Pharmacology Part 6

Question 1

Define depolarization?

Answer 1

the membrane potential becomes less negative (or even positive)

Question 2

Define hyperpolarization?

Answer 2

the membrane potential becomes more negative

Question 3

What causes change in membrane potential?

Answer 3

Ionic movement across the membrane

Question 4

What does the direction of change in potential depend upon?

Answer 4

> The direction of the movement of the ion [into the cell (influx), or out of the cell (efflux)]

> The charge carried by the ion (positive, or negative)

Question 5

What does movement of ions require?

Answer 5

requires ion channels (or transporters)

Question 6

What is passive movement of an ion through a channel driven by?

Answer 6

Electrochemical gradient

Question 7

What happens in response to the opening of cell membrane Na selective channels?

Answer 7

Na flows inwardly;
-concentration gradient is inward

• electrochemical gradient is inward
• the membrane potential is about -80mV which id nrgative to the equilibrium potential for Na which is +60mV
• driving force is Vm-Ena

Question 8

What happens in response to opening of cell membrane K-selective channels?

Answer 8

K+ flows outwardly :

• concentration gradient is outward and has an energy which exceeds that of the electrical gradient, which is inward
• Quantitatively, the membrane potential (Vm, approximately -80 mV for many neurones) is positive to the equilibrium potential for K+ (i.e. EK, about -100 mV) given by the Nernst equation.
• The driving force for K+ efflux is simply (Vm - EK). When positive (in this case +20 mV, note the double negative) outward movement of K+ occurs

Question 9

Describe the action potential in neurones?

Answer 9

brief electrical signals in which the polarity of the nerve cell membrane is momentarily (about 2 msec) reversed

Question 10

How to action potentials propagate along neurones?

Answer 10

propagate along nerve cell axons with constant magnitude and velocity (for a given axon) allowing signalling over long distances

Question 11

When are neurone action potentials generated?

Answer 11

when threshold is reached. They are ‘all or none’

Question 12

What is the ionic basis for action potential?

Answer 12

Inflow of sodium causes hyperpolarization and outflow of K depolarisation

Question 13

Are Na+ and K+ channels selective?

Answer 13

Both are highly selective

Question 14

How are Na+ and K+ channels activated?

Answer 14

activated by membrane depolarization – Na+ channels rapidly; K+ channels with a slight delay

Question 15

Describe the activation of Na+ channels?

Answer 15

self-reinforcing – opening of a few channels causes further channels to open, causes increased depolarization

This is positive feedback

Question 16

Describe the activation of K+ channels

Answer 16

self-limiting –
outward movement of K+ causes repolarization which turns off the stimulus for opening

This is negative feedback

Question 17

What happens to Na channels during depolarisation

Answer 17

initially open rapidly in response to depolarization, they quickly enter a non-conducting, inactivated state during maintained depolarization

Question 18

What is the importance of depolarisation?

Answer 18

required for the channel to enter the closed state in readiness for opening and the generation of a further action potential

Question 19

What does inactivation of Na channels contribute to?

Answer 19

repolarizing phase of the action potential and is responsible for the absolute refractory period

Question 20

What is the absolute refractory period?

Answer 20

no stimulus, however strong, can elicit a second action potential (all Na+ channels inactivated)

Question 21

What is the relative refractory period?

Answer 21

a stronger than normal stimulus may elicit a second action potential (mixed population of inactivated and closed channels, plus the membrane is hyperpolarized)

Question 22

What is the issue with conducting impulses in axons?

Answer 22

nerve cell membrane is ‘leaky’ (i.e. not a perfect insulator): passive signals do not spread far from their site of origin due to current loss across the membrane

Question 23

Describe passive conduction and action potential velocity?

Answer 23

factor in the propagation of the action potential (AP)

The longer the length constant (λ) the greater the local current spread

Greater local current spread increases AP conduction velocity

Question 24

What are the strategies to increase passive current spread?

Answer 24

> Decrease ri (axial resistance of the axoplasm) by increasing axon diameter

> Increase rm (membrane resistance ) by adding insulating material (myelin) provided by Schwann cells in the PNS and oligodendrocytes in the CNS)

Question 25

What kind of axons have the fasted conduction?

Answer 25

Myelinated axons is much faster than nonmyelinated axons of same diameter

Question 26

What is saltatory conduction in myelinated axons?

Answer 26

Action potential jumps from one node of ranvier to the next via voltage activate Na channels