Electrical events of the cardiac cycle

Question 1

What cells are the myocardial pacemaker cells

Answer 1

Cells in SAN
Cells in AVN
Cells in Purkinje fibres
Cells in Bundle of His

Question 2

What are 99% of the cardiac cells

What are 1% of the cardiac cells

Answer 2

Contractile

Autorhythmic

Question 3

What is the natural pacemaker and why

Answer 3

SAN because it has the fastest firing cells (depolarise at a faster rate)

Question 4

In a pacemaker cell (SAN),

what happens to the cells at -60mV

Answer 4

The voltage gated channels are activated

Question 5

What channels open at -60mV and what moves where?

Answer 5

Na+ voltage gated channels open and sodium moves outside to inside

Question 6

What happens when the potential becomes more positive because of the influx of Na+

Answer 6

Calcium channels open because of the threshold potential and Ca2+ moves inside the cell as conc is higher outside

Question 7

What channels also open

-when and what does this do

Answer 7

K+

-When a high mV is reached and the K+ moves out of the cell

Question 8

What slows the excitation and why

Answer 8

AV node so the excitation can spread across the atria

Question 9

What is the rapid spread of excitation through the heart facilitated by

Answer 9

Intercalated discs between fibres which are found in the gap junctions

Question 10

What do the gap junctions provide and what does the cardiac muscle function as

Answer 10

Low resistance pathways

-Cardiac muscle functions as a syncytium (single unit)

Question 11

What does the wave of excitation cause in the other 99% of cells (the contractile cells)

Answer 11

Increase in mV because of the opening of sodium channels so there is a sodium influx

Question 12

Why is there a dip in membrane potential after the increase in membrane potential

Answer 12

Because as the potential becomes more positive, the potassium channels open and there is a potassium eflux so mV becomes more negative

Question 13

Why is there a plateau after the K+ efflux

Answer 13

Because of the influx of calcium

Question 14

Why is the plateau important

Answer 14

It is a refractory period which protects the heart from tetanus

Question 15

What happens after the plateau

-why

Answer 15

There is a fall in the membrane potential

-Calcium channels close and then potassium efflux overtakes and mV becomes more negative and brings it back to -90mV

Question 16

Where does contraction occur from and to

Answer 16

From inside to outside (endocardium to epicardium) and bottom to top (apex to base)

Question 17

What happens during systole

Answer 17

• Influx of Ca2+ during action potential
• Triggers release of further Ca2+ from sarcoplasmic reticulum
• Free Ca2+ activates contraction of myocardial fibres

Question 18

What determines the force of heart contraction

Answer 18

Amount of Ca2+

Question 19

What happens in diastole

Answer 19

• Uptake of Ca2+ by sarcoplasmic reticulum and extrusion of Ca2+ by Na+/Ca2+ exchange and outward Ca2+ pump
• Lowers free Ca2+ allowing relaxation

Question 20

What nerves increases heart rate

Answer 20

Sympathetic nervous system

Question 21

How does the sympathetic nervous system increase heart rate

Answer 21

Adrenaline and noradrenaline activate Beta1 adrenoceptors in SA node to increase heart rate (increase frequency of action potentials)

Question 22

How does the sympathetic nervous system increase the slope of pacemaker

Answer 22

BY increasing the funny current and the influx of Ca2+

Question 23

What nerves decrease heart rate

Answer 23

Parasympathetic via the vagus nerves

Question 24

How does the parasympathetic nervous system decrease heart rate

Answer 24

AcH moves membrane potential to -80mV (hyper polarise) so takes longer to reach threshold to fire off action potential

Question 25

What is the vagus nerve like at rest vs when exercising

Answer 25

Active at rest and then switched off when exercising and sympathetic nervous system is switched on