Week 1 - Pacemaker Cells

Question 1

What are the major differences between pacemaker cells and cardiomyocytes?

Answer 1

• pacemaker cells do not contract
• influenced by parasympathetic system
• not connected via gap junctions
• have funny channels

Question 2

How do you stop a heart from beating when cut out of the body, without cutting any parts of the heart out?

Answer 2

stop providing oxygen and other essential nutrients to it or by not filtering out the waste products (CO₂)

Question 3

Why don’t normal muscle cells have gap junctions to generate APs like cardiac muscle cells?

Answer 3

• specificity
• different types of synapses (excitatory/inhibitory)
• allows a refractory period as the receptor is blocked for some time

Question 4

How are hearts myogenic?

Answer 4

they can create their own action potentials, which spreads to the rest of the heart causing it to beat

Question 5

How do funny channels work?

Answer 5

• they are leaky channels, always allow Na⁺ through
• they are still affected by voltage
• opens wider when voltage increases
• become more leaky when protein kinase binds to it
• protein kinase activated by nor/epinephrine binding by G proteins

Question 6

Why does pacemaking not stop if SAN is removed?

Answer 6

• pacemaking activity starts at the SAN but is present in other cells too, such as AVN, Bundle of His and so on
• but the pacemaking activity slows down throughout
• it usually doesn’t have pacemaking activity as it is overwhelmed by the SAN

Question 7

How and why does the action potential in a pacemaker cell differ from normal cells?

Answer 7

• there is no resting potential in pacemaker cells
• due to the presence of funny channels always allowing Na⁺ through
• keeps the membrane potential higher

Question 8

What is the purpose of Ca²⁺ channels in cardiac muscle?

Answer 8

• they activate ryanodine receptors
• along with Ca²⁺ coming from T-tubules
• these receptors allow even more Ca²⁺ to be released from the sarcoplasmic reticulum
• they can then interact with troponin

Question 9

How are cardiac muscles conductive?

Answer 9

they allow waves to pass through by the gap junctions present

Question 10

What are the two types of Ca²⁺ channels?

Answer 10

T-type: transient (quick)
- only in pacemaker cells
L-type: long term
- in pacemaker cells and cardiomyocytes
- both voltage gated and activated by cAMP