The SA node, also known as the sinus node. It is a group of cells located in the wall of the right atrium of the heart. These cells have the ability to spontaneously produce an electrical impulse (action potential, see below for more details) that travels through the heart via the electrical conduction system, causing it to contract. Read below SA node function.



SA node function

The main role of sinus node cells is to initiate functional potentials. They can pass through the heart and cause spasms. The functional potential is the change in the voltage (membrane potential) across the cell membrane, produced by the movement of charged atoms (ions).


Cells without a pacemaker (including ventricular and atrial cells) have a period immediately following the action potential where the potential of the membrane remains relatively constant; this also knows as the potential of resting membrane.


In the SA node function, this gives a positive change in membrane potential (known as depolarization), which initiates the beginning of the next action potential. However, cancer cells do not have this resting phase. Instead, immediately after one potential action, the membrane potential of these cells automatically begins to depolarize again.


This knows as the potential of the stimulator. When the pacemaker potential reaches a predetermined value, known as a threshold value, then it produces a functional potential. Other cells within the heart (including Purkinje fibers and atrioventricular node, AVN) can also initiate functional potentials. However, they do so slowly, and therefore, if the SA node is working, it usually pierces from AVN.


phase 4

This phase is also known as the stimulator potential. Immediately after the action of potential, when the membrane potential is very negative (hyperpolarized), the voltage slowly begins to increase. This is initially caused by the closure of potassium channels, which reduces the flow of potassium ions.


Phase 0

This is the depolarization phase. When the membrane potential reaches the threshold (about -20 to -50 mV), it begins to depolarise rapidly (it becomes more positive). This is mainly due to the Ca2 + flow through the L-type calcium channels, which are now completely open. The calcium channels and HCN channels deactivate during this stage, T-type.


Phase 3

This phase is the phase of repolarization. This is due to the inactivation of L-type calcium channels (preventing Ca2 + from moving to the cell) and activation of potassium channels, which allows K + outflow from the cell, making the membrane potential more negative.