs of a feather coming off its central rachis (Wikipedia). As a result, these muscles are termed pennate or pinnate muscles, from the Latin for feather (fig. 3).

A pennate muscle where the angle between the central tendon and muscle fibres resemble a feather.

Because of this pennate architecture, the muscle fibres of pennate muscles are often quite short. But…

Length does matter

And it’s definitely to do with how you use it. I’ve just told you that the length of myofibril i.e. the number of end-to-end sarcomeres has no effect on its ability to produce contractile force. You’ll remember that myosin filaments act upon the actin filaments to bring the z-discs closer together and produce muscle contraction. For this to happen the actin and myosin filaments must overlap. So what happens when the length of a single sarcomere changes, when we stretch a muscle, for example? A stretched muscle means that the sarcomeres themselves are stretched resulting in less overlap between actin and myosin filaments. Less overlap means fewer molecular interactions. Fewer molecular interactions means less muscle force. This is why it’s hardest to flex your elbow when your arm is straight and weighed down by something heavy.

Therefore, muscles that change length a lot during, such as jaw opening muscles, use tend to have long sarcomeres to ensure that there is always enough overlap between actin and myosin to produce useful contractile force.

Chewing it over

The jaw opening and closing muscles therefore make a good case study for the advantages and disadvantages of different muscle architecture, as well as the position of muscles relative to the joint (fig. 4).
The jaw closing muscles are pennate with short muscle fibres, but also need to be able to exert high force even when the jaw is fully open. For this reason, they are situated close to the jaw joint meaning that even when your mouth is fully open, the muscles are not stretched too much.

Jaw closing muscles (e.g. masseter) are much closer to the jaw joint than jaw opening muscles (e.g. digastric)
Figure 4. Showing distances between the jaw joint and jaw closing muscle (green, masseter) and a jaw opening muscle (blue, digsatric). The distance is much shorter to the masseter than the digastric. Credit: 3D4 Medical.

The jaw opening muscles need to open the jaw to its full extent and therefore need to have a wide range of motion and must be able to operate even when the jaw is closed and their sarcomeres are fully stretched. These muscles therefore possess longer sarcomeres which ensures that actin and myosin filaments always overlap. These muscles are also very slender which means that they are weak compared to the jaw closing muscles. For this reason the jaw openers are situated far from the jaw joint in order to be able to generate a useful amount of torque and open the jaw.

So muscle strength depends on the number of myofibrils bundled together, while a large range of movement needs long sarcomeres arranged end-to-end. Very few muscles are entirely specialised one way or the other, but both types of muscle architecture are necessary in our day-to-day lives.

Well, I hope that’s been useful, interesting, or otherwise filled a few minutes of your day.
Watch this space for more posts soon!

– James.

References

3D4 Medical. (2014). The muscles of the jaw [digital image]. Available from: 3D4medical.com. [Accessed: 28/09/14].

Azizi, E., Brainerd, E. L., & Roberts, T. J. (2008). Variable gearing in pennate muscles. Proceedings of the National Academy of Sciences of the United States of America, 105, 1745–50. doi:10.1073/pnas.0709212105. Freely available here.

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