Week 1 - Electrical Activity of the Heart Flashcards
How does the function of Ca2+ vary in skeletal muscle vs cardiac muscle?
sarcoplasmic reticulum Ca2+ is enough to fully saturate the myosin heads in skeletal muscle and produce a contraction. In cardiac, Ca2+ entry into cell is controlled depending on the strength of contraction needed. more Ca2+ means stronger contraction
What makes up the intercalated disc?
gap junctions and desmosomes between cardiac muscle cells
How long is the action potential in skeletal muscle vs cardiac muscle?
1-2ms in skeletal, 200-250ms in cardiac. much slower
Why is the action potential of cardiac muscle longer than skeletal?
because in cardiac muscle, there are voltage gated Ca2+ channels as well as Na+ channels, increasing the no. of cross bridges formed between actin and myosin.
How does the refractory period vary in skeletal and cardiac muscle?
Skeletal muscle AP is fast and so it refractory period. this allows tetanic contraction - keep contracting muscle. cardiac muscle AP is slow so refractory period is slow too. No tetanus - one contraction at a time. cant be stimulated again
What is the difference between resting membrane potential in pacemaker and non-pacemaker cells?
pacemaker cells have no resting membrane potential because K+ channels shut immediateluy after AP and Na+ channels open, so they slowly depolarise until threshold at -40mV. Non-pacemaker cells have resting potential of -90mV
Explain the AP, plateau and repolarisation of non-pacemaker cells
- AP occurs when neighbours have AP. Na+ channels and Ca+ channels open.
- At peak, Na+ channels close but Ca+ channels stay open for longer, and K+ channels close, leading to plateau.
- Then Ca+ channels close and K+ leaky channels open, leading to rapid repolarisation.
Explain the pacemaker action potential
- AP happens due to increasing no. of Ca2+ channels open.
- At peak, Ca2+ channels close and leaky K+ open - fast repolarisation
- At repolarisation of -60mV, K+ closes and Na+ open slowly - slow depolarisation before another AP is fired
- At -40mV, Na+ channels close and few Ca2+ transient (not open long) channels open
Where are the strongest pacemaker cells?
Sinoatrial SA node in the right atrium
What is the role of the SA node?
pacemaker of the heart. gradually spreads depolarisation around aorta
What is the annulus fibrosus?
stops depolarisation immediately spreading from atrium to ventricle
What is the significance of the AV node?
only way to spread depolarisation in ventricles. 10X slower than SA node to allow atria time to fully depolarise and squeeze blood into ventricle before the ventricle contracts.
How do we want the ventricle to contract?
short and sharp contraction with pressure.
Why is the contraction of the ventricle rapid?
due to bundle of his and purkinje fibres going down heart and allowing coordinated contraction
What allows us to measure the heart at the periphery?
many myocytes summate and cause large electrical waves which reach the periphery, so we can conduct an ECG
