Autonomic nervous system physiology and its role in functions regulation.

Autonomic (vegetative) nervous system – is a complex of central and perypheral structures which regulate internal environment functional level necessary for organism adequate reaction. Anatomically autonomic nervous system is represented by nuclear structures lying in brain and spine, nervous ganglions and nervous fibres. It is divided morphologically and functionally into 3 parts:

· parasympathetic;

· sympathetic;

· metasympathetic.

Autonomic nervous system reflexes morpho-functional peculiarities. Parasympathetic part. Parasympathetic unit central part is represented by nuclei, located: in midbrain- oculomotor nerve nucleus (III-rd pair of craniocerebral nerves); in medulla oblongata – facial (VII-th pair), glosso-pharyngeal (IX-th) and vagus (X-th pair) nerves nuclei; in spine – lateral corns of sacral part 3 segments. Perypheral part includes: preganglionar fibres – nervous fibres coming from nervous centers, ganglions and postganglionar nervous fibres – innervating effector organs.

Parasympathetic vegetative functions regulation is realized by both highest nervous centers (cerebral and spinal) and by perypheral ones – ganglions. Ganglion is a morphologic and functional unity of neurons. Excitement transduction from preganglionar nervous fibre to postganglionar is realized in parasympathetic ganglions by means of mediator – acetylcholine. When excitation reaches preganglionar fibre therminal, permeability increasing for extraneuronal calcium occurs. Calcium comes in presynaptic membrane zone and activates vesicules transport with acethylcholine to presynaptic membrane. Vesicular membrane is fused with presynaptic membrane. In creates the conditions for mediator releasing into synaptic fissure. Acethylcholine interacts with N-cholinoreceptor on post-synaptic membrane and sodium channels are opened as the result of which EPSP occurs. Acethylcholine is destroyed by enzyme acethylcholineestherase after this interaction. Substances acting like acethylcholine are called agonistes, inhibiting excitement conductance in ganglions – ganglioblockers.

In postganglionar parasympathetic nervous fibres on their endings realization is performed through synapses by means of acethylcholine which in visceral organs (heart, alimentary organs, bronchi et al.) acts through M-cholinoreceptors (muscarine-dependent). Such receptors are not equal. One can differentiate M₁…M₅ receptors. Besides, one can differentiate also N-cholinoreceptors (nicotine-sensitive), located on post-synaptic mebranes of sceletal muscles, in central nervous system. Physiologic effects depend on which receptors acts acethylcholine.

Parasympathetic influences peculiarity on different organs is the following: effect comes quickly because they mainly consist of preganglionar nervous fibres of group “B” where excitement wave spreading velocity is relatively high. But effect also disappears quickly because mediator acethylcholine is destroyed fast. That’s why action of this part of autonomic nervous system is quick and in more extent local (in the place of mediator releasing).

Sympathetic part. Central part is origined from spine nuclei in grey substance beginning fromI-II thoracic till II-IV lumbal segments. Perypheral part is represented by postganglionar neurons beginning from paravertebral and prevertebral ganglions. Excitement conductance in ganglions in this part of autonomic nervous system is realized by mechanisms similar to those in parasympathetic nervous system. Excitation wave is transmitted from postganglionar fiber to effector by means of mediator – noradrenaline (or adrenaline). Noradrenaline produces in body, axonal therminal part and its varicosus dilations. Noradrenaline is located in neuronal vesicles, its part is dissolved in cytoplasm. It is released from vesicules in course of depolarization of presynaptic ending membranes that is accompanied by their permeability changes to calcium ions. Calcium releasing into synaptic fissure occurs by means of exocytosis – vesicular membrane fusing with axonal ending membrane. Noradrenaline or adrenaline reaching postsynaptic membrane interacts with specific receptors which name is adrenereceptors. They are divided into 2 groups – alpha- and beta-adrenoreceptors. In turn, every group is subdivided into subgroups. Alpha-adrenoreceptors leads to skin, mucosas, kidney, abdominal cavity organs, lung, brain, sceletal muscles vessels constriction. At the same time it results in contraction of sphincters smooth muscles and pupil ciliary muscle, causing midriasis (pupil’s dilation).

Beta-adrenoreceptors activation causes vasodilatation in sceletal muscles, coronars, lung, brain, abdominal cavity organs. It also leads to heart beat, freaquency and excitement conductance velocity increasing in typical (working) and atypical myocardiocytes. Other results of such activation – pupillar muscles, biliary tracts smooth muscles relaxation; urinary vesicle tone decreasing.

Autonomic nervous system sympathetic part makes trophyc influence onto different tissues and organs. It means that metabolic processes complex occurs in tissues supporting tissue structure and providing its function and metabolic reactions in it. For example, it enforces energy substances resynthesis processes, changes receptors excitability et al. Biologically active substances – noradrenaline and adrenaline – participate in trophyc processes. They while absorbing into blood are spread to organs and tissues which have no sympathetic innervation and act to them (for example, sceletal muscles).

Comparatively to parasympathetic part, sympathetic one influences more diffusily. It is connected with adrenaline and noradrenaline action because they reach practically all tissues and organs and possess stronger effect in comparison with acethylcholine. Besides, sympathetic nervous system action and influence is more durable.

Metasympathetic part is a complex of structures providing their own nervous regulation of main visceral organs possessing functional automatism (cardiometasympathetic, enterometasympathetic, urethrometasympathetic). Its main functions are as follows as: providing excitement conductance from nervous system structures to effectors, regulatory influences co-ordination performing (of smooth muscles motor activity, alimentary tract organs secretory, excretory and absorbtive activity, local circulation regulation and others).

The base of metasympathetic part are neurons different in their shape, synapses existance, processes amount and length. This system ganglions are located intramurally – in organs walls. Parasympathetic and sympathetic fibres penetrate these ganglions. Central influencings are realized through these fibres. Ganglionar neurons receive and process the information from efectors and are under modulating and correcting influence of impulses coming from brain and spine centers. Information processing is performed in ganglions, excitement transmitting in them is realized with acethylcholine participation (through M- and N-cholinoreceptors) and noradrenaline (through alfa-adrenoreceptors). Impulses are transmitted from postganglionar neurons to effectors by means of such mediators as ATP, serotonine, noradrenaline, acethylcholine, substance “P” and others. Significant role in realizing effects to effector tissues and organs have modulators – kinines, prostaglandines, opioid peptides, renine, angiotensine and others. They change effectors functional answer enforcing or decreasing their activity.

Thus, autonomic nervous system action onto organs and tissues is not equal. Sympathetic part causes their diffuse excitement. This is the system of anxiety, protection, mobilization of reserves necessary for organism interaction with environment. Such mobilization is reached by means of many systems and organs generalized involvement in reaction. Probably, that’s why sympathetic ganglions are situated far from innervated organs and possess the ability to impulses multiplication that provides fast influencing generalization.

Slower but also generalized process appears at adrenaline releasing into blood. Such releasing is considered to be fluid sypmathetic nervous system. Sympathetic impulses activate brain activity, mobilize defence reactions, thermoregulative processes, blood coagulation mechanisms, immune reactions. Sympathetic nervous system excitement is an obligatory condition of emotional state and tension, it is hormonal reactions initial stage (link) at stress. Its influencings have adaptative and trophyc character.

Parasympathetic part and, especially, metasympathetic are the systems of current organism physiologic functions regulation. Such functions provide homeostasis. Metasympathetic neurones possess the features like brain nuclear structures. This system has its own integrative chain for information processing. If parasympathetic system influencings are mainly indirected (although there are also direct influencings to some organs) and more local than in sympathetic, metasympathetic one has only visceral functions (peristalsis saving, absorption, smooth muscles contraction) and it is base, local for these organs.

Vegetative functions regulative centers are practically all parts of central nervous system. Spinal part has segmentary and metameric organization. Its a very important for clinics (hyperaesthesia, hyperalgesia – tactile and nociceptive sensitivity increasing in limited body parts at inner organs diseases). Pains occuring at inner organs diseases are called reflected (Ged’s zones).

In brain stem there are multiple vegetative structures – nuclei and centers of heart activity, vessel tone, respiration, swallowing regulation and others. They must belong such reflexes as olfactory, lacrimal, pupillar, sneeze and others to these reflectory acts.

In dieencephalon particularly in hypothalamus humans have central mechanism of homeostasis, alimentary, respiratory functions, heart-vascular activity, endocrine system, metabolism regulation, thermoregulation.

Somatosensor and other cortical zones are center of localization not only of somatic but also visceral systems.

Autonomic nervous system reflectory reactions. One can differentiate 3 reflexes groups:

· viscero-visceral;

· viscero-somatic;

· viscero-sensor.

Viscero-visceral reflexes are origined and are ended in inner organs. For example, peritoneum receptors in course of their excitement give impulses changing heart activity (Golz reflex, epigastral reflex). Such reflexes may be closed by type of axon-reflex (in limits of one axon branches). It’s necessary to take into account such mechanism of their occuring in clinic practice in course of therapeutical procedures performing (mustard plusters, cupping-glasses, compresses).

Viscero-somatic – include ways on which excitement in addition to visceral reflexes cause also somatic answers (contraction or inhibition of sceletal muscles current activity). Segmentary innervation of some organs (heart, intestines) are on the base of these reflexes. It’s accompanied by integrative reactions of both visceral and somatic organs. For example, abdominal cavity receptors irritation can cause anterior abdominal wall muscles contraction or extremities movement that it is connected with afferent impulses convergence to interneurons of different spine segments. Such segments create common scheme for autonomic and somatic influencings transmission.

Viscero-sensor – include ways in which in answer to autonomic sensor fibres irritation reactions occur not only in inner organs, muscular system but also somatic sensitivity is changed. Due to segmentary organization, autonomic and somatic innervation at inner organs diseases in limited skin locuses tactile and nociceptive sensitivity increasing (reflected pains) is appeared. In course of some diseases (stenocardia, ulcer disease, cholecystitis, pancreatitis et al.) the patients’s complaint is painful sensation in corresponding projectional zones.

 

Lecture 6.