Electrical Events of the Cardiac Cycle

Question 1

Sino-Atrial Node

Answer 1

Spontaneously firing cells of the heart; located at the right atrial wall near the opening of the SVC

Natural Pacemaker

Question 2

Atrio-ventricular cells

Answer 2

Sponteneously firing cells located at the base of the right atrium near the septum, just above the AV junction

Question 3

How does an Action Potential travel along the heart

Answer 3

An impulse begins in the Sinoatrial Node; this sends a wave of excitation across both atria simultaneously

That impulse then travels to the Atrioventricular Node

This action potential then travels through the bundle of His and separates along the septum; traveling down where it reaches the purkinje fibres then the myocardial cells that make up the ventricles

Question 4

Spontaneous Discharge Rates of the heart nodes

Answer 4

SA Nodes: 70-80 Action Potentials/min

AV Nodes: 40-60 Action Potentials/min

Purkinje Fibres: 20-40 Action Potentials/min

Question 5

What happens to the cells of the pacemaker when action potential reaches membrane resting potential (-60mv)

Answer 5

The voltage changes properties of ion channels in cells

Na+ channels open
Ca2+ channels open
K+ channels close

This causes charge to increase as Na+ and Ca2+ are at higher concentrations outside than inside the cell, leading to depolarisation

Question 6

What happens when the voltage of pacemaker cells reach threshold potential

Answer 6

More Ca2+ channels open and the voltage increases even more to about 0mv as a result of the concentration gradient

Question 7

What happens to the cells of the pacemaker when action potential reaches 0mv

Answer 7

Some Na+ and Ca2+ channels begin to close, but more importantly, K+ channels open, causing an efflux that decreases the voltage leading to repolarisation

Question 8

Roughly how long does an impulse take to go through the SA node

Answer 8

Approximately one impulse per second
(look at x-axis)

**60 bpm

Question 9

Describe the rates of impulses that travel during a cardiac impulse

Answer 9

Impulse travels rapidly from the SA node through the atria, then significantly slows down at the AV node by a factor of 20 to cause a delay

After this it rapidly goes through the bundle of his and purkinje fibres where it spreads rapidly through ventricular muscle cells

Question 10

Properties that allow waves of excitation to travel rapidly between myocardial cells

Answer 10

Gap junctions (nexi) that provide low resistance pathways, allowing cardiac muscles to function as a syncitium (single unit)

Question 11

What is the role of the AV delay during cardiac contraction

Answer 11

This allows atrial excitation and contraction to be complete before ventricular contraction, enabling efficient emptying of blood from atria to ventricles

Question 12

How is ventricular excitation kept synchronous

Answer 12

The rapid spread of impulse down the septum (Bundle of His) and through purkinje fibres

Question 13

What is the resting potential of a pacemaker cell versus a ventricular cell

Answer 13

Pacemaker: -60mV
Ventricular: -90mV

Question 14

How does an action potential of a ventricular contractile cell begin

Answer 14

Unlike pacemaker cells, they are not spontaneous at resting potential (-90mV) until extrinsic factors come into play

When a wave of excitation arrives at the cell, Na+ channels open, causing significant Na+ influx

Question 15

What happens to a ventricular cell’s action potential after becoming positive and peaking

Answer 15

K+ channels open and some K+ ions leave the cell

Question 16

What creates the plateau phase of ventricular cells during contraction/when K+ ions begin efflux

Answer 16

Ca2+ ion channels open and create a plateau as Ca2+ influx counteracts the K+ efflux

Question 17

What causes the ending of the plateau phase of ventricular contraction

Answer 17

K+ efflux eventually increases as more channels open, leading to rapid depolarisation back to resting potential

Question 18

Excitation-contraction coupling (Systole)

Answer 18

Influx of Ca2+ moves into myocardial contractile cells during action potential; triggering release of further Ca2+ from sarcoplasmic reticulum

Free Ca2+ activates contraction of myocardial fibres (SYSTOLE)

Amount of Ca2+ determines force of contraction

Question 19

Excitation-contraction coupling (Diastole)

Answer 19

Once Ca2+ reaches a certain level

there is an uptake of Ca2+ by the sarcoplasmic reticulum
AND
An extrusion of Ca2+ by Na+/Ca2+ exchange
AND
Outward Ca2+ pump

This lowers free Ca2+ allowing relaxation
(Diastole)

Question 20

Role of plateau phase of ventricular contractile cells

Answer 20

Cardiac contractile cells have a long refractory period unlike skeletal muscle; this allows the cell to beat then relax as opposed to simply staying contracted as additional stimuli in SkM can cause fused contraction/tetanus

Plateau phase is what provides that long refractory period, preventing tetanus

Question 21

Describe the direction at which cardiac impulse travels

Answer 21

From endocardium to epicardium (in to out)

From apex to base

Question 22

Sympathetic Neuronal Modulation of Heart Rate

Answer 22

Noradrenaline activates the β1 adrenoceptors in SA node which open ion channels, leading to an increase in the funny current and Ca2+ induced impulse

Increases heart rate (Yellow is sympathetic)

Question 23

Funny Current

Answer 23

Mixed sodium–potassium current that causes depolarisation of the heart alongside the calcium induced impulse

Question 24

Parasympathetic Neuronal Modulation of Heart Rate

Answer 24

Parasympathetic nerves release ACh which activate M2 muscarinic receptors in SA node

ACh causes hyperpolarisation of cardiac cells by increasing K+ permeability, making them further away from threshold potential

Additionally, ACh decreases the amount of sodium in the funny current, as well as decreasing Ca2+ moving into the pacemaker cells

Decreases heart rate (green is parasympathetic)