The muscular system is formed by the set of muscles of our body. There are about 600 muscles in the human body; together they represent 40 to 50% of a person's total weight.
Muscles are able to contract and relax, generating movements that allow us to walk, run, jump, swim, write, propel food along the digestive tract, promote blood circulation in the body, urinate, defecate, blink. eyes, laugh, breathe…
Our ability to move depends on the joint action of bones, joints and muscle under the regulation of the nervous system.
In the human body there are large muscles such as those of the thigh and small muscles such as certain muscles of the face. They can be rounded (the eye orbicularis, for example); planes (those of the skull, among others); or fusiform (such as those on the arm).
But in general we can recognize three types of muscle in the human body:
- Unstriated muscle (smooth muscle);
- Skeletal striated muscle;
- Cardiac striated muscle.
The unstriated muscles have slow and involuntary contraction, that is, the movements generated by them occur independently of our will.
These muscles are responsible, for example, for the erection of hair on the skin (“shiver”) and for the movements of organs such as the esophagus, stomach, intestines, veins and arteries, ie muscles associated with peristaltic movements and blood flow in the body.
The skeletal striated muscles attach themselves to the bones usually through fibrous cords called sinews. They have vigorous and voluntary contraction, that is, their movements obey our will. Examples: the muscles of the legs, feet, arms and hands. O striated heart muscle It is the myocardium, the heart muscle, that promotes the heartbeat. Its contraction is vigorous and involuntary.
One of the main properties of muscles is the ability to contract; The contractility; It is she who makes the movements possible.
In the case of skeletal striated muscles, the bones act as levers and allow the movement to be effected. Sometimes movement is possible thanks to the antagonistic work of two muscles. For example, when you bend an arm, the biceps brachii contract, decrease in length, and increase in thickness. At the same time, the triceps brachii relaxes. By stretching the arm, the situation is reversed: the biceps brachii relaxes to normal size, and the triceps brachii contracts.