Excitable tissue – 02

Excitable tissue – 02

Muscles compose up to 40% of total body mass 40%. In general, it provides a variety of motor functions.

1.      There are skeletal (voluntary movements, that are controlled by conscious effort),

2.      cardiac and

3.      smooth muscle tissues (the latter are involuntary since they are innervated by ANS).

Muscle tissue functions:

-          skeletal muscles (movement of the body in three dimensions, movement of parts of the body relative to each other, maintaining the posture, movement of blood and lymph, temperature regulation, pulmonary ventilation, water and salt depots, protective function;

-          smooth muscles (evacuatory function, sphincter function, regulation of blood vessel tone, ligamentous apparatus);

-          cardiac muscle (provides blood flow through the vessels).

 

Muscle fibers have a number of properties: excitability, conductivity, contractility, elasticity

Muscle fibers are characterized by lesser excitability than nerve fibers.

depolarization threshold of muscle and nerve cells is approximately -50 mV, but the resting membrane potential in the muscle fiber is somewhat larger (-90 mV).

Consequently, the strength of the threshold stimulus for the muscle cell must be greater in order to produce excitation.

The duration of the action potential in the muscle cell is somewhat longer than in the nervous cell (in the skeletal muscles 2-3 msec and more).

01. The physiological properties of skeletal muscles. Neuromuscularunits. Features of the excitation of muscle tissue.

Skeletal muscles are characterized by

-          contractility – the ability to change its length during excitation, or to change its tone,

-          conductivity – the ability of an excitable tissue, in this case muscular tissue, to transmit the excitation to a distance,

-          elasticity – the ability of the muscle to restore its original size after the action of the factor causing muscle stretch.

To estimate muscle tissue functions, electromyography is widely used – the method of recording muscle electrical potentials.

There are slow muscle fibers and fast muscle fibers in the body.

-          Slow muscle fibers are the most suitable for long-term aerobic work (able to produce relatively low power for a long period of time),

-          fast muscle fibers are more adapted to perform an anaerobic work (develop short-term high-power efforts in such sports as weightlifting, wrestling, throwing sports, etc.).

 

The neuromuscular unit consists of 1 motoneuron and a group of innervated muscle fibers (their number may be different).

The relationship between the neuron and innervated muscle fibers can vary: for muscles that provide highly coordinated movements neuromuscular unit contains 1-3 muscle fibers (for example, the eye muscle), and for less accurate, but powerful motor processes neuromuscular unit contains up to several hundred muscle fibers (for example, the thigh muscle).

 

02. Types of skeletal muscle contraction (isotonic, isometric and auxotonic contractions).

There are several types of muscle contraction:

-          isotonic – muscle contraction at a constant tension with a change in its length;

-          isometric – muscle contraction without changing the length with increasing tension;

-          auxotonic – with simultaneous change of both muscle tension and length.

In the organism in daily life isotonic contraction is usually preceded by isometric force development.

Such contractions are called mixed contractions (isometric-isotonic-isometric).

 

 

03. Single muscle contraction, its phases. Changes in the excitability of muscle fibers during its excitation and contraction.

Single muscle contraction is characterized by a recorded change in the length of the muscle fiber when it is excited, caused by the action of a single stimulus.

It contains the following phases:

-          latent period,

-          contraction and

-          relaxation.

For a single muscle contraction, the law "all or nothing" is realized.

When the whole muscle is stimulated, the "law of force" is observed,

-          i.e. with an increase in the strength of the stimulus,

-          the force of contraction increases,

-          which is due to the fact that with the action of a stronger stimulus,

-          an increasing number of muscle fibers are involved in the process of contraction.

In different phases of a single muscle contraction, excitability is different.

04. Modern ideas of the mechanism of muscle contraction and relaxation. Theory of sliding filaments. Coupling the muscle excitation and contraction (electromechanical coupling).

all types of muscle tissue the presence of actomyosin chemomechanical complex is characteristic, it converts the energy of chemical bonds of ATP molecules into the process of muscle contraction.

Myofibrils of muscle fibers contain thin (actin) and thick (myosin) myofilaments.

The thick filaments have a diameter of about 15 nm.  They are composed of protein myosin.

The thin filaments have a diameter of about 5 nm. They are chiefly composed of protein actin along with smaller amounts of two other proteins: troponin and tropomyosin.

 

A theory of "sliding filaments" by H. Huxley and J. Hanson was proposed (1953), according to which the shortening of the sarcomere is the result of the interaction of actin and myosin: the myosin head carrying the products of ATP hydrolysis is attached to the corresponding site of the actin filament, which leads to a change in its conformation.

-          The length of the sarcomere decreases by 1%.

-          The myosin head has ATP-ase activity.

-          It is capable of cleaving ATP and using its energy for contraction. This process needs Ca2+ ions participation.

-          The formation of the transverse actomyosin cross-bridges is realized through the Ca2+ dependent mechanism.

-          At rest, the molecules of troponin and tropomyosin block active sites on the actin filaments to which myosin is attached, thereby creating conditions which prevent contraction process.

-          Ions of Ca2+, joining troponin, cause a change in the tropomyosin conformation, which frees the site on the actin filament and allows myosin to contact it.

At rest, the molecules of troponin and tropomyosin block active sites on the actin filaments to which myosin is attached, thereby creating conditions which prevent contraction process.

Ions of Ca2+, joining troponin, cause a change in the tropomyosin conformation, which frees the site on the actin filament and allows myosin to contact it.

T systems, which are inpocketings of the membrane into the interior of the fiber. Due to these formations, the processes of depolarization spread into the cell and can approach to the membrane of the sarcoplasmic reticulum, affecting their functional state.

-          When the muscle fiber is excited, Ca2+ concentration in the myoplasm increases (from 10-7 to 10-3 mol/L) due to Ca2+ exit from the sarcoplasmic reticulum terminal cisternae.

-          It binds to troponin, the conformation of tropomyosin changes and the working cycle of the transverse cross-bridges is initiated.

-          During the relaxation phase, there is a decrease in the level of Ca2+ in the myoplasm due to its active transport back to the terminal cisternae of the sarcoplasmic reticulum by the Ca2+ pump, which is located in its membrane.

-          As a result of a decrease in the level of Ca2+, tropomyosin blocks myosin attachment to actin. This complex of interrelated processes was called electromechanical coupling, i.e. a certain sequence of processes: from the action potential generation to the muscle contraction initiated by it.

 

05. Types of skeletal muscle contraction. Unfused and fused tetanus. The mechanisms of tetanus. Phenomena of optimum and pessimum of stimulation rate and force.

Contraction of the whole skeletal muscle

When several stimuli at a certain frequency act on the muscle (incase that the time interval between single stimuli is less than the duration of a single muscle contraction, i.e. less than 100 msec), the summation of the contractions occurs.

 

With an increasing rate of stimulation it is formed successively:

-          unfused (toothed) tetanus,

-          fused (smooth) tetanus,

-          optimum and

-          pessimum of contraction

These phenomena are based on changes in the excitability of the tissue in different phases of a single muscle contraction (figure 2.9).

Tetanus (tetanic contraction) – sustained continuous contraction of the skeletal muscle with a large amplitude, caused by the action of several stimuli at a certain frequency (from 30 to 100 imp/sec).

There are unfused and fused tetanus.

-          As the rate of stimulation increases, unfused tetanus is replaced by fused tetanus (the rate of stimulation that produces it is called the fusion frequency).

-          The phenomenon of the optimum is due to the fact that each subsequent stimulus (rate is about 100 imp/sec) corresponds to the phase of supernormal excitability, and the phenomenon of pessimum – each subsequent stimulus acting at a higher frequency (more than 150 imp/sec) corresponds to the refractoriness phase (relative or absolute).

 

 

 

06. Muscle strength and muscle work. The dependence of muscle work on the level of the load and the rhythm of muscle contraction. The law of average loads.

A single muscle fiber develops the strength needed to lift a load of 100-200 mg. The strength of the muscles depends on many parameters:

-          the muscle cross section,

-          the functional state,

-          the energy supply,

-          the muscle architecture (parallel, pennate and muscular 56hydrostats).

The contractile capacity of the muscle is characterized by absolute strength.

-          It is known that when lifting loads, there is a definite relationship between the work performed and the weight of the load:

-          at the beginning, the amount of work increases,

-          reaches a maximum and then decreases,

-          i.e. there are medium loads at which the muscles perform the maximum work (law of average load).

 

07. Skeletal muscle tone. Fatigue of working muscles. The mechanism and localization of fatigue of isolated muscle

Fatigue is a decrease in the working capacity of the muscular structure during prolonged active functioning.

It is caused by the accumulation of metabolic products (in particular, lactic acid, depletion of calcium, glycogen, ATP), as well as depletion of energy resources.

It is known that various nervous and humoral factors affect the restoration of working capacity.

In the Orbeli-Genitsiansky experiments on the gastrocnemius muscle of the frog, the protective-adaptive effect of sympathetic innervation on fatigue was observed.

The amplitude of muscle contractions with rhythmic stimulation gradually decreases, reflecting the processes of fatigue.

With subsequent stimulation of the sympathetic nerve, the amplitude of contraction increases, which indicates a universal adaptation-trophic function of the sympathetic division of the autonomic nervous system, which by means of optimization of metabolism, trophicity and excitability provides adaptation of the organism to muscular work.

 

08. The physiological basis of active rest theory (I. M. Sechenov) and sports training.

In 1903 I. M. Sechenov developed active rest theory.

It was found that the restoration of the working capacity of a tired muscle is faster, if it is accompanied by the performance of work by other structures. I. M. Sechenov explained this by fatigue development primarily in the nerve centers.

 

09. Physiological characteristics of smooth muscles. Features of their functioning.

Smooth muscle cells are spindle shaped cells without transverse striation with one nucleus.

They are located in the walls of the internal organs of the body, vessels and skin.

Smooth muscle properties:

-          automaticity (the ability to perform rhythmic, successive reductions without any external influences);

-          plasticity – the ability to maintain a stretch condition without changing the muscle tone;

-          functional syncytium – morphologically individual fibers are separated, but there are special contact areas (nexuses) which provide fast excitation transmission through all muscle fibers;

-          the value of the resting membrane potential is 30-50 mV, the amplitude of the action potential is less than that of the skeletal muscle cells;

-          minimal "critical zone" (excitation occurs if a certain minimal number of muscle elements is excited);

-          for the interaction of actin and myosin, Ca2+ ions are needed, which enter the smooth muscle cell from extracellular fluid during action potentials (unlike skeletal muscle cells, smooth muscle cells have no troponin, tropomyosin or organized sarcoplasmic reticulum);

-          the duration of a single muscle contraction is large (several hundred milliseconds).

This type of cells is, as a rule, a functional syncytium, the excitation propagates fairly quickly from one fiber to another.

It is characterized by slow movements and a prolonged tonic contraction.

Smooth muscle cells have a number of special excitation features.

The duration of a contraction of a single muscular fiber of smooth muscle is about 250 msec or more.

The process of contraction in smooth muscles has a complex mechanism as in skeletal muscles, but there are differences.

Ions of Ca2+ realize their trigger action through interaction with a special calmodulin protein.

Thus, Ca2+ interacts with calmodulin, and the complex is further bound to each of the light chains of myosin, activating its phosphorylation, the lateral bridges of which are attached to the actin filament.

The decrease in the level of Ca2+ occurs due to the mechanisms of its active transport out of the cell.

This process is rather slow, which extends the relaxation phase. It is characterized by relatively low energy expenditure and less fatigue.