Sinusal node

Anatomy

It is about a group of cells located at the top of the right auricle, close to the arrival of the higher Vena cava. This structure is indistinguishable with the naked eye.

Physiology

These autorythmic cells discharge from the potential of cyclic action of manner. During the diastole, the potential of membrane is hyperpolarisé because of the preceding potential of action. During the hyperpolarisation, of other ionic channels permeable with calcium is decontaminated (calcium does not return in the cell, it leaves there thanks to dependant pumps ATP). This hyperpolarisation activates ionic channels of type HCN. The latter have kinetics of slow opening and are permeable with an entering sodium and potassium current (I_h). They thus depolarize slowly the membrane (the interior of the membrane is increasingly positive: it joined the external potential). When diastolic depolarization crosses the threshold of activation of these channels, a calcic Courant of type T (I_T) is generated which initiates the potential of action (calcium returns DNS the cell). During depolarization due to the potential of action, the I_T current is inactivated and decontaminated the I_h current (sodium does not return any more in the cell) (see Biophysique of the ionic channels). Thus the potential of action is again followed hyperpolarisation (hyperpolarisation caused by the re-establishment of ionic balances on both sides of the membrane). Calcium leaves the cell thanks to dependant pumps ATP, and dependant pumps not ATP make return of potassium in the cell and leave sodium. The cell thus reaches again a sufficient level of hyperpolarisation to cause the re-entry of ions sodium in cell etc… The kinetics of the I_h current defines the frequency of this autonomous generator of rate/rhythm ( pacemaker ).

The Dépolarisation of the potential of action is led gradually in the cells of the right and left auricles, being accompanied by a muscular contraction of the fibers . She arrives until the Faisceau of His where the electric impulse is transmitted to the two ventricles.

The sinusal node has its own automation allowing a depolarization a frequency close to 70 with 80/mn at the adult human being. The frequency is controlled by:

the Vagus whose stimulation causes a deceleration of the heart rate, while releasing from the Acétylcholine, which, via the muscarinic activation of receiving, decreases the concentration in AMPc, which slowed down the kinetics of the I_h current, and thus lengthens the diastolic period of depolarization and decreases the heart rate.
the Nervous system sympathetic nerve, of nature humoral (i.e. by Secretion of active substances in blood), implying the Adrenalin and the Noradrenalin, causes an increase in the intracellular concentration of AMPc, via the activation of Récepteur beta-adrénergique. The kinetics of the I_h current is increased by it, which causes the increase in the heart rate.

The irrigation of the sinusal node depends on the coronary Artère right: in the event of Myocardial infarction interesting this artery, the operation of this node can be disturbed.

Exploration

ECG

The ECG visualizes the depolarization of the auricles in the form of a P. wave a succession of sequences P-QRS constitutes a normal rate/rhythm also called sinusal rate/rhythm . In the event of dysfonction (faulty operation) sinusal, one or more waves P can miss. That appears on the layout by pauses. These last can be also visualized on a cardiac Holter which is a recording of the electrocardiogram over one day.

electrophysiological Exploration

It consists in mounting probes by venous way under radiological control until in the heart and to measure the electric activity directly there. One cannot however not visualize the depolarization of the sinusal node directly, but using protocols of stimulation of the auricle followed by an abrupt stop, one can uncover a faulty operation of this last.

Diseases of the sinusal node

See article Turbid cardiac conduction.

An attack of the sinusal node can be called according to various terms which are equivalent: sinusal dysfonction , disease of the sine , sino-auricular block . It appears by a too slow heart (Bradycardie) being able to be either permanent, or by moment ( paroxystic ).

It appears by:

a Lipothymie, simple faintness;
more rarely a syncope (loss of consciousness);
a tiredness ( asthenia ), breathlessness…

The cause can be:

medicamentous (antiarythmic in particular);
ischemic at the time of an attack of the coronary right-hand side;
metabolic, at the time of a Hyperkaliémie (rate of too high Potassium blood) for example.
degenerative (fibrosis due to the age).

The treatment is that of the cause. Where necessary one can propose the installation of a Cardiac pacemaker.

See too

Cardiology

electric System of conduction of the heart

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