Cellular and Molecular Events in the Heart -- 7.1

Question 1

Define the cardiac resting membrane potential and how is the membrane potential achieved?

Answer 1

The potential inside a cardiac cell relative to the extracellular solution. The difference between the two is achieved by the selective permeability of the membrane to different ions, by way of channel proteins.

Question 2

How do you measure the resting membrane potential?

Answer 2

MEasure it as if the the outside of the cell is zero volts and give the inner potential in relation to that value

Question 3

What is the cardiac resting membrane potential value?

Answer 3

~ -90mV

Question 4

What helps to achieve the resting membrane potential value and how is it achieved?

Answer 4

the K+ equilibrium potential of -80mv as the cell membrane off mycocardial cells is mostly permeable to potassium ions
– The ions move out of the cells down their concentration gradient

Question 5

How are electrical events in myocardial cells generated?

Answer 5

Due to pacemaker cells which generate an electrical event, causing and action potential which spreads over the myocardial cells.

Question 6

Describe the changes in the membrane potential in ventricular cells

Answer 6

– In diastole, the MP is close to the EP of K+ (~90mV)

– Depolarisation occurs due to spread of electrical activity from pacemaker cells. This opens fast voltage gated Na channels causing depolarisation towards EP of Na+

– Brief repolarisation (K+ outflow) occurs (MP=0mV)

– Ca channels open due to depolarisation, and the inward flow of Ca maintains depolarisation.

– Ca influx causes the release of more Ca from intracellular stores which causes contraction.

– After ~280ms, Ca channels close and K+ efflux returns the membrane potential to resting.

Question 7

Describe the changes in membrane potential in pacemaker cells

Answer 7

• • Slow, gradual depolarisation occurs due to open Na channels.
• • Once threshold has been reached, Ca2+ channels are stimulated and they open, giving a slow depolarisation.
• • Ca2+ channels close, and the cell repolarises due to K+ efflux

Question 8

What is the spontaneous gradual depolarisation of pacemaker cells known as?

What causes this depolarisation?

Answer 8

pacemaker or ‘funny’ current

Slow Na+ channels

Question 9

How is calcium removed from the cardiac cells once depolarisation has taken place?

Answer 9

Can be taken up by the SR and mitochondria

Question 10

What type of channels are involved with action potentials?

Answer 10

Voltage-gated ion channels

Question 11

What is the force generated in a cell proportional to?

Answer 11

The Ca2+ concentration

Question 12

What does the force on contraction depend on?

Answer 12

The balance between the rate of Ca2+ entry into the cytoplasm and its rate of removal.

Question 13

Give some difference between contraction of skeletal muscle and cardiac muscle

Answer 13

The contraction in cardiac cells is much longer than in skeletal muscle
Skeletal muscle is stimulated by conduction excitation rather than by cells themselves in cardiac muscle

Question 14

How is the heart rate controlled?

Answer 14

By the rate of depolarisation of the pacemaker potential.

Question 15

What would increase the heart rate?

Answer 15

Noradrenaline binding to B1 adrenoceptors would cause the pacemaker potential to happen more quickly, which would increase the heart rate.
Is controlled by the sympathetic nervous system

Question 16

What would decrease the heart rate?

Answer 16

Acetylecholine binding to nicotinic cholinergenic recptors would decrease the slope of the pacemaker potential, therefore decreasing the heart rate.

Question 17

What is hyperkalaemia?

What affect will it have on the membrane potential?

Answer 17

When the extracellular concentration of potassium rises above a critical value

It will make the membrane potential less negative/more positive

Question 18

What is hypokalaemia?

What affect will it have on the membrane potential?

Answer 18

When the extracellular concentration of potassium falls below a critical value

It will make the membrane potential more negative/less positive

Question 19

What is the normal range of values for the extracellular concentration of potassium?

Answer 19

3.5 - 5.5 mmol per litre

Question 20

What is the equilibrium potential?

Answer 20

The hypothetical membrane potential which would develop if it were the only ion that could cross the membrane

Question 21

Define permeability

Answer 21

The ease with which ions can cross the membrane

Question 22

What conducts excitation in the heart?

Question 23

Where are pacemaker cells found?

Answer 23

SAN
AVN
Bundle of His – natural rate is slower than SAN so are overridden (may be important if there is a conduction block

Question 24

Give some characteristics of pacemaker cells

Answer 24

Have a slower AP due to it being a timing signal
Do not have fast Na channels
Upstroke = due to Ca channels
Downstroke = due to K channels

Question 25

When does the pacemaker potential occur?

Answer 25

Between the action potentials

Question 26

What is funny current?

Answer 26

spontaneous gradual depolarisation of pacemaker cells