Excitive tissues functionning general features.

 

Human and animals’ organism has the highest ability to adapt to the constantly varying conditions of external and internal medium. In the basis of adaptive organism reactions lies the universal property of alive tissue - irritability - the ability to respond to the irritating factors action by metabolism change. The irritability is evolutionally the ancient form tissues reaction. During evolution gradual differentiation of tissues participating in adaptive organism activity has taken place. The irritability in these tissues has reached the best expression and has received the name an excitability. The excitability is an ability of a tissue to respond to an irritation specializedly, singlemindedly and with the maximal velocity. Excitation – complex (difficult) biological process expressing by response reaction to an irritation.

A nervous, muscular, epithelial secretory tissue (excitable tissues) have an excitability. The specialized form of response reaction is an excitation process physiological display. A contraction will be a response reaction in any muscular tissue. At a nervous tissue it will be an impulse conduction. At a secretory tissue it will be a synthesis and allocation of biologically active substance.

The excitability of tissues is various. A measure of an excitability is the threshold of stimulation – minimal stimulus force, capable to cause excitation.The stimuli with a size that is less than a threshold one, are called subliminal ones. The stimuli, on force exceeding a threshold of stimulation are called epiliminal ones.

All stimuli can be divided into three groups: physical, chemical and physico-chemical. Physical stimuli - mechanical, temperature, light, sound and electrical ones. Chemical stimuli - acid, alkalis, medicines. Physico-chemical stimuli –osmotic pressure, рН, ion structure changing. Besides, they distinguish biological stimuli - hormones, vitamins and others biologically active substances. They allocate also a group of social stimuli - a word.

All stimuli divide on adequate and inadequate on biological value. Adequate stimuli are such stimuli, acting to the given biological structure under natural conditions and to perception of which it is adjusted specially (e.g., for eye retina photoreceptors the seen part of light is an adequate stimulus). Inadequate stimuli are such, to perception of which the given structure is not adjusted specially (e.g., for a sceletal muscle the adequate stimulus is the nervous impulse, but it can contracts at a mechanical impact too).

Characteristic attribute of exaltation is an electrical current occurrence in tissues (cells). The electrical phenomena (currents or potentials), which arise in organism cells, tissues and organs are named the biological potentials.

The biological potentials arise because there is a difference of potentials between the external and internal party of a cell membrane, which is in a status of rest. Potential, which is registered in a such cell status, is named a membrane potential (resting potential). It is caused by the difference of a potassium, calcium, sodium, chlorine and other ions concentration between intracellular and extracellular medium. So, the potassium ions concentration in a cell exceeds in many times (about 20-40 times) their contents in extracellular medium. The of sodium ions concentration, on the contrary, is lower in intracellular medium in 10-20 times. The ions of chlorine, as well as of a sodium, are mainly concentrated outside of cell membrane, where their content is in 15-20 times more than inside. Their such non-uniform distribution till that and other membrane party provide ion pumps. Ion canals, available in a membrane, can be opened and closed, that depends on a membrane status. So, in a cell which is in a resting status, the sodic canals are closed, and the potassium ones - are opened. Therefore the permeability for different ions is various. If a potassium ions permeability to accept for 1,0, for chlorine it will make - 0,45, and sodium - 0,04. It results that the potassium ions on a concentration gradient diffuse from a cell to extracellular space. The sodium ions counter flow is a very small. In a result the potentials difference between cell internal medium and its outer surface is formed which is from 50 up to 100 mV for different tissues. This potentials difference also refers to as a resting potential or a membrane potential.

At stimulus action there is a membrane status change, ion canals open in it, through which positively charged ions available in excess behind its limits can move in a cell. The "fast" sodic canals opening occured most often. Originally ion current to cell is promoted also by a transmembrane potentials difference. Such process is called depolarization, because it results in this potentials difference reducing. If the stimulus is weaker (subliminal), ion canals are opened a little, therefore the ion current is insignificant. Depolarization occurs slowly. Such changes are named the local depolarization or local potential.

If the of threshold stimulus acts, the depolarization reaches a critical (threshold) level. As a result of it all active electroexcitable ion canals are opened. Depolarization is sharply accelerated and there is even a potential reversion (potential mark change). Thus the positively charged sodium ions flow stops, the appropriate canals are closed. Excessive potassium ions from inside direct outside, resulting to the membrane potential restoration. At first it occurs rather quickly (fast repolarization), and then, when the potassium ions flow decreases, the membrane potential restoration occurs in a slowed-up way (slow repolarization). Further potassium ions exit can proceed and cause a hyperpolarization. Potassium-sodic pump work adducting in initial potentials difference restoration (to polarization) amplifies at this time. All this process from a beginning up to the end is called as an action potential.

As the vital activity of all cells, tissues, organs is accompanied by their electrical activity, the registration of potentials, arising at it, allows to judge processes occurring in them. The diagnostics and control of a treatment of this or that disease is based on it. For example, in a heart such registration of its biological potentials wears the name electrocardiogram (ECG).

In physiology they determine one more property of excitable tissues, which has received the name a lability. It is a functional mobility of tissues, its parameter is the potentials action maximal number, which the excitable tissue is capable to generate per 1 second according to a rhythm of a submitted boring (irritation). The normal size of a lability, e.g., for a nervous tissue makes 500-1000 impulses per second, and for sceletal muscles - 150-200 impulses per second. There is a sceletal muscles lability rising with ageing. It is shown in augmentation of irritation freaquency, at which the gear (incomplete) tetanus turns in smooth. In newborn’s muscles it occurs at a stimulus freaquency 4-20 per second, at adulthood - 50-100 impulses per second.

The general laws of tissues functionning. Between the irritation character and the answer-back reaction of an alive tissue there are close mutual relations, which find expression in the irritation laws.

Irritation force law: the more force of an irritation, the more strong answer-back reaction (up to known limits). The further stimulus force augmentation any more does not lead to the answer-back reaction increasing, and even can cause return reaction, down to its disappearance. It is explained by that each functional unit of tissues (for example, muscular) has its exaltation threshold. That’s why while working the threshold stimulus, those fibers, for which this stimulus is of a such size are only involved in the answer. Others do not react.

At stimulus force augmentation the new fibers are involved, for which the given stimulus is a threshold etc. Further, when the stimulus force will exceed the opportunities of all fibers of the given tissue, its answer-back reaction to the force augmentation will not change (the resources are settled!). Such stimuli, which cause the maximal answer-back reaction, are named in physiology maximal or optimum. At the even greater stimulus force augmentation the answer-back reaction even will decrease, as at such a stinulus force the separate functional fibers of excitable tissues can even be injured. In a result, the answer-back reaction decreases and this phenomenon in physiology is named pessimum, and the stimuli causing it - pessimal.

The law "all" or "anything" is shown, first of all, at the cardiac muscle work analysis. According to this law, subliminal stimuli, acting to a cardiac muscle, do not cause an answer in it (it is "anything"), and threshold and epiliminal stimuli cause answer-back reaction of the same size (it is named "all"). Under the same law the functional unit of any excitable tissue works. Let's take, for example, a muscular fiber and we shall imagine, that threshold stimulus at it is 2В (electrical current strain or voltage). If we act the stimulus of 1V to it, we naturally shall not receive any reaction ("anything"), and if we take the stimulus of 4V, the muscle will give the same answer-back reaction, as well as on 2V ("all"). Naturally, "all" and "anything" are relative concepts, as at the subliminal stimulus action there is a local answer (local potential), therefore it already cannot be treated as "anything".

The law of force-time – with the augmentation of a stimulus force it is required less time of its influence to tissue for answer-back reaction reception. The relation between the duration and force can be expressed by hyperbolic curve, the both branches of which go at any stage in parallel to axes of coordinates. This last circumstance forms the basis that the stimuli of a very small size (less than the threshold) can not cause the answer-back reaction. Threshold stimulus causes answer reaction after several time. This time is called useful time. Each excitatory tissue has its own useful time. But under real conditions it is very hard to be estimated because one can make mistakes while irritation threshold determining (it is connected with the fact that its assessment is based on subjective signs – patients sensations, his skin thickness et al.). And at wrong ziphra of threshold, it leads to strong reducing of real useful time level. That’s why under real conditions one determines not useful time but chronaxy – minimal time necessary for answer reaction receiving if tissue is acted by irritator in 2 thresholds or 2 rheobases. Under such conditions the determination mistake is practically equal to zero (see the curve “force-time”).

Chronaxymetry is widely used in neurology, traumatology and other branches of medicine for nerves and muscles excitability determining. The chronaxy is less, the excitability is bigger and on the contrary. In clinics one assesses these indexes on muscles on extremities: their rheobase is equal to 60-70 V, chronaxy - 0,1-0,7 sec.

 

 

Lecture 2