Lecture 2/3- membrane potentials & action potentials

Question 1

How is the direction of current flow described?

Answer 1

the direction of net movement of positive ions

Question 2

What is the resting membrane potential?

Answer 2

• The difference in voltage between the inside and outside of the cell as measured across the membrane
• -70mv

Question 3

At rest, what ion is the membrane impermeable to?

Answer 3

Na+ ions

Question 4

at rest, how are different ions and non-diffusible molecules (eg proteins) distributed?

Answer 4

• Extra cellular - high Na+ and Cl-
• intracellular - high K+ and proteins (with negatively charged side chains)

Question 5

How is the RMP established?

Answer 5

• Na+K+ ATPase pump - sets up concentration gradients, pumps 3 Na+ out of cell and 2 K+ into cell as it hydrolyses ATP - just with this pump we already have a more (+) outside and (-) inside
• K+ leak channels - At RMP there is an increase of permeability to K+ than Na+ - more leak channels, Leak channels pump K+ out of cell which increases (-) charge in inside

Question 6

What is a graded potential?

Answer 6

• changes in membrane potential
• no threshold or refractory period
• involves ligand/ mechanically gated ion channels
• signaling over short distances

Question 7

Give examples of a graded potential?

Answer 7

• receptor potential (sensory receptors eg hair cells )
• Pacemaker potentials (eg SA node in right atrium)
• Synaptic potential

Question 8

What is the Nernst equation used to calculate?

Answer 8

• it is used to calculate the equilibrium potential for 1 ion

Question 9

What is the Goldmann equation used to calculate?

Answer 9

• it calculates the resting membrane potential, the permeabilities of the ions and the concentrations of the ions
• eg for Na+, K+ and Cl-

Question 10

Why is the Goldmann equation and not the Nernst equation used to calculate RMP?

Answer 10

• this equation takes into account that not all ions are equally permeable to the membrane
• at rest, membrane is most permeable to K+ but there is also a small significant permeability to Cl- and K+

Question 11

What is the equilibrium potential?

Answer 11

• the electrical potential that can balance the concentration difference across the membrane
• electrochemical equilibrium - conc of ion is evenly distributed

Question 12

What is depolarisation?

Answer 12

• potential becomes less negative (more Na+ in cell)

Question 13

What is repolarisation?

Answer 13

• When MP has been depolarised and it goes back to resting value - more (-) again

Question 14

What is hyperpolarisation?

Answer 14

• When the potential is more (-) than the resting value

Question 15

What is an action potential?

Answer 15

• large difference in membrane potential
• large depolarisation
• involves voltage gated ion channels eg Na+, K+ Ca2+ etc
• signaling over long distances
• only occurs in excitable cells eg neurons and muscles

Question 16

Describe the steps of the initiation of an AP

Answer 16

1. RMP close to K+ equilibrium (leak channels)
2. AP begins with depolarising stimulus (eg neurotransmitter binding to receptor)
3. Depol causes voltage gated Na+ channels to open and causes influx of Na+ into cell
4. further depol leads to threshold
5. this opens all VG Na+ channels- rapid depol
6. Rapid Depol overshoots @ peak value ( v +)
7. K+ channels are slow to close - this causes influx of K+ out of cell - repolarisation
8. Closing of K+ channels returns the MP to RMP

Question 17

Compare voltage gated Na+ and K+ channels.

Answer 17

• Na+ -rapid opening and inactivation, responds faster to changes in MP
• K+- opens and closes in response to changes in MP very slowly (delayed response)

Question 18

What is the relative refractory period?

Answer 18

• interval where 2nd AP can be generated only if stimulus is strong enough (after hyperpol)