7. Action Potentials

Question 1

How does the presynaptic cell communicate with the postsynaptic cell?

Answer 1

Movement of ions across the membrane via transmembrane proteins spanning the bilayer

Question 2

What is required for a movement of ions across a membrane?

Answer 2

Open channels

Electrochemical driving force (concentration or voltage gradient)

Question 3

What is the concentration gradient for K+ in a neuron?

Answer 3

Higher INSIDE than outside

Question 4

What attracts K+ ions back into the cell?

Answer 4

When the inside of the cell becomes negative compared to the outside due to leaky K+ channels. More positive charges outside mean inside is more negative than outside

Question 5

Describe equilibrium potential. What is the equilibrium potential of K+?

Answer 5

Equilibrium potential of an ion occurs when its concentration gradient is exactly matched by the membrane potential. Ek of K+ is -94 mV

Question 6

Why is Ek not equal to the resting membrane potential?

Answer 6

Real membranes have multiple channels so are permeable to K+, Na+, and Cl-. Na+ moves into cell due to conc. gradient -> inside of cell becomes less negative so Em increases

Question 7

Why do K+ ions contribute more to the Em than Na+?

Answer 7

K+ are more permeant than Na+. K+ has channels open in resting state (leak channels) while a very small number of Na+ get through

Question 8

Why is the Na/K pump necessary?

Answer 8

Conc. gradients of ions would disappear if not actively moved back down their gradients. 2 K+ in for 3 Na+ out

Question 9

How much of the resting potential is due to the Na/K pump?

Answer 9

6%

Question 10

What are the phases of the action potential?

Answer 10

Rising Phase
Overshoot
Falling Phase
Undershoot

Question 11

What is the “threshold” for an AP?

Answer 11

The level of depolarisation required to cause an increase in Na+ permeability

Question 12

What occurs during depolarisation?

Answer 12

Na+ channels open, large driving forces (-ve potential inside and concentration gradient) cause Na+ entry

Question 13

Why does an overshoot occur?

Answer 13

System is driven towards the equilibrium potential for Na+, which is +ve.

Question 14

What occurs during repolarisation?

Answer 14

High permeability to K+ allows K+ exit (+ve potential inside and concentration gradient)

Question 15

Why does the membrane hyperpolarise?

Answer 15

K+ channels still open, so K+ enters cell and potential is driven to Ek (more -ve)

Question 16

What is the difference between Na and K channels?

Answer 16

Na channels can be inactivated due to an inactivation particle on the intracellular surface. Leads to an absolute refractory and relative refractory period of ~4 ms. K channels do not have inactivation particle and open with a 1 ms delay.

Question 17

What is the use of patch clamp recording?

Answer 17

Allows the precise measurement of the voltage changes across a cell membrane. Can see what times the channels are open and closed.

Question 18

How does axon diameter affect AP conduction velocity?

Answer 18

Increased diameter -> increased conduction velocity. Due to decreased internal resistance

Question 19

Why does myelination increase speed of conduction?

Answer 19

Insulation increases the rate of spread of electro tonic current. Also experience saltatory conduction