Sinoatrial nodule . It is commonly called a heart pacemaker.
Description
The sinoatrial node (abbreviated as SA) or Keith and Flack, is one of the structures that make up the conduction system of the heart; It is commonly called a heart pacemaker.
Normally, this is where the electrical impulse that gives rise to a heartbeat originates. It is located in the anterolateral subepicardium in the upper 2/3 of the terminal sulcus at the junction of the right atrium, under the mouth of the superior vena cava.
It is a fusiform structure from a morphological point of view, its size being proportional to the size of the heart, ranging from 5mm to 30mm, with an average of 15mm in extension by 5mm in thickness, ranging from 1.5mm to 5mm.
Histologically
It is made up of a group of cells (P cells, Transitional cells and Purkinje cells) in close relationship with fibers of the autonomic nervous system and collagen fibers.
The oscilloscopic recording of the action potentials of the cells of this node show two important characteristics:
Absence of resting phase: After repolarization, in phase 4, the membrane potential does not remain stable, but rises slowly, until when it reaches -40 millivolts, a new excitation phase begins spontaneously.
Low speed in the excitation phase: The massive entrance of sodium ions into the cell is not as fast as in the other cardiac cells, but the depolarization phase is established slowly.
The potential change has a speed of 1-2 volts / second, compared to 100-200 volts / second in other cells .
This form of membrane potential explains the automatism of sinus pacemaker cells . The arrival of a stimulus is not necessary to provoke the change in the permeability of the membrane to ions, but said permeability to sodium first and to potassium is established spontaneously and cyclically at a rate of 60 to 100 times per minute.
Although the sinoatrial node works automatically, its operation is regulated by the vegetative nervous system .