Cardiac muscle tissue

Conducting system

Cardiac muscle tissue forms the thick myocardium of the heart wall .

It contains cardiac muscle cells and the connective tissues that surround these cells .

The contractions of cardiac muscle cells pump blood through the heart and into and through the blood vessels of the circulatory system.Cardiac muscle tissue, like skeletal muscle tissue, is striated, and it contracts by the sliding filament mechanism.

cardiac muscle cell is a short, branching cell with one or two large, centrally located nuclei.

Each cell averages about 25 μm in diameter and 120 μm in length .

Adjacent cardiac muscle cells are joined together at their ends to form cellular networks.These branching networks of cardiac muscle cells are called cardiac fibers.The complex junctions that join cardiac muscle cells arecalled intercalated discs.

Intercalated discs have two distinct regions :Transverse regions contain desmosome-like junctions called fascia adherens that function to bind adjacent cells together and transmit the contractile force to adjacent cells.Longitudinal regions contain gap junctions that allow ions to pass between cells, transmitting the contractile signal to adjacent cells.

The free movement of ions between cells allows the direct transmission of an electrical impulse through the entire network of cardiac muscle cells.

This impulse in turn stimulates all the muscle cells in a heart chamber (atria or ventricles) to contract at the same time.

In the intercellular spaces around each cardiac fiber is aloose fibrous connective tissue, the endomysium ,

which aidsto bind adjacent cardiac fibers together and contains the vessels and nerves that serve the muscle cells .

Groups of cardiac fibers form the cardiac muscle bundles in the myocardium .

The connective tissues surrounding the cardiac fibers merge with the fibrous skeleton of the heart and thus function to anchor the muscle cells and transmit the contractile forces produced by the muscle cells, similarto the tendinous origins and insertions of skeletal muscles.

The molecular mechanism for contraction in cardiac muscle is similar to that in skeletal muscle.

Cardiac muscle cells are triggered to contract by ionic calcium (Ca2+) entering the sarcoplasm .

In response to an action potential, a small amount of Ca2+ from the extracellular tissue fluid enters the cardiac muscle cell through the sarcolemma.This rise in intracellular calcium signals the sarcoplasmicreticulum to release its stored Ca2+

As with skeletal muscle, the amount of force that cardiacmuscle cells can generate depends on their length.Significantly, these cells normally remain slightly shorterthan their optimal length. Therefore, when they are stretchedby a greater volume of blood returning to the heart, theircontraction force increases, and they can pump the additional blood.

Unlike cells of skeletal muscle tissue, not all cells of cardiacmuscle tissue are innervated. In fact, an isolated cardiacmuscle cell will contract rhythmically without any innervation at all.

This inherent rhythmicity of cardiac muscle cells isthe basis of the rhythmic heartbeat, as explained next.

Your heart's electrical system includes three important part:

1 .S-A node (sinoatrial node) — known as the heart's natural pacemaker, the S-A node has special cells that create the electricity that makes your heart beat .

2 .A-V node (atrioventricular node) — the A-V node is the bridge between the atria and ventricles .Electrical signals pass from the atria down to the ventricles through the A-V node.

Conducting system

3 .His-Purkinje system — the His-Purkinje system carries the electrical signals throughout the ventricles to make them contract. The parts of the His-Purkinje system include :o His Bundle (the start of the system) o Right bundle branch o Left bundle branch o Purkinje fibers (the end of the system)

The S-A node normally produces 60-100 electrical signals per minute — this is your heart rate, or pulse. With each pulse, signals from the S-A node follow a natural electrical pathway through your heart walls .The movement of the electrical signals causes your heart's chambers to contract and relax.In a healthy heart, the chambers contract and relax in

a coordinated way, or in rhythm .When your heart beats in rhythm at a normal rate ,it is called sinus rhythm.

Step 1: Pacemaker Impulse GenerationThe sinoatrial (SA) node contracts generating nerve impulses that travel throughout the heart wall .This causes both atria to contract .

Step 2: AV Node Impulse ConductionThe atrioventricular (AV) node lies on

the right side of the partition that divides the atria, near the bottom of the right atrium .

When the impulses from the SA node reach the AV node they are delayed for about a tenth of a second. This delay allows the atria to contract and empty their contents first .

Four Steps of Cardiac Conduction

Step 3: AV Bundle Impulse ConductionThe impulses are then sent down the atrioventricular bundle. This bundle of fibers branches off into two bundles and the impulses are carried down the center of the heart to the left and right ventricles.Step 4: Purkinje Fibers Impulse ConductionAt the base of the heart the atrioventricular bundles start to divide further into Purkinje fibers. When the impulses reach these fibers they trigger the muscle fibers in the ventricles to contract

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