Lecture 14

Question 1

How is the distribution of ions maintained separated across plasma membrane?

Answer 1

By ionic pumps and exchangers and so consumes energy (ATP).

Question 2

What does the difference in ion concentration across the plasma membrane generates?

Answer 2

This ionic imbalance cause an electric potential difference between the cytoplasm and the external medium, cells use these electrochemical gradients by allowing ion flow across the plasma membrane under well-controlled conditions.

Question 3

What are ion channels?

Answer 3

Components of the cell membrane which mediate ion transfer across the membrane along an electrochemical gradient.

Question 4

Describe diffusion for dissolved ions:

Answer 4

Dissolved ions distribute evenly over time.

Question 5

What conditions are required for ion to flow down concentration gradient?

Answer 5

• Channels are permeable to specific ions

- Concentration gradient is present across membrane

Question 6

How do you calculate electrica conductance (g)?

Answer 6

R (Resistance)= 1/g

Question 7

State Ohm’s Law:

Answer 7

I=gV

Question 8

How is the electrochemical gradient created?

Answer 8

1) Active transport (Na+ pump)

2) Selective membrane permeability to certain ions and molecules

Question 9

Describe the movement of ions at rest:

Answer 9

At rest sodium will tend to leak in and potassium to leak out. The balance between these determines the Resting Membrane Potential (RMP). The RMP is driven mostly by potassium efflux giving a negative inside value of potential.

Question 10

What is the potassium equilibrium potential?

Answer 10

The membrane potential difference at which movement down concentration gradient equals movement down electrical gradient; electrical gradient equal to and opposite concentration gradient.

Question 11

What prevents RMP from actually reaching Nernstian K+ potential?

Answer 11

There is always some leakage pathways for Na+ and sometimes for Cl- (permeability of K>Na+>Cl- at rest). The finite permeability of the membrane to Na+ (and Cl-) prevents the RMP from actually reaching the Nernstian K+ potential.

Question 12

What defines the extent to which each ion gradient influence membrane potential?

Answer 12

The extent is defined by permeability of the membrane to that ion. i.e Even with a large concentration gradient,, an ion may exert little influence if the Pi value is small.

Question 13

What is at the basis of biolectrice phenomena?

Answer 13

The rapid changes in the ratios of permeabilities for the different ions. To large changes in Vm corresponds minuscles changes in ionic concetrations.

Question 14

What cause the depolarization associated with membrane action potential?

Answer 14

A large increase in PNa (via sodium channels) is the basis for the transient depolarization that triggers action potential.

Question 15

What does Nernst equation displays?

Answer 15

The theoretical balance between electrical and concetration gradients.

Question 16

What does Goldman equation displays?

Answer 16

It refelcts the real world situation of finite variabel ionic permeabilites.

Question 17

Define action potential:

Answer 17

Rapidly propagated ‘all or none’ electrical message that passes along axons of the nervous system. It travels at a constant velocity and amplitude. It is a membrane potential change resulting from the flow of ions through ionic channels present in the PM.

Question 18

Describe a subthreshold potential:

Answer 18

It starts above threshold at its initiation point but decreases in strength as it travels through the cell body. At the trigger zone it is below threshold and doesn’t trigger an AP.

Question 19

Describe a potetial above threshold:

Answer 19

A stronger stimulus creates a graded potential that is still above threshold by the time it reaches the trigger zone, therefore it triggers an AP.

Question 20

How are the relative permeabilities of membrane at rest?

Answer 20

Resting membrane potential is close to Ek because it is mostly permeable to K+. Membrane potential is sensitive to K+ and increased extracellular K+ depolarizes membrane potential.

Question 21

What are the two factors that regulate external potassium concentration?

Answer 21

• Blood brain barrier

- Potassium spatial buffering

Question 22

What is selectivity of an ion channel:

Answer 22

Ion channels are permable to some ions but not to others.

Question 23

What are the two characteristics that allow the permeation of ion channels to be brief and controlled?

Answer 23

Gating

Inactivation

Question 24

What is gating?

Answer 24

The procees by which a channel opens and closes, it is reversible.

Question 25

What are the three properties of gating:

Answer 25

1) It requires minimal energetic interaction between transported ion and channel
2) The ion flux is limited by the status of the channel (open or closed)
3) It is regulated by protein conformational and is independent of permeability control.

Question 26

What is modulation of ion channels?

Answer 26

When the gating process is altered by a second factor that alters the rate of channel gating or the membrane potential at which gating occurs.

Question 27

What are the two possible routes that a conducting channels may take to a non-conducting state again?

Answer 27

• Deactivation: it returns to a closed state

- Inactivation: driven by depolarization to a different non-conducting state.

Question 28

Describe what structural characteristics determine ion selectivity in Kv:

Answer 28

Carbonyl backbone groups of the TVGYG motif in the P loop.

Question 29

Describe what structural characteristics determine ion selectivity in NaV:

Answer 29

DEKA side chains.

Question 30

Describe the peculiarity of Sodium channel activation:

Answer 30

Sodium channel activation is associated with the movement of charge within the structure of the channel itself and this occurs before ion movement (know as gating current).

Question 31

What regulates fast inactivation of Na+ channels?

Answer 31

It is mediated by an intracellular ‘gate’ that binds to the intracellular mouth of the pore (a tethered pore blocker). The inactivation gate is formed by a hydrophobic triad of amino acids (IFM)..