Electrical Activity of the Heart Flashcards
Functional properties of cardiac muscle
Acts as a syncytium: all cells depolarise and contract simultaneously
Gap junctions: electrical connection
Desmosomes physical connection
What is the sarcolemma?
The muscle membrane that goes deep into the muscle cells as T-tubules
What forms an intercalated disc?
Alternating gap junctions and desmosomes
What is the role of Ca in cardiac contraction?
Ca enter to hyper polarise a cell to fire an AP, but the amount that enter is regulated which means the strength of contraction is also regulated
What are the two main differences in cardiac muscle compared to skeletal muscle?
No tetanic contraction:
Cardiac muscle has a long refractory period to allow heart to refill with blood and a long AP tot prevent tetanus (sustained contraction)
Ca entry can regulate contraction:
Ca does not saturate troponin, so regulation of its entry can vary strength of contraction
What are cells called that have unstable resting membrane potentials?
Pacemaker cells
Why is prevention go tetanus import in myocytes?
Cardiac muscle must relax between contractions so that the ventricles can fill with blood (diastole)
How can pacemaker cells generate AP spontaneously?
Their unstable membrane potential - pacemaker potential - which starts at -60mV and slowly drifts upward toward threshold
What makes the resting membrane potential of non-pacemaker cells -90mV?
Leaky K+ channels
If Na+ or Ca2+ channels were open, it would depolarise the cell
Describe the non-pacemaker potential and the permeability to ions
RMP: high resting PK
Initial depolarisation: Increase in PNa
Plateau: increase in PCa2 and decrease in PK
Repolarisation: decrease in PCa and increase in PK
Describe the pacemaker potential and the permeability to ions
The membrane contain If channels which are permeable to K and Na - this causes a decrease in K efflux and greater Na influx
The. net influx of positive charge slowly depolarises the cell
As the pacemaker potential becomes more positive, If channels close and Ca channels (T-type) open
When threshold is met, a different type of Ca channel opens (L-type) which fires the AP
Repolarisation: efflux of K
What determines heart rate?
The AP that initiates exitation-contraction coupling in non-pacemaker cells, originates from the pacemaker cells and spreads to contractile cells through gap junctions
Therefore the speed of pacemaker cell depolarisation determines rate of contraction (HR)
Effect of CCB and cardiac glycosides on electrical activity?
CCB - decrease force of contraction
Cardiac glycoside - increase force of contraction
Temperature effect on electrical activity?
Increase: 10bpm/degree C
Hyperkalaemia effect on electrical activity?
Fibrillation and heart block
