Pacemaker potentials

Question 1

Compare the **% of cells in the heart that are contractile or pace maker** cells

Answer 1

• 99% of cardiac cells are contractile, they are responsible for the mechanical work of pumping
• 1% of cardiac cells are pacemaker cells are specialised non contractile cells

Question 2

What is the function of cardiac pacemaker cells?

Answer 2

• they are responsible for triggering the contraction of cardiac muscle
• exhibit automaticity - spontaneous rhythmic action potential generation
• they set the rhythm of cardiac contraction

Question 3

What is the main location in the heart that contains pacemaker cells? (dominant pace maker)

Answer 3

the Sino Atrial node (SA node)

Question 4

What is the SA node regulated by?

Answer 4

regulated by
* the ANS
* circulating hormones

Question 5

What are the** 2 types of Ca2+ channels** involved in the pacemaker potential?

Answer 5

• T (transient) channel
• L (long type) channel

Question 6

what is the Na+ channel needed for the pacemaker potential?

Answer 6

• Na+ (F)
• funny channel

Question 7

How many phases are there in the pace maker potential (SA node AP)

Answer 7

• 4 phases
• NO PHASE 1
• 4,0,2,3

Question 8

Describe what occurs during phase 4 of SA node AP?

Answer 8

• An **Na+ influx **occurs through the Na+ funny channels
• the F type channels are not selective between K+ and Na+, but Na+ entry is favoured (inward depolarising current)
• delayed recitifier voltage activated K+ channels (Kv) inactivate which reduces the outflow of K+
• opening of the voltage gated T-type Ca2+ channels

Question 9

What are important exchanger channel that is involved in phase 4 of the SA node AP?

Answer 9

The 3Na+/Ca2+ exchanger

Question 10

What does phase 0 involve?

Answer 10

• the opening of the T type Ca2+ channels leads to a membrane depolarisation
• this activates the** L type Ca2+ channels** which cause a** full depolarisation** of the membrane

Question 11

What does phase 2 involve?

Answer 11

• The T type channels close
• the membrane begins to repolarise

Question 12

What does phase 3 involve in the SA node AP?

Answer 12

• The PREVIOUS depolarisation has opened 2 types of K+ channels
• Ikr (rapidly activating delayed outward rectifiying current)
• Iks (slowly activating delayed outward rectifying current)
• The Kv channels activate slowly during the AP and also inactivate slowly

Question 13

What are the** important anatomical parts** of the heart for the cardiac conduction system?

Answer 13

• SA node
• AV node (atrioventricular)
• Bundle of his
• Purkinje fibers

Question 14

What is the function of the bundle of His?

Answer 14

• The bundle of His is an important part of the electrical conduction system in the heart
• they transmit impulses that are made in the AV node down to the ventricles
• the bundle branches out into the left and right branch

Question 15

What is the function of the purkinje fibers?

Answer 15

• The purkinje fibers are extremely efficient at conducting action potentials from the pacemaker cells to the endocardium (the smooth membrane that lines the inside of the heart)
• from the endocardium the impulses travel from myocyte to myocyte via gap junctions

Question 16

What are intercalated discs?

Answer 16

• they are structural formations found between cardiac muscle cells
• they play a role in connecting them together and allowing the passage of signals between them
• they contain desmosomes and gap junctions

Question 17

What are the 3 types of cell junctions that make up the intercalated disc?

Answer 17

1. **Fascia adherens **(anchor actin filaments of the sarcomere)
2. desmosomes (important for cell adhesion via cadherin plaques)
3. gap junctions

Question 18

What is the function of the gap junctions within the intercalated discs?

Answer 18

• they provide electrical coupling for adjacent cardiac muscle cells (allows ions to flow between cells)

Question 19

In summary, how does the SA node AP arise?

Answer 19

• **inflow of Na+ **through F type channels
  * inflow of Ca2+ through T type channels and L type channels
• decreased outflow of K+ through delayed rectifier (Kv) k+ channels

Question 20

In summary, what is the AP depolarisation phase (0) caused by?

Answer 20

It is caused by the inflow of Ca2+ through L type Ca2+ channels

Question 21

In summary, what is the repolarisation phase caused by (2) ?

Answer 21

The repolarisation phase is caused by
* the** K+ outflow** through delayed recitifer (Kv) K+ channels
* and the **closure of the L type Ca2+ channels **

Question 22

Why are the SA node action potentials smaller and slower rising than the ventricular AP’s in cardiac myocytes?

Answer 22

• few fast Na+ channels
• the Ca2+ current through L type channels depolarises the membrane SLOWLY in comparision with the influence of voltage gated Na+ channels in fast response cells