presentation on cardiac muscle

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StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-.

StatPearls [Internet].

Physiology, cardiac muscle.

Rashelle Ripa ; Tom George ; Karlie R. Shumway ; Yasar Sattar .

Affiliations

Last Update: July 30, 2023 .

Introduction

Cardiac muscle also called the myocardium, is one of three major categories of muscles found within the human body, along with smooth muscle and skeletal muscle. Cardiac muscle, like skeletal muscle, is made up of sarcomeres that allow for contractility. However, unlike skeletal muscle, cardiac muscle is under involuntary control.

The heart is made up of three layers—pericardium, myocardium, and endocardium. The endocardium is not cardiac muscle and is comprised of simple squamous epithelial cells and forms the inner lining of the heart chambers and valves. The pericardium is a fibrous sac surrounding the heart, consisting of the epicardium, pericardial space, parietal pericardium, and fibrous pericardium. [1]

The cardiac muscle is responsible for the contractility of the heart and, therefore, the pumping action. The cardiac muscle must contract with enough force and enough blood to supply the metabolic demands of the entire body. This concept is termed cardiac output and is defined as heart rate x stroke volume, which is determined by the contractile forces of the cardiac muscle and the frequency at which they are activated. With a change in metabolic demand comes a change in the contractility of the heart.

Cellular Level

Cardiac muscle cells (cardiomyocytes) are striated, branched, contain many mitochondria, and are under involuntary control. Each myocyte contains a single, centrally located nucleus surrounded by a cell membrane known as the sarcolemma. The sarcolemma of cardiac muscle cells contains voltage-gated calcium channels, specialized ion channels that skeletal muscle does not possess.

Cardiac muscle cells contain branched fibers connected via intercalated discs that contain gap junctions and desmosomes. These interconnections allow the cardiomyocytes to contract together synchronously to enable the heart to work as a pump. [2]

Gap junctions between adjacent cardiomyocytes allow for the propagation of coordinated action potentials from one cell to the next in a phenomenon known as electrical coupling. [3] Cardiac desmosomes are intercellular structures that anchor cardiac muscle fibers together and are vital in maintaining the structural integrity of the heart. [4]

The functional unit of cardiomyocyte contraction is the sarcomere, which consists of thick (myosin) and thin (actin) filaments, the interactions between which form the basis of the sliding filament theory. [5]

The sarcolemma is the cardiomyocyte plasma membrane containing transverse tubules (t-tubules). These t-tubules are highly branched invaginations of the cardiomyocyte sarcolemma that function in excitation-contraction coupling (ECC), action potential initiation and regulation, maintaining the resting membrane potential, and signal transduction. T-tubules regulate the cardiac ECC by concentrating voltage-gated L-type calcium channels and positioning them in close proximity to calcium sense and release channels, ryanodine receptors (RyRs), at the junctional membrane of the sarcoplasmic reticulum. [6]

Development

The development of the heart occurs in various stages. During embryogenesis, the formation of the primitive streak follows the invagination of epiblast cells, indicating the start of gastrulation. Gastrulation divides the embryonic plate, which originally contained two layers between the yolk sac and amniotic cavity, into three germ layers; ecto-, meso-, and endoderm. The mesoderm is situated between the ectoderm and endoderm layers and, during development, spreads laterally and cranially, forming different structures, particularly the heart. [7]

The myocardium begins developing during the second week of gestation in the dorsal mesocardium. After three weeks post-fertilization, the primitive heart begins to develop as a straight tube changing its configuration as time proceeds. This entails folding of the tube, giving rise to bulges that become analogous to the adult heart; truncus arteriosus develops into the aorta and pulmonary artery, bulbus cordis develops into smooth left and right ventricles, primitive ventricle into trabeculated LV/RV, primitive atrium into trabeculated atria and the sinus venosus which develops into the right atrium (sinus venarum) and coronary sinus. [8]

Around the fourth week of development, the heart undergoes a cardiac looping process that establishes the heart's left-sided orientation. This is performed with the help of cilia, a motile structure, and dynein, a protein. [9] If these factors fail to function correctly, dextrocardia will occur, which places the heart on the right side of the chest. This cardiac anomaly is typically seen in Kartagener Syndrome and primary ciliary dyskinesia (PCD). [10]

Further developmental changes occur as the heart is shaped into its proper configuration. The heart begins as a single chamber, but four separate chambers are created through the growth of various septa. The muscular ventricular septum originates from the bottom of the ventricle, with a membranous septum forming shortly after, joining with the aortic-pulmonary septum as its twists down and fuses. The endocardial cushions also appear at this time and separate the left and right atria and ventricles. Any structural changes or defects in these processes can lead to congenital heart disorders.

The primary function of cardiac muscle is to pump blood into circulation by generating sufficient force. The mechanism behind each coordinated contraction involves the cardiac muscle and electrical impulses. These contractile functions of the heart require ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones. Aerobic production is the core utilization process; however, the heart may use anaerobic processes in a limited capacity. [11]

The cardiac action potential lasts approximately 200 ms and is divided into 5 phases: (4) resting, (0) upstroke, (1) early repolarization, (2) plateau, and (3) final repolarization.

Approximate resting membrane potential (RMP): -90 mV

Phase 4 - RMP due to activity of the Na/K ATPase pump. The exchange of three sodium ions out for two potassium ions in maintains the negative intracellular potential.
Phase 0 - depolarization to approximately +52 mv due to sodium influx via fast sodium channels
Phase 1 - partial repolarization due to the closure of fast sodium channels and efflux of potassium and chloride
Phase 2 - plateau phase maintained by the influx of calcium. Potassium efflux also occurs.
Phase 3 - repolarization back to RMP due to potassium efflux and closure of sodium and calcium channels

The generation of a cardiac action potential is involuntary and proceeds via a process known as excitation-contraction coupling (ECC). Action potentials travel along the sarcolemma and into the t-tubules to depolarize the membrane. Voltage-sensitive dihydropyridine (DHP) receptors on t-tubules allow calcium influx into the cell via L-type (long-lasting) calcium channels during the plateau phase (phase 2) of the action potential. This increased intracellular calcium concentration triggers the sarcoplasmic reticulum to release more calcium through the ryanodine receptor, known as calcium-induced calcium-released. [12]

The released calcium attaches to troponin C, causing tropomyosin to detach from the myosin-binding sites on actin. Actin and myosin then form a cross-bridge, and contraction occurs. Cross bridges last as long as calcium is attached to troponin. [13]

Lusitropy is the term used to define the relaxation of the myocardium following ECC. Lusitropy is mediated by the SERCA (sarco-endoplasmic reticulum calcium-ATPase) pump, which sequesters calcium into the sarcoplasmic reticulum, allowing calcium to be removed from troponin-C and returning the myocardium to its relaxed state. [14]

Unlike the cardiac muscle cells, the pacemaker cells' action potential is divided into 3 phases instead of 5, as phases 1 and 2 are absent. Pacemaker cells are comprised of sinoatrial (SA) and atrioventricular (AV) nodes, which are known to fire spontaneously, sending electrical activity throughout the heart, and do not require stimulation to initiate their action.

This autorhythmicity transpires because of funny current channels, which allow sodium ions to leak continuously into the cell (Phase 4), slowly raising the membrane potential until a certain threshold is reached, causing depolarization of the cell. This subsequently opens calcium channels causing calcium ions to enter the cell, further raising the membrane potential (Phase 0). After a positive membrane potential is sensed, potassium channels open, causing an outward flow of ions, returning the membrane potential to its resting potential (Phase 3). [15]

Related Testing

Many clinical tests are utilized to evaluate the function of cardiac muscle. This list discusses high-yield clinical testing and is not meant to be exhaustive.

Echocardiogram: an ultrasound of the heart routinely used to identify cardiac abnormalities. An echocardiogram can assess valvular abnormalities, masses, pericardial disease, congenital abnormalities, and pulmonary hypertension. An echo is also routinely utilized to assess cardiac muscle function and is useful in diagnosing congestive heart failure and cardiomyopathies. An echocardiogram can be performed as a transthoracic echocardiogram (TTE) or a transesophageal echocardiogram (TEE).

TTE: a non-invasive test that utilizes an echocardiography probe placed on the chest wall.
TEE: a specialized probe with an ultrasound transducer at the tip passed into the patient's esophagus, allowing for a posterior view of the heart.

Echocardiogram reports are detailed and offer essential information regarding the heart's function. Echo reports typically include:

Rate and rhythm
Chamber size
Indications of hypertrophy
Right ventricular function
Left ventricular systolic function and ejection fraction
Left ventricular diastolic function
Valvular pathology (if any)
Evidence of mass or thrombus
Congenital abnormalities
Pericardial anomalies
Incidental findings [16]

Electrocardiogram (ECG or EKG): an EKG is a non-invasive test that utilizes electrodes placed on the body's surface to record the heart's electrical rhythms. These electrical rhythms cause depolarization of the heart, which, in turn, leads to the contraction of the myocardium. With this knowledge, one can understand that the EKG indirectly indicates the heart's contraction. [17]

Cardiac biomarkers : blood tests may be performed to identify enzymes and proteins that can indicate heart disease or cardiac damage.

Troponin: measures the levels of cardiac proteins troponin T and troponin I. These proteins are found in cardiac muscle and are released upon damage to cardiac muscle, with troponin I being the more sensitive and specific marker of cardiac injury. [18]
Creatine kinase (CK): This enzyme is released from cardiac and skeletal muscle following damage. The isozyme CK-MB is more sensitive in diagnosing heart damage after a heart attack, with CK-MB levels rising 4 to 6 hours after a heart attack, peaking at 24 hours, and returning to baseline in 48 to 72 hours. [19]
Brain natriuretic peptide (BNP): a hormone secreted by the ventricular myocardium in response to ventricular wall stress (ie, pressure overload or volume expansion). The gold standard in diagnosing heart failure is the measurement of BNP levels, with an elevated level indicating heart failure. [20]
Pathophysiology

The pathophysiology of cardiac muscle is based on damage to cardiac muscle cells, leading to inappropriate contractility.

Cardiomyopathy: A cardiomyopathy is a genetic or acquired disorder of the myocardium associated with cardiac dysfunction. According to the World Health Organization, there exist five main categories of non-ischemic cardiomyopathy: dilated cardiomyopathy (DCM), hypertrophic cardiomyopathy (HCM), restrictive cardiomyopathy (RCM), arrhythmogenic right ventricular cardiomyopathy (ARVC), and unclassified cardiomyopathies.

Dilated cardiomyopathy (DCM): This condition is dilation and impaired contraction of one or both ventricles that cannot be explained by coronary artery disease, valvular disease, or hypertension. Patients with DCM present with decreased systolic contraction and symptoms of heart failure. Inherited forms of DCM have shown mutations in at least 40 individual genes, many of which encode structural components of the sarcomere and desmosome. [21] Nongenetic causes of DCM occur, most often from viral infections leading to inflammation of the myocardium. In addition, certain medications, toxins or allergens, or systemic autoimmune diseases may lead to DCM. [22]
Hypertrophic cardiomyopathy (HCM): This condition is an autosomal dominant disease leading to a thickened (hypertrophied) left ventricle and abnormal heart contraction caused by a mutation in the sarcomere protein genes. This hypertrophied left ventricle leads to outflow obstruction, diastolic dysfunction, mitral regurgitation, and myocardial ischemia. In severe cases, sudden cardiac death may occur. [23]
Restrictive cardiomyopathy (RCM): This condition results in impaired ventricular filling in the setting of nondilated ventricles. RCM is typically characterized by nondilated nonhypertrophied ventricles, with biatrial enlargement secondary to increased atrial pressures. Diseases in which RCM may be seen include sarcoidosis, amyloidosis, and hemochromatosis. [24]
Arrhythmogenic right ventricular cardiomyopathy (ARVC): This is the replacement of the myocardium with fibrofatty tissue leading to an increased predisposition to ventricular tachycardia and sudden cardiac death, especially in young adults and athletes. The release of exercise-induced catecholamines provokes ventricular arrhythmias in predisposed individuals. ARVM typically affects the right ventricle. [25]
Unclassified cardiomyopathies:
Stress-induced cardiomyopathy (Takotsubo cardiomyopathy): A condition known as "broken heart syndrome" presents as a reversible transient ballooning of the apex of the left ventricle leading to wall motion abnormalities brought on by severe emotional or physical stress. [26]
Cirrhotic cardiomyopathy: This is a condition of diastolic and systolic dysfunction, impaired cardiac response to stress, and ECG abnormalities (QT prolongation) in patients with cirrhosis. Cardiac function is typically altered only under stressful conditions. [27]
Ischemic cardiomyopathy (ICM) is a form of dilated cardiomyopathy related to coronary artery disease (CAD). ICM is the decreased ability of the heart to properly pump blood throughout the body due to myocardial damage from cardiac ischemia. The lack of blood supply to the cardiomyocytes leads to cell death, cardiac fibrosis, and left ventricular enlargement and dilation. [28] Globally, ischemic heart disease (IHD) is the leading cause of death, with approximately 7.2 million deaths yearly. [29]

Heart Failure: impairment of ventricular filling and systolic ejection of blood due to structural and functional defects of the myocardium. The left ventricular ejection fraction (LVEF) is crucial in the specific assessment of heart failure. A heart failure patient with an ejection fraction (EF) greater than or equal to 50% is diagnosed as having heart failure with preserved ejection fraction (HFpEF).

An ejection fraction less than or equal to 40% is termed heart failure with reduced ejection fraction (HFrEF), and an ejection fraction of 41 to 49% is diagnosed as heart failure with mid-range ejection fraction (HFmrEF). Distinguishing between these three types of heart failure is of clinical importance, as each has specific treatments and medications. [30]

Myocarditis: an inflammatory disease of the myocardium most commonly caused by acute rheumatic fever or viral infections (Coxsackie virus, parvovirus B19). The long-term outcome of untreated myocarditis is dilated cardiomyopathy with heart failure. [31]

Clinical Significance

Heart disease is the leading cause of death in both men and women in the United States and worldwide. [29] The clinical care of patients with heart disease is based on assessing myocardial function and using interventions to improve cardiac muscle performance and prevent myocardial maladaptations. Based on the pathophysiology previously mentioned, treatment modalities will vary from patient to patient.

As heart failure is seen worldwide, clinicians must understand the physiology and management of this disease. Symptomatic heart failure is typically managed with vasodilators, diuretics, positive inotropes, or digoxin. The ideal therapy would include a diuretic and a vasodilator, such as an angiotensin-converting enzyme (ACE) inhibitor or hydralazine plus isosorbide dinitrate. Depending on the patient's presentation, this therapy would occur with or without digoxin. [32]

Clinicians must talk with each patient about the importance of cardiovascular health. Simply put, the stronger the heart muscles are, the more efficient they become. Regular aerobic exercise is of great importance in the overall health of cardiac muscles and helps strengthen muscle tissue, lowering the risk of stroke and heart attack. Aerobic exercise includes running or walking, swimming, cycling, dancing, and climbing stairs.

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Cardiac Myocyte Action Potential Contributed By Action_potential2.svg: *Action_potential.png: User:Quasarderivative work: Mnokel (talk)derivative work: Silvia3 (Action_potential2.svg) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or GFDL (more...)

Disclosure: Rashelle Ripa declares no relevant financial relationships with ineligible companies.

Disclosure: Tom George declares no relevant financial relationships with ineligible companies.

Disclosure: Karlie Shumway declares no relevant financial relationships with ineligible companies.

Disclosure: Yasar Sattar declares no relevant financial relationships with ineligible companies.

This book is distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0) ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ), which permits others to distribute the work, provided that the article is not altered or used commercially. You are not required to obtain permission to distribute this article, provided that you credit the author and journal.

Cite this Page Ripa R, George T, Shumway KR, et al. Physiology, Cardiac Muscle. [Updated 2023 Jul 30]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-.

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Muscle Tissue

Cardiac Muscle Tissue

OpenStaxCollege

Learning Objectives

By the end of this section, you will be able to:

Describe intercalated discs and gap junctions
Describe a desmosome

Cardiac muscle tissue is only found in the heart. Highly coordinated contractions of cardiac muscle pump blood into the vessels of the circulatory system. Similar to skeletal muscle, cardiac muscle is striated and organized into sarcomeres, possessing the same banding organization as skeletal muscle ( [link] ). However, cardiac muscle fibers are shorter than skeletal muscle fibers and usually contain only one nucleus, which is located in the central region of the cell. Cardiac muscle fibers also possess many mitochondria and myoglobin, as ATP is produced primarily through aerobic metabolism. Cardiac muscle fibers cells also are extensively branched and are connected to one another at their ends by intercalated discs. An intercalated disc allows the cardiac muscle cells to contract in a wave-like pattern so that the heart can work as a pump.

This image is a micrograph of cardiac muscle cells.

View the University of Michigan WebScope to explore the tissue sample in greater detail.

Intercalated discs are part of the sarcolemma and contain two structures important in cardiac muscle contraction: gap junctions and desmosomes. A gap junction forms channels between adjacent cardiac muscle fibers that allow the depolarizing current produced by cations to flow from one cardiac muscle cell to the next. This joining is called electric coupling, and in cardiac muscle it allows the quick transmission of action potentials and the coordinated contraction of the entire heart. This network of electrically connected cardiac muscle cells creates a functional unit of contraction called a syncytium. The remainder of the intercalated disc is composed of desmosomes. A desmosome is a cell structure that anchors the ends of cardiac muscle fibers together so the cells do not pull apart during the stress of individual fibers contracting ( [link] ).

This image shows the structure of the cardiac muscle. A small image of the heart is shown on the top left of the figure and then a magnified view of the cardiac muscle is shown, with the nucleus and the cardiac muscle fiber labeled. A further magnification shows the structure of the intercalated discs with the desmosome and gap junction.

Contractions of the heart (heartbeats) are controlled by specialized cardiac muscle cells called pacemaker cells that directly control heart rate. Although cardiac muscle cannot be consciously controlled, the pacemaker cells respond to signals from the autonomic nervous system (ANS) to speed up or slow down the heart rate. The pacemaker cells can also respond to various hormones that modulate heart rate to control blood pressure.

The wave of contraction that allows the heart to work as a unit, called a functional syncytium, begins with the pacemaker cells. This group of cells is self-excitable and able to depolarize to threshold and fire action potentials on their own, a feature called autorhythmicity ; they do this at set intervals which determine heart rate. Because they are connected with gap junctions to surrounding muscle fibers and the specialized fibers of the heart’s conduction system, the pacemaker cells are able to transfer the depolarization to the other cardiac muscle fibers in a manner that allows the heart to contract in a coordinated manner.

Another feature of cardiac muscle is its relatively long action potentials in its fibers, having a sustained depolarization “plateau.” The plateau is produced by Ca ++ entry though voltage-gated calcium channels in the sarcolemma of cardiac muscle fibers. This sustained depolarization (and Ca ++ entry) provides for a longer contraction than is produced by an action potential in skeletal muscle. Unlike skeletal muscle, a large percentage of the Ca ++ that initiates contraction in cardiac muscles comes from outside the cell rather than from the SR.

Chapter Review

Cardiac muscle is striated muscle that is present only in the heart. Cardiac muscle fibers have a single nucleus, are branched, and joined to one another by intercalated discs that contain gap junctions for depolarization between cells and desmosomes to hold the fibers together when the heart contracts. Contraction in each cardiac muscle fiber is triggered by Ca ++ ions in a similar manner as skeletal muscle, but here the Ca ++ ions come from SR and through voltage-gated calcium channels in the sarcolemma. Pacemaker cells stimulate the spontaneous contraction of cardiac muscle as a functional unit, called a syncytium.

Review Questions

Cardiac muscles differ from skeletal muscles in that they ________.

are striated
utilize aerobic metabolism
contain myofibrils
contain intercalated discs

If cardiac muscle cells were prevented from undergoing aerobic metabolism, they ultimately would ________.

undergo glycolysis
synthesize ATP
stop contracting
start contracting

Critical Thinking Questions

What would be the drawback of cardiac contractions being the same duration as skeletal muscle contractions?

An action potential could reach a cardiac muscle cell before it has entered the relaxation phase, resulting in the sustained contractions of tetanus. If this happened, the heart would not beat regularly.

How are cardiac muscle cells similar to and different from skeletal muscle cells?

Cardiac and skeletal muscle cells both contain ordered myofibrils and are striated. Cardiac muscle cells are branched and contain intercalated discs, which skeletal muscles do not have.

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Healthline - How is Cardiac Muscle Tissue Different from Other Muscle Tissues?
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cardiac muscle , in vertebrates , one of three major muscle types, found only in the heart . Cardiac muscle is similar to skeletal muscle , another major muscle type, in that it possesses contractile units known as sarcomeres ; this feature, however, also distinguishes it from smooth muscle , the third muscle type. Cardiac muscle differs from skeletal muscle in that it exhibits rhythmic contractions and is not under voluntary control. The rhythmic contraction of cardiac muscle is regulated by the sinoatrial node of the heart, which serves as the heart’s pacemaker.

The heart consists mostly of cardiac muscle cells (or myocardium). The outstanding characteristics of the action of the heart are its contractility, which is the basis for its pumping action, and the rhythmicity of the contraction. The amount of blood pumped by the heart per minute (the cardiac output ) varies to meet the metabolic needs of peripheral tissues, particularly the skeletal muscles, kidneys , brain , skin , liver , heart, and gastrointestinal tract . The cardiac output is determined by the contractile force developed by the cardiac muscle cells, as well as by the frequency at which they are activated (rhythmicity). The factors affecting the frequency and force of heart muscle contraction are critical in determining the normal pumping performance of the heart and its response to changes in demand.

Male muscle, man flexing arm, bicep curl.

Cardiac muscle cells form a highly branched cellular network in the heart. They are connected end to end by intercalated disks and are organized into layers of myocardial tissue that are wrapped around the chambers of the heart. The contraction of individual cardiac muscle cells produces force and shortening in these bands of muscle, with a resultant decrease in the heart chamber size and the consequent ejection of the blood into the pulmonary and systemic vessels. Important components of each cardiac muscle cell involved in excitation and metabolic recovery processes are the plasma membrane and transverse tubules in registration with the Z lines, the longitudinal sarcoplasmic reticulum and terminal cisternae, and the mitochondria . The thick (myosin) and thin ( actin , troponin, and tropomyosin) protein filaments are arranged into contractile units, with the sarcomere extending from Z line to Z line, that have a characteristic cross-striated pattern similar to that seen in skeletal muscle.

The rate at which the heart contracts and the synchronization of atrial and ventricular contraction required for the efficient pumping of blood depend on the electrical properties of the cardiac muscle cells and on the conduction of electrical information from one region of the heart to another. The action potential (activation of the muscle) is divided into five phases. Each of the phases of the action potential is caused by time-dependent changes in the permeability of the plasma membrane to potassium ions (K + ), sodium ions (Na + ), and calcium ions (Ca 2+ ).

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Everything to Know About Cardiac Muscle Tissue

Maintenance

Frequently Asked Questions

Cardiac muscle is found in the walls of the heart . It helps the heart perform its function of pumping blood throughout the body. Cardiac muscle tissue is located in the middle of three layers of the heart, called the myocardium . Problems in the myocardium can cause heart failure and arrhythmias or contribute to sudden cardiac death.

This article discusses the role of muscle tissue in the heart and ways to keep your heart's muscle tissue healthy.

manusapon kasosod / Getty Images

Heart Tissue Layers

The heart has three layers of tissue:

Epicardium : The outermost layer of tissue
Myocardium : The middle layer of tissue, made of muscle
Endocardium : The tissue lining the inside of the heart and valves

The pericardium is the sac in which the heart sits.

Cardiac Muscle Tissue Function

The heart can be thought of as a pump. It is responsible for pumping blood throughout the body to provide oxygen and nutrients.

The heart's muscle is stimulated by the electrical system of the heart. Specialized pacemaker cells create an electrical signal that causes contraction, or shortening, of the muscle fibers. This muscle contraction is what causes the heart to squeeze and pump out blood.

At a cellular level, heart muscle tissue is made up of bundles or fibers of interconnected muscle cells, called cardiomyocytes . These cells are packed with units called sarcomeres that are made of proteins called actin and myosin . When stimulated, these two proteins slide against each other to result in contraction of the heart.

Types of Muscle Tissue

The body has three types of muscle tissue. All of them share the ability to contract and have important functions. The tissue types are:

Skeletal muscle tissue provides the function of body movement. It is under voluntary control.
Smooth muscle is found in the digestive tract and in the arteries. It is not under voluntary control.
Cardiac muscle is only found in the heart. It is responsible for pumping blood out of the heart.

Conditions That Affect Cardiac Muscle Tissue

Heart muscle problems have many causes.

Cardiomyopathy , or heart muscle weakness, is a general term for problems with the heart muscle. It can be caused by:

Genetic mutations
Lack of blood flow
Autoimmune or inflammatory conditions
Vitamin deficiency
Damage from toxins

Sometimes the cause is not determined, which is known as idiopathic cardiomyopathy.

Other conditions can affect cardiac muscle tissue. These can cause varying problems, from thickening of the heart muscle to heart failure, arrhythmias, and sudden cardiac death.

Most common causes:

Ischemic heart disease from blocked coronary arteries
Heart attack
High blood pressure
Valvular heart disease, such as aortic stenosis

High Blood Pressure and the Heart

Blood pressure is the force that the heart must pump against to eject blood. When blood pressure is high, the heart must work harder. Just like any other muscle, the heart muscle thickens in response to this increased work. This thickened (hypertrophied) heart muscle can lead to problems with heart filling and heart failure. High blood pressure is one of the more common causes of heart failure.

Other possible issues that could affect heart muscle include:

Myocarditis (inflammation of the heart muscle)
Toxins like alcohol, cocaine, amphetamines
Medications, including certain cancer therapies
Infiltrative disorders (accumulation of abnormal proteins or particles in the heart muscle), including cardiac amyloidosis , cardiac sarcoidosis , or hemochromatosis (iron overload)
Genetic conditions, including left ventricular noncompaction , hypertrophic cardiomyopathy , arrhythmogenic right ventricular cardiomyopathy , glycogen storage disease, or muscular dystrophy
Heart rhythm problems
Congenital heart disease (heart defects present from birth)
Endocrine disorders such as thyroid problems
Extreme stress
Vitamin B1 ( thiamine ) deficiency

When to See a Healthcare Provider

If you are concerned about cardiomyopathy, you should see a healthcare provider for evaluation. Seek medical attention for symptoms like shortness of breath, exercise intolerance, leg swelling, and fatigue, which are signs of heart failure. Even if you don't have any symptoms, if heart failure runs in your family, you should discuss this with your healthcare provider to determine whether screening or genetic testing is needed.

How to Keep Cardiac Muscle Tissue Healthy

While not all types of cardiomyopathy can be prevented, there are things you can do to help keep your heart muscle as healthy as possible.

Living a healthy lifestyle can help keep the heart's muscle tissue healthy by preventing coronary artery disease, high blood pressure, and diabetes. This includes eating a healthy diet , getting regular physical exercise, maintaining a healthy weight, and avoiding tobacco use.

In addition to living a healthy lifestyle, the following can be done to prevent cardiomyopathy:

Controlling blood pressure
Controlling cholesterol levels
Treating coronary artery disease
Avoiding toxins such as drugs and excess alcohol
Controlling blood sugar (recent guidelines recommend sodium-glucose cotransporter-2 (SGLT2) inhibitors for those with diabetes and elevated risk of heart disease)

For those diagnosed with cardiomyopathy, several medications have been proven to prevent or reverse the abnormal remodeling that occurs due to heart disease. These include:

Certain beta-blockers
ACE ( angiotensin-converting enzyme ) inhibitors
Angiotensin receptor blocker/ neprilysin inhibitor
Aldosterone antagonists
SGLT2 inhibitors

Cardiologists (doctors who specialize in heart disease) can prescribe and adjust these medications and provide an individualized treatment plan.

Cardiac muscle tissue is found in the middle of three layers of heart tissue. It enables the heart to pump blood and provide nutrients and oxygen throughout the body. Several things can cause problems with the heart muscle, including ischemic heart disease, heart attack, high blood pressure, and valvular heart disease.

The best ways to prevent cardiomyopathy are to live a healthy lifestyle, control blood pressure, cholesterol, and diabetes, and avoid substances that are known to be toxic to the heart. Those with cardiomyopathy can benefit from effective medications.

A Word From Verywell

The heart is arguably the most important muscle in the body. Keeping cardiac tissue healthy helps the heart function properly and decreases the risk of complications. Knowing your risk and controlling modifiable risk factors such as blood pressure, cholesterol, blood sugar, and smoking are important ways to lower your risk and protect your heart muscle.

Cardiac muscle tissue is a type of muscle tissue found only in the heart. It appears striated (striped) under a microscope due to the presence of sarcomere units that are responsible for its ability to contract. Heart muscle contracts in response to signals from specialized pacemaker cells located in the heart.

Cardiac muscle tissue is located in the middle of three layers of the heart, called the myocardium. It is the thickest of the three layers. On its outer surface, the myocardium is surrounded by a thin, protective layer called the pericardium. On its inner surface, it is lined by the endocardium.

The heart is made up of three layers of tissue. The epicardium is the outer, fibrous layer that lines and protects the heart. The myocardium is the thick muscular layer of tissue. The endocardium lines the inner surface of the heart. The heart also has four valves (aortic, mitral, tricuspid, and pulmonic).

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MedlinePlus. Types of muscle tissue .

Brieler J, Breeden MA, Tucker J. Cardiomyopathy: an overview . AFP . 2017;96(10):640-646.

Heidenreich PA, Bozkurt B, Aguilar D, et al. 2022 AHA/ACC/HFSA guideline for the management of heart failure: a report of the American College of Cardiology/American Heart Association Joint Committee on Clinical Practice Guidelines . Circulation . 2022;145:e895–e1032. doi:10.1161/CIR.0000000000001063

American Heart Association. How to help prevent heart disease at any age .

By Angela Ryan Lee, MD Dr. Lee is an Ohio-based board-certified physician specializing in cardiovascular diseases and internal medicine.

Want to create or adapt books like this? Learn more about how Pressbooks supports open publishing practices.

69 10.7 Cardiac Muscle Tissue

Learning objectives.

By the end of this section, you will be able to:

Describe intercalated discs and gap junctions
Describe a desmosome

Cardiac muscle tissue is only found in the heart. Highly coordinated contractions of cardiac muscle pump blood into the vessels of the circulatory system. Similar to skeletal muscle, cardiac muscle is striated and organized into sarcomeres, possessing the same banding organization as skeletal muscle ( Figure 1 ). However, cardiac muscle fibers are shorter than skeletal muscle fibers and usually contain only one nucleus, which is located in the central region of the cell. Cardiac muscle fibers also possess many mitochondria and myoglobin, as ATP is produced primarily through aerobic metabolism. Cardiac muscle fibers cells also are extensively branched and are connected to one another at their ends by intercalated discs. An intercalated disc allows the cardiac muscle cells to contract in a wave-like pattern so that the heart can work as a pump.

View the University of Michigan WebScope at http://virtualslides.med.umich.edu/Histology/Cardiovascular%20System/305_HISTO_40X.svs/view.apml to explore the tissue sample in greater detail.

Chapter Review

Review Questions

1. Cardiac muscles differ from skeletal muscles in that they ________.

are striated
utilize aerobic metabolism
contain myofibrils
contain intercalated discs

2. If cardiac muscle cells were prevented from undergoing aerobic metabolism, they ultimately would ________.

undergo glycolysis
synthesize ATP
stop contracting
start contracting

Critical Thinking Questions

1. What would be the drawback of cardiac contractions being the same duration as skeletal muscle contractions?

2. How are cardiac muscle cells similar to and different from skeletal muscle cells?

Answers for Review Questions

Answers for Critical Thinking Questions

An action potential could reach a cardiac muscle cell before it has entered the relaxation phase, resulting in the sustained contractions of tetanus. If this happened, the heart would not beat regularly.
Cardiac and skeletal muscle cells both contain ordered myofibrils and are striated. Cardiac muscle cells are branched and contain intercalated discs, which skeletal muscles do not have.

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Cardiac Muscle

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Cardiac Muscle Definition

Cardiac muscle, also known as heart muscle, is the layer of muscle tissue which lies between the endocardium and epicardium . These inner and outer layers of the heart, respectively, surround the cardiac muscle tissue and separate it from the blood and other organs. Cardiac muscle is made from sheets of cardiac muscle cells. These cells, unlike skeletal muscle cells, are typically unicellular and connect to one another through special intercalated discs . These specialized cell junction and the arrangement of muscle cells enables cardiac muscle to contract quickly and repeatedly, forcing blood throughout the body.

Cardiac Muscle Structure

Cardiac muscle exists only within the heart of animals. It is a specialized form of muscle evolved to continuously and repeatedly contract, providing circulation of blood throughout the body. The heart is a relatively simple organ. Through all the twists and turns and various chambers, there are only three layers. The outer layer, known as the epicardium or visceral pericardium , surround the cardiac muscle on the exterior. This helps protect it from contact with other organs. The parietal pericardium attaches to this outer layer creates a fluid-filled layer which helps lubricate the heart. The inner layer, or endocardium , separates the muscle from the blood it is pumping within the chambers of the heart. In between these two sheets lies the cardiac muscle. Cardiac muscle is sometimes referred to as myocardium . This can be seen in the image below.

When we look a bit closer at cardiac muscle, we can see that it is arranged in sheets of cells, which are connected to each other in a lattice-work fashion. Where two cells meet a specialized junction called an intercalated disc locks the two cells into place. While this region looks like a dark disc under the microscope, it is actually the interlocking of hundreds of finger-like projections from each cell. These projections have small holes in them, gap junctions , which can pass the impulse to contract to connected cells. Interlaced between and around these cells are nerves and blood vessels, which carry signals and oxygen to the cardiac muscle.

At the microscopic level, cardiac muscle is organized much like skeletal muscle. Both muscle tissues are striated , meaning they show dark and light bands when viewed under a microscope. These band are created by the highly organized sarcomeres . A sarcomere is a bundle of protein fibers which respond to a signal and contract. In both skeletal and cardiac muscle, these sarcomeres are made of actin and myosin and are supported by the same proteins. Tropomyosin is a protein which wraps actin and stops myosin from binding to it. Troponin is a protein which holds tropomyosin in place until a signal to contract has been received. These proteins are the same in both skeletal and cardiac muscle.

Function of Cardiac Muscle

As in skeletal muscle, the signal to contract is an action potential. However with skeletal muscle this signal usually comes from the somatic , or voluntary, nervous system. Cardiac muscle is controlled by the autonomous nervous system . Cells in your brain and cells embedded throughout your heart act to release well-timed nervous impulses which signal your heart cells to contract in the correct pattern. While the source of the signals is different, the reception of the signal and the rest of contraction are very similar.

The action potential , or nerve impulse, on the surface of the cell stimulates a specialized organelle to release calcium ions (Ca 2+ ). This organelle is called the sarcoplasmic reticulum , and is derived from the endoplasmic reticulum found in a general cell. The Ca 2+ ions released into the cytoplasm affect the protein troponin, causing it to release tropomyosin. Tropomyosin shifts position and myosin is allowed to attach to actin. Myosin then used the energy stored in ATP molecules to walk along the actin filaments and shorten the length of each sarcomere. When the impulse is gone, the Ca 2+ is reabsorbed quickly into the sarcoplasmic reticulum. Troponin reattaches to tropomyosin, and the cardiac muscle cells release. This general process happens every time your heart beats.

As all the muscle cells work in unison, a force can be exerted in the chambers of the heart. The sheets of cardiac muscle are laid so they run perpendicularly to one another. This creates the effect that when the heart contracts, it does so in multiple directions. The ventricles and atria of the heart shrink from top to bottom and from side to side as these multiple layers muscle fibers contract. This produces a strong pumping and twisting force in the ventricles, forcing blood throughout the body.

Lodish, H., Berk, A., Kaiser, C. A., Krieger, M., Scott, M. P., Bretscher, A., . . . Matsudaira, P. (2008). Molecular Cell Biology 6th. ed. New York: W.H. Freeman and Company. Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., & Jackson, R. B. (2014). Campbell Biology, Tenth Edition (Vol. 1). Boston: Pearson Learning Solutions. Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

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Properties of Cardiac Muscle

Published by Jeremy Watkins Modified over 6 years ago

Presentation on theme: "Properties of Cardiac Muscle"— Presentation transcript:

The Heart: Conduction System

Electrophysiology (Conduction System of Heart)

Conductive System Of Heart

CONDUCTIVE TISSUE OF HEART

Heart –Electrical Properties

Aims Introduction to the heart.

Microscopic Anatomy of Heart Muscle

CARDIAC CYCLE Renee Anderson.

Cardiovascular System Block Cardiac electrical activity (Physiology)

Electrical Properties

The Electrical System of the Heart. Cardiac Muscle Contraction Depolarization of the heart is rhythmic and spontaneous About 1% of cardiac cells have.

Heart tissue Endocardium (internal layer) Myocardium (middle – muscle)

Electrical Activity of the Heart

Pages  Cardiac muscle cells contract:  Spontaneously  Independently  Two systems regulate heart rhythm:  Intrinsic Conduction System  Uses.

Conduction System of the Heart & Electrocardiography

The Heart Control of the Heart Beat The Heart Beat The heart is made up of cardiac muscle. Cardiac muscle is myogenic, which means it naturally contracts.

Anatomy & Physiology/Cardiovascular System. About the size of a an adult fist Hollow and cone shaped Weighs less than a pound Sits atop the diaphragm.

Electrophysiology (Conduction System of Heart) Dr. Mohammed Sharique Ahmed Quadri Assistant Prof. physiology Al maarefa college 1.

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Cardiac Muscle and Heart Function

Apr 07, 2019

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Cardiac Muscle and Heart Function. Cardiac muscle fibers are striated – sarcomere is the functional unit Fibers are branched; connect to one another at intercalated discs . The discs contain several gap junctions Nuclei are centrally located Abundant mitochondria

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less abundant
membrane potential
chronotropic agents compounts
cardiac conduction
normal bradycardia tachycardia
nervous system

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Cardiac Muscle and Heart Function • Cardiac muscle fibers are striated – sarcomere is the functional unit • Fibers are branched; connect to one another at intercalated discs. The discs contain several gap junctions • Nuclei are centrally located • Abundant mitochondria • SR is less abundant than in skeletal muscle, but greater in density than smooth muscle • Sarcolemma has specialized ion channels that skeletal muscle does not – voltage-gated Ca2+ channels • Fibers are not anchored at ends; allows for greater sarcomere shortening and lengthening

How are cardiac contractions started? Cardiac conduction system • Specialized muscle cells “pace” the rest of the heart; cells contain less actin and myosin, are thin and pale microscopically • Sinoatrial (SA) node; pace of about 65 bpm • Internodal pathways connect SA node to atrioventricular (AV) node • AV node could act as a secondary pacemaker; autorhythmic at about 55 bpm • Bundle of His • Left and right bundle branches • Purkinje fibers; also autorhythmic at about 45 bpm ALL CONDUCTION FIBERS CONNECTED TO MUSCLE FIBERS THROUGH GAP JUNCTIONS IN THE INTERCALATED DISCS

Why are fibers of the conducting system autorhythmic? If channels How does the depolarization in these cells affect cardiac muscle cells? Superimpose changes in the muscle cell’s membrane potential on this graph Membrane potential of SA nodal cells

Changes in ion concentrations in a cardiac muscle fiber following depolarization What causes the muscle resting membrane potential to change initially? What would be happening with a skeletal muscle at this point?

The refractory period is short in skeletal muscle, but very long in cardiac muscle. • This means that skeletal muscle can undergo summation and tetanus, via repeated stimulation • Cardiac muscle CAN NOT sum action potentials or contractions and can’t be tetanized

Autonomic nervous system modulates the frequency of depolarization of pacemaker • Sympathetic stimulation (neurotransmitter = ); binds to b1 receptors on the SA nodal membranes • Parasympathetic stimulation (neurotransmitter = ); binds to muscarinic receptors on nodal membranes; increases conductivity of K+ and decreases conductivity of Ca2+ How do these neurotransmitters get these results?

ECG examines how depolarization events occur in the heart • If a wavefront of depolarization travels towards the electrode attached to the + input terminal of the ECG amplifier and away from the electrode attached to the - terminal, a positive deflection will result. • If the waveform travels away from the + terminal lead towards the - terminal, a negative going deflection will be seen. • If the waveform is travelling in a direction perpendicular to the line joining the sites where the two leads are placed, no deflection or a biphasic deflection will be produced.

The electrical activity of the heart originates in the sino-atrial node. The impulse then rapidly spreads through the right atrium to the atrioventricular node. (It also spreads through the atrial muscle directly from the right atrium to the left atrium.) This generates the P-wave • The first area of the ventricular muscle to be activated is the interventricular septum, which activates from left to right. This generates the Q-wave • Next the bulk of the muscle of both ventricles gets activated, with the endocardial surface being activated before the epicardial surface. This generates the R-wave • A few small areas of the ventricles are activated at a rather late stage. This generates the S-wave • Finally, the ventricular muscle repolarizes. This generates the T-wave

Since the direction of atrial depolarization is almost exactly parallel to the axis of lead II (which is from RA to LL), a positive deflection (P wave) would result in that lead. • Since the ventricular muscle is much thicker in the left than in the right ventricle, the summated depolarization of the two ventricles is downwards and toward the left leg: this produces again a positive deflection (R-wave) in lead II, since the depolarization vector is in the same direction as the lead II axis. • Septal depolarization moves from left to right, the depolarization vector is directed towards the - electrode of lead II (RA), and therefore a negative deflection (Q-wave) is produced.

Arhythmias can be detected with an ECG draw the others Normal Bradycardia Tachycardia Atrial fibrillation Ventricular fibrillation Compounds that increase or decrease rate are called chronotropic agents Compounts that increase or decrease force of contraction are called inotropic agents

Cardiac Cycle http://www.physiology.wisc.edu/phys335/Cycle_11-14-99.swf

100 50 Atrial volume (ml)

Cardiac Output = Q = volume of blood ejected from the heart each minute What would I have to know to be able to determine what the cardiac output is at rest? What is the stroke volume? What determines the stroke volume? How can I alter cardiac output?

What causes this increase in stroke volume?

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Muscles and muscle tissue

Author: Declan Tempany, BSc (Hons) • Reviewer: Christina Loukopoulou, MSc Last reviewed: October 30, 2023 Reading time: 26 minutes

Muscle is defined as a tissue primarily composed of specialized cells /fibers which are capable of contracting in order to effect movement. This can relate to movement of the body or body parts with our external environment, however did you know that almost all movement of blood, food and other substances within the body is often the result of muscle contraction?

Depending on the type, the primary function of muscle is to move the bones of the skeleton. However, muscles also enable the heart to beat and can be found in the walls of hollow organs, such as the intestines, uterus and stomach. In this article, we will explore the many functions of muscle in the human body as well as its basic structure, types and classifications.

Key facts about muscle tissue
Types	Striated muscle (skeletal, visceral striated, cardiac) Non striated (smooth) muscle
Skeletal muscle	Striated muscle; formed of long, multinucleate, unbranched myocytes Attached at one or either ends to a bony attachment point
Cardiac muscle	Striated muscle; formed of short, uninucleate, branching myocytes which connected at intercalated discs Specialized muscle of the heart → myocardium
Smooth muscle	Non striated muscle; formed of short, uninucleate, spindle shaped myocytes Located in the walls of internal organs, blood vessels etc.
Connective tissues	loose connective tissue surrounding muscle cells/fibers Fibrous sheath which divides muscle tissue into fascicles Fibrous sheath which surrounds entire skeletal muscles

Structure of muscle tissue

Connective tissues of muscle, gross anatomy, visceral striated muscle, pacemaker cells and innervation, smooth-muscle-like cells, dermatomyositis and polymyositis, muscular dystrophy, muscle atrophy, sleep twitches, related articles.

Muscle tissue has four main properties:

Excitability: an ability to respond to stimuli
Contractibility: an ability to contract
Extensibility: an ability to be stretched without tearing
Elasticity: an ability to return to its normal shape

Through these properties, the muscular system as a whole performs several important functions. These include the production of force and movement, support of body stature and position, stability of joints , production of body heat (to maintain normal body temperature), as well as, provision of form to the body.

Regardless of its morphology or type, muscle tissue is composed of specialized cells known as muscle cells or myocytes (myo- [muscle, Greek = mys]), commonly referred to as muscle fibers (all of these terms are interchangeable); this is due to their extensive length and appearance. Myocytes are characterized by protein filaments known as actin and myosin that slide past one another, producing contractions that move body parts, including internal organs. Interestingly, these proteins are not exclusive to muscle cells; actin and myosin are commonly found as cytoskeletal elements in many cell types and are involved in cellular functions relating to the changing of cell shape (e.g. cell movement, phagocytosis etc.). Myocytes however, are characterized by a particular abundance of these proteins within their cytoplasm, so much so that they occupy most of their interior. Furthermore, in the case of myocytes, actin and myosin filaments are generally oriented along a single axis, thereby eliciting movement in a linear fashion. At its most basic level, muscle tissue is classified as either striated or non-striated/smooth based on the presence or absence of ‘striations’ (i.e. stripes/furrows) seen at a microscopic level; these are formed due to a particular arrangement of actin and myosin filaments within the myocyte (discussed below). Striated muscle can be further subdivided into three classifications based on its location and morphology:

skeletal muscle
visceral striated muscle
cardiac muscle

Skeletal muscle

Skeletal muscle is the most common type of muscle tissue found in the body and consists of highly elongated, multinucleate, non branching cells which are arranged in a parallel manner. Skeletal myocytes often measure several centimeters, or tens of centimeters in length, with the number of nuclei contained within being proportional to their length. They are biologically classified as ‘ syncytia ’; cells which are formed by fusion of several smaller, mononuclear cells. In the case of skeletal muscle, the cells which merge to form myocytes are known as myoblasts .

Skeletal muscle is often referred to as ‘voluntary’ muscle due to the fact that we think of its contraction being under conscious control; this is a misconception however, as skeletal muscle is involved in various movements which are under a subconscious level (e.g. breathing).

The cytoplasm of each skeletal muscle fibers/myocytes (referred to as sarcoplasm (sarco-, commonly used to denote muscle-related terms [flesh, Greek = sarx])) is largely occupied by subunits known as myofibrils which extend the length of the cell. Myofibrils are essentially chained structures composed of repeating units of contractile units known as sarcomeres . These are primarily composed of two protein myofilaments , thin actin filaments and thick myosin filaments and are responsible for muscular contraction. The arrangement of actin and myosin filaments within each sarcomere is very regular and contributes to the formation of distinct striations (i.e. stripes) of dark A bands and light I bands when viewed under light microscope.

Myosin is visible as the A band of the sarcomere. Actin filaments are anchored to a structure known as the Z disc (or Z line) located at either end of the sarcomere; they are present across the entire length of the I band (region of actin filaments only, no myosin) and a portion of the A band (region where actin and myosin filaments overlap). The A band is especially important when considering the dynamics of muscle contraction, as it is the location where filament movement occurs. Actin filaments do not extend completely into the A bands, leaving a central region, known as the H zone , which appears slightly lighter than the rest of the A band due to the fact that it does not contain both myofilaments. The center of the H zone has a vertical line, known as the M line , which connects myosin filaments to each other.

During contraction, sarcomere length shortens due to ‘walking’ of the myosin filaments along the actin towards each z disc, pulling them centrally; this action in turn reduces the size of the H zone. This mechanism for contraction is known as ‘ sliding filament theory ’.

Hypertrophy (growth) of adult skeletal muscle occurs due growth of existing muscle fibers:

growth in the girth of muscle fibers is thought to take place through ‘splitting’ of existing myofibrils as a result of stress placed on sarcomeres during physical activity, thereby increasing the mass of the muscle as whole.
growth in the length of muscle fibers occurs as a result of new sarcomeres being added to the end of existing myofibrils.

Key terms related to sarcomeres
Actin	Thin filaments of the sarcomere
Myosin	Thick filaments of the sarcomere
I band	Light band of the sarcomere; contains only actin (thin) filaments (I band= isotropic) (memory aid: is the l ght band)
Z disc	a cross-striation which bisects the I band, marking the beginning and end of one sarcomere; serves as an anchoring point actin filaments ( = wischenscheiben / ’in between’ discs)
A band	Dark band of the sarcomere; contains both actin (thin) and myosin (thick) filaments (A band = anisotropic band) (memory aid: is the d rk band)
H zone	Lighter zone of the A band which lacks actin filaments; contains only myosin ( zone = ellezone / ‘light zone)
M line	A fine line in the center of the H zone/A band which connects myosin filaments to one another ( line = ittelscheibe / ‘ iddle disc’)

Skeletal muscle fibers are bound together by loose areolar tissue, known as endomysium (endo- = within) which contains the usual complement of cells such as fibroblasts and macrophages, as well as small nerve and vascular branches. Bundles of striated muscle fibers which tend to work together to perform a specific function are enclosed by a thicker layer of connective tissue known as perimysium (peri- = around), to form muscle fascicles . These fascicles, in turn, are grouped together by a final outer covering of dense connective tissue, known as epimysium (epi- = outside), which forms the muscle [organ] as a whole. The endomysial, perimysial and epimysial coverings merge where muscles attach to adjacent structures forming tendons, fasciae or aponeuroses.

Skeletal muscle is found in many sizes and various shapes. The small muscles of the eye may contain only a few hundred myocytes/muscle fibers, while the vastus lateralis muscle of the thigh may contain hundreds of thousands of myocytes.

The shape or form of muscle is generally dependent on its fascicular architecture and fiber length which also helps to define the muscle’s function e.g. some muscles, such as the gluteal muscles , have numerous thick, short fascicles, while others such as the sartorius muscle have a lesser amount of long and relatively slender fascicles. These differences in muscle shape and fiber arrangement permit skeletal muscle to function effectively over a relatively wide range of tasks.

Fascicles or bundles (group of muscle fibres) of skeletal muscles can be arranged into four basic structural pattern, circular , parallel , convergent , and pennate . This difference in fascicular arrangement also accounts for the different shapes and functional capabilities of various skeletal muscles.

Circular muscles (also known as skeletal sphincter muscles) have a fascicular pattern where the fascicles are arranged in concentric rings. Muscles with this arrangement surround external body openings, which they close by contracting. Muscles with this fascicular arrangement may be termed as orbicular muscles (Latin: orbiculus = small disc), such as the orbicularis oculi (which covers the eye) and orbicularis oris (which surrounds the mouth).

A convergent (a.k.a. triangular) muscle has a broad origin with fascicles converging toward a single tendon of insertion. Such a muscle is triangular or fan shaped. One example is the pectoralis major muscle of the anterior thorax .

In a parallel arrangement, the length of the fascicles run to the long axis of the muscle. There are three types of parallel muscles:

quadrilateral muscles , which have a short, flat form e.g. thyrohyoid muscle
strap muscles , that have a narrow belt- or strap-like belly e.g. sartorius muscle
fusiform muscles , with a spindle-shaped and extended belly, e.g. biceps brachii muscle

In a pennate pattern, the fascicles are short and they attach obliquely to a central tendon that runs the length of the muscle. Pennate muscles are of three forms:

Unipennate , in which the fascicles insert into only one side of the tendon, as in the extensor digitorum longus muscle of the leg ;
Bipennate , in which the fascicles insert into the tendon from opposite sides. The tendon is central giving the muscle a resemblance of a feather. The rectus femoris of the thigh is bipennate;
Multipennate , which looks like many feathers side by side, with all their quills inserted into one large tendon. The deltoid muscle , which forms the roundness of the shoulder is multipennate.

Want to quickly master the names of all major muscles in the body? Build the foundations of your muscular system knowledge with our free muscles quiz guide .

Visceral striated muscle is structurally identical to skeletal muscle (i.e. looks the same in microscopic preparations). However, it is limited to a number of soft tissue structures, namely the tongue, pharynx and upper third of the esophagus.

Test your knowledge on the histology of the skeletal muscle with this quiz.

Cardiac muscle

Like skeletal muscle, cardiac muscle is striated and hence is composed of similar contractile proteins which are also structurally arranged into sarcomeres (discussed above). It is here however, where most similarities between these muscle tissue types ends. Cardiac muscle cells/fibers (cardiomyocytes) are much shorter and broader compared to those found in skeletal muscle and are branched at their ends. They are generally uninucleate (i.e. have one nucleus each), however sometimes may be binucleate. The nucleus is centrally located compared to those seen peripherally in skeletal myocytes, with the myofibrils passing on either side, leaving a clear zone of perinuclear sarcoplasm around the nucleus.

Striations in cardiac muscle are not as defined as those seen in skeletal muscle as they are slightly obscured by relatively large amounts of mitochondria and other organelles present in the cell (reflecting the higher metabolic demands of this tissue compared with skeletal muscle). Cardiomyocytes are connected at their ends by specialized junctional complexes known as intercalated discs; these serve to functionally couple all cardiomyocytes, thus allowing rapid propagation of signals for contraction across the heart muscle tissue. In light microscopy, they are identified as dark staining, irregularly arranged short lines which cross the cardiac muscle tissue, perpendicular to the fiber direction.

Cardiomyocytes are surrounded by fine, loose connective tissue , similar to endomysium seen in skeletal muscle albeit less organized. Condensations of perimysial-like dense connective tissue can be observed dividing groups of cardiac muscle cells/fibers into fascicles which, unlike that seen in skeletal muscle, whirl and spiral in a multidirectional manner (except in the case of the papillary muscles). As a result, cut sections of cardiac muscle tissue will usually present various orientations of muscle fibers adjacent to one another.

Cardiac pacemaker cells (a.k.a. ‘stimulating’ cardiomyocytes, cardiac conducting cells) are highly specialized/modified cardiomyocytes which are capable of generating and carrying contractile signals across the myocardium . These are arranged into nodes ( sinuatrial and atrioventricular ), atrioventricular ‘bundles’ (of His) and subendocardiac conducting networks (commonly referred to as Purkinje fibers ) to form the conducting system of the heart.

These networks are regulated by both parasympathetic and sympathetic divisions of the autonomic systems which send branches to the nodes mentioned above; sympathetic input increases heartbeat, while parasympathetic signals slow it down. Impulses carried to the heart by these autonomic nerve endings do not initiate contractile impulses but rather modify heart rate by their influence over cardiac pacemaker cells.

Learn more about the conducting system of the heart in this study unit.

Smooth muscle

Smooth muscle is most commonly found in the walls of tubular structures (e.g. vessels, gut, ducts, bronchi etc.) as well as hollow organs (e.g. urinary bladder , uterus ) and principally functions to modify the diameter/size of these structures in order to propel/expel the contents within (or alternatively to contain contents within an organ, in the case of sphincters). When compared to skeletal/cardiac muscle, smooth muscle is morphologically and functionally much more diverse and is subject to subconscious/involuntary control. Therefore, its arrangement varies from organ to organ; nevertheless there are several common attributes which will be discussed below.

Smooth muscle cells are generally uninucleate and are much smaller and shorter than those seen in skeletal muscle. They are spindle-shaped with long tapered ends and are usually packed together with their long axes parallel to neighboring cells in an ‘interdigitating’ manner. Each cell is enveloped by a basement membrane and other connective fibers which bridge the spaces between adjacent cells; condensations of these extracellular structures, known as dense plaques , provide a region of attachment for the smooth muscle cells. Two adjacent dense plaques allow for cell–to-cell attachment, providing mechanical stability to the tissue. The components of the extracellular matrix are produced by the smooth muscle cells themselves, rather than fibroblasts as seen in skeletal muscle.

One of the primary differences between smooth muscle and skeletal/cardiac muscle cells is the fact that the contractile proteins (actin/myosin) are not organized into sarcomeres; therefore they lack striations as seen in other muscle tissue types. Instead, actin (thin) and myosin (thick) filaments are scattered across the sarcoplasm of the cell. Actin filaments are attached to condensations of cytoskeletal intermediate filaments known as dense bodies (which therefore are functionally equivalent to Z-discs seen in skeletal muscle) as well as dense plaques mentioned above. Myosin filaments are located between actin filaments. During contraction they cause actin filaments to slide past each other, causing the cell to shorten mainly along its long axis.

Cell-to-cell communication is largely facilitated via gap junctions which are located close to openings in the basement membrane, allowing the regulation and propagation of contractile signals across the smooth muscle tissue.

Learn more about smooth muscle in the following study unit.

It’s also worth noting there are several other contractile cells which resemble smooth muscle cells (either morphologically and/or functionally) however are not classed as such. Examples include:

pericytes (found in the capillary walls)
myoid/peritubular cells ( testis ),
myoepithelial cells (often found in glandular epithelium) and
myofibroblasts ( skin , wound healing).

Clinical correlations

Dermatomyositis and polymyositis cause inflammation of the muscles. They are rare disorders, affecting only about one in 100,000 people per year. More women than men are affected. Although the peak age of onset is in the 50s, the disorders can occur at any age.

These disorders are characterized by muscle weakness that usually worsens over several months, though in some cases symptoms come on suddenly. The affected muscles are close to the trunk (as opposed to in the wrists or feet), involving for example the hip, shoulder, or neck muscles. Muscles on both sides of the body are equally affected. In some cases, muscles are sore or tender. In some cases, the muscles of the pharynx (throat) or the esophagus (the tube leading from the throat to the stomach ) are involved, causing problems with swallowing . In some cases, this leads to food being misdirected from the esophagus to the lungs , causing severe pneumonia.

Muscular dystrophy is a group of muscle diseases that weaken the musculoskeletal system and hamper locomotion. Muscular dystrophies are characterized by progressive skeletal muscle weakness, defects in muscle proteins, and the death of muscle fibres (muscle cells) and tissue.

It is a group of inherited diseases in which the muscles that control movement progressively weaken. The prefix, dys-, means abnormal, while the root, -trophy, refers to maintaining normal nourishment, structure and function. The most common form in children is called Duchenne muscular dystrophy and affects only males. It usually appears between the ages of 2 to 6 and the afflicted live typically into late teens to early 20s.

Muscle atrophy is also called “ muscle wasting ”. The majority of muscle atrophy in the general population results from ‘disuse’. People with sedentary jobs and senior citizens with decreased activity can lose muscle tone and develop significant atrophy. This type of atrophy is reversible with vigorous exercise. Bed-ridden people can undergo significant muscle wasting. Astronauts, free of the gravitational pull of Earth, can develop decreased muscle tone and loss of calcium from their bones following just a few days of weightlessness.

Muscle atrophy resulting from disease rather than disuse is generally one of two types, that resulting from damage to the nerves that supply the muscles, and disease of the muscle itself.

Examples of diseases affecting the nerves that control muscles would be:

poliomyelitis ;
amyotrophic lateral sclerosis (ALS or Lou Gehrig's disease);
Guillain-Barre syndrome .

Examples of diseases affecting primarily the muscles would include:

muscular dystrophy ;
myotonia congenita ;
myotonic dystrophy ;
as well as other congenital, inflammatory or metabolic myopathies.

The twitching phenomenon that happens in the early stage of sleep is called a hypnagogic massive jerk , or simply a hypnic jerk . It has also been referred to as a sleep start . There has been little research on this topic, but there have been some theories put forth.

When the body drifts off into sleep, it undergoes physiological changes related to body temperature, breathing rate and muscular tone. Hypnic jerks may be the result of muscle changes. Another theory suggests that the transition from the waking to the sleeping state signals the body to relax. But the brain may interpret the relaxation as a sign of falling and then signal the arms and legs to wake up. Electroencephalogram studies have shown sleep starts affect almost 10 percent of the population regularly, 80 percent occasionally, and another 10 percent rarely.

Muscle movement or twitching also may take place during the Rapid Eye Movement , or REM, phase of sleep. This also is the time when dreams occur. During the REM phase, all voluntary muscular activity stops with a drop in muscle tone, but some individuals may experience slight eyelid or ear twitching or slight jerks. Some people with REM behavioral disorder, or RBD, may experience more violent muscular twitching and full-fledged activity during sleep. This is because they do not achieve muscle paralysis, and as a result, act out their dreams.

References:

K.M. Baldwin and F. Haddad: The muscular system: Muscle plasticity. History of Exercise Physiology, (2014) p. 337
K.L. Moore and T.V. N. Persaud: The developing human (Clinically oriented embryology), 8th edition, (2007), p.357-363.
K.L. Moore and A.F Dalley: Clinically Oriented Anatomy, 4th edition, (1999), p.26-32.
M.H. Stone, M. Stone and W.A. Sands: Principles and Practice of Resistance Training, 1st edition, (2007), p. 175-182.
R.M.H McMinn: Last's anatomy (Regional and Applied), 9th edition, Ana-Maria Dulea (2014), p. 5-8.
Structure of a sarcomere: Colorado Community College System, https://pressbooks.ccconline.o... , CC4)
Actin-myosin filaments: (Boumphreyfr, https://en.wikipedia.org/wiki/ ..., CC BY-SA 3.0, modified)

Illustrators:

Macro- and microscopic view of a muscle - Paul Kim
Orbicularis oculi muscle (anterior view) - Yousun Koh
Pectoralis major muscle (anterior view) - Yousun Koh
Biceps brachii muscle (lateral-right view) - Yousun Koh
Deltoid muscle (posterior view) - Yousun Koh

Articles within this topic:

Cardiac muscle tissue
Muscle fascicle
Myofilament
Neuromuscular junction: Structure and function
Skeletal muscle histology
Striated musculature
Types of muscle cells

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Cardiac Muscle and Heart Function

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Aging of Cardiac Muscle and Cardiac Failure

Cardiac Muscle is only found in the heart

Cardiac muscle

Muscle Function

CARDIAC MUSCLE

Cardiac Muscle versus Skeletal Muscle

Cardiac Function

Cardiac Muscle

Working Group of Heart Failure and Cardiac Function

Cardiac Muscle Contraction

Cardiac Muscle I

the Heart Muscle Cell ( Cardiac striated muscle )

CARDIAC MUSCLE: CONTRACTILE MECHANISM OF CARDIAC MUSCLE

Muscles and muscle tissue

Structure of muscle tissue

Skeletal muscle

Cardiac muscle

Clinical correlations

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