Muscular contractions regimes.

At a muscle irritation by single stimulus the single muscular contraction arises. One can distinguish the latent period (from irritation beginning to answer-back reaction beginning), shortnening period (actually contraction) and relaxation period. In reply to a rhythmic irritation (namely the such one our muscles are received) the muscle is reduced lengthly (for a long time). Such contraction has received the name tetanic or summarized. If each subsequent pulse approaches to a muscle in the period, when it began to be relaxed, there is an infused or incomplete tetanus. If the interval between irritations decreases so, that each subsequent pulse comes to a muscle, at that moment, when it is in a contraction phase, there is a smooth tetanus.

In a certain degree the tetanus formation mechanism is explained by superposition phenomenon. However, it can be caused by excitability changing as well. And if to take into account, that the excitability changes are caused by membrane potential change features during exaltation, then it is easy to explain smooth tetanus occurrence and its size. Let's try to understand this phenomenon together. If to render an irritation to muscle during its contraction (smooth tetanus) or relaxation (incomplete or infused tetanus), it is necessary on that moment the excitability increasing existance. Why it’s so? At this time the slow depolarization phase develops in a muscle, when the membrane potential is lower, than in rest state, but is higher, the than threshold potential. That’s why even subthreshold (subliminal) stimulus will cause the depolarization acceleration (i.e. the excitability at this time in a muscle is raised - supernormal excitability). Fast depolarization beginning results in the situation when the tissue loses ability to react to an irritation. This phase refers to as absolute refracterity (absolute inexcitability). At repolarization time the excitability is restored. This period refers to as relative refracterity. An excitability at this moment is below than the initial one, and only strong (epiliminal) stimuli can cause the answer-back reaction. Then when the restful (remainded) repolarization develops, the excitability grows and becomes above initial. This phase refers to as exaltation (hyperexcitability). During its occurrence even subliminal stimuli can cause the answer-back reaction. Precisely at this moment the threshold stimuli also cause the phenomenon of a tetanus (both infused, and smooth). That’s why this reaction is more on size, than the single muscular contractrion. Further a membrane hyperpolarization comes and the excitability falls, it is a a subnormal excitability phase. At this moment the epiliminal stimulus is required to cause the answer-back reaction.

Under natural (physiological) activity conditions in human being organism the muscle shortness degree can be various.

One can differentiate the following types of muscular contraction according to the shortness size:

1) isotonic is the muscular contraction, at which its fibers are shortened at a constant external load (under real conditions such type is practically absent);

2) isometric is a muscular activation type, at which it develops a strain (tension) without the length change, it underlies the static work;

3) auxotonic is a regimen, in which the muscles develop a tension and are shortened, such reductions are the characteristic of walking, run, sailing.

The muscles have the certain force. The myodynamia (muscle force) is the greatest load size, which it can lift. There is a concept of an absolute muscle force -it is a maximal load, which the muscle lifts on 1 sm of transversal physiological section. For example, at a masseter it makes - 10,0 kg /sm². Besides there is a concept of a relative muscle force. It is the muscle ability to rise of a load on unit of a muscle anatomic section (is measured in kg / sm²).

The muscular force grows during all period of a childhood, but especially intensively - in young age. At the second childhood period beginning the force of the majority of muscular groups in boys and girls does not differ. By at 12-15 years of age, the muscles force in boys becomes approximately on 30 % more, than in girls. With age especially after 8 years, the ability to performance of long muscular work – endurance - is enlarged. It is higher in boys.

The muscular work is determined by product of mass of the lifted load on muscle shortage size. All human muscles useful action coefficient is equal to 15-25 %, at trained people it is higher - 35 %. There is a law of average loadings, at which the muscle is working for a long time at average loads in an optimum (average) contraction rhythm. At long-termed exercise the working muscular hypertrophy is developed. There occurs the whole musculation mass and each muscular fiber mass augmentation. At a hypodynaemia muscles atrophy comes. At long mode of operations of muscles weariness comes - subjective status, and then the fatigue develops. The objective attributes of ability to work hard decreasing join to the feeling of weariness: the force, endurance, rate of impellent (motor) reactions falls. One can distinguish the acute fatigue - the result of a hard work (for example, sport competitions) and the chronic fatigue - the result of repeated regular influence of loads without regular rest.

Fatigue reasons:

1) accumulation metabolites (lactic, pyruvic and other acids, ions suppressing an action potential) in muscular tissue;

2) power (energy) musclular stocks exhaustion (glycogen, ATP);

3) infringement as a result of a muscular circulation tension;

4) nervous centers efficiency (capacity for work) change. The efficiency is quickly restored at active rest, when there is activity kind change or change of working bodies (organs).

In musclular work there can be two statuses:

1) dynamic - there is a load moving and movement of bones and joints;

2) static - the muscular fibers develop a strain (tension), but are not shortened almost (deduction or restraining of a load). The static work is more tiring, than the dynamic one.

In a whole, the skeletal muscles play an important role not only in body moving in space, parts of a body opposite each other, pose maintenance, but also they take part in blood and lymph movement, heat producing, an inspiration and exhalation (expiration) act, they are the depot of liquids and salts, glycogen, provide mechanical protection of cavitary bodies (organs). And, at last, the movements caused by sceletal musculation work, are the powerful antistressful factor.

 

Smooth muscles. They are located in visceral organs and blood vessels walls. They can perform relatively slow movements and durable tonic contractions. First ones - determine hole organs content transfer (stomach, intestines, urinary vesicle); second ones – prevent gall bladder, urinary vesicle content exit (thus, these contractions are expressed in sphincters).

Smooth muscle contraction duration (comparatively to sceletal one) is more significant. It reaches several seconds or even several minutes. Relaxation occurs especially slow. Smooth muscle possesses big force at such slow contraction. Smooth muscle comes into state of strong durable contraction (like sceletal muscles tetatus) due to its slowed contraction. That’s why they contract without fatigue development. Besides, energy expenditures in them are little. Smooth muscles relaxation is performed slower than in sceletal muscles, because sarcoplasmic reticulum development in them is less than in sceletal muscles and calcium ions are transported slow through cellular membrane.

Smooth muscles functional classification:

1. Muscles possesing spontaneous activity – they can contract while stimuli absence (rhythmic intestinal muscles contractions).

2. Muscles that have no any spontaneous activity.

Spontaneous muscular activity is linked with their stretching that causes muscular fiber membrane depolarization and action potentials occurence. Smooth muscles that have no any spontaneous activity are contracted under impulses from vegetative nervous system (arterias, spermatic ducts, iris myocytes).

 

Cardiac muscle. Main cardiac muscle peculiarities:

· automatism;

· excitability;

· conductance;

· contractility.

Automatism – is ability to self-excitation under impulses occuring in myocardium itself. Its nature is not yet clear but there are some data about its connection with cells- pacemakers activity located in heart nodes. Systolic node is the first order pacemaker. Sinus node biopotentials distinguishing features: repolarization phase doesn’t result in membrane potential restoration but transforms into secondary (dyastolic) depolarization which after threshold potential reaching causes new action potential occurence. Automatism possess all heart conductive system elements (atrio-ventricular node, Purkin’e fibers). It is decreased with impuls passage from heart base to its apex (from heart venous end to its arterial end). This regularity is known as Gaskell’s law (rule, gradient).

Excitability alsohas its peculiarities in cardiac muscle. Myocardium is contracted with maximal force to threshold stimuli i.e. heart contraction force doesn’t depend on irritation force (law ”everything or nothing”). One can differentiate contractive (working, typical) myocardiocytes and conductive (atypical). Contractive myocardium possesses excitability but doesn’t possess automatism. During dyastole resting potential of these cells is stable and its level is higher than in pacemakers (80-90 mV). Action potential in these cells occurs under pacemakers excitement. It reaches cardiomyocytes and causes depolarization of their membranes.

Working myocardium action potential consists of following phases:

· fast depolarization;

· initial fast repolarization;

· slow repolarization (plato phase);

· fast ending repolarization.

Important myocardium activity peculiarity is the following: cardiomyocytes action potential duration is about 300-400 msec that corresponds to myocardium contraction duration.

There is correlation between cardiac muscle excitement and contraction. Myocardial contraction trigger is action potential like in sceletal muscle. Depolarization phase coincides absolute refractiveness phase. But as absolute refractiveness is very long in cardiac muscle (up to 0,3 sec) than cardiac muscle excitability is absent in course of all contraction (shortening) period. That’s why cardiac muscle doesn’t give smooth tetanus. Relaxation period corresponds to fast repolarization period and relative refractiveness period. That’s why it also doesn’t give infused tetanus. During relative refractiveness phase superliminal stimuli can cause myocardium excitement and its contraction out of turn – extrasystole – appears as answer reaction.

Contractiveness peculiarity is also cardiac muscle subjugation Frank-Starling’s law: the more heart is stretched in course of dyastole, the stronger its contraction is in course of systole. Besides, as it was explained above, the second law of heart muscle activity is law “everything or nothing”.

Conductance – is cardiac muscle ability to conduct excitement both through working myocardium fibers and conductive system.

Excitement wave conductance velocity through heart different parts:

· muscular contractive atrial fibers – up to 0,8-1,0 m/sec;

· in atrio-ventricular node – 0,02-0,05 m/sec;

· in Gis’s fasciculus – 1,0-1,5 m/sec;

· in Purkin’e fibres – 3,0-4,0 m/sec.

Slow excitement conductance in atrio-ventricular node is called atrio-ventricular lack. It is equal to 0,04-0,06 sec.

 

Lecture 3