Editing Heart (section)

===Wall===
{{Further|Cardiac muscle}}
[[File:2004 Heart Wall.jpg|thumb|Layers of the heart wall, including visceral and parietal pericardium]]

The heart wall is made up of three layers: the inner [[endocardium]], middle [[myocardium]] and outer [[epicardium]]. These are surrounded by a double-membraned sac called the pericardium.

The innermost layer of the heart is called the endocardium. It is made up of a lining of [[simple squamous epithelium]] and covers heart chambers and valves. It is continuous with the [[endothelium]] of the veins and arteries of the heart, and is joined to the myocardium with a thin layer of connective tissue.<ref name="CNX2014"/> The endocardium, by secreting [[endothelins]], may also play a role in regulating the contraction of the myocardium.<ref name="CNX2014"/>

[[File:2006 Heart Musculature.jpg|200px|thumbnail|The swirling pattern of myocardium helps the heart pump effectively]]The middle layer of the heart wall is the myocardium, which is the [[cardiac muscle]]—a layer of involuntary [[striated muscle tissue]] surrounded by a framework of [[collagen]]. The cardiac muscle pattern is elegant and complex, as the muscle cells swirl and spiral around the chambers of the heart, with the outer muscles forming a figure 8 pattern around the atria and around the bases of the great vessels and the inner muscles, forming a figure 8 around the two ventricles and proceeding toward the apex. This complex swirling pattern allows the heart to pump blood more effectively.<ref name="CNX2014"/>

There are two types of cells in cardiac muscle: [[cardiomyocyte|muscle cells]] which have the ability to contract easily, and [[cardiac pacemaker|pacemaker cells]] of the conducting system. The muscle cells make up the bulk (99%) of cells in the atria and ventricles. These contractile cells are connected by [[intercalated disc]]s which allow a rapid response to impulses of [[cardiac action potential|action potential]] from the pacemaker cells. The intercalated discs allow the cells to act as a [[syncytium]] and enable the contractions that pump blood through the heart and into the [[great arteries|major arteries]].<ref name="CNX2014"/> The pacemaker cells make up 1% of cells and form the conduction system of the heart. They are generally much smaller than the contractile cells and have few [[myofibril]]s which gives them limited contractibility. Their function is similar in many respects to [[neuron]]s.<ref name="CNX2014"/> Cardiac muscle tissue has [[autorhythmicity]], the unique ability to initiate a cardiac action potential at a fixed rate—spreading the impulse rapidly from cell to cell to trigger the contraction of the entire heart.<ref name="CNX2014"/>

There are specific [[Bioinformatics#Gene and protein expression|proteins expressed]] in cardiac muscle cells.<ref>{{Cite web|url=https://www.proteinatlas.org/humanproteome/heart|title=The human proteome in heart |series=The Human Protein Atlas|website=www.proteinatlas.org|access-date=2017-09-29|archive-date=9 November 2018|archive-url=https://web.archive.org/web/20181109191813/http://www.proteinatlas.org/humanproteome/heart|url-status=live}}</ref><ref>{{Cite journal|last1=Uhlén|first1=Mathias|last2=Fagerberg|first2=Linn|last3=Hallström|first3=Björn M.|last4=Lindskog|first4=Cecilia|last5=Oksvold|first5=Per|last6=Mardinoglu|first6=Adil|last7=Sivertsson|first7=Åsa|last8=Kampf|first8=Caroline|last9=Sjöstedt|first9=Evelina|s2cid=802377|date=2015-01-23|title=Tissue-based map of the human proteome|journal=Science|language=en|volume=347|issue=6220|pages=1260419|doi=10.1126/science.1260419|issn=0036-8075|pmid=25613900}}</ref> These are mostly associated with muscle contraction, and bind with [[actin]], [[myosin]], [[tropomyosin]], and [[troponin]]. They include [[MYH6]], [[ACTC1]], [[TNNI3]], [[CDH2]] and [[Plakophilin-2|PKP2]]. Other proteins expressed are [[MYH7]] and [[LDB3]] that are also expressed in skeletal muscle.<ref>{{Cite journal|last1=Lindskog|first1=Cecilia|last2=Linné|first2=Jerker|last3=Fagerberg|first3=Linn|last4=Hallström|first4=Björn M.|last5=Sundberg|first5=Carl Johan|last6=Lindholm|first6=Malene|last7=Huss|first7=Mikael|last8=Kampf|first8=Caroline|last9=Choi|first9=Howard|date=2015-06-25|title=The human cardiac and skeletal muscle proteomes defined by transcriptomics and antibody-based profiling|journal=BMC Genomics|volume=16|issue=1 |pages=475|doi=10.1186/s12864-015-1686-y|pmid=26109061|pmc=4479346|issn=1471-2164 |doi-access=free }}</ref>