Muscle Physiology

skeletal muscle physiology

ALL YOU NEED TO KNOW ABOUT MUSCLE CONSTITUTION

Muscles… Here they are. It would seam, what’s that difficult about skeletal muscles? Everything about them had been studied and discovered long ago. But this statement is rather deceptive.

The outward simplicity of skeletal muscle physiology is followed by the two problem questions that science hasn’t yet answered: how do muscle grow and at the cost of what do they contract? Sure, we can answer these questions in general; we know the factors that influence these processes and can control them to some extent; but this control is not complete (otherwise we’d look like Olympic gods). For years we had to try different wokout techniques searching for the most efficient ones; and this shooting at random has rather questionable efficiency. In order to increase it, let’s discuss what we really know about skeletal muscle and their physiology.

There are three types of muscle tissue: skeletal, unstriated and cardiac. The function of cardiac tissue is clear due to its name; so we’ll skip it. Unstriated muscles contract vascular walls helping blood running; they also contract our bowels assisting the process of food movement and perform other important functions.

Finally, skeletal muscles, that are most important for us, are responsible for moving of different skeletal parts against each other. Let’s take a closer look at their constitution.

Skeletal muscle anatomy

 

A basic contractive unit of any skeletal muscle is a muscle fiber – a big stretched (up to 7 inches) cell that has the shape of a prolonged cylinder with pointed edges. This fiber (like any other cell) is surrounded with covering – sarcolemma. Groups of muscle fibers constitute bundles that are united into one muscle attached by its edges to bones by means of tendons. With the help of tendons bones receive force of muscle contraction; and thus we can move.

muscle fiber  anatomy

Control over the process of contraction muscles is performed by nerve cells (axons). Each of them (like an octopus) has lots of appendices that are attached to separate muscle fibers. One nerve cell activates a group of muscle fibers forcing them to work as a unit.

Not all nerve cells are active at the same time, however. That is why usually only a small part of a muscle is working but not the whole muscle. That is its main peculiarity: muscle cannot contract entirely but only in parts what allows us to regulate the force and speed of its contraction. The weaker the brain signal is the fewer muscle fibers are contracting. That is why our mental impulse for each training session is so important.

It is also important to mention that each skeletal muscle has a restricting mechanism to control muscle tension. When tendon receptors register critical tensions they start hampering the process of contraction. In critical situations a person is said to be able to ‘switch off’ the tension control and demonstrate superpower.

Contraction muscle

 

The process of skeletal muscle contraction depends on muscle fibers physiology – basic construction material of contracting muscles. A muscle fiber is a very interesting cell; it has two special features.

skeletal muscle physiologyFirst of all, any skeletal muscle fiber is multinuclear; ‘reserve’ nuclei appear to be satellite cells that, in comparison with muscle fibers, are capable of segmentation throughout the whole period of their life. They enable skeletal muscle fibers to increase their mass and regenerate. That is why a wholly damaged muscle can regenerate: in case of a trauma satellite cells segment and transform into new muscle fibers.

Second, a muscle fiber cytoplasm has specific thin fibrils (myofibrils or contractile elements) that are situated along the cell parallel to each other. They are able to reduce their length according to nerve impulses and due to this means they can constrict their muscle fiber. Myofibril has diametrical striation (its dark and light strips interchange); when contracting ‘light’ strips reduce their length almost disappearing in case of complete contractions.

Interchange of light and dark strips in the myofibril thread is determined by ordered disposition of thick threads of myosin and thin threads of actin. Contraction of a muscle is performed by means of retraction of thin actin threads between thick myosin ones. The slip of actin threads along myosin ones is possible because myosin threads have lateral appendices called bridges.

Movement of myosin bridges can be compared with strokes of galley oars. Like a galley moves on the water due to oar strokes, slip of threads is possible due to stroke movements of myosin bridges. The only difference is that the movement of bridges is asynchronous.

Muscle physiology — energy profile

So, muscle contraction is performed due to movement of myosin bridges and requires a lot of energy. Energy reserve (of ATP molecules) in a muscle is limited; that’s why muscles always require continuous energy supply when working. Any muscle has three sources of energy supply:

  • decomposition of phosphocreatine;
  • glycolysis;
  • oxidation of organic substances in mitochondrions.

 

Decomposition of phosphocreatine

skeletal muscle physiologyAn ATP molecule is a universal energy source of all living organisms. An ATP molecule, by turning into ‘useless’ ADP, gives out energy in the form that’s the most appropriate for energy consumption.

ATP + H2O = ADP and acid + energy

But the miracle lies in the fact that a ‘useless’ ADP molecule is able to turn back into a ‘useful’ ATP one if our organism gets enough phosphocreatine.

ADP + phosphocreatine = ATP + creatine

It’s important to mention that our organism needs only several minutes to recover its phosphocreatine supply and that recovery can take place only after muscles stop working. If phosphocreatine supply could be recovered with muscles working we would be able to work for a long time with heavy weights and numerous repetitions.

Glycolysis

It is the process of decomposition of one glucose molecule into two molecules of lactic acid followed by liberation of energy that’s enough for ‘changing’ of two ATP molecules. This process also takes place in muscle fibers with the help of 10 special ferments.

1 molecule of glucose + ferments + ADP = 2 molecules of lactic acid + 2ATP + H2O

The process of glycolysis doesn’t require oxygen (such processes are called anaerobic ones) and allows a muscle to recover its ATP supply very quickly.

Oxidation

skeletal muscle growthThis process takes place in mitochondrions (energy stations of a cell) and requires oxygen (and, consequently, the time to deliver it). Such processes are called aerobic. At the beginning, oxidation goes on up to the glycolysis stage (see above). In the result of this reaction molecules of pyruvate appear; they penetrate into mitochondrions and turn there by means of further oxidation into CO2 and H2O. This process is followed by liberation of energy that’s enough for making of 36 ATP molecules (Krebs cycle). Schematically, it looks like this:

glucose + oxygen + 38ADP = carbonic gas + water + 38ATP

So, aerobic decomposition of glucose liberates enough energy for replenishment of 38 ATP molecules. It means that oxidation is 19 times more effective than glycolysis but it requires a lot of time due to the necessity to deliver oxygen.

Types of muscle fibers (skeletal muscle anatomy)

Skeletal muscles and muscle fibers that constitute them differ according to numerous characteristics: contraction speed, fatigability, diameter, colour, etc. Traditionally, all skeletal muscle fibers are divided into red and white, slow and fast, glycolytic and oxidizing ones.

Oxidizing or red muscle fibers are of small diameter and surrounded by numerous capillaries; they contain a lot of myoglobin (this protein is the true reason of red colour of these fibers). They receive energy by means of mitochondrion oxidation of carbohydrates and fatty acids.

Glycolytic or white muscular fibers have bigger diameter and contain a lot of glycogen that serves as their reserve nutrient. Glycogen dissociates into glucose that serves as glycolysis ‘fuel’.

All skeletal muscles consist of two main types of muscle fibers: those that are able to contract very quickly (power ones) and those that contract rather slowly (endurance ones). Contraction speed of a muscle fiber is determined by the type of myosin (the contracting cell part). There are different types of this protein: some of them provide quick contraction, other – high endurance; the third – a certain combination of both factors.

workout exercisesIt is an interesting fact: ‘quick’ fibers prevail in muscles of weightlifters and ‘slow’ ones – in muscles of marathoners. We still don’t know the reason of such strange ‘priority’. Some scientists say that it is due to sportsmen’s genetics, other – that the reason lies in changes that undergo their muscles when training. I personally believe that fibers can really change under influence of corresponding loads.

Now I need to say something really important. Exactly ‘quick’ fibers are capable of considerable hypertrophy; that is why those who have more ‘quick’ fibers in their skeletal muscles are able to increase their total muscle mass very quickly. I call such guys genetically gifted or just lucky men. As a rule, they are strong but possess weak endurance if they don’t train it additionally.

It is extremely important to define what type of fibers prevails in your skeletal muscles to choose the optimal training regime. ‘Your body is your own lab. Experiment with it and it’ll find its way!’

Written by: Dennis Borisov
© April 2010
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