Excitable cells

Question 1

What is the role of the intercalated disks in cardiac muscle cells?

Answer 1

allow changes in membrane potential to travel directly to neighbouring cells without the need for a chemical synapse

Question 2

What makes cells excitable?

Answer 2

Difference in ion concentrations represents stored energy which powers excitable cells

Question 3

When solutes are charged and the membrane is selectively permeable, there will be a conflict between chemical and electrical energy. True or false?

Answer 3

True

Question 4

Action potentials of specialised myocytes and contractile myocytes are faster than neurons. True or false?

Answer 4

False - slower and rely on Ca2+ gated channels rather than Na+

Question 5

A cardiac contractile cell fires an action potential only when it is depolarised to threshold. True or false?

Answer 5

True

Question 6

Pacemaker cells can’t fire APs in the absence of an external stimulus. True or false?

Answer 6

False

Question 7

What are pacemaker potentials?

Answer 7

The slow depolarisation that leads up to each AP

Question 8

In pacemaker cells and other cardiac muscle cells, electrical signals are caused by changes in plasma membrane ______ ________ brought about by the opening and closing of specific ion channels

Answer 8

ion permeability

Question 9

In cardiac muscle, the most important permeability changes involve, K+, Ca2+ and Na+ respectively. True or false?

Answer 9

False - Na+, K+ and Ca2+ respectively

Question 10

The intracellular fluid is rich in sodium, but poor in potassium and calcium in comparison to the extracellular fluid. True or false?

Answer 10

False - rich in potassium, poor in calcium and sodium

Question 11

As membrane permeability to a particular ion increases, the membrane potential moves towards the equilibrium potential of that ion. True or false?

Answer 11

True

Question 12

In intracellular fluid, the equilibrium potential of potassium is negative and is positive for calcium and sodium. What happens to the equilibrium potentials of each ion when there is an increase in permeability of them all?

Answer 12

Equilibrium potential of calcium and sodium becomes more positive
Equilibrium potential of potassium becomes more negative

Question 13

What triggers electrical signals in pacemaker cells?

Answer 13

changes in permeability of ions

Question 14

If the SA node is prevented from generating the electrical impulse, other components of the conductive system are able to take over. True or false?

Answer 14

True - ectopic pacemakers

Question 15

What connects pacemakers and conducting cells of the heart to contractile cells?

Answer 15

Gap junctions

Question 16

What four things do contractile myocytes contain?

Answer 16

Actin
myosin
troponin
sarcoplasmic reticulum

Question 17

Sarcoplasmic reticulum contains stores of calcium. True or false?

Answer 17

True

Question 18

What is the sequence of events for the electrical activity of pacemaker cells?

Answer 18

slow depol at start due to closing of K+ channels and opening of funny channels (Na+)
Initial depol triggers T-type Ca2+ channels to open
L-type Ca2+ channels open as threshold is reached
Depol triggers opening of K+ channels - leading to reducing in membrane potential
Reduction in membrane potential leads to closing of Ca2+ channels
Membrane repolarised as a result of reduction in ca2+ into cell and increase in potassium leaving cell

Question 19

What is the sequence of events in electrical activity of cardiac contractile cells?

Answer 19

depolarisation of membrane triggers opening of Na+ channels
Na+ channels become inactivated but membrane potential only drops a small amount because inital depolarisation triggers closing of K+ channels and opening of L-type Ca2+ channels
There is a plateau period where K+ remain closed and Ca2+ channels remain open
Increase in K+ leaving cell as K+ channels open in response to depolarisation.
Ca2+ channels close so less Ca2+ enters cell - membrane repolarised
Resting potential restored where K+ is dominant to Na+ and Ca2+ - membrane potential is -90mV

Question 20

When is contraction due to actin-myosin cycling enabled?

Answer 20

When cytosolic Ca2+ binds to troponin making it active and capable of interacting with myosin

Question 21

The fall in cytoplasmic Ca2+ makes troponin inactive and the actin-myosin fibres relax. How does this happen?

Answer 21

After repolarisation, active transport of Ca2+ returns it to the sarcoplasmic reticulum and across the cell membrane to the extracellular fluid