Nervous and muscular excitability determining in dentistry.

Chronaxymetry - at electrical current application on muscle current is coming also through nervous fibers located in it. That is why by chronaxy level determining one can tell about motor nerve fibers injure. Irritation threshold (rheobase) and chronaxy of nervous fibers are lower than in muscles. That is why at normal muscle chronaxy determining we in fact measure chronaxy of nervous fiber innervating it. If nerve is injured than nervous fiber is degenerated and under such conditions electrical stimulus applying to the muscle shows muscular fibers chronaxy (such chronaxy is longer in time).

Temperature methods (cooling, warmth) and mechanical one (percussion) stimuli are used for pulp excitability determining. But they are difficult to be dosed.

Electroodontodiagnostics. Electrical current is acted to pulp through enamel and dentine. Tooth electroexcitability assessment is in fact the investigation of corresponding sensitive nerves and tooth pulp. Healthy teeth have equal excitability and react to current force 2-6 mcA. If tooth threshold is less than 2 mcA it testifies to hyperexcitability (at parodontosis). At pulpits on the contrary nervous fiber threshold is bigger than the threshold of muscle and is more than 6 mcA. Excitability reducing up to 100-200 mcA is pulp death sign.

Such data are important in neurology. Chronaxy is longer in paralyzed muscles than in normal muscle. And, in progressive neural diseases, chronaxy is prolonged gradually.

2. Study aims:

To know: excitable tissues physiological investigative methods; excitable tissues main features and activity laws; electrical current usage advantages.

To be able to: make nervous-muscular preparation, use electrical stimulation and contraction registration.

3. Pre-auditory self-work materials.

3.1.Basic knowledge, skills, experiences, necessary for study the topic:

Medical biophysics	Measurement units system, tissues electrical features	To get acquainted to structure and to manage the skills of work with electrical devices, investigate electrical phenomena nature in excitable tissues
Medical biology	Substances passage into the cell (regularities), biological membranes structure and function, irritability as reflexion form, irritability general features, irritability in animals at evolutiona scale different floors
Histology	Cells membrane structural-functional characteristics, cell barrier-receptory and transport system, excitable tissues general features and activity laws, possibilities of chronaxymetry method usage in neurological practice.	To draw cellular membranes structure and to analize their functions

 

3.2. Topic content.

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 of 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) biological maximal velocity. Excitement – is a complex (complicated 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. Physical-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 photoceptors the seen part of light is an adequate stimulus). Non-adequate stimuli are such, to perception of which the given structure is not adjusted specially (e.g., for a skeletal muscle the adequate stimulus is the nervous impulse, but it can contracts at a mechanical impact too).

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 is 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 the fact 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 optimal. At the even greater stimulus force augmentation the answer-back reaction even will decrease, as at such a stimulus force the separate functional fibers of excitable tissues even can 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 "nothing" or "everything" (“all” or “nothing”) 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 "nothing"), and threshold and epiliminal stimuli cause answer-back reaction of the same size (it is named "everything"). 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 2V (electrical current strain or voltage). If we act the stimulus of 1V to it, we naturally shall not receive any reaction ("nothing"), and if we take the stimulus of 4V, the muscle will give the same answer-back reaction, as well as on 2V ("all"). Naturally, "nothing" and "everything" 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 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 the augmentation 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.

The excitability curve demonstrates the exact relationship between the strength and the duration of a stimulus. So, it is also called the strength—duration curve (Fig. 1).

Characteristic Features of the Curve

The shape of the curve is similar in almost all excitable tissues. Following are some of the important points to be studied in the excitability curve:

– Rheobase: This is the least possible, i.e. minimum, strength (voltage) of stimulus which can excite the tissue. The voltage below this cannot excite the tissue, whatever may be the duration of stimulus.

– Utilization time: It is the minimum time required for a rheobasic strength (threshold strength) to excite the tissue.

– Chronaxie: It is the minimum time, at which a stimulus with double the rheobasic strength (voltage) can excite the tissue.

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Fig.1. Strength—duration curve.

 

Importance of Chronaxie

The value of chronaxie is used to compare the excitability in different tissues. The measurement of chronaxie determines the excitability of tissue. Longer the chronaxie, lesser is the excitability. Chronaxie in human skeletal muscles varies from 0.08 milliseconds to 0.32 milliseconds. In frog's skeletal muscle, it is about 3 milliseconds.

Chronaxie is 10 times more in skeletal muscles of infants than in the skeletal muscles of adults.

Chronaxie is shortened by increased temperature and prolonged in cold temperature. It is shorter in homoiothermic animals than in poikilothermic animals. Chronaxie is shorter in red muscles than in white muscles.

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 skeletal muscles - 150-200 impulses per second. There is a skeletal muscles lability rising with ageing. It is shown in augmentation of irritation frequency, at which the gear (incomplete) tetanus turns in smooth. In newborn’s muscles it occurs at a stimulus frequency 4-20 per second, at adulthood - 50-100 impulses per second.