The ECG

Question 1

what is funny current?

Answer 1

a slow, inward Na+ current that is activated on hyperpolarisation and has properties suitable for generating repetitive activity and modulating spontaneous rate

Question 2

What determines the steepness of phase 4 depolarisation? (and hence AP frequency)

Answer 2

degree of activation of funny current

Question 3

What is phase 4 in the cycle of SAN myocytes?

Answer 3

spontaneous depolarisation that triggers AP once membrane potential reaches between -40 and -30mV

Question 4

What is phase 0 in the cycle of SAN myocytes?

Answer 4

depolarisation phase of the AP

Question 5

What is phase 3 in the cycle of SAN myocytes?

Answer 5

repolarisation

Question 6

What brings about changes in membrane potential during the different SAN myocyte phases?

Answer 6

changes in movement of Ca2+ and K+ ions across the membrane, through If, IKr, IKs, ILCa and ITCa

Question 7

What triggers contraction in skeletal muscles? Describe the mechanism.

Answer 7

when an AP reaches a myocyte, the signal spreads across the sarcolemma including the T tubules, and the cell is excited and depolarisation occurs

vg Ca2+ channels open, Ca2+ enters the myocyte and binds to RyR => Ca-I CR from SR

Ca2+ binds to troponin C in the troponin complex, allowing conformational change to move tropomyosin out of the way allowing for myosin and actin binding and contraction

Question 8

How is the SERCA pump regulated?

Answer 8

by phosphorylation caused by beta agonists and the cAMP > PKA cascade

Question 9

Why is AP in SAN cells primarily dependent on changes in Ca2+ and K+ ?

Answer 9

equilibrium potential for
Ca2+ ~134mV
K+ ~ -96mV

Question 10

What do funny currents do?

Answer 10

cause the membrane to spontaneously depolarise, initiating phase 4

Question 11

Describe how a pacemaker action potential is generated

Answer 11

funny currents cause the membrane to spontaneous depolarise from -60mV (4)

by -50mV, T-type Ca2+ channels open and new inward Ca2+ currents further depol the cell

by -40mV, L-type Ca2+ channels open, further increasing Ca2+ conductance

by -30mV, threshold potential is reached (0)

peak is reached and K channels open, allowing K+ out of the cell and causing repol and hyperpol

Question 12

Why is the hyperpolarised state needed for the pacemaker current to occur?

Answer 12

without hyperpol (<-50mV), pacemaker channels remain inactivated, which suppresses the pacemaker current and decreases the phase 4 slope

S4 moves from resting position to inward down position to open HCN4 channels

to channel opens and allows Na+ and K+ to flow through: responsible for spontaneous depol in pacemaker APs

cAMP is also an agonist for I(f)

Question 13

what does HCN channels stand for?

Answer 13

hyperpolarization-activated cyclic nucleotide-gated channels