CS: Conduction System

Question 1

Describe cardiac muscle cells

Answer 1

Autorythmic and spread action potentials (APs) to contractile cells

Question 2

What is SAN?

Answer 2

Sinoatrial node

Question 3

What does the SAN do?

Answer 3

SAN initiates the sequence and the cardiac conduction system- anatomical pacemaker- by causing atrial muscles to contract. Contraction initiated and dictated by APs.

Question 4

What is the role of the SAN?

Answer 4

SAN initiates the normal electrical pattern followed by the contractions of the heart

Question 5

How do the APs from the SAN activate the AVN?

Answer 5

atrioventricular node (AV node). The internodal pathways consist of three bands (anterior, middle, and posterior) that lead directly from the SA node to the next node in the conduction system, the AV node.

Question 6

What are the 3 bands of the internodular pathway?

Answer 6

Anterior, middle, and posterior

Question 7

What is the AVN?

Answer 7

Atrioventricular node

Question 8

What is the trigger for cardiac muscle contraction?

Answer 8

The electrical event, the wave of depolarization, is the trigger for muscular contraction. The wave of depolarization begins in the right atrium, and the impulse spreads across the superior portions of both atria and then down through the contractile cells. The contractile cells then begin contraction from the superior to the inferior portions of the atria, efficiently pumping blood into the ventricles.

Question 9

What is conduction of the APs delayed at the AVN?

Answer 9

To allow the completion of atrial contraction

Question 10

How are the APs delayed at the ANV?

Answer 10

The septum prevents the impulses from spreading directly to the ventricles without passing through the AVN.

Question 11

What happens to the APs after hey reach the AVN?

Answer 11

Delayed then APs are allowed to propagate and travel down the Bundle of His and its bundle branches, to the Purkinje Fibres, to eventually cause ventricular contractions. Contraction is initiated from the bottom up.

Question 12

What are the Purkinje fibres?

Answer 12

The Purkinje fibers are additional myocardial conductive fibers that spread the impulse to the myocardial contractile cells in the ventricles. They extend throughout the myocardium from the apex of the heart toward the atrioventricular septum and the base of the heart.

Question 13

How is cardiac muscle contraction initiated?

Answer 13

Action potentials are generated by autorythmic conductive cells, and trigger depolarisation. Waves of depolarisation spread to contractile cells via gap junctions.

Question 14

What do ECGs show?

Answer 14

The movement of ions, across cell membranes of cardiac muscle cells, causes a current to flow throughout bodily fluids which can be detected at the skin surface to generate ECG traces.

Question 15

What does the P wave show?

Answer 15

Atrial depolarisation/ systole

Question 16

What does the QRS complex show?

Answer 16

Ventricular depolarisation (systole) and atrial diastole

Question 17

What does the T wave show?

Answer 17

Ventricular repolarisation/ diastole

Question 18

What is cardiac output?

Answer 18

The volume of blood pumped by each ventricule per minute

Question 19

How is cardiac output calculated?

Answer 19

Cardiac output = stroke volume x heart rate

Question 20

How is heart rate controlled?

Answer 20

Heart rate is regulated by the sinoatrial node (SAN) and constant nervous and hormonal influences exert control over heart rate. The autonomic nervous system (ANS) controls the involuntary responses to heart rate: sympathetic (fight or flight response) and parasympathetic (rest and digest response; homeostasis).

Question 21

What is the predominant hormone that influences heart rate?

Answer 21

Adrenaline

Question 22

Where is adrenaline produced and what does it do?

Answer 22

Adrenaline is produced by the adrenal glands and binds to the same B1 adrenergic receptors that noradrenaline binds to.

Question 23

What controls the movement of ions into and out of cardiac conductive cells?

Answer 23

Transport proteins embedded in the sarcolemma

Question 24

Describe the depolarisation and repolarisation process in cardiac conductive cells

Answer 24

Cardiac conductive cells contain a series of sodium ion channels that allow a normal and slow influx of sodium ions that causes the membrane potential to rise slowly from an initial value of −60 mV up to about –40 mV. The resulting movement of sodium ions creates spontaneous depolarization (or prepotential depolarization). At this point, calcium ion channels open and Ca2+ enters the cell, further depolarizing it at a more rapid rate until it reaches a value of approximately +5 mV. At this point, the calcium ion channels close and K+ channels open, allowing outflux of K+ and resulting in repolarization.

Question 25

How are cardiac contractile cells triggered?

Answer 25

Cardiac contractile cells rely on conductive cells to generate APs; positive calcium ions move through the gap junctions from depolarised conductive cells to generate a small voltage change in contractile cells to initiate depolarisation

Question 26

Describe the depolarisation and repolarisation process in cardiac contractile cells

Answer 26

Contractile cells demonstrate a much more stable resting phase than conductive cells at approximately −80 mV for cells in the atria and −90 mV for cells in the ventricles. Despite this initial difference, the other components of their action potentials are virtually identical. When stimulated by an action potential, sodium voltage-gated channels rapidly open, beginning the positive-feedback mechanism of depolarization. This rapid influx of positively charged ions raises the membrane potential to approximately +30 mV, at which point the sodium channels close. Depolarisation is followed by the plateau phase, in which membrane potential declines relatively slowly. This is due in large part to the opening of the slow Ca2+ channels, allowing Ca2+ to enter the cell while few K+ channels are open, allowing K+ to exit the cell; calcium influx briefly balance potassium efflux and calcium ions initiate contraction. Once the membrane potential reaches approximately zero, the Ca2+ channels close and K+ channels open, allowing K+ to exit the cell. At this point, membrane potential drops until it reaches resting levels once more and the cycle repeats.