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You are watching: Thickest layer of the heart wall

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


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Introduction

The love muscles heat the myocardium, or middle layer that the heart walls, and are responsible for the contractile role of the cardiac pump. Created of cardiomyocytes, the heart muscle has actually distinctive cellular and physiological features allowing it to generate pressure to preserve adequate tissue and organ perfusion transparent the whole body. Love muscle renders up among the more quickly functioning embryonic guts that proceeds to contract and also beat from development throughout a lifetime. Offered by a facility system the coronary vasculature, cardiac lymphatics, and also autonomic innervation, cardiomyocytes line the thickest layer of every chamber of the heart. Cardiovascular diseases are the leading reason of mortality worldwide. A vast variety of these illness involve the heart muscles with diverse mechanisms of pathophysiology, causing contractile dysfunction, cabinet damage and also death, and cardiac pump failure. However, plenty of interventions, treatment options, and therapies room aimed at minimizing damage, restoring functionality, preventing the occurrence, and reducing the danger of cardiovascular disease.


Structure and Function

Three distinct layers comprise the love walls, native inner come outer:


Endocardium
Myocardium

The muscles of the heart, termed the myocardium, make up the middle and also thickest class of the love wall. This great lies between the single-cell endocardium layer, which lines the inside chambers, and also the external epicardium, which provides up part of the pericardium that surrounds and protects the heart. Histologically, love muscles room composed that cells referred to as cardiomyocytes that have unique structures and properties correlating to their contractile function.<1> Cardiomyocytes room striated, uninucleate muscle cells found exclusively in the heart muscle. A distinctive cellular and physiological function of cardiomyocytes are intercalated discs, which contain cell adhesions such as space junctions, to facilitate cell-cell communication. These discs reduce inner resistance and also allow activity potentials come spread quickly throughout the whole heart muscle via the passage of fee ions. Thus, the love muscle acts together a useful syncytium with rapid synchronized contractions that space responsible for pumping blood throughout the body. Functionally, the heart muscles depend on electrochemical gradients and the potentials to generate contractile pressure for every heartbeat.

The sinus node, situated within the appropriate atrial myocardium, spontaneously depolarizes and thus determines the heart rate. This depolarizations room currents of ion influx that are carried from the sinus node come the heart muscle via conducting cells. As soon as the depolarization reaches the heart muscle, voltage-gated sodium networks open, enabling a rapid influx of sodium ions into the cardiomyocytes, depolarizing the cells. The hopeful membrane potential cause voltage-gated potassium and then calcium channels to open, enabling potassium to sirloin out and also calcium to sirloin in. The initial influx of calcium is vital for the second release of calcium native the sarcoplasmic reticulum found within the love muscle cells. The accumulation of intracellular calcium ions binds come troponin C, relocating tropomyosin beside to allow actin-myosin binding and cross-bridge cycle responsible for muscle contraction.<1> The quantity of calcium exit is directly proportional come the lot of actin-myosin interaction permitted and thus correlates with the contractile pressure of the love muscle generated. Physiologically, this coincides with parameters such as stroke volume, ejection fraction, and cardiac output offered to assess heart function. At the end of every cycle, calcium gets restored to the sarcoplasmic reticulum via SERCA (Sarco(endo)plasmic illusion (SER) Ca2+ ATPase) pumps if sodium-potassium and also sodium-calcium ATPase pumps gain back the cardiomyocyte membrane potential therefore the cycle have the right to repeat with the following incoming depolarization.<1>


Embryology

The heart muscle originates native the mesoderm layer and also begins forming during the 3rd week that embryonic development. The mesoderm serves together the primary source for myocardial precursor cells, which comprise the cardiogenic or major heart field throughout early development. A primitive, horseshoe-shaped endothelial heart tube is formed and begins contracting come facilitate the embryo’s early on circulation system. Within the next several weeks, the proliferation that cardiomyocytes is crucial for expanding the myocardial layer and generating the multichambered mechanism of the mature heart.<2> if existing cardiomyocytes add to the expansion of the myocardium via proliferation and also organization, new heart muscle cell are additionally recruited from adjacent mesenchymal layers the further increases the muscle layer.<2> following myocardial development, the heart walls undergo additional maturation, compaction, and trabeculation. Dilatations or swellings that the heart tube embryonic structures in addition to neural crest cell migration facilitate the advancement of the chambers and also inflow/outflow tracts. This processes result in a maturation and fully functional, contracting heart by the eighth embryonic week and throughout adulthood.


Blood Supply and also Lymphatics

The heart 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 best coronary artery (RCA), branch straight off the aorta via the coronary Ostia. These arteries and also their branches it is provided tributary arteries that operation perpendicular come the love surface and also transverse from the epicardium, v the myocardium, and down to the endocardium.<3> The LCA easily branches right into the left anterior to decrease (LAD) coronary artery and also the left circumflex (LCX) coronary artery. The LAD runs vertically down the interventricular groove towards the apex and also supplies blood to the anterior left ventricular myocardium, the anterior two-thirds that the interventricular septal myocardium, and also the anterolateral papillary muscle connecting the mitral valves. The LCX courses horizontally along the atrioventricular groove and also gives climb to the left obtuse marginal coronary artery, together giving the lateral and also posterior left ventricular myocardium. The RCA operation horizontally along the best atrioventricular groove and gives climb to the best acute marginal coronary artery, which gives the ideal ventricular myocardium. The RCA additionally gives rise to the posterior to decrease artery (PDA) in about 90% of the human populace (the PDA comes from the LCX in the other roughly 10%), which gives the posterior myocardium the both ventricles, the posterior one-third that the interventricular septal myocardium, and also the posteromedial papillary muscle the the mitral valves.<3> Blood flow via the coronary arteries come the myocardium occurs throughout diastole and also ventricular be safe via the passive circulation of blood into the aortic Ostia. During systole and ventricular contraction, the coronary arteries end up being compressed, and also thus impedes myocardial blood flow.

The venous mechanism of the love muscles operation parallel come the coronary arteries. Venous drainage that the left ventricular myocardium is perfect by the interventricular vein and the an excellent cardiac vein, i m sorry drains right into the coronary sinus, discovered in the posterior right atrioventricular groove, which climate drains into the right atrium.<3> The anterior cardiac veins are responsible for draining blood indigenous the right ventricular myocardium directly into the appropriate atrium.<3>

The cardiac lymphatic drainage system is comprised of lymphatic capillaries and pre-collector vessels organized in plexuses within every of the heart wall surface layers.<4> this lymphatic vessels and also plexuses circulation from subendocardium, v the myocardium, up v the subepicardium, right into the mediastinal lymph nodes, and also ultimately draining into both left and also right venous angles between the internal jugular veins and also the subclavian veins. The source of flow for lymphatic drainage comes from contractions the the myocardium, i beg your pardon generate pressure to propel fluid movement with the device to the lymph nodes.


Nerves

Heart muscles space innervated generally by 2 nerves, the accelerans nerve and also the vagus nerve, which carry out sympathetic and parasympathetic stimulation native the autonomic concerned system, respectively. Intrinsic ganglia for the myocardium are present in the epicardium, i beg your pardon receives signals from post-ganglionic sympathetic connections coming from the accelerans nerve and pre-ganglionic parasympathetic relationships from the vagus nerve.<5> many post-ganglionic sympathetic relationships synapse straight with the heart muscle cells, publication norepinephrine as the primary neurotransmitter.<5> ~ above binding, norepinephrine stimulates beta-adrenergic receptor to rise contractility the the myocardium via enhancing calcium influx. Preganglionic parasympathetic fibers synapse first with the epicardial intrinsic ganglia and also then post-ganglionic neurons straight synapse through the myocardium.<5> Acetylcholine is the major neurotransmitter for myocardial parasympathetic signals, acting on muscarinic (M2) receptors on the cardiomyocytes.


Muscles

The muscle class of the heart is termed the myocardium and is comprised of cardiomyocytes. The myocardium is discovered in the walls of all four chambers the the heart, despite it is more thick in the ventricles and also thinner in the atria. This disparity is due to the difference in the generation the the force of contraction necessary for propelling blood in between the atria and the ventricles, with ventricles inquiry much much more power.


Physiologic Variants

Chronic main hypertension is a common and prevalent an illness affecting a big percentage of the United says population. Over long periods, chronic systemic hypertension can result in structural and functional transforms to the love muscle wall. As result of chronic boosts in afterload, the press in i beg your pardon the left ventricular muscle have to contract against increased average arterial pressure, the love muscle responds through compensatory cardiomyocyte hypertrophy. The ventricular muscle wall surface thickens come reduce wall surface stress v a resulting rise in the wall surface thickness come the room diameter ratio.<6> Left ventricular hypertrophy (LVH) for this reason characteristically displayed a geometry that concentric thickness and is a physiological solution to and also a typical complication of also mild chronic hypertension.<6> physical examination result of LVH may encompass an enlarged point of maximal impulse (PMI) and an S4 gallop upon cardiac apex auscultation.

Normal aging processes transform heart muscle structurally and physiologically. Arteries end up being less compliant and stiffen end time, and in larger years, this outcomes in boost in afterload as result of greater pressure versus which the love muscle must contract. Together a compensatory response, left ventricular thickness boosts from cardiomyocyte hypertrophy. End time, cardiomyocytes to decrease with period from apoptosis, necrosis, or autophagy bring about an overall decrease in cardiomyocyte number in aged heart muscle.<7> together a compensatory mechanism, the remaining cardiomyocytes might hypertrophy or experience pathological remodeling. This changes result in a decrease in cardiac compliance and rise in wall stiffness. Within the heart muscle cells, age-related alters induce a transition from alpha-myosin heavy chain come the beta-myosin hefty chain with diminished cross-bridge cycle activity.<7> This condition ultimately leader to contractile decrease and diastolic dysfunction in the aging love muscle. Calcium homeostasis is also affected during aging processes as result of a reduced ability of SERCA pumps and also the sodium-calcium pumps to properly restore relaxing membrane potential calcium levels.<7> Disrupted calcium homeostasis affects heart muscle be safe mechanics and also thus leads to diastolic dysfunction.


Surgical Considerations

A significant, however underrecognized, love muscle complication ~ non-cardiac surgery is perioperative myocardial injury (PMI), which is distinctive from myocardial infarction. Risk factors for PMI before and also after surgery incorporate age greater than or equal to 65 years and also a preexisting background of atherosclerotic disease. PMI is an acute rise in high-sensitivity cardiac troponin T (hs-cTn) plasma concentrations.<8> PMI often presents there is no chest pain, dyspnea, or other common cardiac injury symptoms and is therefore routinely missed throughout clinical workups perioperatively. However, PMI correlates with a significantly increased hazard of 30-day mortality post-noncardiac surgery.<8> To successfully diagnose PMI, hs-cTn screening have to be provided perioperatively come detect and also quantify cardiomyocyte injury to alleviate the risk of short and long-term mortality.


Clinical Significance

Coronary artery an illness (CAD), also termed ischemic love disease, is the many prevalent cardiovascular disease and the leading reason of worldwide mortality, with far-reaching implications and after-effects for love muscle functionality. CAD is characterized by the development of atherosclerotic plaques within the coronary arteries bring about a to decrease in blood flow and oxygen and nutrient distribution to the myocardium. Manifestations the CAD space termed acute coronary syndromes (ACS) and also include steady angina, turbulent angina, and also others from the supply-demand mismatch of insufficient oxygen perfusion native at least 70% occlusion the the myocardial vascular supply. The most severe manifestations that CAD are myocardial infarctions, colloquially termed “heart attacks.” Myocardial infarctions (MIs) divide right into ST-segment key myocardial infarctions (STEMI) and also non-ST-segment elevation myocardial infarction (NSTEMI), escape on complete or near-total coronary vascular occlusions, respectively, and subsequent result on electrocardiogram (EKG).

The place of myocardial infarctions in ~ the heart can be generally localized via which specific leads are influenced with abnormal ST-segment and/or T-wave morphologies on an EKG. MIs demonstrate cardiomyocyte necrosis, and diagnostic findings incorporate positive cardiac troponin (cTn) tests and creatinine kinase myocardial tape (CK-MB) tests.<9> following MIs, the impacted heart muscle wall is often weaker than it was prior to the MI from cabinet injury and also death, inflammation, and also fibrotic replacement that decreases love muscle contractile function, which increases the hazard for love failure, free wall ruptures, arrhythmias, suddenly cardiac death, and other complications. Therapy for MIs has pharmacotherapy such as antiplatelet drugs and also fibrinolysis, interventions such as percutaneous coronary interventions (PCIs), or surgeries such together coronary artery bypass grafting (CABG) aimed in ~ restoring perfusion and vascularization the the myocardium.<9>

Cardiomyopathies space inherited or acquired diseases of the love muscle that impact structure and also functionality in the lack of other cardiovascular disease processes, such as CAD. Hypertrophic cardiomyopathy (HCM) is the most usual genetically inherited heart an illness and adheres to an autosomal leading inheritance pattern. The pathophysiology that HCM comes from mutations in myocardial sarcomere genes bring about myofibrillar disarray, serious thickening, hypertrophy of the heart muscle wall, and diastolic dysfunction. Manifestations that HCM encompass dyspnea, syncope, palpitations, and sudden death, and most typically presents in adolescence. Dilated cardiomyopathy (DCM) is identified by eccentric hypertrophy, dilation the the cardiac muscle wall, and also systolic dysfunction.<10> DCM is the final myocardial an answer to a selection of genetic and also environmental stress such as chronic alcohol abuse, famous or helminth infections, specific chemotherapies, and other etiologies v the most common being idiopathic DCM.<11> therapy for cardiomyopathies consists of pharmacotherapy to minimize heart muscle demand such together calcium-channel blockers or beta-blockers, implantable cardioverter-defibrillator (ICD) placements to protect against life-threatening arrhythmias or heart transplantation.

Myocarditis is the term because that inflammation that the love muscle and also has number of etiologies. The most common etiology in the Western people is viral infections (infectious myocarditis), but other causes include toxicity reactions or allergy to drugs, autoimmune diseases, or various other infections (bacterial, fungal, parasitic). Myocarditis involves damage and also loss of both cardiomyocytes and cardiac vascular endothelial cells, both of i m sorry are typical targets of infection from inflammatory processes and also white blood cabinet (often lymphocytic) infiltration right into the heart muscle wall.<12> Complications encompass interstitial cardiac fibrosis, wall motion abnormalities, arrhythmias, heart failure, myocardial infarctions, reduced ejection fraction, and also sudden cardiac death.<12> Myocarditis has a varied presentation through non-specific symptom 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 the cardiac pump dysfunction from a selection of etiologies, including plenty of diseases affecting the heart 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 distinct clinical characteristics. HF is essentially characterized as the i can not qualify of the love to pump adequately causing congestion, reduced organ perfusion, and also functional impairment.<1> Etiologies and also risk components for HF space diverse and also encompass processes such as myocardial injury or infarction, CAD, chronic hypertension, valvular dysfunction, arrhythmias, cardiomyopathies, and numerous various other pathways. The pathophysiology resulting in HF involves complex interactions of solution such as neurohormonal activation, peripheral vascular effects, and physiological processes within the love muscle itself.

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<1> treatment for HF entails pharmacotherapy such as ACE inhibitors and also beta-blockers to decrease heart stress and workload or inotropes to rise contractility. Therapies for progressed heart failure encompass mechanical circulatory support such as ventricular assist devices (VADs) or heart transplantation.