Pacemaker Cells

Question 1

What 2 things can gate ion channels?

Answer 1

Voltage-gated, ligand-gated

Question 2

What triggers the contraction in myocardial cells?

Answer 2

1. Rise in internal Ca2+ (from extracellular and sarcoplasmic reticulum)
2. Troponin binds to Ca2+ exposing the myosin-binding site on actin
   Systole occurs as a result of internal calcium conc rising

Question 3

What determines the resting membrane potential

Answer 3

The permeability of the membrane to different ions

Question 4

What distributes Na+ and K+, what drives this?

Answer 4

Na+-K+ ATPase; 3 Na+ out, 2K+ in

1. Driven by the need to return to RMP by keeping the interior of the cell negative (3 positives out and 2 positives in)
2. Cell must keep the concentration gradients for both ions (more Na+ is always outside and more K+ is always inside)

Question 5

What are the resting membrane potentials for the following ions…
K+
Na+
Cl-

Answer 5

K+ is higher inside the cell: -90 mV (if you could let all the K+ out, the RMP would become -90mV)

Na+ higher outside cell: +50 mV

Cl- higher outside cell: -90 mV

Question 6

What determines the equilibrium?

Answer 6

Balance of concentration and electrochemical charge gradient

Question 7

What is meant by depolarisation, repolarisation and hyperpolarisation?

Answer 7

Depolarisation: making the cell less negative
Repolarisation: making cell negative again (back to resting membrane potential)
hyperpolarisation: Making the cell overly negative due to K+ channels being open for too long

Question 8

What do the pacemaker cells initiate?

Answer 8

The electrical activity that starts depolarisation of cardiac cells

Question 9

What happens in depolarization of cardiac cells?

Answer 9

Na+ channels open, since Na+ has a higher conc outside the cell sodium floods inwards and the membrane potential peaks to almost +50 mV

Question 10

What happens after depolarization in cardiac cells?

Why is systole 250 ms?

Answer 10

Opening of the calcium channels: again calcium follows its conc gradient and flows inwards keeping the membrane potential positive (not repolarising it)
The inward flow of Ca2+ stimulates the Sarcoplasmic reticulum to release calcium stores

Systole is 250 ms as the calcium channels stay open for 250 ms, (everything else occurs in diastole)

Question 11

What happens when the calcium channels close in cardiac cells?

Answer 11

The K+ channels open: there is a much higher conc of K inside the cell, so K+ will flow outward and bring the membrane potential back to its RMP (-90 mV)

Question 12

What shape is the pacemaker’s AP, why is it different than a normal cardiac cell’s?

Answer 12

Short and triangular; it has no fast sodium channels to quickly depolarise
The upward diagonal: calcium channels stay open slowly depolarising to reach the calcium equilibrium

Triangle peak: when Calcium channels shut rapidly

Downward diagonal: K+ channels open allowing a gradual decline of membrane potential towards the K+ equilibrium until it enters hyperpolarisation

Question 13

What stops hyperpolarisation in pacemaker cells from never-ending?

Answer 13

Pacemaker cells open up HCN channels: which opens up allowing a small amount of Na+ into the cell which is called the funny current. Therefore the membrane potential rises again until it reaches the threshold and Ca2+ channels reopen

Question 14

Explain what sympathetic and parasympathetic innervation will do to the depolarisation slope of pacemaker cells

Answer 14

Sympathetic: NA steepens the slope, meaning the next AP is generated faster

Parasympathetic: Ach shallows the slope, HR in turn lowers