Cardiac Muscle Intricacies: Desmosomes' Role Explored

Cardiac muscle tissue is only found in the heart. It is made up of individual heart muscle cells (cardiomyocytes) that are connected by intercalated discs, allowing them to work as a single functional unit. These intercalated discs contain desmosomes, which are cell structures that act as binders during the contraction of cardiac muscles.

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Intercalated discs contain desmosomes

Intercalated discs, also known as lines of Eberth, are complex structures that connect adjacent cardiac muscle cells. They are found at the ends of each cardiac muscle cell, forming a zigzag interconnection between them and allowing them to work as a single functional unit.

The three types of cell junction that make up an intercalated disc are desmosomes, fascia adherens junctions, and gap junctions. Desmosomes, also known as macula adherens (plural: maculae adherentes), are cell structures that act as binders during the contraction of cardiac muscles. They prevent separation during contraction by binding intermediate filaments, anchoring the cell membrane to the intermediate filament network, and joining the cells together.

Fascia adherens junctions are anchoring sites for actin, connecting to the closest sarcomere. They are ribbon-like protein structures that function as connectors and binders of cardiac muscle 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 them. This produces depolarization of the heart muscle.

All three types of junctions work together as a single unit called the area composita. Mutations in the intercalated disc gene can lead to various cardiomyopathies, resulting in heart failure.

Desmosomes prevent separation of cardiac muscle during contraction

Cardiac muscle tissue is only found in the heart. It is made up of individual heart muscle cells, or cardiomyocytes, that are connected by intercalated discs. Intercalated discs are complex structures that connect adjacent cardiac muscle cells. They contain three types of cell junction: desmosomes, fascia adherens junctions, and gap junctions.

Desmosomes are intercellular junctions specialized for strong adhesion. They act to prevent the separation of cardiac myocytes during individual fiber contraction by binding to intracellular intermediate filaments (cytoskeleton). This binding joins cells together, increasing the mechanical strength of the cardiac muscle tissue.

Desmosomes create tighter and smaller junctions compared to fascia adherens. They serve as a localized adhesion site that resists shearing forces as the cardiac muscles contract and pump blood. These structures prevent the separation of adjacent cardiac muscles by binding the intermediate filaments, increasing the mechanical strength of the cardiac muscle in the process. As a result, the intercellular and intracellular connections tighten between each cardiac muscle cell, preventing them from separating and pulling apart during contraction.

The cardiac desmosome is composed of transmembrane cadherins desmoglein-2 (DSG2) and desmocollin-2 (DSC2), which are anchored to the armadillo proteins plakoglobin (JUP) and plakophilin-2 (PKP2), and desmoplakin (DSP). Desmosomes are critical adhesion structures in cardiomyocytes, and their mutation or loss is linked to the heritable cardiac disease arrhythmogenic right ventricular cardiomyopathy (ARVC).

Cardiac muscle is striated and found only in the heart

Cardiac muscle, also known as myocardium, is one of three major categories of muscles in the human body, the other two being smooth muscle and skeletal muscle. Cardiac muscle is found only in the heart, specifically in the heart's middle layer or myocardium, which is sandwiched between the inner endocardium and the outer epicardium or pericardium.

Cardiac muscle is striated, which means it appears striped when viewed under a microscope. This striated appearance is caused by the arrangement of lighter I bands composed mainly of actin and darker A bands composed primarily of myosin. These bands are organised into sarcomeres, the fundamental contractile units of muscle cells. Cardiac muscle cells are also referred to as cardiomyocytes, and they contain T-tubules, which are microscopic tubes or pouches of cell membrane that run from the cell surface to the cell's interior. These T-tubules play a crucial role in improving the efficiency of contraction by concentrating voltage-gated L-type calcium channels and positioning them near calcium sense and release channels.

The contractile forces of cardiac muscle cells, along with the frequency of their activation, determine what is known as cardiac output, which is defined as heart rate multiplied by stroke volume. These involuntary muscle cells are highly branched and connected to one another at their ends by intercalated discs, which contain gap junctions and desmosomes. The presence of intercalated discs allows cardiomyocytes to contract together in a synchronised manner, enabling the heart to work as a pump.

The contractions of the heart, or heartbeats, are controlled by specialised cardiac muscle cells called pacemaker cells, which directly control heart rate. These pacemaker cells respond to signals from the autonomic nervous system and various hormones that modulate heart rate to control blood pressure.

Pacemaker cells control heart rate

Cardiac muscle tissue is only found in the heart. The heart's natural pacemaker is a small mass of specialised cells known as pacemaker cells, which are concentrated in the sinoatrial (SA) node. The SA node is the primary pacemaker of the heart and its cells have the quickest rate of spontaneous depolarization, which is why they initiate action potentials the quickest.

Pacemaker cells produce electrical impulses, known as cardiac action potentials, which control the rate of contraction of the cardiac muscle, that is, the heart rate. The pacemaker cells respond to signals from the autonomic nervous system (ANS) to speed up or slow down the heart rate. They 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.

The key to the rhythmic firing of pacemaker cells is that, unlike neurons, these cardiomyocytes will slowly depolarize by themselves and do not need any outside innervation from the autonomic nervous system to fire action potentials. In all other cells, the resting potential is caused by a continuous outflow or "leak" of potassium ions through ion channel proteins in the membrane that surrounds the cells. However, in pacemaker cells, this potassium permeability (efflux) decreases over time, causing a slow depolarization.

In the event that the heart's natural pacemaker is defective, a mechanical device called an artificial pacemaker may be used to produce these impulses synthetically. Most pacemakers work only when they’re needed (demand pacemakers). They have a sensing device that shuts the pacemaker off if the heartbeat is above a certain rate. The sensors (electrodes) at the end of the wires (leads) detect abnormal heartbeats and deliver electrical impulses to return the heart to its normal rhythm.

Cardiac muscle cells are branched

Cardiac muscle cells, also known as cardiomyocytes, are branched. They are only found in the heart, forming the contractile walls of the organ. These cells are much smaller than skeletal muscle cells, with a diameter of 10-20 μm and a length of 50-100 μm. They are also structurally distinct from skeletal muscle cells, which are long and multinucleated. In contrast, cardiac muscle cells are branched and usually contain a single nucleus located in the central region of the cell.

The branched nature of cardiac muscle cells is important for their function. These cells are connected to each other at their ends by intercalated discs, which are complex structures composed of desmosomes, fascia adherens junctions, and gap junctions. The gap junctions allow for the rapid propagation of action potentials across the entire myocardium, enabling the heart to contract and relax as a single unit, or functional syncytium. This coordinated contraction is essential for the heart to work effectively as a pump, supplying blood to the circulatory system.

The desmosomes within the intercalated discs provide a tight mechanical connection between cardiac muscle cells. They bind intermediate filaments and anchor the cell membrane to the intermediate filament network, joining the cells together. This prevents separation during contraction, ensuring the structural integrity of the heart muscle.

Additionally, the branched structure of cardiac muscle cells contributes to their ability to generate sustained depolarization plateaus. This is due to the entry of Ca++ through voltage-gated calcium channels in the sarcolemma, which is a cell membrane unique to cardiac muscle cells. The sustained depolarization results in longer contractions compared to skeletal muscle. Overall, the branching of cardiac muscle cells, along with their intercalated discs and specialized cell properties, is crucial for the synchronized and efficient functioning of the heart.

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