Phsyiology - Membrane potentials

Question 1

What are the layers of the heart?

Answer 1

Endocardium - inner surface thin layer of cells

Myocardium - muscle and conducting tissue

Epicardium - outer surface of mesothelium

Pericardial space - Contains lubricating fluid

Pericardium - thin fibrous sac

Question 2

What are cardiac myocytes?

Answer 2

Muscle cells making up the myocardium

Similar to normal myocytes

Small, rough cyclindrical myocytes are jined by intercalated discs making them into fibres

Question 3

Describe the action potentials of cardiac myocytes

Answer 3

Na+ influx causes depolarisation through opened gates which close at peak of depolarisation

Plateau phase due to slow Ca2+ influx and K+ efflux

• Ca2+ channels open just after peak
• Ca2+ influx triggers Ca2+ release from sarcoplasmic reticulum => contraction

Repolarisation due to K+ efflux => Ca2+ channels close also => rapid repolarisation

Hyperpolarisation
- Na+ slowly begins to flow into cell

Question 4

Describe SAN and AVN action potentials

Answer 4

No Na+ channels - relied wholly on Ca2+ influx

No plateau

Slow depolarisation

Decaying resting potential due to slowing efflux of K+

Slower cell-cell conductance - very important in AVN