Muscle contraction

Muscle contraction is a complex process with one simple objective of enabling the movement to an organism. This complex process can be studied from many different levels, and the knowledge about the musculo-skeletal system reaps benefits in a number of different areas or professions. Generally, the human health during the lifespan is very closely related to normal and effective function of this particular system.

 

D I V I S I O N S

 

Two basic types of muscle contractions are isotonic and isometric. With isotonic, the muscle gets shorter during the contraction, and with isometric, it stays the same lenght. The special type of excercise called isometry is using isometric contractions for developing the strenght of muscles and joints. It's used during the rehabilitations, like for example after having a cast for certain period due the broken bone. The muscle is feeble and significantly smaller so normal exercise is out of question.

 

It's also benefitial for those confined to bed temporarily or long-term. Isometry is basically flexing the muscles without moving or twisting of arms or legs during the contraction. It can be executed by pressing against unmovable object ( for example wall or bed), or simply by pressing two hand against each other. The best thing is that it doesn't require any equipment or skill, and it can be done anywhere and by anyone.  

 

Another special type of involuntary muscle contractions would be cramp or twitching. Stretching and massage are usually very effective in calming down the muscle, as those things usually happen after prolonged exercise or fasting, where the fine balance of electrolytes in the body is disturbed.

 

Muscle contraction works on all-or-none basis as there is no such thing as partial contraction. Plus, it's followed by so-called refractory period, during which the muscle is getting back ready for another contraction.

 

There are three basic types of muscle tissue in the body:

  a) cardiac - muscle of the heart

  b) smooth - muscle in the gut / involuntary

  c) striated - skeletal / voluntary muscles

 

Striated muscles then divides further into two types:

  a) tonic - slow fibres holding mostly the posture

  b) phasic - twitch fibres of two types:

        1 - slow (red fibres)

        2 - fast (white fibres)

 

Another division could be seen depending on the conditions around the muscle contraction. More specifically, it can be done with the presence or the absence of oxygen.

  a) aerobic - with oxygen / using oxygen

  b) anaerobic - without oxygen / creating oxygen dept

 

This oxygen dept is then compensated in the body by flooding the muscle with toxic compound called lactic acid. Accumulation of this substance in the muscle gives that characteristic feeling of burning in the muscle, during and at the end of very vigorous exercise. Lactic acid needs to be quickly eliminated that's why the burning stops soon after you stop moving. Because it's toxic, it's presence is dangerous to the organism, which is dealing with it three possible ways:

 

a) oxidize to pyruvic acid

b) convert to carbs

c) neutralized and excreted

 

Lactic acid and it's effect will make you stop moving, as any further movement at that precise point is litterally painful. It can flood the blood and liver and destroy the living tissue. Regular aerobic exercise and intake of quality food increase the resistance of body towards lactic acid, and more lean muscle tissue the person possess, longer the person can excercise without being stopped by accumulated lactic acid.

 

This diagram shows nicely how muscle contraction actuallly work on the tissue level. Actin and myosin myofilaments work together to create the complex called actomyosin. This complex is then capable of sliding the actin and myosin against each other, and sort of fold them up, shortening the overall lenght of the muscle fibre. Sarcomeres is a fancy term for the muscle cells, and all this model is called sliding filament hypothesis.

 

For muscle contraction to occur, there is a number of conditions that needs to be met, and there is actually a stream of insights coming from knowing those conditions. For the sake of simplicity, I created a simple list:

 

- the presence of the protein called actin

- the presence of the protein called myosin

- the presence of Ca

- the presence of Mg

- the presence of ATP

- the presence of creatine phosphate

- calcium pumps has enough of Ca pumped in

 

Simply put, calcium pumps located on the plasma membrane, pumps in calcium, which triggers all the other processes. Creatine phosphate is another, alternative source of energy in the muscles. They chip in when there is a shortage of ATP, or the exercise is so vigorous, that all energy sources are in the action.
 

3 FORMS OF ENERGY

On molecular level, the muscle has three big sources of energy to reach for. All three are continously available at all times, what differ is the ratios between them, which correlate with the state or activity of body. Plus they differ in how long they can be available, and the time for their recovery. Three form of energy are:

1. fats - triglycerides
2. glucose - monosaccharides
3. creatine phosphate

Fats are the biggest contributor supplying heart and all energy needed for keeping the posture, along with very slow movement. Fat are continuously used by muscles even if you don't move at all. Technically, fats can never run out completelly from the whole body, but their usage by muscles is limited, because it depends on the oxygen input. And that, in turn, depends on the state of your lungs, heart, and basically the whole body. So to burn more fat than you usually do, you need to improve the state of almost whole body.

Glucose is the type of monosaccharide and serves as main energy source for the brain with whole nervous system, which runs only on glucose. Then, muscles use glucose along with its secret source of it - the glycogen. This is sort of animal version of complex carbohydrates and it can be stored in muscles and liver. From there it can be used at any point, and when you then eat carbs again, you refill it.

Interesting thing is the ratio of fats and glucose in which muscles use them. Supposing you are lying down on the bed without moving. Fat / glucose ration would be about 70/30. But the moment you start moving fat goes down and glucose up. The quicker you will move, further the fats will go down, and glucose up in the ratio. Get the picture?

Creatine phosphate is very interesting substance stored in the muscles only. It start working only when we move very fast, or for longer time. It's the additional source of energy that is available for the muscles in the times of shortage of both fats and glucose. There is quite limited amount of creatine phosphate in muscles, so suppose you start running as fast as you can, it can last for about ten seconds. When you stop, and not move for another 30 seconds, it will be replaced with freshly-new made one.

The storage of this substance can be increased by involving the body or muscles in heavy exercise in short bursts. This is the case of sprinters or bodybuilders for example. They train muscles to develop bigger creatine phosphate stores in order to achive greater performance, or appearance. Those three types of energy can be then combined to bring about the movement or sport we need.

 

 This very nice flow diagram describes the muscle contraction cycle in four steps. The diagram is pretty much self-explanatory, so it's all the question of reading and trying to understand. First I recommend to map out the symbols from the key in the grey box, and then reading the steps should make easily a lot of sense on closer observation and study. Good luck and enjoy.
 

ATP - cell's rechargeable battery

ATP, or adenosine triphosphate, could be put in an analogy of a mini rechargeable battery that cells are using to feed the energy for many different processes within the body. It contains three basic parts.  The adenine molecule, sugar molecule (ribose), and three phosphate groups - which are really one phosphorus atom surrounded by four oxygen atoms.

The energy we talk about here is in the form of high-energy bond between those phosphate groups. When they separate, the energy is released, and then used. To put back on that last phosphate group, and therefore recharging it, it requires the energy which the body needs to get from the food we eat. This principle is explained on this picture.

 

When you take away that last phosphate group, you create the molecule called ADP - adenosine diphosphate (only 2 phosphate groups). That would be the representation of empty battery in this analogy. Rotating those two chemical reactions, then provides the means of an empty and full battery, which are moving from the place of using up to the place of recharging it. And back again and again cyclically.

 

For example, one molecule of glucose can bring up the production of as much as 38 ATPs, provided there is an oxygen present in the process. This is called an aerobic respiration. In the absence of oxygen, however, only two ATPs can be made - the process called anaerobic respiration. For more detailed view of how and where ATPs are made in the body, this flow diagram does a great job.

 

Here you can see that some ATPs are made in mitochondrion, some in the cytosol, and some in the gut - where happens the basic breaking down of food from the meals we eat. In the gut, the proteins, complex carbohydrates, and lipids are digested into the final products, which are in form of amino acids, glucose, fatty acids, and glycerol respectively.

 

First round of freshly made ATPs is happening in the cytosol, where the glucose and the glycerol are used for creating it. Cytosol is semi-fluid matrix inside of the cells, in which all the organelles are suspended. Mitochondrion is a special organelle for making ATPs and many other functions. There in the middle, there is a cyclical sequence of chemical reactions called TCA, or Krebs cycle (or citric acid cycle). This represents the biggest system for ATP production in the body.

 

Stress response

The stress response, commonly known as the fight-or-flight reaction, is the first part of so-called General Adaptation Syndrome (GAS). It's a short lasting physical reaction of the body (or organism) towards the real (or even any imagined) threat. This response can be seen from multiple of levels - molecular, cellular, anatomic or atomic. It can be defined by physical or/and chemical processes which are triggered by it, or by any physical symptoms on the perception level. Or by some other means.

Three phases of GAS are:

1. stress response (fight-or-flight reaction)
2. resistance
3. exhaustion

So in the hypothetical situation where your life is in danger and you are forced to run away, the first part is very explosive and effective accumulation of energy at your disposal. If the threat persists, the next phase called resistance, is much longer sustainable state of increased level of disposable energy, but decreasing with time. After a while, depending on physical condition of that particular person, the third phase called exhaustion will follow. This is the state where the most of the energy is already spent, and the body is no longer in a position to fight or flight anymore. 

But from those three phases the first one is by far most interesting, so let's go deeper into particular physiological and chemical changes that are triggered by the stress response. It's all happening in real time, so the stress response is occurring on the observable level (to a certain degree of course). The consequences of the stress response could be put into two broad categories:

a) short-term consequences

b) long-term consequences

and those two differ substantially, so let's elaborate both of them.

SHORT-TERM

In the short-term, the stress response can be useful and helpful by presenting some information (a lesson learned), the impulse for the body to improve, or get stronger. In the short-term, it can be even perceived as a pleasurable event, providing it's happening within well-known boundaries (like for example in adrenalin sports). But it can be harmful in the case when the impulse is too strong to handle, and it slips the person into any negative experience of stress. And in the big proportions we talk about trauma (on mental level) or the injury (on physical level).

The principle behind the stress response in the short-term is to provide the organism the opportunity to improve, or strengthen itself in some way. Providing there is the sufficient and quality recovery time, and handling the stress is still manageable (to the comfortable level), the result should be the improvement of the body in dealing with that particular type of stress or condition.

The analogy could be used of learning how to drive the car. The frustration and stress associated with handling new circumstances evoke the stress response on the number of occasions. But if handled, it leads into the ability of driving the car, which in turn becomes a very useful skill in life.

LONG-TERM

By the long-term consequences of stress response I don't mean cyclical and continual short-term stress response with recovery time on day-to-day basis. But mainly it means some degree of continual residue of stress, which hasn't been handled within the manageable (or comfortable) level. Or, in the case when there isn't sufficient recovery time provided to the organism, even if the impulse itself is handled well. This stress is not really used for an improvement of the organism, but instead it builds up as some form of toxic substances within the body, or it can manifest as the lack of useful (energy) substances in the body.

Now here, we really talk about quite negative consequences of the stress response. Those can be recognizable on physical level, as well as on the perception level. It's roughly summarized in the flow diagram bellow:

Noise, heat, infection, isolation, aggression, and many more other factors persisting in the experience of the individual, can lead into some sort of negative long-term consequences of the stress response. Concrete physical symptoms then follow, for example the memory loss, the rapid (unhealthy) weight loss, ulcers, heart diseases, loss of sexual function, and many many others.

DISSECTION OF STRESS RESPONSE:

Once we have cleared the function of it, and the differences between long-term and short-term, we can now go deeper into many particular changes within the body. For easier grasping of so many processes, I created a simple list of direct physical and/or chemical processes that follow immediate stress response. In brackets, I present the particular hormone, which is responsible for that particular physical response - even if only in those cases (of course) where it can be determined. So simply put, the stress response causes:

- increased heart rate (adrenalin)
- increased blood pressure (noradrenalin)
- decreased pain sensitivity (endorphines)
- stopped/slowed down the activity of digestive system (cortisol)
- stopped/slowed down the activity of reproductive system (cortisol)
- alerted mind (adrenalin)
- enriched blood with:
    a) glucose  (glucagon)
    b) fatty acids (cortisol)
- started up/kicked in of the immune system (ACTH)
- started up the proliferation (building up) of T-cells
- inhibited production of GH (growth hormone)
- stimulated release of CRH from hypothalamus into blood
- stimulated anterior pituitary to release ACTH
- stimulated cortisol synthesizing from cholesterol in adrenal cortex
- increased carbohydrate metabolism (glucagon, cortisol)
- stopped/slowed down the tissue repair (less GH)
- increased rate of blood circulation (adrenalin+noradrenalin)
- enhanced memory formation (adrenalin)
- increased signalling of sympathetic part of nervous system
- suppressed signalling of parasympathetic part of nervous system
- prepared body to deal with challenges (adrenalin)
- inhibited inflammatory response (cortisol)
- suppressed stomach activity
- reduced secretion of digestive acids
- decreased blood flow to the walls of the stomach
- slowed down epithelial cell replacement in the gut
- reduced thickness of mucous membrane in the gut
- reduced production of reproductive hormones
- loss of sexual desire (less testosterone)
- disruption of sexual performance (less testosterone)
- enhanced brain activity - agility - learning
- more relaxed lung muscles - prepared for more oxygen getting in
- increased volume of lungs / their effectiveness