Lecture 7: Generation of Action Potentials

Question 1

3 phases of AP

Answer 1

1. depolarization
2. repolarization
3. after-hyperpolarization

Question 2

absolute refractory period

Answer 2

• period where cell is incapable of generating a new AP

- caused by inactivation of VG Na+ channels

Question 3

Relative refractory period

Answer 3

• more difficult to generate a new AP
• occurs at after-hyperpolarization until resting membrane potential is reestablished
• only bigger stimulus/GP will generate a new AP
• K+ efflux and inactivation of Na+ channels causes RRP

Question 4

How is AP generated?

Answer 4

Voltage gated channels are controlled by membrane potential, and changes in MP will open or close them
-2 channels in the axon hillock : Na+ and K+

Question 5

Depolarization

Answer 5

Occurs when Vm reaches threshold potential and Na+ channels open first, Na+ entering the cell causes depolarization

Question 6

Repolarization

Answer 6

K+ channels open more slowly than Na+ channels, so when Na+ close K+ remain open
-K+ leaving causes repolarization

Question 7

After-hyperpolarization

Answer 7

• occurs when voltage-gated K+ channels close

- represents time needed for extra K+ near outside membrane to disperse

Question 8

Ion movement during AP

Answer 8

• only a few ions move in or out of the cell during AP
• approx. 1 of every 100,000 K+ ions move to change Em from +30 to -70
• any noticeable change in movement is restored by Na+/K+ ATPase

Question 9

What re-establishes resting Vm?

Answer 9

by the Na+/K+ ATPase, but not during AP
-the duration of the after-hyperpolarization represents the time required for extra K+ near outside membrane to diffuse away

Question 10

VG Na+ channel has two gates

Answer 10

has activation gate that is voltage dependent, and an inactivation gate that is time dependent
-act. gate opens due to depolar and inact closes after a certain period of time, then repolar. closes the act. gate and time opens the inact., returning the channel to rest position

Question 11

Why don’t we see AP in dendrites and cell bodies?

Answer 11

density of VG Na+ channels must be high for positive feedback mechanism to function, which they are at axon, but not elsewhere

Question 12

What does “excitability” mean?

Answer 12

-ability to generate a new AP when stimulated
-the easier it is to generate an AP, the more excitable the membrane is
The more excitable, the lower the threshold stimulus needed to generate an AP

Question 13

definition of threshold stimulus and threshold MP***

Answer 13

• the min stimulus that needs to be applied on the neuron

- min MP needed to be reached to open the critical number of Na+VG channels to start the positive feedback loop

Question 14

Signal Coding def

Answer 14

how info about duration and strength of stimulus is passed on to postsynaptic axon

Question 15

signal coding and frequency of AP

Answer 15

AP frequency codes the strength of stimuli, therefore AP frequency also increases with stronger stimuli (how frequent they are=how strong stim is)
-duration of a series of AP codes for duration of the stimulus (how many there are=duration)

Question 16

Suprathreshold stimulus…

Answer 16

can trigger an AP during the relative refractory period

Question 17

Why is the movement of relatively few ions across the membrane sufficient to change the membrane potential?

Answer 17

the ions cause a LOCAL change in potential that quickly conducts through the cytosol