origin and regulation of the heart and cardiac cycle

cardiac conduction system

SA node→R and L atrial cardiomyocytes→AV node→bundle of his→purkinje fibre network→R and L ventricular cardiomyocytes

non contractile cardiac cells

• larger than standard cardiomyocytes

• fewer contractile protein

• more glycogen and mitochondria

• connected by desmosomes and gap juntions

• no intercalated disks

pacemaker action potentials

• pacemaker cells are myocardial cells of the SA and AV nodes

• SA nodal cells depolarise and set the rate

• synchronisation- the action potential is conducted down the conduction system as an electrical impulse and between cardiomyocytes via gap junctions

rate of depolarisation

• sa node - 75 bpm

• av node- 50 bpm

• bundle and purkinje- 30bpm

features of cardiac muscle

Cardiac muscle is a functional syncytium - cells (myocytes) are electrically and mechanically coupled.

• Self-contracting.

• Single nucleus

. • Intercalated discs located between cardiac muscle cells

. • Gap junctions provide passage for ions and small molecules.

• Desmosomes and adheren junctions provide mechanical continuity.

High mitochondrial density - a continuous supply of ATP is needed to support contraction

ventricular action potential

• 4- resting potential

• 0- rapid depolarisation. sodium channels open and membrane potential positive

• 1- slow release potassium current, small drop in membrane potential

• phase 2- plateau phase. calcium influx balances out potassium efflux. calcium influx stimulates intracellular calciium ions releasing initiating contraction, no further action potential can occur- refracotry period

• 3-Repolarisation Rapid K+ efflux returns membrane potential back to -70 mV.

cardiac contraction

• action potential conducted to contractile myocytes through gap junctions

• AP travels between sarcomeres activating calcium ion channels in the t tubules leading to the influx of calcium ions

• DHPRs are voltage gated calcium channels in skeletal muscle initiate relase from the SR.

• Conformational change of the DHPRs in excitation contraction coupling causes nearby RYRs to open releasing stored calcium ions.

• calcium ions binds to troponin c which exposes actin binding site allowing contraction

cardiac relaxation

• calcium removed through sr uptake and efflux through membrane channels

• calcium removal returns troponin complex to inhibitng position on actin, ending contraction, actin returns to original

• enzyme SERCA 2a returns calcium ions back into SR

ECG

• p wave- atrial depolarisation

• qrs- ventricle depolarisation

• t wave- repolarisation of ventricles

• pr interval- av conduction time

• st segment- isoelectric period- both ventricles completely depolarised

• qt interval- time for both ventricular depolarisation and repolarisation to occur- average duration of ventricular action potential

• rr interval- measure of cardiac cycle length

what can an ecg show

• heart rate

• heart rhythm

• origin of excitation

• anatomical orientation of heart

• relative size of heart chambers

• spread of impulse

• decay of excitation

• no info about contractile properties and pumping action

1. SA node

2. AV node

3. bundle of his

4. left bundle branch

5. left posterior fascicle

6. left anterior fascicle

7. left ventricle

8. ventricular septum

9. right ventricle

10. right bundle branch

cardiac cycle

1. atrial contraction

2. isovolumetric contraction (AV valve closure and increase in pressure without any volume change)

3. rapid ejection

4. reduced ejection

5. isovolumetric relaxation (period after ventricular contraction where aortic valve has closed but mitral valve has not opened yet)

6. rapid filling

7. reduced filling