The muscular system is what is responsible for all movement of the body. This includes movement of blood, movement of the skeleton, and even movement of food trough the GIT.
Smooth Muscle
This is found in the wall of organs and blood vessels, and is responsible for moving the contents of whichever organ is in question. The most notable of these would be the sequential contraction of the walls of the GIT, to move the food bolus along.
Cardiac Muscle
Highly specialized type of smooth muscle, the muscle of the heart is responsible for pumping the blood around the body, as discussed earlier.
Skeletal Muscle
These are responsible for gross and fine motor functions of the body. In other words, all of the movements we choose to make.
Fibre Structure
Skeletal muscle is made up of groups of fibres. Much like a thick steel cable, these fibres are grouped together to form strands, and the strands are then grouped together to form the muscles as we view them. At the base of these fibres is what is known as myofibrils. Within these myofibrils is where all the action takes place. This is where fuels are broken down for energy (chiefly by the mitochondria), as well as where the message received at the neuro-muscular junction is converted into a chemical reaction leading to muscular contraction.
Muscular Contraction
In order to cause movement, sequential contractions of the skeletal muscles must occur. As these muscles contract (shorten) and relax (lengthen) they well cause movement of the bones, about their joints. There are three types of muscular contractions, which work together to create these movements:
Isometric – (same length) this type of contraction is when there is an increase in tension, by not change in length. In other words, the muscle does not lengthen, nor shorten, under the tension. This leads to stabilization.
Isokinetic – (same speed) this contraction is where the rate of change in length is kept constant. In other words, the tension is varied in order to keep the rate of movement the same.
Isotonic – (same tension) this is when movement is caused by the tension in the muscles. And, this is further divided into two types.
- Concentric contractions are when the muscles shortens under tension
- Eccentric contractions are when the muscles lengthen under tension
Sliding Filament Theory
The sliding filament theory is a relatively complex sequence of chemical reactions, all leading towards something as simple as myofilaments sliding over one another. When the acetyl choline is received by the muscle cells, from the terminal end of the motor neuron, it is taken up, and causes calcium ions (Ca++) to be released within the cell. This, in turn, leads to actin filaments binding with myosin filaments. The heads of the myosin filaments then twist and collapse onto the body of the myosin, using ATP as fuel for this movement. When acetyl-choline sterase is released into the neuro-muscular junction, Ca++ returns to where it is stored within the muscle cell, and the actin and myosin release their bind, and the muscle returns to its original length. Or, it relaxes.
When the resistance force is greater than what the muscle is used to, tiny micro tears develop in the myosin, at the bases of the heads. In instances where the force is great enough, the heads can even be torn off. This leads to what is known as delayed onset muscle soreness (DOMS). When these micro tears heal, the myosin is thickened, and made stronger. This forms the basis for hypertrophy, which will be discussed later.
Types of fibres
The types of muscle fibres can be broken down into 3 main categories. They are separated by rate and force of contraction, appearance, and fuel use.
Type I – highly vascular and red in colour, these fibres contract relatively slowly. They can, however, contract innumerable times, as long as fuel is supplied, and waste byproducts are removed. They use fats and carbohydrates from the bloodstream, as well as from within the muscle, for fuel, with oxygen.
Type IIa – less vascular than type I, and therefore have a pink appearance. These fibres contract significantly more quickly. As a result, they also fatigue fairly soon after contractions begin. Their contractions are also more powerful than Type I fibres. As a result of the greater force and speed of the contractions, they use fuel more quickly, and produce large amounts of waste byproducts. Their main fuel is carbohydrate stored within the muscle.
Type IIb – the least coloured of the 3 main types, as it has the least vascularity, and is often referred to as being white. These are the fastest contracting of the fibres, and also the most forceful. As has already been demonstrated, faster contractions lead to more rapid exhaustion. Because the rate of contraction is so rapid, they have to use a fuel that is stored within the muscle fibre, itself.
While distinct types of fibres are discernible, this shouldn’t be seen as exclusivity when it comes to contractions. All types of fibres are involved in muscular contractions. But, the ratio of how much work is done by which type of fibre is dictated by the nature of the activity. In other words, an activity requiring faster contractions will use a great amount action from Type II fibres.