Skeletal muscle: structure, function, contraction

Most men would certainly like a well-defined muscular physique that they can show off at the pool. But few of us actually, ask ourselves how this appealing tissues is structured and how it functions.

Skeletal muscle, also called voluntary muscle, comprises approximately 40% of bodyweight and contains 50-75% of all proteins in a human's body. It is the most plastic and dynamic tissue we have. Muscle mass is mainly determined by the balance between protein synthesis and breakdown. Hormonal activity, nutrition, physical activity and other variables influence this balance. 
 
Muscles attach to bones or other muscles by tendons. There are approximately 650 named muscles in the body and nearly a quarter of these are in the head and neck alone. Let's now investigate what is the primary purpose of this fascinating tissue.

​The functions of the skeletal muscular system
 
Being voluntary, skeletal muscles are the only tissue in our body that is under our conscious control and has the ability to contract. This allows it to fulfil its main function, which is to create movement. In most cases during movement opposing muscles work in pairs to allow the movement. The prime mover (agonist) contracts, while the opposing muscle (antagonist) relaxes.
 
A function that relates to movement is the transport of some substances. Muscles assist greatly in venous return and lymphatic circulation, for example.
 
Another important function of skeletal muscles is to assist joint stability and maintain posture. Although ligaments play a major role, it is the muscles that keep joints stable and aligned during movement. Maintaining a precise balance between contraction and relaxation, called muscle tone, keeps the body stable in space. This function also helps with preventing the development of dysfunctional imbalances and minimising the risk of injuries.
 
A lot of heat is produced during muscular contractions. The majority of which is used to maintain body temperature. Shivering and dilation (widening) of capillaries are another way in which muscles assist thermal control.
 
Muscles also play an important role in protecting organs, particularly the ones in the abdominal cavity. They also help organs function more efficiently. A wide, open chest and well-aligned spine held in place by functional muscles allow better breathing. This makes the job of the lungs and the heart easier.
 
Skeletal muscles can also affect the autonomic nervous system. For example, we can will ourselves to slow our breath rate and extend our exhalation with the help of the diaphragm and the intercostal muscles. This would stimulate our parasympathetic nervous system. As a result, we'll experience a sense of calmness, slower pulse, and lower blood pressure almost instantly.
 
Muscles are also interrelated with our endocrine system. Having well-developed helps with blood sugar control. This makes the job of the insulin-releasing pancreas easier. As another example, increasing our muscle mass will put greater demands on our metabolism. This would inevitably have some effect on the thyroid gland.
 
More recently, research suggests that exercised muscles can have a significant role in cancer protection. There are at least two possible mechanisms. The first one includes the normalising effect on growth factors that muscle activity has. The second has to do with the release of anti-inflammatory chemicals.

​The structure of a skeletal muscle
 
Skeletal muscle is made of approximately 75% water, 20% protein and 5% fats, minerals and stored sugar in the form of glycogen. Thinking of 'muscles' as individualised units is an illusion which collapses when investigating cadavers. In reality, muscle tissue is distributed across the body and held and organised by the fascia.
 
If we conceptually isolate some skeletal muscle tissue as a recognisable muscle, we can consider it as an organ of the muscular system. As such, it is made of muscle, connective, nerve, blood and vascular tissue.
 
What we normally call a muscle is wrapped in a collagen covering called the epimysium. As we go deeper in it, we discover that the muscle is composed of smaller units called fascicles. Again, they are wrapped by collagen fibres forming what the perimysium.
 
Fascicles are made of individual muscles fibres (cells), grouped and wrapped in endomysium. Nerve axons travel through the fascicles and branch out to innervate (form a neuromuscular junction) the individual fibres.
 
Next to each muscle fibre, there are satellite cells. These can either mature to a fibre or fuse with damaged fibres to repair them. Looking into the fibre itself, we see that there are cylindrical strands of contractile proteins. These are called myofibrils and there are hundreds of them within a section of a muscle fibre.
 
Looking into the fibre itself, we see that almost its entire cross section consists of long, cylindrical strands of contactile proteins. These are called myofibrils and there are hundreds of them within a section of a muscle fibre.
 
Myofibrils, themselves are divided by Z disc marks into contractile units of muscle called sarcomeres. The sarcomeres are composed of thin and thick myofilaments. The thin ones attach to one end of the Z disc while the thick ones lie at the centre overlapping the thin ones.
 
The thick filaments are composed of myosin, which is an elongated protein. Thin filaments are composed of actin. 
 
The sliding filament theory of muscle contraction
 
The structure of sarcomeres, as described above, enables them to contract and create muscle shortening. Let's find out how this happens.
 
Myosin molecules are shaped like golf clubs with the head pointing away from the surface of the thick filament. When this structure binds to the thin filament a cross-bridge is formed. This link is enabled by Ca++ ions binding to troponin, another protein found at intervals.
 
This enables the cross-bridges to pull on the thin filament, which creates a shortening of the sarcomere. Then the actin is released from the cross-bridge and this completes one pull cycle. The metabolic cost of this is one ATP molecule. This cycling can continue in the presence of Ca++ as until the muscle reaches its full contraction.
 
During the described process of contraction, muscles become shorter and thicker in appearance. The shortening of the individual fibres causes the whole muscle to shorten and pull on the bone through the tendon. This is how movement is generated.

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The ‘all-or-none’ law of muscle contraction
 
As mentioned before, nerve axons innervate each muscle fibre. The motor neuron from which these axons stem together with all the fibres that they innervate form a motor unit.  Usually, groups of these units work in harmony to orchestrate the contraction of a single muscle.
 
Skeletal muscles contract when they receive an electrical signal called action potential. This signal travels along the nerve cells innervating the fibres. When they reach the cell, action potentials trigger a chain of chemical reactions.
 
The motor unit then responds as a single unit. If the stimulus is strong enough to cause a response, all the fibres within the unit will contract. If it is not sufficient, not a single fibre will respond. This behaviour of motor units is called the all-or-none law.
 
The degree to which the muscle as a whole contracts, then, will be determined primarily by the number of motor units recruited through the signal coming from the central nervous system (CNS). The greater the intensity of the contraction, the greater the number of motor units recruited. Performing a deadlift with 100 kg will recruit more units compared to 50 kg.
 
Here is an example that illustrates how the two principles described above work together. Imagine that you are trying to lift a heavy box from the floor. You squat down, hold it firmly try to lift it above the floor and it wouldn’t move. You stand up, reset your position, squat down, take a deep breath and voila… This time you are able to get up with the box in your hands.
 
What just happened is that on your second attempt you recruited more motor units by sending a stronger signal from your brain. The apparent increase in your strength did not happen because more fibres within the same number of units responded, which would be a violation of the all-or-none law.
 
Now, consider a slightly different scenario. When you failed to lift the box the first time instead of taking a pause and trying again you just kept trying by progressively increasing your effort. That way you are sending impulses one after the other to the muscle with no rest in between. In this case, the sum of all stimuli occurs which increases the strength of the contraction. This process called multiple wave summation is what allows you to eventually lift the box as you try harder and harder. Of course, the weight of the box must be within your current strength reach.