Electrical Properties of the Heart

Question 1

What do gap junctions do

Answer 1

Permit cell-cell conduction of excitation hence myocardial cells form a functional syncytium
=> ALL OR NONE CONTRACTION

Question 2

What is the RMP due to

Answer 2

Increased concentration of K+ in IC fluid

Increased permeability of cell membrane to K+ as compared to other ions

Question 3

What is the RMP of a cardiac cell

Answer 3

-90 mV which is close to the equilibrium potential for K+ (-94mV) but there is an inward background current of Na+

Question 4

What is happening during plateau phase

Answer 4

Cell is ELECTRICALLY INEXCITABLE (absolute refractory period) for 200-400 ms - active contraction only lasts 200-250 ms
(fused series of contraction in cardiac muscle would be fatal)

Question 5

What does the plateau phase do

Answer 5

Initiates contraction
AND influences strength of contraction
Ca2+ entry => stronger force of contraction

Question 6

What is the relative refractory period

Answer 6

Period of gradual return -> full excitability (50ms)

Question 7

m vs h gate of Na+ channel

Answer 7

m - activation

h - inactivation

Question 8

Stage 4 of pacemaker AP = PRE-POTENTIAL/PACEMAKER POTENTIAL

Answer 8

Initial potential of SA node = -60/-70mV

Spontaneously and slowly becomes more +ve

Question 9

Name of stage 0

Answer 9

Upstroke

Question 10

Name of stage 3

Answer 10

Repolarisation

Question 11

What is the decay of pacemaker potential over time caused by

Answer 11

1. Na+ current (if) progressively depolarising cell
2. Gradually falling membrane permeability to K+
   => ik decreases progressively allowing if to dominate

Question 12

How is the AP triggered

Answer 12

threshold of -50 to -40mv is reached iCaT contributes to final 1/3 of pacemaker potential (channels are voltage gated)

Question 13

Why is upstroke (0) slow in rising and small in amp

Answer 13

Cells of SA and AV nodes lack functionally fast Na+ channels and also slow ICa
Progressive depolarisation is opposed by iK

Question 14

What is the late repolarisation (3) due to

Answer 14

iK

Question 15

How are changes in rates of SA node discharge achieved

Answer 15

Primarily by autonomic nervous activity

Question 16

What is a change in HR controlled by

Answer 16

Frequency of SA pacemaker discharge

Question 17

Parasympathetic innervation of SA node

what does it do

Answer 17

Vagus nerve - decreases HR

decreases slope of pacemaker potential and hyperpolarises SA node => threshold reached later => HR slows

Question 18

Sympathetic innervation of SA node

Answer 18

Cardiac nerves - increase HR

Question 19

What happens to pacemaker potential following sympathetic stimulation

Answer 19

The slope of the pacemaker potential increases
Threshold is reached faster
Spontaneous rate of SA node firing increases HR
Noradrenaline is released from sympathetic nerve endings and binds to beta 1 adrenergic receptor

Question 20

What conductances increase

Answer 20

GNa and GCa

therefore iCa and if increase

Question 21

IC signalling of pacemaker potential

Answer 21

• NA binds to beta 1 adrenergic receptor
• causes an increase in conc of cAMP
• cAMP increases if, which is the inward Na+ current
• cAMP activates PKA which phosphorylates the Ca2+ channel and thereby increases iCa

Question 22

IC signalling of hyperpolarisation

Answer 22

• ACh binds to its receptor (Muscarinic M2)
• activation of ACh-sensitive K+ channel
• Gk increases
• decreased IC conc of cAMP
• decreased effect of sympathetic stimulation
• decreased if, iCa => decreased slope of pacemaker potential