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You are watching: The thickest layer of the heart wall is

StatPearls . Endowment Island (FL): StatPearls Publishing; 2021 Jan-.


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Introduction

The heart muscles heat the myocardium, or center layer of the heart walls, and are responsible because that the contractile function of the cardiac pump. Composed of cardiomyocytes, the heart muscle has actually distinctive cellular and also physiological features enabling it come generate force to keep adequate tissue and organ perfusion transparent the whole body. Love muscle renders up one of the faster functioning embryonic guts that continues to contract and beat from development throughout a lifetime. Offered by a complicated system the coronary vasculature, cardiac lymphatics, and also autonomic innervation, cardiomyocytes line the thickest great of every chamber of the heart. Cardiovascular conditions are the leading cause of mortality worldwide. A vast variety of these diseases involve the love muscles with varied mechanisms of pathophysiology, bring about contractile dysfunction, cabinet damage and also death, and cardiac pump failure. However, many interventions, treatment options, and therapies space aimed at minimizing damage, restoring functionality, avoiding the occurrence, and also reducing the hazard of cardiovascular disease.


Structure and Function

Three distinctive layers comprise the heart walls, from inner to outer:


Endocardium
Myocardium

The muscles of the heart, termed the myocardium, consist of the middle and also thickest great of the heart wall. This class lies in between the single-cell endocardium layer, i beg your pardon lines the inside chambers, and also the external epicardium, which renders up component of the pericardium the surrounds and also protects the heart. Histologically, love muscles room composed that cells dubbed cardiomyocytes the have distinctive structures and also properties correlating to your contractile function.<1> Cardiomyocytes space striated, uninucleate muscle cells discovered exclusively in the heart muscle. A unique cellular and physiological feature of cardiomyocytes room intercalated discs, i m sorry contain cell adhesions together as gap junctions, to facilitate cell-cell communication. This discs reduce inner resistance and allow activity potentials come spread quickly throughout the entire heart muscle via the i of charged ions. Thus, the love muscle acts together a sensible syncytium with rapid synchronized contractions that are responsible for pumping blood throughout the body. Functionally, the heart muscles count on electrochemical gradients and the potentials to generate contractile pressure for each heartbeat.

The sinus node, located within the right atrial myocardium, spontaneously depolarizes and thus identify the love rate. This depolarizations are currents of ion influx the are brought from the sinus node to the love muscle via conducting cells. Once the depolarization reaches the love muscle, voltage-gated sodium networks open, allowing a quick influx of sodium ions right into the cardiomyocytes, depolarizing the cells. The positive membrane potential cause voltage-gated potassium and then calcium channels to open, allowing potassium to rush out and calcium to sirloin in. The initial influx of calcium is crucial for the 2nd release the calcium native the sarcoplasmic reticulum found within the love muscle cells. The accumulation of intracellular calcium ions binds come troponin C, moving tropomyosin beside to allow actin-myosin binding and also cross-bridge cycle responsible for muscle contraction.<1> The lot of calcium released is straight proportional to the quantity of actin-myosin interaction allowed and thus correlates through the contractile force of the love muscle generated. Physiologically, this corresponds with parameters such together stroke volume, ejection fraction, and also cardiac output provided to assess love function. In ~ the finish of every cycle, calcium gets revived to the sarcoplasmic delusion via SERCA (Sarco(endo)plasmic delusion (SER) Ca2+ ATPase) pumps if sodium-potassium and also sodium-calcium ATPase pumps reclaim the cardiomyocyte membrane potential therefore the cycle deserve to repeat through the following incoming depolarization.<1>


Embryology

The love muscle originates native the mesoderm layer and also begins forming during the 3rd week the embryonic development. The mesoderm serves together the primary resource for myocardial precursor cells, which make up the cardiogenic or primary heart field throughout early development. A primitive, horseshoe-shaped endothelial heart tube is formed and also begins contracting come facilitate the embryo’s beforehand circulation system. In ~ the following several weeks, the proliferation the cardiomyocytes is essential for expanding the myocardial layer and also generating the multichambered system of the mature heart.<2> if existing cardiomyocytes contribute to the expansion of the myocardium via proliferation and organization, new heart muscle cells are likewise recruited from surrounding mesenchymal layers that further expands the muscle layer.<2> adhering to myocardial development, the heart wall surfaces undergo more maturation, compaction, and also trabeculation. Dilatations or swellings of the heart pipe embryonic structures together with neural crest cell migration facilitate the development of the chambers and inflow/outflow tracts. These processes result in a mature and fully functional, contracting love by the eighth embryonic week and also throughout adulthood.


Blood Supply and also Lymphatics

The love muscles’ blood supply comes straight from the system of coronary arteries the runs within the epicardial layer. Two key coronary arteries, the left coronary artery (LCA) and the ideal coronary artery (RCA), branch directly off the aorta via the coronary Ostia. These arteries and their branches supply tributary arteries that run perpendicular to the heart surface and transverse native the epicardium, v the myocardium, and down come the endocardium.<3> The LCA quickly branches into the left anterior diminish (LAD) coronary artery and also the left circumflex (LCX) coronary artery. The LAD operation vertically down the interventricular groove in the direction of the apex and supplies blood come the anterior left ventricular myocardium, the anterior two-thirds of the interventricular septal myocardium, and the anterolateral papillary muscle connecting the mitral valves. The LCX process horizontally follow me the atrioventricular groove and also gives rise to the left obtuse marginal coronary artery, together supplying the lateral and posterior left ventricular myocardium. The RCA operation horizontally along the best atrioventricular groove and also gives increase to the appropriate acute marginal coronary artery, which supplies the best ventricular myocardium. The RCA also gives rise to the posterior descending artery (PDA) in around 90% the the human populace (the PDA originates from the LCX in the other around 10%), which provides the posterior myocardium the both ventricles, the posterior one-third of the interventricular septal myocardium, and the posteromedial papillary muscle that the mitral valves.<3> Blood circulation via the coronary arteries to the myocardium occurs during diastole and also ventricular be sure via the passive flow of blood into the aortic Ostia. Throughout systole and also ventricular contraction, the coronary arteries end up being compressed, and thus impedes myocardial blood flow.

The venous mechanism of the heart muscles runs parallel to the coronary arteries. Venous drainage the the left ventricular myocardium is perfect by the interventricular vein and the an excellent cardiac vein, i beg your pardon drains into the coronary sinus, discovered in the posterior ideal atrioventricular groove, which climate drains into the right atrium.<3> The anterior cardiac veins are responsible for draining blood native the right ventricular myocardium directly into the best atrium.<3>

The cardiac lymphatic drainage system is made up of lymphatic capillaries and pre-collector vessels arranged in plexuses within every of the heart wall surface layers.<4> these lymphatic vessels and also plexuses circulation from subendocardium, with the myocardium, up v the subepicardium, into the mediastinal lymph nodes, and ultimately draining into both left and right venous angles in between the internal jugular veins and also the subclavian veins. The source of flow for lymphatic drainage comes from contractions that the myocardium, which generate force to propel fluid movement v the mechanism to the lymph nodes.


Nerves

Heart muscles space innervated mainly by 2 nerves, the accelerans nerve and the vagus nerve, which administer sympathetic and also parasympathetic stimulation indigenous the autonomic worried system, respectively. Intrinsic ganglia because that the myocardium are current in the epicardium, i beg your pardon receives signals from post-ganglionic sympathetic connections coming native the accelerans nerve and pre-ganglionic parasympathetic connections from the vagus nerve.<5> most post-ganglionic sympathetic relationships synapse straight with the love muscle cells, releasing norepinephrine together the main neurotransmitter.<5> upon binding, norepinephrine stimulates beta-adrenergic receptor to boost contractility that the myocardium via increasing calcium influx. Preganglionic parasympathetic fibers synapse very first with the epicardial intrinsic ganglia and then post-ganglionic neurons straight synapse through the myocardium.<5> Acetylcholine is the major neurotransmitter because that myocardial parasympathetic signals, acting on muscarinic (M2) receptor on the cardiomyocytes.


Muscles

The muscle class of the heart is termed the myocardium and also is made up of cardiomyocytes. The myocardium is uncovered in the wall surfaces of all 4 chambers the the heart, though it is more thickness in the ventricles and thinner in the atria. This disparity is because of the distinction in the generation that the force of contraction required for propelling blood in between the atria and the ventricles, v ventricles inquiry much an ext power.


Physiologic Variants

Chronic main hypertension is a common and prevalent condition affecting a huge percentage that the United states population. Over long periods, chronic systemic hypertension can an outcome in structural and also functional alters to the heart muscle wall. Because of chronic boosts in afterload, the press in which the left ventricular muscle must contract against increased average arterial pressure, the heart muscle responds with compensatory cardiomyocyte hypertrophy. The ventricular muscle wall thickens come reduce wall stress with a resulting increase in the wall thickness to the room diameter ratio.<6> Left ventricular hypertrophy (LVH) hence characteristically displayed a geometry that concentric thickness and is a physiological an answer to and also a common complication of even mild chronic hypertension.<6> physical examination result of LVH may include an enlarged point of maximal impulse (PMI) and an S4 gallop ~ above cardiac apex auscultation.

Normal aging processes transform heart muscle structurally and physiologically. Arteries become less compliant and stiffen end time, and also in larger years, this results in boost in afterload due to greater pressure versus which the heart muscle have to contract. As a compensatory response, left ventricular thickness boosts from cardiomyocyte hypertrophy. Over time, cardiomyocytes lessen with age from apoptosis, necrosis, or autophagy resulting in an all at once decrease in cardiomyocyte number in aged heart muscle.<7> together a compensatory mechanism, the continuing to be cardiomyocytes may hypertrophy or experience pathological remodeling. These changes result in a decrease in cardiac compliance and boost in wall surface stiffness. In ~ the love muscle cells, age-related changes induce a shift from alpha-myosin heavy chain come the beta-myosin heavy chain with reduced cross-bridge to ride bicycle activity.<7> This problem ultimately leader to contractile decrease and diastolic dysfunction in the aging heart muscle. Calcium homeostasis is also affected during aging processes because of a reduced capability of SERCA pumps and the sodium-calcium pumps to properly restore relaxing membrane potential calcium levels.<7> Disrupted calcium homeostasis affects love muscle relaxation mechanics and thus leader to diastolic dysfunction.


Surgical Considerations

A significant, yet underrecognized, heart muscle complication after ~ non-cardiac surgery is perioperative myocardial injury (PMI), which is unique from myocardial infarction. Risk components for PMI before and also after surgery encompass age higher than or same to 65 years and a preexisting background of atherosclerotic disease. PMI is an acute boost in high-sensitivity cardiac troponin T (hs-cTn) plasma concentrations.<8> PMI often presents without chest pain, dyspnea, or other typical cardiac injury symptoms and also is therefore routinely missed throughout clinical workups perioperatively. However, PMI correlates through a significantly increased danger of 30-day mortality post-noncardiac surgery.<8> To successfully diagnose PMI, hs-cTn screening have to be used perioperatively to detect and also quantify cardiomyocyte injury to minimize the threat of short and long-term mortality.


Clinical Significance

Coronary artery an illness (CAD), additionally termed ischemic heart disease, is the many prevalent cardiovascular disease and the leading cause of worldwide mortality, with far-ranging implications and aftermath for love muscle functionality. CAD is identified by the development of atherosclerotic plaques in ~ the coronary arteries causing a diminish in blood flow and also oxygen and also nutrient delivery to the myocardium. Manifestations the CAD room termed acute coronary syndromes (ACS) and also include secure angina, turbulent angina, and others native the supply-demand mismatch of inadequate oxygen perfusion indigenous at the very least 70% occlusion that the myocardial vascular supply. The most severe manifestations the CAD are myocardial infarctions, colloquially termed “heart attacks.” Myocardial infarctions (MIs) divide into ST-segment key myocardial infarctions (STEMI) and also non-ST-segment elevation myocardial infarction (NSTEMI), depending on full or near-total coronary vascular occlusions, respectively, and also subsequent result on electrocardiogram (EKG).

The place of myocardial infarctions within the heart deserve to be generally localized via which certain leads are influenced with abnormal ST-segment and/or T-wave morphologies on an EKG. MIs show cardiomyocyte necrosis, and diagnostic findings encompass positive cardiac troponin (cTn) tests and also creatinine kinase myocardial band (CK-MB) tests.<9> adhering to MIs, the influenced heart muscle wall surface is often weaker 보다 it was before the MI from cabinet injury and death, inflammation, and also fibrotic replacement the decreases heart muscle contractile function, which boosts the threat for love failure, cost-free wall ruptures, arrhythmias, suddenly cardiac death, and other complications. Therapy for MIs includes pharmacotherapy such together antiplatelet drugs and also fibrinolysis, interventions such as percutaneous coronary interventions (PCIs), or surgeries such as coronary artery bypass grafting (CABG) aimed at restoring perfusion and vascularization that the myocardium.<9>

Cardiomyopathies are inherited or acquired diseases of the heart muscle that influence structure and functionality in the lack of other cardiovascular condition processes, such together CAD. Hypertrophic cardiomyopathy (HCM) is the most typical genetically inherited heart condition and follows an autosomal dominant inheritance pattern. The pathophysiology of HCM comes from mutations in myocardial sarcomere genes resulting in myofibrillar disarray, serious thickening, hypertrophy the the love muscle wall, and diastolic dysfunction. Manifestations of HCM encompass dyspnea, syncope, palpitations, and sudden death, and most commonly presents in adolescence. Dilated cardiomyopathy (DCM) is defined by eccentric hypertrophy, dilation that the cardiac muscle wall, and systolic dysfunction.<10> DCM is the last myocardial an answer to a range of genetic and environmental stresses such together chronic alcohol abuse, viral or helminth infections, specific chemotherapies, and other etiologies with the most common being idiopathic DCM.<11> therapy for cardiomyopathies consists of pharmacotherapy to reduce heart muscle demand such together calcium-channel blockers or beta-blockers, implantable cardioverter-defibrillator (ICD) placements to prevent life-threatening arrhythmias or heart transplantation.

Myocarditis is the term for inflammation the the love muscle and has number of etiologies. The most typical etiology in the Western civilization is viral infections (infectious myocarditis), yet other causes include toxic reactions or allergies to drugs, autoimmune diseases, or other infections (bacterial, fungal, parasitic). Myocarditis involves damage and also loss that both cardiomyocytes and also cardiac vascular endothelial cells, both of i m sorry are typical targets of epidemic from inflammatory processes and also white blood cell (often lymphocytic) infiltration right into the love muscle wall.<12> Complications encompass interstitial cardiac fibrosis, wall surface motion abnormalities, arrhythmias, love failure, myocardial infarctions, lessened ejection fraction, and sudden cardiac death.<12> Myocarditis has a diverse presentation with non-specific symptoms such together chest pain, dyspnea, flu-like manifestations, yet can likewise be symptomless.

Heart failure (HF) is the common end-stage pathway and clinical manifestation of cardiac pump dysfunction native a range of etiologies, including many diseases affecting the love muscle. HF deserve to be categorized into various groups such together acute vs. Chronic, appropriate heart vs. Left heart, systolic (reduced ejection fraction) vs. Diastolic (preserved ejection fraction), every presenting with unique clinical characteristics. HF is essentially characterized as the i can not qualify of the heart to pump adequately resulting in congestion, decreased organ perfusion, and functional impairment.<1> Etiologies and also risk determinants for HF space diverse and encompass procedures such as myocardial injury or infarction, CAD, chronic hypertension, valvular dysfunction, arrhythmias, cardiomyopathies, and numerous various other pathways. The pathophysiology causing HF involves facility interactions of solution such together neurohormonal activation, peripheral vascular effects, and physiological processes within the heart muscle itself.

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<1> therapy for HF requires pharmacotherapy such together ACE inhibitors and also beta-blockers to decrease love stress and also workload or inotropes to increase contractility. Therapies for progressed heart failure include mechanical circulatory support such as ventricular assist gadgets (VADs) or love transplantation.