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 also has 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.