Physiology: Electrical Properties of the Heart

Question 1

What is different about the RMP of Pacemaker cells?

Answer 1

RMP is less negative

RMP is unstable - generates APs spontaneously and rhythmically at varying rates

Question 2

Name the 3 phases of the SA node action potential.

Answer 2

Phase 4: pre-potential or pacemaker potential
Phase 0: upstroke
Phase 3: repolarization

Question 3

What is the initial potential of the SA node?

Answer 3

-60 to -70mV and it declines spontaneously to become less negative

Question 4

What causes the initial decay of the pacemaker potential?

Answer 4

Pacemaker current = inward Na+ current
Declining potassium conductance
Small inward current of Ca ions at -50mV

Question 5

Why is the upstroke of the AP slow rising?

Answer 5

Cells lack the functional fast sodium channels

Question 6

What mainly causes the 0 phases?

Answer 6

Slow inward current of Calcium

Opposed by outward K+ current

Question 7

What causes Phase 3: Late Repolarization?

Answer 7

K+ exits bringing the membrane back to threshold

Question 8

What controls the HR?

Answer 8

Frequency of SA pacemaker APs

Autonomic inputs control rate of SA node discharge

Question 9

Name the fibres innervating the SA node.

Answer 9

Parasympathetic fibres –> vagus nerve

Sympathetic fibres –> cardiac nerves

Question 10

What does sympathetic and parasympathetic stimulation do?

Answer 10

Sympathetic stimulation increases HR

Parasympathetic stimulation reduces HR

Question 11

What does sympathetic innervation do to the curve?

Answer 11

Slope of Phase 0 is increased (pacemaker potential) = spontaneous rate of SA node depolarization is increased leading to an increased HR

Question 12

What does parasympathetic stimulation do to the curve?

Answer 12

Reduces the slope of the pacemaker potential and hyperpolarizes the SA node –> HR slows

Question 13

Sympathetic

Answer 13

NA binding to B1 adrenergic receptors
Rise in cAMP
PKA - phosphorylates the Ca channel (increase Ca) and K channel (increasing repolarization = shorten duration of AP)

Question 14

Parasympathetic

Answer 14

ACh binds to muscarinic (M2) receptors 
Decreases cAMP 
Reduces sympathetic stimulation effects 
Hyperpolarization - activation of ACh sensitive potassium channel - potassium permeability is increased = hyperpolarization (RMP more negative) 
Longer to get to threshold = HR slows

Question 15

Which system dominates at rest?

Answer 15

PSNS - vagal tone/vagal breaking

Question 16

What does bradycardia/tachycardia do?

Answer 16

Bradycardia - increase PSNS

Tachycardia - increase SNS/decrease PSNS

Question 17

What is the dominant pacemaker?

Answer 17

SA node

Greater rate of SA node suppresses others (AV node and conducting system)

Question 18

Name the structures involved in the excitation conduction system.

Answer 18

SA node
AV node
Bundle of His 
Left and right bundle branches 
Purkinje fibres

Question 19

What must the AP pass through to get to the ventricles?

Answer 19

Atrioventricular ring

Question 20

What join myocardial cells?

Answer 20

Joined by structures called intercalated discs which contain gap junctions
Gap junctions permit cell-to-cell conduction of excitation = functional syncytium
ALL or NONE contraction

Question 21

What is the function of the AV node in the conduction system?

Answer 21

Permits a delay which allows full depolarization and contraction of the atria before the ventricles are depolarized

Question 22

What are Purkinje cells?

Answer 22

Modified cardiac cells

Question 23

What ensures that cardiac muscle does not produce tetany?

Answer 23

The electrical and mechanical activity overlap considerably in time - cardiac contractile force cannot summate.