Nervous tissue physiology (receptors, nervous fibres, synapses).

Nervous tissue in organism is represented by different structures that are united in morphological and functional aspect and are nervous system base. All nervous system structures have row of common features and peculiarities:

· neuronal structure;

· synaptical connection between neurons and others.

Nervous system co-ordinates all organs and system activity providing its effective adaptation to changeable environmental conditions and formes purposeful behaviour.

Information about internal or external environment state is percepted by nervous system elements – receptors.

Receptors – are specialized structures necessary for stimuli perception and their transformation into nervous impuls.

2 main receptors types:

1. Sensor - providing different external or internal irritators perception:

a) primary – sensitive (simple) – nervous endings of sensory neurons afferent conductors. They are located in skin, mucosae, blood vessels et al.

b) secondary-sensitive (complicated) – specialized cells; as a rule they are in composition of sense organs – vision, gustation, hearing.

2. Cellular – providing perception of information transported by molecules of chemicals – mediators, hormones et al.

Other receptors classification (according to origin of percepted information):

1. Interoreceptors – receptors that percept sygnals about internal environment irritations and are located in internal organs:

· pressoreceptors (baroreceptors);

· chemoreceptors;

· thermoreceptors;

· noceoceptors;

· proprioreceptors of bones, ligaments, joints, muscles, vestibular apparatus;

· tissular receptors – localized in intersticium and cellular microenvironment.

2. Exteroreceptors – percept irritations of external stimuli:

a) contact – located in skin and mucosae:

· tactile;

· thermoreceptors;

· gustatory (tasty);

· noceoceptors;

b) distant:

· phono-;

· photo-;

· olfactory.

Common task for all sensor receptors is irritation transforming into biopotential. Irritator while its action for example on receptor cell increases its membrane permeability to sodium ions. It leads to creation of so-called local or receptor potential in it. It encourages mediator releasing, which acts to nervous ending. As a result of this analogous potential occurs but it is named as generatory potential. Later it generates nervous impuls. Further, due to charges difference in nervous fiber ending and through all its longitude action potential (nervous impuls) appears and then it is diverged through all nervous fiber.

Receptors features:

1) Specificity – ability to percept only definite, i.e. adequate for the given receptor, stimulus. This receptor ability has been formed in course of evolution.

2) High sensitivity – ability to answer to very small by intensivity parameters of adequate stimulus.

3) Rhythmical excitement impulses generation in answer to the stimulus action.

4) Adaptation – ability to adapt to stimulus action which is expressed in receptor activity and excitement impulses generation freaquency reducing.

5) Functional mobility – increasing or decreasing of functionalreceptors amount dependently of environmental conditions and organism functional state.

6) Specialization of receptors to adequate stimulus definite parameters. Receptors in perypheral analizator part composition are unequal as for their attitude to stimulus. One of them answer only to the origin of its action, others – on it stoppage, third – on intensivity change.

Nervous fibres and nerves physiology.

Nervous fibres possess excitability and according to morphologic principle they are divided into myeline and myeline-free. Nervous fibres form nerve or nervous stem, consisting of great amount of them. Nervous fibres transmitting excitation from receptors to central nervous system (CNS) are called afferent; from CNS to the effector organs – efferent. Nervous fibres possess: excitability, conductance, lability. Nervous tissue excitability is higher than muscular one. It is various in different nervous fibres. Myeline (thick) nervous fibres is significantly higher than myeline-free (thin).

Excitement conductance through nervous fibres obeyes definite laws.

Physiological integrity law

tells that excitation conductance through nervous fibre is possible only in a case of its non-interrupted anatomical structure and physiological features.

Excitement conductance two-sided law

at irritation application on nervous fibre the excitement is diverged through it in both sides from irritation place (at tooth nerve irritation pain is stretched not only on local tissues but also irradiates in other body parts).

Excitement isolated conductance law

excitation through nervous fibres being in a composition of mixed nerves (for example, vagus) is diverged separately, i.e. it doesn’t transmit through one nervous fibre to another.

Excitement conductance velocity is different in nervous fibres. It depends on their diameter and structure (myeline membrane existance). All nervous fibres are divided into 3 main types according to their conductance velocity. Type “A”fibres – are covered by myeline membrane (sceletal muscles motor fibres), excitement wave conductance velocity is up to 120 m/sec. Type “B”fibres – vegetative nerves myeline fibres, excitement wave conductance velocity is up to 18 m/sec. Type “C”fibres – myeline-free nervous fibres (vegetative or autonomic nervous system postganglionar fibres), excitement wave conductance velocity is up to 3 m/sec.

Excitement conductance mechanism through nervous fibres. Excitement spreading through nervous fibres is based on bioelectrical potentials ion generation mechanisms. At excitement spreading through type “C” fibre local electrical currents occuring between excited locus, charged electronegatively, and unexcited, charged electropositively, cause simultaneouse membrane depolarization till its critical level with further action potential generation in every membrane point through all the stretching of nervous fibre. Such excitement conductance is called uninterrupted.

Myeline membrane presence, possessing high resistance, and membrane locuses, not having it, creates conditions for “saltatory” excitement conductance through myeline nervous fibres of types “A” and “B”. Local electrical currents occur between neighboring Ranvier’s nodes because excited membrane of node becomes electronegative as for the surface of neighbouring unexcited node. Local currents depolarize membrane of unexcited node till critic level and action potential occurence. Thus, excitation “jumps over” nervous fibre locuses covered by myeline, from one node to another. Such excitement conductance velocity reaches 120 m/sec. At the same time, such excitement wave conductance is more economic than the uninterrupted one.

Nervous fibres possess lability – the ability to reproduce definite number of excitation cycles in time unit according to the rhythm of applied irritations. Lability measure is maximal excitation freaquency which nervous fibre can reproduce in time unit according to the rhythm of received irritations. Nervous fibre lability is the highest and is approximately 1000 impulses per second.

Important characteristics of nervous fibre is its relative indefatigueability, which depends in many aspects on the fact that energy losses in it are insignificant in course of excitement and repair processes pass quickly. Besides, nervous fibre pass excitement wave with large underloading (it can transmit up to 100 impulses/sec but in the most cases transmits less for normal physiological reactions).

Synapses are excitement transmission place from one neuron process to other neuron body or process. There are 3 main synaptic parts:

· presynaptic membrane;

· synaptic fissure;

· postsynaptic membrane.

Such transmission may be performed by 2 ways: electrical or chemical. Main mechanism of information transmission between neurons is chemical. Information transfer in chemical synapses are realized by means of mediators (acetylcholine, noradrenaline, serotonine et al.). There are 2 types of chemical synapses: exciting and inhibiting. Exciting or inhibitng character of synapse is determined by corresponding mediator. Any mediator under impulse coming to presynaptic ending is released into synaptic fissure, where it goes into contact with special receptors on postsynaptic membrane. Result of such interaction: postsynaptic membrane permeability increasing as for sodium ions and membrane’s partial depolarization named as exciting post-synaptic potential (EPSP). At multiplied mediator coming EPSE is summarized and action potential occurs.

Synapse is a morphological structure. Its physiological analogue is nervous center.

CNS is a complicated structure consisting of large amount of interacting nervous centers. Anatomically nervous center is an integrity of neurons located in a definite brain part and are essential for definite reflex performing. Physiologically nervous center – is a complicated functional unity of many nervous centers located in different CNS parts and providing difficult reflectory acts and organism functions regulation due to their integrative activity. Examples – respiratory center, heart-vascular center et al.

Nervous centers features. Nervous centers possess a row of character features and peculiarities of excitation conductance, whcih significantly are determined by synaptic formations presence and structure of neuronal chains forming these centers. These synapses transmitting excitation received the name exciting. Some functional features are characteristics for them. They are also nervous centers features.

One-sided excitement conductance in nervous center is determined by its one-sided conductance through synapses.

Excitement conductance lack – is connected with the fact that excitation wave is transmittered slower in synapse than through nervous fibre (it’s necessary time for mediator accumulation and exciting post-synaptic potential EPSP forming). EPSP – size on which membrane potential of post-synaptic membrane in decreased while mediator portion acting on it.

Excitement summation – can be temporary or simultanenous (it is connected with EPSP accumulation in one synapse) and space (linked with EPSP accumulation in different synapses of one and the same neuron).

Excitement rhythm transformation – impulses number increasing or decreasing on neuron “exit” in comparison with impulses number which it receives on “entrance”.

Afteraction. Reflectory acts are ended not at the same time with stimulus action stoppage but they are lasted for long after action stoppage.

High sensitivity to hypoxy and different chemical substances. It gives opportunity to well-directed brain functions pharmacological regulation.

High fatigue is a result of nervous centers low lability and mediator consumption for EPSP forming.

There are also special inhibitory synapses in CNS the role of which are to inhibit excitation wave conductance. The same processes in comparison to exciting synapses take place in inhibitory ones. Main inhibitory mediator is gamma-aminooleic acid. It increases postsynaptic membrane permeability for potassium and chlorum ions causing postsynaptic membrane hyperpolarization. The difference is that inhibitory mediators cause in such synapses membrane inhibitory post-synaptic potential (IPSP) occurence. IPSP – is that size on which post-synaptic membrane membrane potential is increased while action inhibitory mediator on it. Such inhibition is called postsynaptic.

Presynaptic inhibition is realized due to axo-axonal synapses. It is expressed as presynaptic membrane depolarization and exciting mediators releasing inhibiting. Presynaptic inhibiting mediator is glycine.

One of variant of neuronal integration is the possibility to regulate the size of coming information due to feed-back connection. Axonal collaterals can establish synaptic contacts with special associative neurons. For example, impulse occurence in motoneuron not only activates muscular fiber but also excites special neurons (inhibitory Renshow cells) through collaterals. Such Renshow cells establish synaptic contacts with motoneurons and inhibit them (this is so-called recurrent inhibition).

Main principles of reflectory activity co-ordination.

All reflectory activity is rather co-ordinative. Reflexes are tightly connected one to another in nervous system integrated reaction to the irritation. Inhibition in CNS is of great importance. First, it performs co-ordinative role, i.e. directs excitation on a definite way to the definite nervous centers. As a result of such action well-directed elective excitation irradiation occurs (irritation of hand definite locuses causes definite fingers flexing and leaving). Sometimes irradiation becomes diffuse, non-directed, realized on different ways simultaneously(at epilepsy). Irradiation is the simplest mechanism of such co-ordination. Irradiation means spreading. The stronger stimulus is the more receptors participate in answer reaction and finally more central neurons.

Excitation in nervous centers due to irradiation can converge from different origins to one and the same neurons. Convergency is the next principle of co-ordination.

Opposite to convergency – there is divergency - one neuron contact with great number of neurons of higher order. Role: given sygnal influence sphere becomes wider. Example: in course of physical exercises information flow is strongly enforced from proprioreceptors to next nervous system parts that leads to ending result increasing.

Occlusion is more complicated co-ordinative mechanism. It is the interference of one reflectory act influence spheres with the same of other. That’s why at simultaneouse coming of several afferent sygnals to one and the same neurons they become ending result reducing.

Due to interrelations between excitation and inhibition processes in CNS dominanta principle is expressed in its work. It is main working principle for nervous centers activity which is expressed in temporary dominant excitation locuses occurence. Such dominant locuses have increased excitation, they are stable. There are 2 main reasons of dominanta: increased impulse coming from efferent organs (alimentary or urinary dominanta) and hormones or other biologically-active substance activity increasing (sexual dominanta).

Feed-back afferentation principle is in information conductance (return) from effector (working organ) ahead to nervous center structures where it is processed and thus nervous center performs control under effectiveness, properity and optimal level of reflectory activity.

All co-ordinative mechanisms are directed on doing answer reaction faster, more purposeful and more adequate and its performance - without excessive energy expenditures.

Besides very important role in reflectory activity co-ordination, inhibition performs important protective role or defencive function. Multiple organism reactions are formed with obligatory participation of different CNS parts on the basis of excitement and inhibition processes interaction.

 

Lecture 4