L47 - Cardiac Output: Electrical events and Heart rate

How do you calculate cardiac output?

Cardiac output = Heart rate x Stroke volume

Name of the components of the Conduction System of the Heart

• Pacemaker cells (SAN and AVN)

• Right bundle branch

• Purkyne fibres

• Left bundle branch

• Cardiomyocytes (contraction)

• Bundle of His

Sequence of cardiac excitation

What causes the pacemaker cells of the SAN to trigger an action potential?

• Low resting membrane potential (-60 to -70 mV)

• Na+ leaking in through F-type channels and calcium ions moving in through the T-type channels causes to depolarisation

• Rapid opening of voltage gated calcium L type channels leads to rapid depolarisation

• Reopening of potassium channels and closing of calcium channels leads to repolarisation

What is the mechanism of contraction of the ventricular cardiomyocytes?

• Rapid opening of voltage gated sodium channels leads to rapid depolarisation

• The prolonged opening of voltage gated calcium channels and closure of potassium channels leads to the prolonged plateau of depolarisation

• Opening of potassium channels results in the repolarization phase

How does calcium produce contraction of the cardiac muscles?

Excitation-Contraction Coupling in Cardiac Muscle

Why does cardiac muscle have a prolonged refractory period?

To allow for ventricles to fill with blood prior to pumping

• Refractory period = time required before it is possible to re-stimulate muscle contraction

Snio-atrial is regulated by the

autonomic nervous system

What regulates the rate of depolarisation in the SAN?

Parasympathetic and sympathetic nervous system

How does the parasympathetic nervous system regulate the rate of depolarisation in the SAN?

• Parasympathetic neurones release acetylcholine

• m2 muscarinic receptors of DAN

• Increase in K+ efflux

• Decrease in Ca2+ influx

• Hyperpolarizes cell and decreases rate of depolarisation

• Decrease in heart rate

How does the sympathetic nervous system regulate the rate of depolarisation in the SAN?

• Sympathetic neurones release noradrenaline

• b1 adrenergic receptors of SAN

• Increase in Na+ and Ca2+ influx

• Increases rate of depolarisation

• Increases heart rate

How do you measure an electrocardiogram?

Using electrodes

What is an ECG?

A summation of the spread of action potentials through the various sections of the heart

What does each wave mean?

P = Atrial depolarisaiton

QRS = Ventricular depolarisation

T = Ventricular depolarisation

PQ segment = Atrial contraction

QT segment = Ventricular contraction

WHy are ECGs important?

Provides valuable information concerning the electrical but NOT mechanical activity of the heart

Allows for diagnosis of diseases