Logan Steele
Cardiac muscles are regulated by muscarinic receptors, which are acetylcholine receptors that form G protein-coupled receptor complexes in the cell membranes of certain neurons and other cells. The M2 receptor is believed to be the predominant muscarinic receptor subtype expressed in cardiac muscle. Acetylcholine, the neurotransmitter of the parasympathetic nervous system, slows the heart rate by activating the M2 muscarinic receptor (M2R) that, in turn, opens the acetylcholine-activated potassium channel (IK, ACh) to slow the firing of the sinus node.
| Characteristics | Values |
|---|---|
| Cardiac muscle subtype | M2 receptor |
| M2 receptor function | Regulates heart rate by modulating acetylcholine (Ach) activated K+ current IK, ACh through dissociation of G-proteins |
| M2 receptor activation | Ach |
| M2 receptor activation leads to | Bradycardia, salivation, bronchoconstriction, miosis, and increased gastrointestinal motility and secretion |
| M2 receptor inhibition | Atropine |
| M2 receptor inhibition leads to | Increased heart rate |
| M2 receptor location | Heart and lungs |
| M2 receptor effect on contractile forces of the atrial cardiac muscle | Reduces it |
| M2 receptor effect on conduction velocity of the atrioventricular node | Reduces it |
| M3 receptor location | Smooth muscles of the blood vessels and lungs |
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What You'll Learn
- Cardiac muscle cells (cardiomyocytes) are striated, branched, and involuntary
- Pacemaker cells in the sinoatrial node and atrioventricular node are smaller and conduct slowly
- Coronary arteries supply cardiac muscle with blood, and cardiac veins drain it
- Cardiomyopathies are diseases of the heart muscle, including ischemic conditions like angina
- Cardiac muscle cells are joined by intercalated discs to form long fibres
Cardiac muscle cells (cardiomyocytes) are striated, branched, and involuntary
Cardiac muscle, also known as myocardium, is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that constitutes the main tissue of the wall of the heart. The myocardium forms a thick middle layer between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium). The cardiac muscle is responsible for the contractility of the heart and, therefore, the pumping action.
Cardiac muscle cells, also called cardiomyocytes, are the contractile myocytes of the cardiac muscle. They are short cylindrical, branching cells about 80 µm in length and 15 µm in diameter with a single, centrally placed nucleus (or occasionally two nuclei). Cardiomyocytes contain t-tubules, pouches of cell membrane that run from the cell surface to the cell's interior, which help to improve the efficiency of contraction. The 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.
The generation of a cardiac action potential is involuntary and proceeds via 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 release. 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.
The cardiac muscarinic receptor (M2R) is believed to be the predominant muscarinic receptor subtype expressed in cardiac muscle. Acetylcholine, the neurotransmitter of the parasympathetic nervous system, effects cellular responses in cardiac muscle cells via activation of muscarinic cholinergic receptors. Acetylcholine slows the heart rate by activating the M2R that, in turn, opens the acetylcholine-activated potassium channel (IK,ACh) to slow the firing of the sinus node.
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Pacemaker cells in the sinoatrial node and atrioventricular node are smaller and conduct slowly
The cardiac pacemaker is the heart's natural rhythm generator. It employs pacemaker cells that produce electrical impulses, known as cardiac action potentials, which control the rate of contraction of the cardiac muscle, that is, the heart rate. These pacemaker cells are found in the sinoatrial node (SA node) and the atrioventricular node (AVN or AV node). The SA node is the primary pacemaker of the heart and is located in the upper right atrium near the superior vena cava entrance. The AVN acts as a secondary pacemaker and is located between the atria and ventricles, within the atrial septum.
The SA node is a cluster of specialized cardiomyocytes known as pacemaker cells that can spontaneously generate cardiac action potentials. These cells are constantly generating electrical impulses, setting the heart's normal rhythm and rate. The SA node controls the rate of contraction for the entire heart muscle because its cells have the quickest rate of spontaneous depolarization, thus they initiate action potentials the quickest. The normal resting heart rate in adult humans is about 70 beats per minute, with the SA node depolarizing at an intrinsic rate between 60 and 100 beats per minute.
The AVN is also endowed with automaticity and can generate viable pacemaker activity in case of SA node failure. However, AVN pacemaking is intrinsically slower than that of the SA node due to lower densities of certain ionic currents and higher expression of others. The cells of the AVN normally discharge at about 40-60 beats per minute. The AVN is an important structure in the heart's electrical conduction system, and its function has been extensively studied due to its pivotal role in cardiac conduction.
The cardiac muscarinic receptor (M2R) plays a crucial role in regulating heart rate variability and vulnerability to atrial arrhythmia. Acetylcholine slows the heart rate by activating the M2 muscarinic receptor, which in turn opens the acetylcholine-activated potassium channel (IK,ACh) to slow the firing of the sinus node. This regulation of heart rate is dependent on the delicate interplay between parasympathetic and sympathetic nerve activity at the level of the sinus node.
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Coronary arteries supply cardiac muscle with blood, and cardiac veins drain it
The heart is a muscular, four-chambered organ that is responsible for distributing blood throughout the body. The continuous activity of the heart creates a high demand for nutrients to be delivered to cardiac tissue and for waste to be removed. The coronary arteries supply oxygenated blood to the heart muscle, while the cardiac veins drain away the deoxygenated blood.
The right and left coronary arteries are the first branches of the ascending aorta. The right coronary artery supplies blood to the right atrium, portions of both ventricles, and the heart conduction system. Normally, one or more marginal arteries arise from the right coronary artery inferior to the right atrium. The marginal arteries supply blood to the superficial portions of the right ventricle. On the posterior surface of the heart, the right coronary artery gives rise to the posterior interventricular artery, also known as the posterior descending artery. It runs along the posterior portion of the interventricular sulcus toward the apex of the heart, giving rise to branches that supply the interventricular septum and portions of both ventricles.
The coronary veins that remove the deoxygenated blood from the heart muscle include the great cardiac vein, the middle cardiac vein, the small cardiac vein, the smallest cardiac veins, and the anterior cardiac veins. The smallest cardiac veins, also known as Thebesian veins, directly drain into all cardiac chambers. The middle cardiac vein collects blood from the areas supplied by the posterior interventricular artery. The small cardiac vein drains the blood from the posterior surfaces of the right atrium and ventricle. The anterior cardiac veins drain the anterior surface of the right ventricle and may also receive blood from the right marginal vein.
The anterior cardiac veins may drain independently into the right atrium or into the coronary sinus. The great cardiac vein joins the coronary sinus on the posterior surface of the heart, which empties directly into the right atrium. The Thebesian veins may also receive some of the anterior cardiac veins before draining into the coronary sinus. The arrangement of the drainage pathway of the coronary veins is less predictable than that of the arterial supply.
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Cardiomyopathies are diseases of the heart muscle, including ischemic conditions like angina
Cardiomyopathies are diseases of the heart muscle or myocardium, the thick layer of muscle tissue that makes up the bulk of the heart. The heart wall is a three-layered structure with the myocardium sandwiched between the inner endocardium and the outer epicardium. Cardiomyopathies can cause the heart to stiffen, enlarge, thicken, or form scar tissue, resulting in a loss of its ability to pump blood effectively. This can lead to further heart problems, including arrhythmias (irregular heartbeats), heart failure, heart valve disease, and cardiac arrest.
Cardiac muscle cells, also called cardiomyocytes, are the contractile cells of the myocardium. They are responsible for the contractility of the heart and its pumping action. The cells are rectangular in shape and are joined at their ends by intercalated discs to form long fibres. Each cell contains myofibrils, specialised protein contractile fibres of actin and myosin that slide past each other during contraction. This sliding motion results in the heart's squeezing action, allowing it to pump blood.
The contraction of cardiac muscle cells is coordinated by electrical impulses carried by pacemaker cells in the sinoatrial and atrioventricular nodes. These pacemaker cells are spontaneously active and send electrical signals throughout the heart without requiring external stimulation. The coordination of these contractions is crucial for efficient blood pumping. If this coordination breaks down, the heart may not pump properly, even if individual cells are contracting.
Cardiomyopathies can be caused by various factors, including genetic mutations, coronary artery disease, autoimmune diseases, infections, heart inflammation, thyroid disease, muscular dystrophy, high cholesterol, and stress. Treatment options include medications, lifestyle changes, and medical devices to improve blood flow and manage symptoms, but there is currently no cure for cardiomyopathies.
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Cardiac muscle cells are joined by intercalated discs to form long fibres
The cardiac muscarinic receptor (M2R) plays a crucial role in regulating heart rate variability and vulnerability to atrial arrhythmia by modulating the acetylcholine (ACh)-activated K+ current IK, ACh. Acetylcholine, the neurotransmitter of the parasympathetic nervous system, affects cellular responses in glands, smooth muscle, and cardiac muscle cells via the activation of muscarinic cholinergic receptors.
The three types of cell junctions that make up an intercalated disc are desmosomes, fascia adherens junctions, and gap junctions. Desmosomes prevent separation during contraction by binding intermediate filaments, anchoring the cell membrane to the intermediate filament network, and joining the cells together. Fascia adherens are anchoring sites for actin and connect adjacent cells. Gap junctions connect the cytoplasms of neighbouring cells electrically, allowing cardiac action potentials to spread between cardiac cells by permitting the passage of ions between cells, producing depolarization of the heart muscle.
All of these junctions work together as a single unit called the area composita. Mutations in the intercalated disc gene are responsible for various cardiomyopathies that can lead to heart failure.
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Frequently asked questions
Cardiac muscles are muscles in the heart.
Muscarinic acetylcholine receptors (mAChRs) are acetylcholine receptors that form G protein-coupled receptor complexes in the cell membranes of certain neurons and other cells. They are mainly found in the parasympathetic nervous system.
Yes, cardiac muscles contain muscarinic acetylcholine receptors.
