Cardiac 2 (BP) Flashcards
What are skeletal muscles like?
- Muscle cells made up of sarcomeres that cont thick filaments (composed of myosin) and thin filaments (composed of actin)
- Shortening of sarcomere occurs via sliding filament mechanism, where actin filaments slide along adjacent myosin filaments
What are cardic muscles like?
- Similar to skeletal muscle in terms of composition & function
- DIFFERENT in terms of how cells interact
- Cardiac muscle acts like a syncytium - a single, multinucleated cell formed from many fused cells
- A wave of depolarisation is followed by atrial & ventricular contraction, push the blood the same time into other areas
- Its purpose is to all work together - all muscles need to work at once
What are the parts of the heart involved in cardiac excitation?
- Interatrial pathway
- Sinoatrial (SA) node
- Right branch of bundle of His
- Atrioventricular (AV) node
- Left branch of bundle of His
- Purkinje fibres
How is the heart able to generate its own rhythm?
- 1% cardiac cells - with pacemaker activity (set the pace of the heart) (auto-rhythmic cells)
- 99% cardiac cells → contractile function
What is the “natural pacemaker” of the heart?
The Sinoatrial (SA) node
How big is the Sinoatrial (SA) node?
Approx 8mm long & 2mm thick
What is the Sinoatrial (SA) node?
It is a specialised region in the right atrial wall at the junction between the superior vena cava & the right atrium
What are cells within the SA node like?
They normally fire very FAST, generate heart beat
Do cells in the SA node have a resting potential?
Do not have resting potential (don’t rest, always moving); transmembrane potential ‘less negative’ than in ventricular cardiomyocytes
What are the phases of the SA node?
- Phase 0 = upstroke of action potential is less steep than myocyte
- Phase 3 = Plateau is not sustained
- Phase 4 = Membrane potential deviates from K+ equilibrium potential
What are the stages of the action potential of SA node?
- Slow depolarisation = Na+ influx (slow), Ca2+ influx; reduced K+ efflux
- Rapid depolarisation = Ca2+influx
- Repolarisation = K+ efflux
What is the potential of ventricular cells (the 99%)?
No pacemaker potential - cells remain essentially at rest (-90mV) until excited by electrical activity propagated from the pacemaker
What are the stages of an AP being made in the ventricular cells (the other 99%)?
- Rapid depolarisation: Na+ influx
- Plateau phase: Ca2+ influx
- Repolarisation: K+ efflux
What happens between the upstroke & downstroke of the ventricular cell?
There is a refractory period –> cannot be constantly firing
What is needed for cardiac excitation?
Needs to be efficient & co-ordinated
- Action potentials generated at SA node
- Rapid excitation thru both atria
- Excitation reaches AV node where conduction is SLOW
What does cardiac excitation allow for?
Allows atria to contract & empty blood into ventricles
How is cardiac excitation an efficient & co-ordinated pump?
Excitation spreads rapidly down the bundle of His & Purkinje fibres to ensure almost simultaneous of ventricular cells → efficient, co-ordinated pump
Why must cardiac exitation be efficient & co-ordianted?
Must be efficient & co-ordinated, if not blood will start to back up (e.g. into the lungs can cause difficulty breathing) can lead to heart failure
How can caridac excitation be recorded?
Can be recorded from the surface of the body with an electrocardiogram (ECG)
What is recorded in an ECG?
4 events but only 3 are visible
1 - Atrial depolarisation = P wave
2 - Ventricular depolarisation = QRS complex
a. Masks atrial repolarisation (lub-dub sound)
3 - Ventricular repolarisation (resetting & waiting to start again) = T wave
What do systole & diastole mean?
Systole = contract
Diastole = relax
What are the 3 key parts of cardiac contraction?
1 - Atrial systole
2 - Ventricular systole
3 - Ventricular diastole
What happens in atrial systole?
(Cardiac contraction)
a) A-V valves open & atria empty blood into ventricles
b) Atrial excitation & contraction should be complete before onset of ventricular contraction
What happens in ventricular systole?
(Cardiac contraction)
a) Part 1: ventricles contract → rise in pressure forces closed A-V valve (stops backflow of blood)
b) Part 2: Pressure in ventricle rises above aortic pressure → aortic valve opens & blood is ejected from heart
