Skeletal Muscles I - Structure and Function
People generally think of muscles as being a singular mass of tissue that we use to move our bodies around, but behind the scenes they are much more complex than that! Muscles are important, allowing us to move, breathe, and live active lives. Let's discuss the makeup of skeletal muscles, the muscles that we use to control our skeleton and move our arms and legs.
Muscle fibres and myofibrils
A muscle is made up of thousands of muscle fibres, which are like elastic bands that all pull together to cause that one muscle to contract and move. These muscle fibres are surrounded by blood vessels which supply the muscle fibre with fresh oxygen and nutrition.
Big muscles like the ones in your legs (quads, hamstrings) have lots of muscle fibres, while smaller weaker muscles (like the ones in your wrist) have less. There is truly strength in numbers!
But if we dive even deeper, we can see that muscle fibres themselves are made up of a bunch of other strands. Muscle fibres themselves are made up of many smaller “elastic bands”, at this level they are called myofibrils.
But wait, there's more!
The base unit: Sarcomere
A myofibril gets all its power from its base unit: The Sarcomere (fig.1). One myofibril has many sarcomeres lined up in series (see fig. 2). These sarcomeres all generate a small pull, but when they are connected together, the force from their pulls are added up to generate quite a large amount of tension! A bunch of sarcomeres pull, causing the myofibril to pull, which causes the muscle fibre to pull, which in turn causes the muscle itself to contract; a chain reaction to generate large forces.
But still, to find the source of this force, we must dive one layer deeper and understand what makes up a sarcomere.
The origin of force: Actin and Myosin
And now we’ve reached the origin of force, this is where all muscular force is created. The sarcomere gets its force from actin and myosin. A sarcomere itself is made up of 1 Myosin filament, and 2 Actin filaments.
Myosin (thick filament) forms attachments (crossbridges) and pulls on Actin (thin filament) and shortens the entire length of the sarcomere. Think of Myosin like a strong person in a tug-o-war, pulling on the rope (actin).
This is happening across hundreds and thousands of individual sarcomeres. Every time you think to move your arm or leg, your brain sends electrical signals through your nerves (like electrical wires) to the muscles. These electrical signals are then turned into chemical signals, as the nerve releases calcium ions which tell the myosin filaments to pull on the actin. Thousands of small people (myosin filaments) then pull on their own individual ropes (actin filaments) and cause the overall muscle to shorten and contract.
All these myosin filaments need a lot of energy to continue to play tug-o-war and contract your muscles! Therefore we end up burning a lot of energy when we exercise and often need to eat more to replenish our energy stores.
What is muscle damage?
When you exercise. Your muscle tears at a microscopic level. The spaces between sarcomeres are slightly damaged, and the body responds by adding more sarcomeres to help share the load. This gives you bigger and longer muscles, and greater ability to generate force next time!
But when you overstretch and strain your muscles, you end up tearing your muscle fibres macroscopically and it ends up being a painful injury that you might see a physiotherapist for.
If you have any further questions on muscle function and how you might be able to improve painful and dysfunctional muscles, or how to manage muscle strains, please give us a call at 3061 7128 and talk to one of our physiotherapists.
Blog and videos by UQ Physiotherapy student undertaking clinical placement, supervised by principal physiotherapist, Winnie Lu.
References:
Geeves, M. A. (1991). The dynamics of actin and myosin association and the crossbridge model of muscle contraction. Biochemical Journal, 274(1), 1-14.
Pollard, T. D., Weihing, R. R., & Adelman, M. R. (1974). Actin and myosin and cell movemen. CRC critical reviews in biochemistry, 2(1), 1-65.
Rayment, I., Holden, H. M., Whittaker, M., Yohn, C. B., Lorenz, M., Holmes, K. C., & Milligan, R. A. (1993). Structure of the actin-myosin complex and its implications for muscle contraction. Science, 261(5117), 58-65.
Margaritelis, N. V., Theodorou, A. A., Baltzopoulos, V., Maganaris, C. N., Paschalis, V., Kyparos, A., Nikolaidis, M. G.. Muscle damage and inflammation after eccentric exercise: can the repeated bout effect be removed? Physiol Rep, 3 ( 12), 2015, e12648, doi: 10.14814/phy2.12648