Electrical Activity of the Heart pt. 2 Flashcards
What is excitation-contraction coupling?
An action potential triggers a myocyte to contract, followed by subsequent relaxation.
What is a cardiac action potential?
Sequential flow of electrons across ion channels in cardiac cell membranes resulting in electrical activation of myocardial cells
What is excitation-contracting coupling in the heart?
The relationship between electrical signals of action potentials and mechanical changes in the heart muscle cells (Cardiomyocyte) that lead to contraction of the heart muscle
What is a sacromere composed of and what are they responsible for?
Two main protein filaments (thin actin and thick myosin filaments) which are the active structures responsible for muscular contraction.
What is the sarcoplasmic reticulum responsible for?
Intracellular calcium storage
What is a functional syncytium present in the excitation-contraction coupling in cardiac muscle?
A network where the cell works together with adjacent cells (not connected to each other) to undergo excitation; the cells are connected electrically (cardiac cells) and mechanically working in coordination.
The syncytial network allows the heart cells to work together as a unit.
What connects cells electrically?
Gap junctions
Eg - if one cell depolarises, the depolarisation will spread to the neighbouring cell and it will depolarise too
What connects cells physically?
Desmosomes
Stitch all cardiac cells together so they act as one big muscle
What allows the myocardium act as a syncytium?
Desmosomes and gap junctions at intercalated discs
What is the membrane of the muscle cell called?
Sarcolemma
How does the length of the action potential vary in skeletal and cardiac muscle?
Skeletal: very very short, 1-2ms
Cardiac: much longer, 200-250ms
Because as well as being mediated by voltage gated sodium channels there are also voltage gated calcium channels which open when cell depolarises and calcium floods into the cell
What happens due to cardiac muscles having a long action potential?
Cannot exhibit tenatic contraction
Can calcium be regulated in excitation-contraction coupling?
Ca2+ entry from outside cell can regulate contraction as Ca2+ release does not saturate the troponin, so regulation of Ca2+ release can be used to vary the strength of contraction
Where does the signal from myocardial contraction come from?
Auto-rhythmic cells aka pacemakers because they set the rate of the heartbeat
What causes the rapid depolarisation phase of the cardiac action potential?
Na+ entering the cell
What causes the rapid repolarisation phase of the cardiac action potential?
K+ leaving the cell
What is the main difference between cardiac and skeletal muscle action potentials?
The myocardial cell has a longer action potential due to Ca2+ entry in phase two
What are the four phases of a cardiac action potential?
0 - Na+ channels open
1 - Na+ channels close
2 - Ca2+ channels open; fast K+ channels close
3 - Ca2+ channels close; slow K+ channels open
4 - Resting potential
Happens in following order:
4, 0, 1, 2 , 3, 4
What happens in phase four (resting membrane potential) of the cardiac action potential?
Myocardial contractile cells have a stable resting potential of -90mV
What happens in phase zero (depolarisation) of the cardiac action potential?
Membrane potential becomes more positive
Voltage gates Na+ channels open, allowing Na+ to enter the cell and rapidly depolarise
What happens in phase one (plateau) of the cardiac action potential?
Initial repolarisation is very brief.
Action potential flattens for two reasons:
Decrease in K+ permeability
Increase in Ca2+ permeability
What happens to voltage gated Ca2+ channels during phases 0 and 1?
Voltage gated Ca2+ channels have been slowly opening
When they finally open, Ca2+ enters the cell while some fast K+ channels close - this combination causes action potential to flatten out into a plateau.
What happens in phase three (rapid repolarisation) of the cardiac action potential?
Plateau ends when Ca2+ channels close and K+ permeability increases once more.
Slow K+ channels open, K+ exits rapidly, retuning the cell to its resting potential
What are pacemaker cells?
Group of cardiac muscle cells with the ability to spontaneously create action potentials (automaticity)
What influences pacemaker cells?
Sympathetic and parasympathetic nervous systems
What pacemaker cells determine heart rhythm?
Pacemaker cells differ in speed of spontaneous depolarisation
Cells with fastest rate of depolarisation at any given time determine heart rhythm, - remaining/slower cells are latent pacemakers
What is the SA node and where is it located?
Collection of pacemaker cells and is located in the upper wall of the right atrium, at the junction where the superior vena cava enters.
How does the sympathetic nervous system impact the SA node?
Increases firing rate of the SA node and increases heart rate
How does the parasympathetic nervous system impact the SA node?
Decreases firing rate of the SA node, and thus decreases heart rate.
What do pacemaker cells do
Spontaneously generate electrical impulses.
The wave of excitation created by the SA node spreads via gap junctions across both atria, resulting in atrial contraction (atrial systole) – with blood moving from the atria into the ventricles.
Where are latent pacemaker cells found?
AV note - base of right atrium near septum
AV node and bundle of His
What is the rate of pacemaker cells v latent pacemaker cells?
Pacemaker: 60-100bpm
Latent:
AV node: 40-60
Bundle of His: 20-40
Purkinje fibres: 20-40
What is the function of the AV node?
Transmits impulses from atria to ventricles and coordinates their contraction
Node of specialized cardiac muscle cells
Where is the AV node located?
Posteroinferior interatrial septum wall, within triangle of atrioventricular node (Koch’s triangle) near the coronary sinus on the interatrial septum
Where do electrical impulses go after they spread across the atria?
They converge at the atrioventricular node
Why does the AV node delay impulses?
To ensure the atria have enough time to fully eject blood into the ventricles before ventricular systole.
The wave of excitation then passes from the atrioventricular node into the atrioventricular bundle.
What is another term for the atrioventricular bundle?
Bundle of His
What is the atrioventricular bundle?
A continuation of the specialised tissue of the AV node,
What does the atrioventricular bundle divide into?
Right and left branches that travels through septum between ventricles
What does it mean to say that pacemaker cells have natural automaticity?
Generate their own action potentials
How is parasympathetic activity of the heart mediated?
Acetylcholine acting on M2 muscarinic receptors at the SAN, This lengthens the interval between pacemaker potentials, hence slowing heart rate.
How is sympathetic activity of the heart mediated?
Noradrenaline acting on B1 adrenoceptors. This shortens the interval between impulses by making the pacemaker potential steeper, hence increasing the heart rate.
Pathway of electrical conduction in the heart
SA node
Atrial internodal fibres
AV node
bundle of His
Purkinje fibres
Ventricular myocytes
What is the annulus fibrous?
Ring of non-conducting tissue.
Its job is to prevent the depolarisation from immediately spreading from the atria to the ventricles.
How slowly does the AV node conduct?
0.005m/s - 10x slower than the SA
What is the role of the conducting system of the heart?
Coordinated contraction of the heart and explains why there are specific events that take place
What are the three waves shown on an ECG?
P wave
QRS complex
T wave
What does the P wave represent?
Atrial depolarisation
What does the QRS complex represent?
Ventricular depolarisation
When does the T wave represent?
Ventricular repolarisation
Why is there no wave for atrial repolarisation on an ECG?
AR happens at the same time as the ventricles depolarise
What does the ECG tell us about?
Special conducting system of the heart and heart rhythm
Normal ecg is known as sinus rhythm
What ECG change is seen in 1st degree heart block?
PR interval has increased
What change is seen in 2nd degree heart block?
Overtime PR interval increases, QRS wave has dropped
What happens in 3rd degree heart block?
No atrial conduction
What is hyperkalemia?
Higher potassium levels in blood
What is hypokalaemia?
Lower potassium levels in blood
What can too much or too little potassium cause?
Potassium levels below 3,0 mmol/l cause significant Q-T interval prolongation with subsequent risk of ventricular fibrillation and sudden cardiac death.
Potassium levels above 6,0 mmol/l cause peaked T waves, wider QRS complexes and may result in bradycardia, asystole and sudden death.
What does Einthoven’s triangle describe?
The three limb electrodes, I, II and III form a triangle (Einthoven’s Equilateral Triangle), at the right arm (RA), left arm (LA) and left leg (LL).
