Electrical activity of the heart Flashcards
How does an action potential come about in cardiac muscle?
- Propagated along the sarcolemma mediated by voltage gated sodium channels
- Down into tubules
- Magic interaction and calcium is released from stores in sarcoplasmic reticulum
- Calcium binds binding sites on troponin
- Allowing actin and myosin component to interact and form cross bridges
- Myosin grabs actin and pulls it = muscle cell contraction
What are the differences between skeletal and cardiac muscle?
- In skeletal muscle there is a true syncytium being formed but in cardiac muscle there isn’t a true syncytium but a functional syncytium
- Length of action potential
- Refractory periods
Explain the true syncytium in skeletal muscle?
Multiple muscles fuse together to form a larger muscle fibre.
Each fibre is independent from another
Explain the functional syncytium in cardiac muscle?
Cardiac muscle cells are independent but all work together
They are joined together physical and electrically
Always have gap junction then desmosome, then gap junction then desmosome (alternating)
How are cardiac muscle cells connected electrically?
Gap junctions
- narrow channels through centre connecting cytoplasm of one cell to cytoplasm for another
- some small cells can pass through this channel
- this electrically connects cells as when one depolarises this spreads to the next and so on.
How are cardiac muscle cells connected physically?
Desmosomes
- Stitch all cardiac muscle cells together so they contract as one big muscle
What are intercalated discs?
Alternating gap junction then desmosome then gap junction then desmosome ect.
How does length of action potential differ between skeletal and cardiac muscle?
Skeletal = very short
Cardiac = much longer
Why is AP longer in cardiac muscle cells?
Long plateau phase as mediated by voltage gated calcium channels in addition to v-gated sodium channels.
How does AP work in cardiac cells?
Open when cell depolarises and calcium flows into the muscle cell
- this depolarises the cell and also binds to binding sites on troponin and increases the number of cross-bridges formed between myosin and actin
Why doesn’t this happen in skeletal muscles?
Release of Ca from stores in sarcoplasmic reticulum is enough to fully saturate the binding sites on the troponin and you always get a full twitch contraction
What can be done in cardiac muscles with Ca?
You can modulate how much Ca comes into the cell from outside affecting No. cross bridges and changing strength of contraction
What is the refractory period for skeletal muscle and what does this do?
Very short and gives consecutive twitch contractions which can summate giving tetanic contractions (tetanus)
What is the refractory period like for cardiac muscle and why?
Very long
Needs to contract and relax continuously
What happens during the refractory period?
During refractory period you cant stimulate muscle and fire another action potential so heart wont contract = relaxed
Why is a long refractory period necessary for cardiac muscle?
We need the heart to contract (pump blood around the body) and relax (time for heart to fill with blood so it can contract again)
What muscles exhibit tetanus?
Skeletal muscle
NOT cardiac muscle
What does tetanic muscle contractions allow for?
Continued contraction of a muscle
e.g. if you want to hold a heavy object your muscles will have to continuously contract.
How is contraction of cardiac muscle regulated?
Ca2+ entry from outside the cell regulates contraction
- Ca2+ release does not saturate the troponin, so regulation of Ca2+ release can be used to vary strength of contraction.
What is the resting mem potential in non-pacemaker cells?
-90mV
What is the resting mem potential of pacemaker cells?
No stable resting mem potential
What does an AP for a non-pacemaker cell look like?
Rapid depolarisation
Long plateau
Repolarises quite rapidly to RMP
(see lecture notes for diagram)
What does an AP for a pacemaker cell look like?
Spontaneous depolarisation to threshold of -40mV
Fires action potential
Repolarise
(see lecture notes for diagram)
What are the 3 channels that cause AP in normal cells?
Leaky K channels
V-gated Na channels
V-gated Ca2+ channels
How do leaky K channels maintain RMP?
Open at rest and allow K inside the cell to leak down its conc. gradient to outside the cell.
Makes the cell more negative
- Builds up electrical gradient so K is pulled back into cell
- Reaches equilibrium where rate of K leaking out by conc. gradient = rate of K pulled back in by electrical gradient
- RMP = -90mV
How does an AP come about?
V-gated Na channels open when cell reaches threshold and Na flood into cell
- Conc and electrical gradient pull Na into the cell
- Depolarises the cell
V-gated Ca2+ channels open when cell reaches threshold
- Ca2+ floods into the cell down conc. and electrical gradients
- Depolarises further
What causes the plateau during an AP?
The voltage gated Ca channels
- They take longer to open and Ca takes longer to move into the cell
What maintains RMP in pacemaker cells?
Leaky K channels
How is an AP initiated in non-pacemaker cells?
Told to depolarise by neighbouring cell
What happens in rapid depolarisation phase of an AP in non-pacemaker cells?
Rapid depolarisation phase
- V-gated Na channels open
- Na flood quickly into cell = depolarisation
- Closes quickly - decrease mem potential
What happens in the plateau phase of an AP in non-pacemaker cells?
Plateau phase
- Permeability of cell to K decreases (as some K channels close)
- Opening of Ca2+ channels which take longer to open and will stay open for longer (permeability for Ca increases)
What happens in rapid repolarisation phase of an AP in non-pacemaker cells?
Leaky K channels open again
Ca2+ channels close
Brings cell back to RMP
Summarise AP in non-pacemaker cells
Resting mem potential
- High resting pK
Initial rapid depolarisation
- Increase in pNa
Plateau
- Increase in pCa and decrease in pK
Repolarisation
- Decrease in pCa and increase in pK
What brings about an AP in pacemaker cells?
Increase in permeability of cell to calcium
What is depolarisation of pacemaker cells like and what causes it?
Spontaneous depolarisation
- Gradual decrease in permeability of K
- Early increase in pNa
- Late increase in pCa
What causes gradual decrease in pK in pacemaker AP?
Some leaky K channels will close
What causes early increase in pNa in pacemaker AP?
V-gated Na channels
- Open due to repolarisation phase of previous AP
- Na ions flow into the cell
What causes the late increase in pCa in pacemaker AP?
V-gated Ca channels (T-type)
- Not open very long and don’t allow a lot of Ca into the cell
- Causes tiny depolarisation
What do pacemaker cells allow the heart to do and what is this called?
Keep beating despite being out of the body
- Auto-rhythmicity
Summarise auto-rhythmicity
Will spontaneously depolarise to threshold and evoke AP
Depolarisation spreads to neighbours via electrical connections known as gap junctions
Whole heart contracts as one muscle
(see notes for diagram)
What is the special conduction system of the heart?
Ensures coordinated contraction of the heart
What does a signal pass through in the special conducting system
Sinoatrial node
Annulus fibrous
Atrioventricular node
Bundle of His and Purkinje fibres
Where are pacemaker cells found and which ones start the signal?
Found throughout the heart
- Fastest in the sinoatrial node
What do the pacemaker cells in the sinoatrial node do?
Depolarise first making them the pacemaker cells for the rest of the heart
What happens after pacemaker cells depolarise?
Depolarisation wave spreads from cell to cell via gap junctions through the atria.
Causes the atria to contract and blood to be squeezed into ventricles.
What are the steps to the special conducting system?
Sinoatrial node pacemaker cells
Annulus fibrosis
AV node
Bundle of His
Purkinje fibres
Ventricular contraction
What is the role of the annulus fibrosis and how does it do this?
Prevents depolarisation spreading directly from atrium to ventricle.
- It is non-conducting
How is the depolarisation wave spread from atrium to ventricle?
Via the atrioventricular node.
What does the AV node do?
Gives time for the atria to depolarise, contract then squeeze blood into ventricles before the ventricles contract.
What happens in ventricular contraction?
Depolarisation spreads through ventricle
- Bundle of His: right and left, go down septum and around heart before splitting into smaller fibres.
- Purkinje fibres
What is the characteristic of ventricular contraction and why?
Short and sharp - to create a big pressure to push blood into pulmonary/systemic circulations
How do large extracellular electrical waves occur?
Lots of small extracellular electrical potentials evoked by many cells depolarising and repolarising at the same time summate.
What are the different waves on an ECG?
P-wave
QRS complex
T wave
What happens at P-wave?
Atrial depolarisation
What happens during the QRS complex?
Ventricular depolarisation
What happens at T-wave?
Ventricular repolarisation
What does and ECG tell us about?
Special conducting system of the heart
The hearts rhythm
What are some disorders of conduction?
Heart block
- 1st, 2nd, 3rd degree
What is heart block?
Disruption in the conduction of the depolarisation in the AV nodes and along to the ventricles.
What happens in 1st degree heart block?
p-r interval has increased (normal should be 0.2s or less)
Still getting conduction from atria to ventricles just taking longer.
What happens in 2nd degree heart block?
Some failure of conduction
- Over time p-r interval gradually increasing until ultimately that conduction fails
- A QRS complex is dropped and then another
Depolarisation isn’t spreading from atria to AV node as it should and thus causing ventricles to contract.
What happens in 3rd degree heart block?
No atrial ventricular conduction
- Have normal p waves as sinoatrial node is doing its job as a pacemaker
- Depolarisation spread throughout atria and they contract but doesn’t spread to ventricles.
Still have QRS complexes as other pacemakers in the ventricles that take over, cause it to depolarise and V contraction.
How is 3rd degree heart block treated?
Installing an electrical pacemaker device into the heart
What are some disorders of rhythm?
Atrial flutter
Atrial fibrillation
Ventricular fibrillation
What happens in atrial flutter?
Atria depolarise and contract faster than they should.
Supraventricular tachycardia
What does atrial flutter look like on an ECG?
Lots of P-waves (6-7 before a QRS complex)
- Sawtooth appearance
What happens in atrial fibrillation?
Failure of pacemaker to spread wave of depolarisation through atria and leading to coordinated atrial contraction
Cells depolarise and contract at different times
- Uncoordinated contraction of atria
What does atrial fibrillation look like on an ECG?
No clear P-waves
QRS complex happening at random intervals
What happens in ventricular fibrillation?
Uncoordinated contraction of ventricles
- Can be coarse or fine
What is the severity of ventricular fibrillation and the consequences?
Very serious as heart is not effectively pumping blood around the body
Quickly lose consciousness as blood not getting to the brain
How can ventricular fibrillation be treated?
Shockable rhythm
- Defibrillator shocks heart and depolarises all cells at the same time, all enter refractory period at the same time
- Allows sinoatrial node to restart sinus rhythm
- Factory reset
