generating an action potential

Steps of generating an action potential

1: resting state

2: depolarization

3: repolarization

4: hyperpolarization

1: resting state

all gated Na+ and K+ channels are closed

membrane is polarized

during the resting state

only leakage channels for Na+ and K+ are open to maintain resting membrane potential

membrane polarity is -70 mV

2: depolarization

Na+ channels open → Na+ enters the cell

Na+ influx causes

more depolarization, which opens more Na+ channels → ICF becomes less negative

at threshold (-55 to -50 mV), positive feedback causes

opening of all Na+ channels

what does the opening of all Na+ channels result in

large action potential spike

membrane polarity jumps to +30 mV

3: repolarization

Na+ channels inactivate, K+ channels open → membrane returns to resting membrane potential

During the repolarization stage

membrane permeability to Na+ drops to resting state; AP spike stops rising

voltage-gated K+ channels open: K+ leaves cell

Hyperpolarization

some K+ channels remain open, and Na+ channels reset

In hyperpolarization, some K+ channels remain open, this allows for

excessive K+ efflux

the inside of the membrane becomes more negative than in resting state

BIG PICTURE

1: resting state - no ions move through voltage-gated channels

2: depolarization - caused by Na+ flowing INTO the cell

3: repolarization - caused by K+ flowing OUT OF the cell

4: hyperpolarization - caused by K+ continuing to leave the cell

repolarization resets

electrical conditions, not ionic conditions

after repolarization

Na+/K+ pumps in axon restore ionic conditions

pump out 3 Na+/pump in 2 K+

Absolute refractory period

time period during which another stimulus will have no effect

During absolute refractory period, voltage-gated sodium channels are

inactivated

what effect does inactivating the voltage-gated sodium channels have

prevents AP from moving backwards

relative refractory period

another stimulus could trigger another AP, but it would have to be stronger than normal

During the relative refractory period, what is the polarity of the cell

the cell is partially hyperpolarized (farther from threshold)

propagation

allows action potential to be transmitted from origin down entire axon length toward terminals

depolarization is one area causes

depolarization in neighboring areas

APs start at the initial segment (by axon hillock)

AP “moves” down the length of the axon

one-way forward movement because of Na+ channel inactivation

In action potential conduction velocity, larger fibers =

faster impulse

continuous conduction

slow; occurs in unmyelinated axons

saltatory conduction

30x faster; occurs only in myelinated axons

What does myelin do

insulates and prevents leakage of charge

where are voltage-gated Na+ channels located

myelin sheath gaps

APs are generated only at

gaps

electrical signal appears to

jump rapidly between gaps