Cardiac Muscle Cells + Cardiac Cycle

Question 1

Cardiac Muscle Cell Histology

Answer 1

• Mono-nucleated
• Striated (sarcomeres)
• Intercalated Discs: allow depolarisation to travel b/w cardiac myocytes via gap junctions

Question 2

SA Node vs AV Node

Answer 2

SA NODE:
- Right Atrium
- Intrinsic pacemaker for heart via spontaneous depolarisation (due to slow leak of sodium across membrane)
- Faster than AV node

AV NODE:
- Right Atrium
- Secondary pacemaker for heart
- Slower than SA node = delayed transmission of impulse to allow full ventricular filling

Question 3

Excitation-Contraction Coupling (Cardiac Myocyte contraction)

Answer 3

1. AP propagates down T-tubules
2. Triggers opening of ca+ channels
3. Ca+ influx from SR into myocyte
4. Ca+ binds to troponin = contraction via actin-myosin coupling
5. Ca+ pumped back into SR = contraction finishes

Question 4

Autonomic Innervation of Heart

Answer 4

SYMPATHETIC:
- T1-5 preganglionic fibres
- Increase HR, PR and contractility
- Reduce AVN ERP (refractory period of AV node /blength of time after one signal to conduct another)

= reduction means less time b/w contraction
= increased HR + contractility

PARASYMPATHETIC:
- Vagus nerve
- Reduce HR and contractility
- Increase AVN ERP

= longer time b/w contraction
= slower HR

Question 5

Fast Response vs Slow Response AP:
- Depolarisation Rate
- Plateau Phase
- Refractory Period

Answer 5

Fast = Ventricular
Slow = Atrial

DEPOLARISATION RATE
- Fast: na+ channels for rapid depolarisation
- Slow: ca+ channels for slower depolarisation

PLATEAU PHASE
- Fast: prolonged due to ca+ influx and K+ efflux balance
- Slow: shorter

REFRACTORY PERIOD
- Fast: Longer = sustained ventricular contraction to allow complete ventricular filling = efficient blood ejection
- Slow: shorter = allow faster HR in atria and quicker emptying of blood into ventricles