19.3 Structure of Cardiac Muscle / Cardiac Rhythm / Electrical Behavior / Electrocardiogram

Cardiac Muscles/ Cardiomyocytes

• striated

• short, thick, branched cells

• ends are branched like a log with a notch at the end

• is in contact with other cardiomyocytes, forming one network in the atria and one in the ventricles

• single, centrally placced nucleus, up to 1/3 have two or more nuculei

  • joined together by intercalated discs

Intercalated discs of myocytes

• interdigitating folds - plasma membrane at ends of cells, folded like bottom of an egg carton, the folds of adjoining cells interlock with each other and increase surface area of intercellular contact

• mechanical junctions - cells are tightly joined by two types of mechanical junctions fascia adherens and desmosomes. the fascia adherens is extensive, broad band analogous to a strip of velcro, where actin of the thin myofilaments is anchored to the plasma membrane and each cell is linked to the next by transmembrane proteins. fascia adherens in interrupted here and there by desmosomes (patches of mechanical linkage) that prevent contracting cardiomyocytes from pulling apart

• electrical junctions - intercalated discs contain gap junctions which form channels that allow ions to flow from cytoplasm of one cardiomyocyte directly into the next. they enable each cardiomyocyte to electrically stimulate its neighbors. entire myocardium of the two atria behave almost like a single cell, as well as all the myocardium of the two ventricles. unified action is essential for effectively pumping heart chambers

What happens if cardiomyocytes are damaged?

There is no framework to repair or replace.

If it is damaged it is almost entirely by fibrosis (scarring)

Little capacity for mitosis and regeneration

What type of respiration does cardiac muscle use?

Aerobic respiration to make ATP

Very rich in myoglobin (short term source of oxygen for aerobic respiration) and glycogen (for stored energy)

What does the cell metabolism look like?

Huge mitochondria that fill about 25% of the cell

Adaptable to the organic fuels that can be use

At rest, the heart gets 60% of its fuel from fatty acid, 35% from flucose and 5% from other fluids like ketones, lactate, and amino acids

More vulnerable to oxygen deficiency than it is to lack of a specific fuel

No prone to fatigue

What is the cardiac conduction system?

• internal pacemaker and nervelike conduction pathways of internal pacemakers and nervelike conduction pathways through the myocardium

• it creates rhythmic electical signals by:

• 1) sinuatrial (SA) node - patch of modified cardiomyocytes in the right atrium, under the epicardium near the superior vena cava - the “pacemaker”, it initiates all heartbeats and keeps the heart rate

• 2) signals from the SA node spread throughout the atria as shown by the red arrows in the figure

• 3) atrioventricular (AV) node is located at the lower end of the inerartirial septum near the right AV valva. This node acts as an electrical gateway to the ventricles, the fibrous skeleton acts as an insulator to prevent currents from getting to the ventricles by any other route

• 4) atrioventricular (AV) bundle - pathway by which signals leave the AV node. The bundle then forks into right and left bundle branches, entering the interventricular septum and descend toward the apex

• 5) subendocardial conducting network - consist of processes that arise from lower end of bundle branches, although nervelike in action, composed of modified cardiomyocytes specialized for electrical conduction rather than contraction. at the apex of the heart they turn upward and ramify throughout the ventricular myocardium, distributing electrical excitation to the cardiomyocytes of the ventricles. They form a more elaborate network in the left ventricle than in the right. once they have delivered the electrical signal to their limits, the cardiomyocytes perpetuate it by passing ions from cell to cell through gap junctions

What is systole and diastole?

Systole - heart contraction

Diastole - heart relaxation

What is Sinus Rhythm?

Normal cardiac rhythm, triggered by the sinuatrial (SA) node

In resting adult 70-80 bpm, but can range from 60 to 100

What is Ectopic Focus?

Regions of spontaneous firings other than the SA node

If SA node is damaged an ectopic focus can take over the governance of heart rhythm

What happens if the SA node is damaged?

An ectopic focus would take over, most common is the AV node, which would produce a slower heart rate of 40-50 bpm, called nodal (junctional) rhythm

If neither the SA nor AV node is producing, another ectopic foci can fire at 20 - 40 bpm, this speed can keep you alive but is too little flow of blood to the brain

This is often the reason for someone to have a pacemaker put in

Pacemaker Physiology

• The SA node doesn’t have a stable resting membrane potential

• The membrane potential starts at -60mV and drifts upward, leading to gradual depolarization called pacemaker potential prepotential, this results primarily from slow inflow of Na+ without compensating outflow of K+

• When pacemaker reaching a threshold of -40mV, voltage gated calcium channels open and Ca2+ flow in from extracellular fluid → produces rising (depolarizing) phase of action potential, peaking slightly above 0 mV

• Then K+ channels open → K+ leave the cell

• This make cytosol increasingly negative and creates the falling (repolarizing) phase of action potentials

• When repolarization is complete the K+ channels close and the pacemaker potentials starts over on its way to producing the next heartbeat

• Each depolarization of the SA node sets off another heartbeat, when SA node fires it excites the other components in the conduction system, thus the SA node serves as the system’s pacemaker

• at rest it typically fires every 0.8 second, creating about 75 bpm

How do impulses travel in the heart?

The SA node firing excites atrial cardiomyocytes and stimulates the two atria to contract simultaneously

The signal travels at 1m/s through atrial myocardium and reached av node in about 50 ms

in av node signal slows to 0.05m/s because it is thinner but they have few gap junctions to be transmitted between, this delays the signal for about 100 ms but is needed because that is the time that the ventricles need to be able to fill with blood

In ventricular myocardium speed is 0.3 - 0.5 m/s, however since this is slower and the heart wouldn’t beat similtaneously, so there re other signals traveling as well

The signal going through the AV bundle and subendocardial conduction network moves at 4m/s and is the fastest in the conduction system because they have many gap junctions

entirity of the ventricular myocardium depolarizes within 200 ms after SA node fires, allowing it to beat as 1

Ventricular systole (contraction) begins at apex of heart, first to be stimulated and goes upward pushing blood toward semilunar valves. the spiral arangement of the vortex of the heart, the ventricles twist slightly as they contract

Electrical behavior of Myocardium

1) voltage-gated Na+ channels open

2) Na+ inflow depolarizes membrane and triggers the opening of still more Na+ channels, creating positive feedback cycle and rapidly rising membrane voltage

3) Na+ channels close when the cell depolarizes and the voltage peaks at nearly 30+ mV

4) Ca2+ entering through slow Ca2+ channels prolong depolarization of membrane, creating a plateau, the plateau falls slightly because of some K+ channels remaining closed until end of plateau

5) Ca2+ channels close and Ca2+ is transported out of the cell, K+ channels open and rapid K+ outflow returns membrane to its resting potential

What are slow calcium channels?

• channels that a small amount of Ca2+ comes in from the extracellular fluid into the cell itself

• it binds to ligand-gated Ca2+ channels on the sarcoplasmic reticulum, which opens them and a greater amount of Ca2+ comes into cytosol. The second wave of Ca2+ brinds to troponin and triggers contraction

• SR provides 90-98% of Ca2+ needed for myocardial contraction

What is arrhythmia?

Deviation from regular SA node driven sinus rhythm

What is ventricular fibrillation?

A type of arrhythmia, hall mark of a heart attack

Usually occurs in patients that have history of coronary artery disease

The electrical impulse continues to try to stimulate the same area over and over instead of the impulse dying

No blood is pumping

Looks like a bag of worms

What is defibrillation?

Emergency procedure where the heart is given a shock with pair of paddle electrodes

This is an attempt to depolarize entire myocardium and stop fibrillation