Lecture 9- The excitable heart Flashcards
What are the two cell types in the heart?
- Electrical
- Contractile
What cell type is most common in the heart?
- Contractile make up 99%
- Electrical make up 1%
What do electrical cells in the heart look like and how does this relate to their function?
- ‘pale’ striated appearance with low actin and myosin content
- This is because they communicate via an ion signal (calcium) if they has lots of actin and myosin cross bridges would form preventing the signal passing through as rapidly
What do contractile cells in the heart look like and how does this relate to their function?
- Striated appearance with high actin and myosin content
- Cross bridges form (triggered by calcium) and this causes the shortening of the sarcomeres and ultimately contraction of heart muscles
What are two examples of electrical cells in the heart?
- Purkinje cells
- AV nodal cells
How does the electrical signal pass from cell to cell in the heart?
- Depolarisation starts at the sinoatrial node (SAN)
- This signal spreads to neighbouring cells
- It spreads rapidly between adjacent conduction cells via gap junctions and is also passed down to contractile cells to trigger contraction
- The contraction cells also have gap junctions between them to help the spread of the calcium signal
What do gap junctions sit within?
-Within the intercalated discs that connect most of the cells within the heart
What does the fast nature of transferring the electrical signal through the heart allow?
- Allows many cells to operate as one unit
- This is known as a functional syncytium
What are the steps in the conduction pathway of the heart?
- SA node
- Internodal bundles
- AV Node (atrioventricular)
- AV bundle (Bundle branches)
- Purkinje fibers
What is the SA node?
-It is a small collection of cells which act as the pacemaker of the heart i.e. tells it when to beat (where electrical signal originates)
Which three places does the SA node send the electrical signal?
- Directly into cells of right atrium
- Across interatrial bundle to the left atrium (this causes the two atria to contract together first)
- Through the internodal bundles to the AV node
Where is the SA node?
Right atrium
What is the role of the AV node?
- Collects the electrical signal from SA node and pauses it.
- This allows time for atria to relax so the ventricles can contract afterwards (separate to atria contraction)
Where are the AV bundle branches located?
The septal wall that separates the left and right sides of the heart.
Why does the electric current of the heart go all the way down and then back up via the Purkinje fibers?
- Think toothpaste, the most efficient way to squeeze out blood is to start at bottom and work up.
- Works as the pulmonary artery and aorta are located at the top
Where does the conduction pathway start?
SA node
Where does the conduction pathway end?
Purkinje fibers
If the left atrium of the heart is contracting, which of the following is most likely to be TRUE?
A. The left ventricle is also contracting.
B. The right ventricle is also contracting.
C. The right atrium is also contracting.
D. The vena cava is also contracting.
E. No other chamber of the heart is also contracting.
C. The right atrium is also contracting
What is Depolarisation? How does it relate to contraction?
- Electrical signal arriving
- Contraction is occurring
What is Repolarization? How does it relate to contraction?
- Electrical signal leaving
- Resetting back to normal state, relaxing
In the atria systole phase which chambers of the heart are repolarizing/ depolarizing?
Both atria’s are fully depolarized and contracting
In the isovolumetric ventricular systole phase which chambers of the heart are repolarizing/ depolarizing?
- Atria repolarize and relax, while AV node sends excitation to ventricles
- Ventricles fully depolarized and contract
In the ventricular ejection phase which chambers of the heart are repolarizing/ depolarizing?
- Move into ventricular ejection phase when valves open
- No change in depolarisation
In the Isovolumetric ventricular relaxation phase which chambers of the heart are repolarizing/ depolarizing?
-Ventricles begin to repolarize and relax
In the ventricular diastole phase which chambers of the heart are repolarizing/ depolarizing?
-Ventricles fully repolarized and relaxed, heart is
back to quiescence (not contracting or relaxing just sitting there)
What does an ECG measure?
- Measures the depolarisation and repolarization of cardiac cells
- It doesn’t measure the exact electrical signal you are producing but instead measures changes in relation to the rest of body
- Often very hard to interpret
What electrical event is generating a P wave?
Depolarization of atria, initiated by the SA node
What electrical event/s is generating a QRS complex?
- 2 electrical events
- Both the atria are repolarizing and the ventricles are depolarizing at the apex
What electrical event is generating a T wave?
Repolarization of ventricles at apex
What mechanical events are happening at the P wave of the ECG?
Atria are contracting
What mechanical events are happening at the QRS complex of the ECG?
Atria are relaxing, ventricles are contracting
What mechanical events are happening at the T wave of the ECG?
Ventricles are relaxing
On the diagrams typically used to show depolarization +repolarization what colour represent what?
Yellow= Depolarisation Red= repolarization
How do you work out heart rate from an ECG trace?
Take the R-R interval (time in seconds). Then divide 60 by this number to get the beats per minute.
When on moves from reclining to standing what does the cardiovascular system detect and initate to counteract?
Cardiac myocytes are less stretched as less blood will return to the heart (work against gravity). This means according to starlings law that there will be decreased contraction +stroke volume. Heart rate will then go up to keep the CO constant in order to maintain arterial blood pressure
