Editing Sinoatrial node (section)

==Structure==
The sinoatrial node is an [[Ellipse|oval-shaped]] structure that is approximately 15&nbsp;[[millimetre|mm]] long, 3&nbsp;mm wide, and 1&nbsp;mm thick, located directly below and to the side of the [[superior vena cava]].<ref name="Hall">{{cite book |last1=Hall |first1=John E. |title=Guyton and Hall textbook of medical physiology |date=2011 |location=Philadelphia, Pa. |isbn=9781416045748 |page=115 |edition=Twelfth}}</ref> The size can vary but is usually between 10-30&nbsp;mm long, 5–7&nbsp;mm wide, and 1–2&nbsp;mm deep.<ref>{{Cite journal|last1=Csepe|first1=Thomas A.|last2=Zhao|first2=Jichao|last3=Hansen|first3=Brian J.|last4=Li|first4=Ning|last5=Sul|first5=Lidiya V.|last6=Lim|first6=Praise|last7=Wang|first7=Yufeng|last8=Simonetti|first8=Orlando P.|last9=Kilic|first9=Ahmet|date=2017-03-01|title=Human Sinoatrial Node Structure: 3D Microanatomy of Sinoatrial Conduction Pathways|journal=Progress in Biophysics and Molecular Biology|volume=120|issue=1–3|pages=164–178|doi=10.1016/j.pbiomolbio.2015.12.011|issn=0079-6107|pmc=4808362|pmid=26743207}}</ref><ref>{{cite journal | last1 = Chandler | first1 = N. | last2 = Aslanidi | first2 = O. | last3 = Buckley | first3 = D. | last4 = Inada | first4 = S. | last5 = Birchall | first5 = S. | last6 = Atkinson | first6 = A. | last7 = Kirk | first7 = D. | last8 = Monfredi | first8 = O. | last9 = Molenaar | first9 = P. | last10 = Anderson | first10 = R. | last11 = Sharma | first11 = V. | last12 = Sigg | first12 = D. | last13 = Zhang | first13 = H. | last14 = Boyett | first14 = M. | last15 = Dobrzynski | first15 = H. | year = 2011 | title = Computer three-dimensional anatomical recon-struction of the human sinus node and a novel paranodal area | journal = Anatomical Record | volume = 294 | issue = 6| pages = 970–9 | doi=10.1002/ar.21379| pmid = 21538926 | doi-access =  | s2cid = 45464625 }}</ref>

===Location===
The SA node is located in the wall ([[epicardium]]) of the [[atrium (heart)|right atrium]], laterally to the entrance of the [[superior vena cava]] in a region called the [[sinus venosus|sinus venarum]] (hence ''[[wikt:sino-#Prefix|sino-]]'' + ''[[wikt:atrial#Adjective|atrial]])''.<ref name="Dorlands">{{Citation |author=Elsevier |author-link=Elsevier |title=Dorland's Illustrated Medical Dictionary |publisher=Elsevier |url=http://dorlands.com/ |postscript=.}}</ref> It is positioned roughly between a groove called the [[crista terminalis]] located on the internal surface of the [[heart]] and the corresponding [[terminal sulcus (heart)|sulcus terminalis]], on the external surface.<ref name=Monfredi/> These grooves run between the entrance of the [[superior vena cava]] and the [[inferior vena cava]].{{cn|date=May 2025}}

===Microanatomy===
[[File:Sinoatrial node 2 low mag.jpg|thumb|Figure 2: Low magnification [[H&E stain|stained]] [[histology|image]] of the SA node (center-right on image) and its surrounding tissue. The SA node surrounds the [[sinoatrial nodal artery]], seen as the open [[lumen (anatomy)|lumen]]. Cardiac muscle cells of the right atrium can be seen to the left of the node, and fat tissue to the right.]]
The cells of the SA node are spread out within a mesh of [[connective tissue]], containing [[nerves]], [[blood vessels]], [[collagen]] and [[fat]]. Immediately surrounding the SA node cells are paranodal cells.<ref name=Monfredi/> These cells have structures intermediate between that of the SA node cells and the rest of the [[atrium (heart)|atrium]].<ref>{{cite journal | last1 = Chandler | first1 = NJ | last2 = Greener | first2 = ID | last3 = Tellez | first3 = JO | last4 = Inada | first4 = S | last5 = Musa | first5 = H | last6 = Molenaar | first6 = P | last7 = Difrancesco | first7 = D | display-authors = etal | year = 2009 | title = Molecular architecture of the human sinus node: Insights into the function of the cardiac pacemaker | journal = Circulation | volume = 119 | issue = 12| pages = 1562–1575 | doi=10.1161/circulationaha.108.804369| pmid = 19289639 | doi-access = free }}</ref> The connective tissue, along with the paranodal cells, insulate the SA node from the rest of the atrium, preventing the electrical activity of the atrial cells from affecting the SA node cells.<ref name=Monfredi/> The SA node cells are smaller and paler than the surrounding [[atrium (heart)|atrial cells]], with the average cell being around 8 micrometers in diameter and 20-30&nbsp;micrometers in length (1 micrometer= 0.000001 meter).<ref>{{cite journal | last1 = Honjo | first1 = H. | last2 = Boyett | first2 = M.R. | last3 = Kodama | first3 = I. | last4 = Toyama | first4 = J. | year = 1996 | title = Correlation between electrical activity and the size of rabbit sino-atrial node cells | journal = The Journal of Physiology | volume = 496 | issue = 3| pages = 795–808 | doi = 10.1113/jphysiol.1996.sp021728 | pmid = 8930845 | pmc = 1160865 }}</ref> Unlike the atrial cells, SA node cells contain fewer [[mitochondrion|mitochondria]] and [[myofibers]], as well as a smaller [[sarcoplasmic reticulum]]. This means that the SA node cells are less equipped to contract compared to the [[atrium (heart)|atrial]] and [[ventricle (heart)|ventricular]] cells.<ref name=Boyett>{{cite journal | last1 = Boyett | first1 = Honjo | last2 = Kodama | first2 = I. | year = 2000 | title = The sinoatrial node, a heterogeneous pace-maker structure | journal = Cardiovascular Research | volume = 47 | issue = 4| pages = 658–87 | doi=10.1016/s0008-6363(00)00135-8| pmid = 10974216 | doi-access = free }}</ref>

Action potentials pass from one [[heart|cardiac cell]] to the next through pores known as gap junctions. These gap junctions are made of proteins called [[connexin]]s. There are fewer gap junctions within the SA node and they are smaller in size. This is again important in insulating the SA node from the surrounding atrial cells.<ref name=Monfredi/><ref name=Boyett/>

===Blood supply===
The sinoatrial node receives its blood supply from the [[sinoatrial nodal artery]]. This blood supply, however, can differ hugely between individuals. For example, in most humans, this is a single [[artery]], although in some cases there have been either 2 or 3 sinoatrial node arteries supplying the SA node. Also, the SA node artery mainly originates as a branch of the [[right coronary artery]]; however in some individuals it has arisen from the [[Circumflex branch of left coronary artery|circumflex artery]], which is a branch of the [[left coronary artery]]. Finally, the SA node artery commonly passes behind the [[superior vena cava]], before reaching the SA node; however in some instances it passes in front. Despite these many differences, there doesn't appear to be any advantage to how many sinoatrial nodal arteries an individual has, or where they originate.<ref>{{cite journal | last1 = Vikse | first1 = J. | last2 = Henry | first2 = B.M. | last3 = Roy | first3 = J. | last4 = Ramakrishnan | first4 = P.K. | last5 = Hsieh | first5 = W.C. | last6 = Walocha | first6 = J.A. | last7 = Tomaszewski | first7 = K.A. | year = 2016b | title = Anatomical variations in the Sinoatrial Nodal artery: A Meta-Analysis and clinical considerations | journal = PLOS ONE | volume = 11 | issue = 2| page = e0148331 | doi = 10.1371/journal.pone.0148331 | pmid = 26849441 | pmc = 4743947 | bibcode = 2016PLoSO..1148331V | doi-access = free }}</ref>

===Venous drainage===
There are no large [[veins]] that drain blood away from the SA node.  Instead, smaller [[venule]]s drain the blood directly into the [[atrium (heart)|right atrium]].<ref>{{cite journal | last1 = Anderson | first1 = K.R. | last2 = Ho | first2 = S.Y. | last3 = Anderson | first3 = R.H. | year = 1979 | title = Location and vascular supply of sinus node in human heart | journal = Heart | volume = 41 | issue = 1| pages = 28–32 | doi = 10.1136/hrt.41.1.28 | pmid = 426954 | pmc = 514694 }}</ref>