physiology

Skeletal muscle

After analyzing the main characteristics of the muscles of the human body and the various types of muscle tissue, let's focus on skeletal muscle.

Among the three (besides the one we remember the smooth one and the cardiac one), the skeletal muscle tissue is the most abundant, so much so that in an adult man it constitutes about 40% of the body weight. As the name suggests, skeletal muscle is connected to the bones; its movement of contraction and relaxation causes the bone segments into which it is inserted to modify the reciprocal position.

The bones are the passive component of the movement, while the skeletal muscles represent the active component, as they possess the ability to contract under nervous stimulation and to generate motive power.

Constituents of skeletal muscle
  • Water (about 75%)
  • Proteins (about 20%). The most important are myosin and actin.
  • Glycides (0.5-1.5%). The most important is glycogen.
  • Neutral fats, cholesterol and phospholipids.
  • Mineral salts (about 5%).
  • Enzymes.
  • Nitrogenous extractive substances (eg: creatine and urea) and non-nitrogenous extractive substances (eg lactic acid).
  • Pigments (ex: myoglobin)

The muscle transmits its strength to the bones by means of tendons, very resistant and slightly elastic fibrous structures. Tendons appear as cords or as fibrous sheets, depending on whether they are associated with long muscles or large muscles; in any case they are tightly welded to the muscle regions adjacent to them. In fact, the connective tissue of the muscle merges with the bundles of tendon collagen, constituting the so-called myotendinous junction. It is a particularly solid and resistant union, so that tendon injuries rarely occur at this level, while it is easier for the tendon to detach from the bone fragment where it is inserted.

THE MUSCLE PULLS THE BONES, BUT DOES NOT PUSH THEM!

For example, the biceps brachialis muscle, which allows us to flex the forearm, is not able to extend it.

Since the muscle cannot make the movement contrary to the one to which it is a member, the muscles work in pairs or groups of antagonists. In other words, each muscle corresponds to another with an opposite function. Returning to the previous example, the extension of the forearm is guaranteed by the contraction of the triceps.

In order for movement to occur, it is necessary that during the contraction and the shortening of one, the other relaxes and relaxes. Precisely for this reason, biceps and triceps are a classic example of antagonist muscles.

On the basis of their function, the muscles that collaborate in the execution of a movement are called AGONISTS, those ANTAGONISTS that oppose the reciprocal movement (for example, flexors and extenders are antagonistic to each other).

Similarly, there are muscles that have a synergistic action, as in the case of the brachialis and the biceps or the anconeus and the triceps; in this case we speak of agonist muscles.

A further distinction can be made between agonists and synergists; the first term, in fact, is up to those muscles that together allow a certain movement to be performed; Instead, those muscles that assist (facilitate) the movement generated by the agonists are given the synergistic adjective.

Skeletal muscles are never completely relaxed. Even during sleep there is a weak permanent contraction, called MUSCULAR TONE.

A little "nomenclature" :

There is talk of flexion when the centers of the bones attached to the muscle approach each other; vice versa, we talk about extension.

In relation to the movement it performs, we speak of the origin of a muscle to indicate the tendinous end closest to the most stable trunk or bone; instead, the insertion represents the most distal or mobile point of implantation (pulls the bony head behind). The two tendons of the brachial, for example, are inserted, respectively, in the lower half of the anterior aspect of the humerus (arm) and on the tuberosity of the ulna ("upper part of the forearm"). Since the main action of this muscle is to flex the forearm, the insertion point on the tuberosity of the ulna is called insertion.

The central portion of the muscle, generally shaped like a spindle, appears as a fleshy mass and is called a muscular belly. The contractile force depends on the volume and the fleshy part, but not only on it (in general, the greater its development and the greater the force produced during the contraction of skeletal muscle).