Themuscle is a contractile organ that performs a motor function in animals. It consists of muscular tissue capable of contracting, determining the intrinsic movement of some organs or the movement of limbs, and, in general, locomotion. This property is due to the structures of the muscle tissue, both morphological and chemical. There are three types of muscle tissue (smooth,striated,cardiac) (Fig. 1.1), all of which have, to some extent, elongated, contractile and elastic cells, calledmuscle fibers.
Smooth muscle tissue is present in involuntary muscles, andstriated tissue in voluntary muscles;cardiac muscle tissue is an exception, as it is striated and involuntary.
Figure 1.1 Types of muscle tissue.
Striated muscle is made up of a long series offibers. Each of these is made up of other thinner filaments, called myofibrils, which are composed of two proteins,actin andmyosin. At the ends of the muscle bundle, there are tendons, which connect the muscle to the bones.
When we decide to make a movement, we send an electrical impulse to the muscle through the nervous system. This impulse comes from the brain and causes the actin and myosin filaments to slide. In this way, the filaments shorten, causing the contraction of the entire muscle bundle that moves the bones.
The muscle exerts effort, thus requiring energy expenditure. This energy is supplied by a phosphorus compound, called adenosine triphosphate (ATP), produced by a cellular organelle called the mitochondria starting from the sugars that reach the muscle through the blood. The mitochondrion can produce ATP in two different ways: in the first case it uses oxygen, which is aerobic work; in the second case it resorts to lactic acid, thus resulting in an anaerobic workout. There aremore than 600 muscles and they make up about 40% of our body weight (Fig. 1.2); from the large muscles of the shoulders and legs to the small muscles located in the eye sockets that move the eye,each one contributes to the body’s extraordinary range of motion and have the ability tocontract andrelease successively.
Figure 1.2 The major muscles of the body.
Muscles are connected to the bones through robust formations of fibrous tissue, calledtendons, which shorten when the muscle contracts, resulting in the convergence of the two bones on which they are inserted and, consequently, movement.
Muscles can be subdivided intodeep muscles when they are connected/attached only to bones, andsuperficial muscles, when one of the attachment points is the skin. The latter are the muscles of the face, or mimics, as their contraction changes the construct of the skin of the face, producing facial expressions.
Skeletal muscles arevoluntary muscles, which means that they contract on our command, although they can also contract following an involuntary reflex stimulus.
On the other hand,smooth muscles areinvoluntary and found in visceral structures.Without our intervention, they allow automatic motility of the organs, that is, the one that regulates the progression of food in t
