cardiac muscle

What is a syncytium

• Clusters of cardiomyocytes With multiple nuclei

How is cardiac muscle a syncitium

• Cardiomyocytes are connected end-to-end by intercalated discs

• They have the same structure and contract the same as skeletal muscle

• Instead of completely parallel/consistent organisation, they have:

  • Branched-like structures

Importance of gap junctions in cardiac muscle

• Low resistant = connected cytoplasm of adjacent cardiomyocytes

• Rapid passage of action potentials between cells

• Quick wave wave of depolarisation

Importance of desmosomes in cardiac muscle

• Strong mechanical junctions = physically link cardiomyocytes together

  • Provide structural integrity

  • Stops cells pulling apart during heart contractions

What is the importance of the heart as a syncytium

• Rapid co-ordinated contraction

• All-or-nothing principle

• Effective pumping

• Protection = separately contracts atria and ventricle

Describe the process of conduction of the heart

1. SA node = primary pacemaker region

2. Action potentials between cells spreads across the atria

3. AV node use the secondary pacemaker

   • Delays conduction to the ventricles

   • Natural physiological lag = refilling can occur

4. Conduction propagates slowly through AVN

5. Propagates along ventricular system so stimulate the purkinje fibres

   • Purkinje fibres send AP down sternum of the heart = contracts from the base up

   • No residual blood in the ventricles = more efficient

How does the refractory period help with blood circulation

• Slow depolarisation of the membrane = lag time = refilling needed for the heart

How does the membrane potential of the SA node affect contraction

• Membrane potential of the SA node is much higher than other cell types

  • Maintain maximum diastolic resting potential

• When threshold is reached it will trigger a contraction

Describe the process of calcium induced calcium release

1. AP travels down T-tubules

2. Depolarisation opens voltage-gated Ca channels

3. Causes an influx of Ca

4. RYR opens dues to Ca influx

5. Ca from the SR enters the cytoplasm via RYR

   • Calcium induced calcium release

6. Contraction

7. Ca rise is short lived =

   • Ca is pumped into the SR by SERCA

   • Ca is removed by Na/Ca exchanger

Describe the Frank-starling law of the heart

• The amount of stretch in the cardiac muscle determines the amount of force generated during contractions

  • Amount of stretch in sarcomeres determines force

  • When blood enters the heart

    • Increased contractile potential

    • Greater capacity to squeeze ventricles

Name some positive inotropic drugs and how they work

• B-adrenoceptor agonists =

  • Adrenaline

  • Noradrenaline

  • Dobutamine

• Increase cAMP and activate PKA

• Increase Ca currents and Ca release from the SR

• Adrenaline treats shock and cardiac arrest

• Dobutamine can treat heart failure

Name some negative inotropic drugs and how they work

• B-adrenoceptor antagonists =

  • Propranolol, metoprolol

• Ca channel blockers

  • Verapamil, dilitiazem, nifedipine

• = treat cardiac arrhythmias, angina, hypertension