Lecture 9: THE EXCITABLE HEART Flashcards
What are the types of cells in the heart?
Electrical/conduction and contractile
What % of cells in the heart are electrical/conduction?
1%
What % of cells in the heart are contractile?
99%
What are the types of electrical/conduction cells?
Purkinje cells and AV nodal cells
What is the appearance of electrical/conduction cells?
Pale striated
What is the actin and myosin content of electrical/conduction cells?
Low
What is the aim of electrical/conduction cells?
To transmit an electrical signal as quickly as possible from cell to cell
What % of cells in the heart are contractile?
99%
What are the names of contractile cells?
Myocardial cells
What is the appearance of contractile cells?
Striated
What is the actin and myosin content of contractile cells?
HIgh
What is the aim of contractile cells?
To contract the heart
Where does the action potential in the heart propagate?
Along the surface membrane of electrical and contractile cells
Where does depolarisation start?
At the sinoatrial node (SAN)
Where’d does the signal spread?
To neighbouring cells (contractile and conduction)
What does increased cytosolic calcium levels do in a contractile cell?
Cause cross bridge attachment and contraction
What do intercalated discs do?
Connect most cells of the heart
What do intercalated discs contain?
Gap junctions
What are gap junctions?
Pores with low resistance to ionic current (electrical ionic current passes through easily)
What do gap junctions allow?
Current to flow between adjacent cells
Where do gap junctions spread the impulse?
Along the conduction pathway
Where does the impulse spread between?
Electrical and contractile cells and also between contractile cells
What do gap junctions increase?
The speed of the impulse throughout the heart
What do the millions of cardiac cells have to do?
Behave as one
What is a functional syncytium?
Many cells cooperating as one unit
What is the SA node?
The pacemaker for the heart which tells it when to beat (does not require the brain or nerves)
Where is the SA node found?
On the top of the right atrium
Where does the conduction pathway start?
At the SA node
Where does the SA node send the signal?
3 places - right atrium, left atrium via the intertribal bundle and to the AV node down the intertribal bundle
When does contraction of the atria occur?
When the signal reaches them and while the rest of the heart is still yet to receive the signal
What does the AV node do?
Collects the signal from the SA node and pauses it for a moment before sending it along the AV bundle
Why does the AV node pause the signal?
So that the atria can contract and relax before the ventricles begin to contract
What are the AV bundle branches?
2 branches of the AV bundle
What do the AV bundle branches do?
Take the signal to the bottom of the heart down the septum and then to the purkinje fibres
What do the purkinje fibres do?
Take the signal up the sides of the heart wall
Where does the signal start in the ventricles?
At the bottom and moves its way upwards as this is the most efficient way to remove a lot of blood out the aorta and pulmonary arteries
What is the first step of the excitation and conduction pathway?
The signal is spread from the SA node and depolarisation of the atria begins
What happens after the signal is spread from the SA node and depolarisation of the atria begins?
The atria are fully depolarised and contract. Signal has reached the AV node and is being held there
What happens after the atria are fully depolarised and contract. Signal has reached the AV node and is being held there?
The atria repolarise and relax, while the AV node sends excitation (depolarisation) down the septum to the ventricles
What happens after the atria depolarise and relax, while the AV node sends excitation (depolarisation) down the septum to the ventricles?
Atria are fully relaxed while the ventricles are fully depolarised and contract (isovolumetric contraction and ventricular ejection)
What does ECG stand for?
Electrocardiogram
What is the lead in an ECG?
A virtual line between two surface electrodes
What does a single lead do?
Detects a difference between electrodes (depolarisation and repolarisation)
What are the features of an ECG line?
P wave, QRS complex and T wave
What causes the P wave?
Atrial depolarisation initiated by the SA node
When is the impulse first delayed?
When atrial depolarisation is complete and the signal is being held at the AV node
What causes the QRS complex?
Ventricular depolarisation beginning and atrial repolarisation occurring
What happens in the gap between the QRS complex and the T wave?
Ventricular contraction is complete
What causes the T wave?
Ventricular repolarisation begins
What happens after the T wave?
Ventricular repolarisation is complete
What is the first heart sound?
Lubb
What happens during lubb?
The AV valves snap shut
What is the second heart sound?
Dupp
What happens during dupp?
The semilunar valves close
