<P> There are no large veins that drain blood away from the SA node . Instead, smaller venules drain the blood directly into the right atrium . </P> <P> The main role of a sinoatrial node cell is to initiate action potentials, so that it can pass throughout the heart and cause contraction . An action potential is a change in voltage (membrane potential) across the membrane of the cell, produced by the movement of charged atoms (ions). Non-pacemaker cells (including the ventricular and atrial cells) have a period, immediately after an action potential, where the membrane potential remains relatively constant; this is known as a resting membrane potential . This resting phase (see cardiac action potential, phase 4) ends when another action potential reaches the cell . This produces a positive change in membrane potential (known as depolarisation), which initiates the start of the next action potential . Pacemaker cells, however, don't have this resting phase . Instead, immediately after one action potential, the membrane potential of these cells begins to depolarise again automatically, this is known as the pacemaker potential . Once the pacemaker potential reaches a set value, known as the threshold value, it then produces an action potential . Other cells within the heart (including the purkinje fibers and atrioventricular node; AVN) can also initiate action potentials; however, they do so at a slower rate and therefore, if the SA node is working, it usually beats the AVN to it . </P> <P> Outlined below are the 3 phases of a sinoatrial node action potential . In the ventricular action potential, there are 5 phases (labelled 0 - 4), however pacemaker action potentials don't have an obvious phase 1 or 2 . </P> <P> This phase is also known as the pacemaker potential . Immediately following an action potential, when the membrane potential is very negative (it is hyperpolarised) the voltage slowly begins to increase . This is initially due to the closing of potassium channels, which reduces the flow of potassium ions (I) out of the cell (see phase 3, below). Alongside the deactivation of the potassium channels, channels known as hyperpolarisation - activated cyclic nucleotide--gated (HCN) channels, are activated . Activation of these channels at very negative membrane potential is an unusual property for ion channels, therefore the flow of sodium (Na) and some potassium (K) through the activated HCN channel is referred to as a funny current (I). This funny current causes the membrane potential of the cell to gradually increase, as the positive charge (Na and K) is flowing into the cell . Another mechanism involved in pacemaker potential is known as the calcium clock . Here, calcium is released spontaneously from the sarcoplasmic reticulum (a calcium store) into the cell, this is known as a spontaneous Ca spark . This in - crease in calcium within the cell then activates a sodium - calcium exchanger (NCX), which removes one Ca from the cell, and exchanges it for 3 Na into the cell (therefore removing a charge of + 2 from the cell, but allowing a charge of + 3 to enter the cell) therefore in - creasing the membrane potential . The calcium is later pumped back into the cell via calcium channels located on the cell membrane and SR membrane . The increase in membrane potential produced by these mechanisms, activates T - type calcium channels and then L - type calcium channels (which open very slowly). These channels allow a flow of calcium ions (Ca) into the cell, making the membrane potential more positive . </P>

Any region of spontaneous firing other than the sa node is