3. Origin of the heartbeat and the cardiac cycle Flashcards
(43 cards)
Where is the primary pacemaker region?
Sinoatrial node (SA)
- generates an electrical signal that spreads through cellular gap junctions
- spontaneously depolarises
What are Purkinje fibres?
Wiring system
- modified myocytes not nerves
Which ventricle has more bundles?
Left as it is thicker
What is the order of conduction in the heart?
SA node → Atrial muscle → AV node (small pause) → Bundle of His → Purkinje fibres → ventricular muscle
During pacemaker potential (heart is relaxed) describe the movement of ions
K+ leaves cell
Na+ (leaky channels) and Ca2+ enter the cell
During rapid depolarisation describe the movement of ions
Ca2+ enters the cell through voltage sensitive Ca2+ channels (T channels)
- at threshold of 2nd set of Ca2+ channels are opened = L type calcium channels
During repolarisation describe the movement of ions
K+ leaves the cell
- potassium channels open and the cell is able to repolarise
What other autonomic foci are involved in spontaneous depolarisation?
- atrial foci (60-80 bpm)
- junctional foci (40-60 bpm)
- ventricular foci (20-40 bpm)
Why is the SA node faster?
80-100 bpm
- essentially overrides the rest of the foci between the muscle
How would you describe cardiac muscle?
Functional syncytium - essentially all of the cells are acting as one cell
- electrically and mechanically coupled
What are the principal features of cardiac muscle?
Intercalated discs, central nuclei (usually one but sometimes two) with perinuclear space, branched fibres, good blood supply
What is If?
Funny or pacemaker current - Na+
What is phase 4 of the action potential?
Resting membrane potential
What is phase 0 of the action potential?
Rapid depolarisation
- fast Na+ channels open = voltage sensitive
What is phase 1 of the action potential?
Notch
- fast Na+ channels close
What is phase 2 of the action potential?
Plateau
- Ca2+ enters (voltage - sensitive gated channels) - L type Ca2+ channels
- K+ permeability is low
What is phase 3 of the action potential?
Repolarisation
- K+ leaves (K+ channels open; calcium channels close)
What makes cardiac muscle contract?
Needed for systole
- calcium moves through the T-tubules
- some moves into the sarcoplasmic reticulum
- Calcium induced calcium release
- Ca2+ + troponin C = contraction
What makes cardiac muscle relax?
Needed for diastole
- facilitated transport (Na+/Ca2+ exchanger, 3:1)
- pumps Ca2+ ATPase
Why is a long plateau phase and timing of tension development important?
- Relatively long refractory period (due to plateau phase)
- Inability to respond to further stimulation
- Allows the ventricles sufficient time to empty and refill before the next contraction
Define Systole/systolic
Chambers emptying/contracting
Define Diastole/diastolic
Chambers filling/relaxing
What are the phases of the cardiac cycle?
- Atrial contraction = diastole
- Isovolumetric contraction = systole
- Rapid ejection = systole
- Reduced ejection = systole
- Isovolumetric relaxation = diastole
- Rapid filling = diastole
- Reduced filling = diastole
How are heart sounds produced?
Valves closing
