Spread of Excitation through the Heart and ECG Flashcards
What is the ultimate role of the heart?
To provide adequate blood flow to all the organs and tissues of the body
What does the right atrium do?
It receives blood returning from the systemic circuit
What does the left atrium do?
Receives blood returning from the pulmonary circuit
What does the right ventricle do?
Pumps blood to the pulmonary circuit
What does the left ventricle do?
Pumps blood to the systemic circuit
What does myogenic mean?
Signals itself
What does autorhythmicity mean?
Generates own rhythm
What do pacemaker cells do?
Spontaneously generate action potentials
Where are pacemaker cells located?
In the Sinoatrial Node (upper right atrium)
In the Atrioventricular Node (near the tricuspid valve)
What do conduction fibres do?
Conduct action potentials through the myocardium
They have a low electrical resistance
which two components make up the conduction system of the heart?
the pacemaker cells and the conduction fibres
How are rapid transmission of action potentials possible in the heart?
As all the cardiac cells are connected by gap junction inside the intercalated disks
What are desmosomes?
They are protein fibres which form physical bonds between cells to resist mechanical stress
Where are desmosomes found?
Inside intercalated disks
State the steps involved in the initiation and conduction of an impulse during a heart beat
- Impulse is initiated in the SAN
- Impulse travels to the AVN via conduction fibres and through most of the atrial muscle
- AVN transmits action potential less rapidly (delayed by 0.1s to allow ventricles to fill with blood)
- AVN transmits electrical signal dow the atrioventricular bundle (bundle of His) in inter-ventricular septum
- Signals split into the left and right branches to the ventricles
- Signal travels through Purkinje fibres which spread through the ventricular myocardium
Describe how action potentials spread through the heart
- An action potential is initiated in the SAN
- Action potentials are conducted from the SAN to the atrial muscle
- Action potentials spread through the atria to the AVN where conduction slows to allow filling of ventricles with blood
- Action potentials travel rapidly through the conduction system to the apex of the heart
- Action potentials spread upwards through the ventricular muscle
- Eventually heart returns to resting state until the next AP is initiated
What is pacemaker potential?
It is the slow, positive increase in voltage across the cell’s membrane
What do pacemaker cells do immediately after an action potential?
They immediately depolarise until membrane potential reaches threshold which triggers the next action potential
What are contractile cells responsible for?
Cells responsible for the contraction of the heart
What are the 5 steps involved in the contraction of the heart?
- Depolarisation
- Repolarisation
- Plateau
- Repolarisation
- Resting potential
What events occur during depolarisation?
Increased permeability to Na
So more Na move into the cell causing a massive rise in membrane potential
MP increases from -90mV to +40mV
What events occur during the first repolarisation?
Membrane permeability:
Na decreases
K decreases
Ca increases
Na channels inactivated
Membrane potential is more negative
Ca channels open
MP repolarises to counteract decreases Na permeability
What events occur during plateau?
K channels stay shut and Ca channels stay open, keeping the membrane depolarised
What events occur during the second repolarisation?
Membrane permeability of K increases (due to the delayed rectifier channels
More K moved out of cell
MP is more negative causing the inward rectifiers to open
Ca channels close
membrane is depolarised ending the AP
When are the cells at resting potential?
When K, Na and Ca are all at resting values
K>Na and Ca
membrane potential is at -90mV
What does an electrocardiogram (ECG) do?
Records the spread of electrical current through the heart as a function of time through the cardiac cycle
How is the electrical current for an ECG measured?
Recorded by means of electrodes
How is electrical activity generated by muscle tissue spread throughout the body?
As body fluids act as conductors
What does a large amplitude on the ECG mean?
The activity is more synchronised
Where should electrodes be placed according to Einthoven’s triangle?
Right arm
Left arm
Left leg
Then the electrodes should be connected to an oscilloscope
What does the P wave indicate on an ECG?
Atrial depolarisation
What does the QRS complex indicate on an ECG?
Ventricular depolarisation
What does the T wave indicate on an ECG?
Ventricular repolarisation
What does the P-Q interval indicate on an ECG?
AVN conduction time
What does the Q-T interval indicate on an ECG?
Ventricular systole
What does the T-Q interval indicate on an ECG?
Ventricular diastole
What does the R-R interval indicate on an ECG?
Time between heart beats
Define Arrhythmias
A condition in which the heart beats with an irregular or abnormal rhythm.
EG to fast (more than 100bpm) or to slow (less than 50bpm)
Describe a third degree heart block
When there is no conduction through the AVN
This causes dissociation between the atrial and ventricular contractions
This condition is potentially lethal
Describe pre mature atrial contraction (PAC)
PACs are early electrical impulses that are generated within the atria, but that are not from the SAN. PACs momentarily interrupt the normal sinus rhythm by inserting an “extra” heartbeat. There is usually a short pause before the next normal heartbeat occurs, so PACs are often perceived as a “skip” in the heartbeat.
Describe Ventricular fibrillation
When heart beats are no longer synchronised so blood isn’t pumped around the body efficiently
This condition is deadly
Describe what atrial fibrillation can lead to
Leads to weakness but is not deadly as ventricles can still contract sufficiently
