Electrical properties of the heart

Question 1

The SA node + general info

Answer 1

• The heartbeat is generated by auto-active (or pacemaker) cells in the SA node
• All nodes function is to generate specialised electrical properties
• The cells in the SA node can generate this auto-activity
• The inside of the cells is fluctuating from -60 inside the cell to +20 outside the cell
• auto-active cells do not have a true resting membrane potential

Question 2

Membrane potential of an auto-active cell

Answer 2

There are 2 parts to the membrane potential of an auto-active cell

 i. the action potential → “activation of the cell”
 ii. the pacemaker potential: a slower rise in membrane potential towards threshold = which then initiates the action potential - this is in place of a true resting membrane potential seen in other cells (including the normal contractile cardiac muscle cells)

• It’s at the threshold where the proper action potential itself gets initiated
  
  • At a threshold you get an opening of a voltage-gated channel (often a Na+ channel but doesn’t have to be) = rapid influx of sodium into the cell (sodium ions are positively charged) = so the inside of the cell starts to become more positive and hence the membrane potential rises from a negative position to a more positive position

Question 3

Pacemaker potential

Answer 3

• Pacemaker potential is generated by the movt of sodium ions diffusing into the cell = increase in permeability to sodium ions = due to a sodium channel opening-up
• The funny channel (opened during pacemaker potential) is a voltage-gated Na+ channel (allows the movt of sodium) that activates with repolarization (at ~ -50 mV) = which is very rare (and funny?)

Question 4

Action potential

Answer 4

Action potential is generated by the movt of calcium ions diffusing into the cell = increase in permeability to calcium ions = due to a calcium channel opening-up as well as an movt of potassium ions diffusing out of the cell = which causes the dropn

Question 5

depolarisation + repolarisation

Answer 5

• Depolarises = movt in a positive direction (from negative to positive) = can have slow and fast depolarisation
• Repolarisation = downward direction (from positive to negative)

Question 6

ION MOVEMENTS IN AUTO-ACTIVE CELLS

Answer 6

• Will have more positively charged ions on the outside because of the Na-K ATPase pump = membrane potential
• Is also a calcium pump that pumps calcium out of the cell
• Pacemaker potential = the diffusion of sodium into the cell through the funny channel which generates this rise in membrane potential up to threshold
• At threshold, the action potential is activated due to a calcium channel opening up
• Movt of calcium = much more rapid than sodium = why there is a steep incline
• At +20 a potassium voltage gated channel opens up and K leaves the cell resulting in a drop
• So when K+ leaves the cell the cell becomes more negatively charged which results in membrane potential dropping
• A negative membrane potential means the inside is more negative than the outside
• A positive membrane potential means the inside is more positive than the outside
• The Vm rising in a depolarising direction and reaching +20 Vm prompt the K+ channel to open up
• It’s only the existence of these funny channels that allow that auto-activity in the heart
• Pumps = move ions more slowly than channels → are also continually working as long as have ATP

Question 7

Other pacemaker cells

Answer 7

• Pacemaker (auto-active) cells are located in four main sites (SA node, AV node, AV bundle & Purkinje fibres):
• However, these other regions discharge more slowly = the slope of the pacemaker potential = flatter = takes longer for these cells to reach threshold = slower to activate action potential
• The other cells do not reach auto-activate: as in practice, these other cells are activated by cell-to-cell propagation of action potentials. In reality they never auto-activate
• The SA node fires off and before any of the others can fire off through auto-activity, the electrical signal generated in the SA node gets transmitted rapidly through your heart and causes the other cells to get activated
• The SA node is called the “natural pacemaker” because it “paces” the normal, healthy heart = the one that normally causes the heart to contract

Question 8

What happens if the SA node is damaged (eg. an infarct in that region of the heart)?

Answer 8

• As we have other areas that have auto-active cells e.g. AV node etc = what happens is instead of your heart being initiated by the SA node (which has been damaged), it starts to get initiated in the AV node instead = is fine = can continue down the line e.g. to the AV bundle if the AV node gets damaged etc
• Heart rate would slow down enormously but you would still be living

Question 9

Heart Rate

Answer 9

• The inherent rate of discharge in a SA node cell is approx. once every ~0.8 sec.
• This determines the HR, meaning the inherent HR is (1 beat/0.8 sec) or 75 beats/min (average resting heart rate in an adult is ~ 75 BPM)
• This can be increased or decreased by the autonomic nervous system & cardiotropic hormones (such as adrenaline).
• The speed at which pacemaker cells in the SA node depolarize determines the rate at which the heart contracts (typical about every 0.8 s).
• These changes in ion conductance affect the rate of depolarisation of the pacemaker cells, & there affect heart rate
• HR is modulated by neural and hormonal input.

Question 10

How does heart rate (HR) increase?

Answer 10

• HR is modulated by neural and hormonal input.
• Increases in heart rate: the sympathetic neurotransmitter is noradrenaline (Nad), and adrenaline(Ad) = a hormone released from the adrenal gland.
• Both of these activate adrenergic receptors (ß1type) in the SA node causing an increase in Na+ conductance through the funny channel.
• Sympathetic stimulation = increase in slope in pace-maker potential so can reach threshold more quickly = increase in sodium permeability

Question 11

How does heart rate (HR) decrease?

Answer 11

• Decrease in heart rate: the parasympathetic (vagal) neurotransmitter is acetylcholine (ACh).
• ACh activates cholinergic receptors (muscarinic type) in the SA node causing a decrease in Na+ conductance & an increase in K+ conductance = more slowly

Question 12

CONTRACTILE CELL

Answer 12

• There is a true resting membrane potential in contractile cells = no pacemaker potential (no ramp)
• 90 to +30
• Initiation of depolarisation requires a stimulus: to activate these cells there is a stimulus = stimulus leads to a slight depolarisation until the cell reaches threshold and then it fires off the action potential
• The rising phase is due to opening of voltage-gated Na+ channels
• Rapid influx of Na+
• Membrane potential becomes more positive
• NOTE: this is different to the nodal cells where the action potential is due to Ca2+ influx
• At approx. +30mV Na+ channels close
• A K+ channel opens briefly → K+ efflux. The membrane potential begins to fall but after a few msec, there is (i) a decrease in the number of open K+ channels and (ii) Ca2+ channels open
• There is a balance of charge (efflux of K+ matches influx of Ca2+)
• This is called The Plateau phase
• After 175ms, the Ca2+ channels close
• More K+ channels open → increase efflux of K+
• Positive charge leaves the cell, the inside becomes more negative
  → membrane potential becomes more negative
  → repolarisation (restoration of resting Vm)