THE MYOCARDIUM

ADEBOYE OLUWAJUYITAN

The myocardium is the muscular wall of the heart, or the heart muscle. It contracts to pump blood out of the heart and then relaxes as the heart refills with returning blood.

The myocardium's smooth outer membrane is called the epicardium. Its inner lining is called the endocardium.

WORD Origin

Myo – Muscle Cardio – Heart ‘-ium’ – Tissue, Structure Myocardium – muscular tissue of the heart

Myo – Muscle Cardio – Heart ‘-cyte’ – cell Cardiomyocyte – Cardiac Muscle Cell ‘pathy’ – disease Cardiomyopathy – disease (chronic) of the

heart muscle

THE HEART MUSCLE

The MYOCARDIUM, or cardiac muscle, is the thickest section of the heart wall and contains CARDIOMYOCYTES, which are the contractile cells of the heart.

The thickness of the myocardium determines the strength of the heart's ability to pump blood.

PROPERTIES OF CARDIAC MUSCLE

Striations

T-tubules

Intercalated disks

MYOCYTE

A myocyte (also known as a muscle cell) is the type of cell found in muscle tissue (myocardium).

There are two types of cells within the heart: the Cardiomyocytes and the Pacemaker cells. 

Cardiomyocytes make up the atria and the ventricles.

Pacemaker cells in the conduction system are specialized cardiomyocytes that generate and conduct electrical impulses.

CARDIOMYOCYTES

a)  Make up the muscular walls of the atrium and ventricles of the heart

b)  Possess specific properties(1)  contractility – the ability of the cell to shorten and lengthen its fibers(2)  extensibility – the ability of the cell to stretch

ELECTRICAL CELLS

a)  Make up the conduction system of the heart

b)  Are distributed in an orderly fashion through the heart

c)  Possess specific properties▪ automaticity – the ability to spontaneously generate and discharge an electrical impulse▪ excitability – the ability of the cell to respond to an electrical impulse▪ conductivity – the ability to transmit an electrical impulse from one cell to the next

MYOFIBRIL

A myofibril (also known as a muscle fibril) is a basic rod-like unit of a muscle cell. Muscles are composed of tubular cells called myocytes, known as muscle fibers in striated muscle, and these cells in turn contain many chains of myofibrils.

Myofibrils are composed of long proteins including actin, myosin, and titin, and other proteins that hold them together. These proteins are organized into thick and thin filaments called myofilaments, which repeat along the length of the myofibril in sections called SARCOMERES. 

Muscles contract by sliding the thick (MYOSIN) and thin (ACTIN) filaments along each other.

MUSCLE TYPES

There are 3 types of muscle tissue: Skeletal muscle tissue, Cardiac muscle tissue, and Smooth muscle tissue. The functions of muscle tissues depend on the

type of muscle tissues and their locations in the body.

Cardiac muscle fibres are essentially long, cylindrical cells with one (or sometimes two) nuclei.  These are centrally located within the cell.

Each muscle fiber connects to the plasma membrane (sarcolemma) with distinctive tubules (T-tubules).

At these T-tubules, the sarcolemma is studded with a large number of calcium channels which allow calcium ion exchange.

The flux of calcium ions into the muscle cells stimulates an ACTION POTENTIAL, which causes the cells to contract.

Between the ends of adjacent cardiac muscle cells are specialised intercellular junctions called INTERCALATED DISKS. These are irregular transverse thickenings of the sarcolemma that contain structures called DESMOSOMES. Desmosomes are like spot-rivets, that hold adjacent cardiac muscle fibres together. 

The intercalated discs also act as points of anchorage for the contractile proteins, and they contain important channels called GAP JUNCTIONS.  These connect the cytoplasm of adjacent cardiac muscle fibres and permit the  extremely rapid low-resistance spread of action potentials from one cell to another. 

SARCOMERE

SARCOMERE is the contractile unit of the myocardial cell. Sarcomeres are composed of long, fibrous proteins that slide past each other when the muscles contract and relax.

Two of the important proteins found in sarcomeres are MYOSIN, which forms the thick filament, and ACTIN, which forms the thin filament. Myosin has a long, fibrous tail and a globular head, which binds to actin.

Two other proteins present in sarcomeres are TROPONIN and TROPOMYOSIN.

TROPONIN is attached to the protein TROPOMYOSIN and lies within the groove between actin filaments in muscle tissue. In a relaxed muscle, tropomyosin blocks the attachment site for the myosin cross-bridge, thus preventing contraction.

TROPONIN

Troponin is a complex of three polypeptides found in striated muscle fibres.

One polypeptide (TnI) binds to actin, another (TnT) binds to tropomyosin, and the third (TnC) binds to calcium ions.

When calcium ions bind to troponin, the troponin changes shape, forcing tropomyosin away from the actin filaments. This allows myosin cross-bridges to attach onto the actin, enabling contractions to occur.

Cardiomyocyte Perfusion

Contain the protein myoglobin, which stores oxygen.

Adapted to be highly resistant to fatigue. Cardiomyocytes have a large number of mitochondria, enabling continuous aerobic respiration.

Large blood supply relative to its size, which provides a continuous stream of nutrients and oxygen, while providing ample removal of metabolic waste.

Myocardial Thickness

The myocardium has variable levels of thickness within the heart. Chambers of the heart with a thicker myocardium are able to pump blood with more pressure and force compared to chambers of the heart with a thinner myocardium. The myocardium is thinnest within the atria, as the atria fill largely through passive blood flow.

The thickness of the myocardium may change in some individuals as a compensatory adaptation to disease. The myocardium may thicken and become stiff, or it may become thinner and flabby.

Myocardial Thickness and Disease

Cardiac hypertrophy is a common result of hypertension (high blood pressure) in which the cells of the myocardium enlarge as an adaptive response to pumping against the higher pressure. Eventually it may become so severe that heart failure occurs when the heart becomes so stiff that it can no longer pump blood.

A flabby heart is typically the result of myocardial infections (myocarditis), in which the heart muscle becomes so weak that it cannot efficiently pump blood, which also leads to heart failure. 

Functions of the Myocardium

Providing a scaffolding for the heart chambers Assisting in contraction and relaxation of the

cardiac walls so that blood can pass between the chambers.

Conducting electro-stimulation through its own tissues and into the epicardium (The Conducting system of the heart).

FRANK-STERLING LAW OF THE HEART

The greater the initial length of cardiac muscle fibers, the greater the strength of contraction

The Frank-Starling mechanism describes how the heart changes its force of contraction, and therefore stroke volume, in response to venous blood return.

Greater venous blood return results in an increase in ventricular filling and preload. In turn, the length of cardiac muscle fibers increases (they are stretched as the heart fills with blood), resulting in greater strength of contraction.

Conduction System of the Heart

This pathway is made up of 5 elements: The Sino-atrial (SA) node The Atrio-ventricular (AV) node The Bundle of His The Left and Right Bundle Branches The Purkinje fibers

SINOATRIAL NODE

Inherent firing rate is the rate at which the SA NODE or another pacemaker site normally generates electrical impulses

SA Node Dominant or primary pacemaker of the heart Inherent rate 60 – 100 beats per minute Located in the wall of the right atrium, near the

inlet of the superior vena cava Once an impulse is initiated, it usually follows a

specific path through the heart, and usually does not flow backward

BACHMANN’S BUNDLE As the electrical

impulse leaves the SA node, it is conducted through the left atria by way of the Bachmann's bundle, through the right atria, via the atrial tracts.

ATRIO-VENTRICULAR JUNCTION1.  AV node

a)  Is responsible for delaying the impulses that reach it

b)  Located in the lower right atrium near the interatrial septum

c)  Waits for the completion of atrial emptying and ventricular filling, to allow the cardiac muscle to stretch to it's fullest for peak cardiac output

d)  The nodal tissue itself has no pacemaker cells, the tissue surrounding it (called the junctional tissue) contains pacemaker cells that can fire at an inherent rate of 40 – 60 beats per minute

BUNDLE OF HISS

a)  Resumes rapid conduction of the impulses through the ventricles

b)  Makes up the distal part of the AV junction then extends into the ventricles next to the interventricular septum

c)  Divides into the Right and Left bundle branches

Hiss bundle have a rate is 30-40bpm; Bundle branches 20-30bpm.

PURKINJE FIBERS

a)  Conduct impulses rapidly through the muscle to assist in depolarization and contraction

b)  Can also serve as a pacemaker, discharges at an inherent rate of 15 – 20 beats per minute or even more slowly

a)  Are not usually activated as a pacemaker unless conduction through the bundle of His becomes blocked or a higher pacemaker such as the SA node or AV junction do not generate an impulse

b)  Extends form the bundle branches into the endocardium and deep into the myocardial tissue

ACTION POTENTIAL

medical definition of ACTION POTENTIAL :  a momentary reversal in the potential difference across a plasma membrane (as of a nerve cell or muscle fiber) that occurs when a cell has been activated by a stimulus—called also spike potential.

(Merriam-Webstar)

Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and endocrine cells, as well as in some plant cells.

 In muscle cells, an action potential is the first step in the chain of events leading to contraction

In animal cells, there are two primary types of action potentials. One type is generated by voltage-gated sodium channels, the other by voltage-gated calcium channels.

Sodium-based action potentials usually last for under one millisecond, whereas calcium-based action potentials may last for 100 milliseconds or longer.

In cardiac muscle cells, an initial fast sodium spike provides a "primer" to provoke the rapid onset of a calcium spike, which then produces muscle contraction

Cardiac Action Potential The action potential of a cardiac muscle fiber can be

broken down into several phases:0- depolarization,1- initial rapid repolarization,2- plateau phase,3- late rapid repolarization,4- baseline.

DEPOLARIZATION & REPOLARIZATION 1.  Cardiac cells at rest are considered polarized, meaning no

electrical activity takes place. 2.  The cell membrane of the cardiac muscle cell separates

different concentrations of ions, such as sodium, potassium, and calcium.  This is called the resting potential.

3.  Electrical impulses are generated by automaticity of specialized cardiac cells.

4.  Once an electrical cell generates an electrical impulse, this electrical impulse causes ions to cross the cell membrane and leads to an action potential, also called DEPOLARIZATION.

5.  The movement of ions across the cell membrane through sodium, potassium and calcium channels, is the drive that causes contraction of the cardiac cells/muscle.

6.  Depolarization with corresponding contraction of myocardial muscle moves as a wave through the heart.

7.  REPOLARIZATION is the return of the ions to their previous resting state, which corresponds with relaxation of the myocardial muscle.

8.  Depolarization and repolarization are electrical activities which cause muscular activity.

9.  The action potential curve shows the electrical changes in the myocardial cell during the depolarization – repolarization cycle.

10.  This electrical activity is what is detected on ECG, not the muscular activity.

Key Points All cells in the heart can spontaneously generate action

potential (AP).

The heart is myogenic, it does not require the nervous system to work, in contrast to the skeletal muscles which are neurogenic.

Not all heart cells generate APs at the same speed.

The cells that have the fastest rate of intrinsic activity/automaticity are called the Pacemaker Cells of the heart.

All heart cells are electrically joined together by intercalated disks (Gap Junctions).

That means that once one heart muscle cell generates an AP, it just spreads to the others.

Thus APs generated by pacemaker cells normally spread throughout the heart before the other cells have a chance to generate an AP.

So the heart functions as a single unit.

What are the Autonomic Neurons for?

To VARY the rate of activity of the heart.

The ANS modulates the HR by speeding it up (SANS) when you exercise or slowing it down (PANS) when you go to sleep.

Sympathetic nervous system (or Adrenergic)

1.  Accelerates the heart 2.  Two chemicals are influenced by the

sympathetic system – epinephrine & norepinephrine

3.  These chemicals increase heart rate, contractibility, automaticity, and AV conduction

Parasympathetic nervous system ( or Cholinergic)

1.  Slows the heart 2.  The Vagus nerve is one of this systems nerves, when

stimulated slows heart rate and AV conduction.

THE ACTION POTENTIAL CURVE

It consists of 5 phases

Ions and Concentration Gradients

Na+ – Higher in E.C.F. (Outside cell)

K+ – Higher in I.C.F. (Inside cell)

Ca+ – Higher in E.C.F. (Outside cell; Inside the cell it is stored in Sarcoplasmic reticulum)

Types of Ion Channels

Leakage–gated Ion Channels

Voltage–gated Ion Channels

Ligand–gated Ion Channels

PHASE 4 - REST

1)  this is the cells resting phase (2)  the cell is ready to receive an electrical

stimulus

PHASE 0 – UPSTROKE

(1)  is characterized by a sharp, tall upstroke of the action potential

(2)  the cell receives an impulse from a neighbouring cell and depolarizes

(3)  during this phase the cell depolarizes and begins to contract

PHASE 1 - SPIKE

(1)  contraction is in process (2)  the cell begins an early, rapid,  partial

repolarization

PHASE 2 - PLATEAU

(1)  contraction completes, and the cell begins relaxing

(2)  this is a prolonged phase of slow repolarization

PHASE 3 - DOWNSLOPE (1)  this is the final phase

of rapid repolarization

(2)  repolarization is complete by the end of phase 3

PHASE 4 - REST

(1)  return to the rest period (2)  the period between action potentials

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