Generating the Cardiac Rhythm

Question 1

What is the meant by myogenic rhythmicity or autorhythmicity?

Answer 1

Cardiac muscle cells depolarise and contract rhythmically without nerves, conduction then occurs in a highly coordinated way

Question 2

What are the 3 broad types of cardiac action potentials?

Answer 2

SA and AV nodes
Atrial muscle
Purkinje fibres and ventricular muscle

Question 3

In what three ways can cardiac action potentials differ from each other and other types of AP’s?

Answer 3

duration
shape
underlying ionic basis (can use different ions)

Question 4

Draw and describe the shape of the action potential at the SA node.
What is the frequency of action potentials from here?
What is the length of each action potential?

Answer 4

There is no 'resting' membrane potential
There is the pacemaker potential 
This is held close to the threshold as the cells are highly excitable 
100 AP's/min
200ms - long lasting

Question 5

What is the SA node?

Answer 5

An area of 5000 densely packed excitable cells (not muscle cells)
They fire 100 AP’s a minute when there is no neuronal control

Question 6

Which ions are responsible for the pacemaker potential at the SA node?

Answer 6

1. inward movement of Na+ leads to slow depolarisation (not fast voltage gated current)
2. Inward movement of Ca2+
3. Decreased conductance of K+ (closing channels)

Question 7

Which ions are responsible for the action potential at the SA node?

Answer 7

1. Inward movement of Ca2+ (different Ca channels to those involved in pacemaker potential)
2. Further decrease conductance of K+ (less important)
3. Outward movement movement of K+ to repolarise cell

Question 8

The intrinsic rate at the SA node is 100 AP’s per min, so why is resting heart rate 60-70 beats/min?

Answer 8

A lot of sympathetic innervation to ventricles, sparse vagal innervation to ventricles

Question 9

In terms of ions, describe how parasympathetic (vagal) activity decreases HR.

Answer 9

Autonomic control of HR occurs by altering pacemaker potential.
ACh at M2 receptors:
1. hyper polarises cell (opens K+ channels)
2. reduces slope of pacemaker potential (takes longer to get to threshold so number of AP’s is reduced)
This is a negative chronotropic effect

Question 10

In terms of ions, describe how sympathetic activity increases HR.

Answer 10

NorAd and Ad at B1 receptors
1. Increases slope of pacemaker potential (increases Na+ and Ca2+ conductance).
This makes the cell more excitable and more likely to reach threshold.

Question 11

What is meant by a functional syncytium?

Answer 11

A group of cells connected by gap junctions that contract as one as electricity spreads quickly

Question 12

Describe the shape of the action potential in atrial muscle
Describe the shape of the action potential in ventricular muscle
How do these differ?

Answer 12

Stable resting potential 
 Vm = -80mV in atria, Vm = -90mV in ventricles 
Very fast depolarisation 
Plateau
Depolarisation back to resting potential

Question 13

How fast is the deplatisatio of the atria?

Answer 13

0.5m/s
complete within 0.09 seconds of SA firing
VERY FAST!

Question 14

Describe the shape of the action potential in the AV node
How fast is the conduction rate through the AVN?
What is the pacemaker rate?
Why is this important?

Answer 14

less steep pacemaker potential,
approx 0.05 m/s (poor/slow)
40 bpm
AV delay ensures atrial depolarisation, contraction and ejection before ventricles depolarise

Question 15

How fast is conduction through the following structures?:

• Atria
• AV node
• Bundle of His
• Left and right bundle branches
• Purkinje fibres
• Ventricular muscle

Answer 15

• 0.5 m/s
• 0.05 m/s
• 2 m/s
• 2 m/s
• 4 m/s
• 0.5 m/s

Question 16

What is the pacemaker rate of the SAN?
What is the pacemaker rate of atria?
What is the pacemaker rate of the AVN?
What is the pacemaker rate of the ventricles?

Answer 16

100bpm
60-80bpm
40bpm
20-40bpm

Question 17

Which ions are responsible for the action potential in ventricular myocytes?

Answer 17

Rapid depolarisation - inwards Na+ movement (fast voltage gated channels)
Slight repolarisation - inactivation of Na+ channels at +10mV
Plateau - inward Ca2+ movement AND outward movement of K+
Repolarisation - outward movement of K+

Question 18

Explain how skeletal muscle can go into tetany

Answer 18

The twitch in skeletal muscle lasts much longer than the electrical event.
Temporal summation means if repeated AP’s are fired then the cell doesn’t relax.
The RP is short so the cell can be stimulated before the muscle has had a chance to relax.

Question 19

Explain why cardiac muscle cannot go into tetany

Answer 19

Cardiac action potential are longer.
The twitch in skeletal muscle last as long as the electrical event.
This means the muscle is relaxing during repolarisation.
By the time the AP is over, the muscle has relaxed so it cannot hold a sustained contraction