50. Thetanus and its species.

Change in contraction force is observed with high-frequency rhythmic stimulation of skeletal muscles. The strong and lasting contraction that arises in this case is called the tetanus.

If the skeletal muscle is irritated with such a frequency of rhythmic impulses at which each subsequent irritation will fall into the muscle relaxation period from the previous irritation, then the so-called incomplete summation of single muscle contractions occurs and a jagged tetanus is formed . At the same time, the force of contraction gradually increases, and characteristic teeth and depressions are recorded on the contraction curve.

If the frequency of irritation is increased to such an extent that each subsequent irritation will fall into the period of shortening or development of muscle tension from the previous irritation, then the so-called total summation of single contractions occurs . In this case, a smooth tetanus arises, in which the contraction force increases faster, smoothly and to more significant values.

 

 

51. The mechanism of occurrence of tetanic contractions.

When a second stimulus is applied during a period of shortening or development of muscle tension, the summation of two successive contractions occurs and the resulting response in amplitude becomes much higher than with a single stimulus; if the muscle fiber or muscle is stimulated with such a frequency that repeated stimuli will fall on the period of shortening, or the development of tension, then the total summation of single contractions occurs and a smooth tetanus develops. Thetanus is a strong and lasting muscle contraction. It is believed that the basis for this phenomenon is an increase in the concentration of calcium inside the cell, which allows the reaction of actin and myosin to interact and the generation of muscle strength by the transverse bridges for a fairly long time. With a decrease in the frequency of stimulation, an option is possible when a second stimulus is applied during the relaxation period. In this case, the summation of muscle contractions will also occur, however, a characteristic retraction on the muscle contraction curve will be observed - incomplete summation, or dentate tetanus.

With tetanus, the summation of muscle contractions occurs, while PD of muscle fibers do not stack

 

52. The dependence of the amplitude of the tetanus on the frequency of irritation

 

53. Muscle tone and its difference from tetanus.

 

54. Work and muscle power. Types of work: dynamic (overcoming and inferior) and static (holding). The law of medium loads.

With contractions of the skeletal muscles in vivo, mainly in the isometric contraction mode, for example, with a fixed pose, they talk about static work, and when making movements - about dynamic work.

The contraction force and the work performed by the muscle per unit time (power) do not remain constant during static and dynamic work. As a result of prolonged activity, the performance of skeletal muscles decreases. This phenomenon is called fatigue contraction and a period of relaxation.

Static operation is more tedious than dynamic. Fatigue of an isolated sk . muscle  is due primarily to the fact that in the process of doing work in the muscle fibers accumulate oxidation products - lactic and pyruvic acid.

According to the laws of physics, work is the energy expended on the displacement of the body with a certain strength to a certain distance: A = the F * the S . If muscle contraction occurs without load (in isotonic mode), then mechanical work is zero. If, at maximum load, muscle shortening does not occur (isometric mode), then the work is also zero. In this case, the energy of the working muscle is completely converted into thermal energy.

According to the law of medium loads, the muscle can perform maximum work with medium loads.