Unit 3 - Action Potential

Electronic Spread

Passive localization of conduction of electrical charge across a cell’s membrane

Properties of AP

1. Threshold for initiation

2. All or none

3. Self regenerative

4. Refractory period

Electrotonic Decay

Decrease in amplitude as it moves through a neuron

Current strength depends on

Conductance and Driving Force

As cell depolarizes, what happens to K driving force and conductance?

Driving gets further away, and therefore conductance wants to increase

V_m is closer to which ion?

K because permeability is less for Na than K

Na and Ca Voltage Channel

Has 1 large pore forming subunits with 4 domains

K Voltage Channel

Made of 4 pore forming subunits

S4

Voltage sensor that’s positively charged

S4 Opening

1. At rest, positive channel attracted to V_m

2. Depolarization repells positive charge in alpha helices, changing structure to pull up

3. Channel returns with V_m below threshold

m-gate

Na activation gate

h-gate

Na inactivation gate

n-gate

K activation gate

Why do the gates open at different times?

K and Na inactivation gate are slower, they all begin to open at the same time

Giant Squid Axon

Discovered ionic concentrations

Voltage Clamp Experiments

Discovered threshold potential using electrodes

Voltage Clamp

Measures ionic current and holds it at a potential with current injector

Feedback Loop

Compares potentials with comman potential

Capacitive Current

“current” measured from positive charge outside membrane being repelled from depolarization, making it appear a positive charge has gone through the membrane

Gating Current

“current” measured from positive charge of Na gate moving up and opening

Na doesn’t contribute to the

Delayed outward current

AP Ionic Movement

1. Capacitative and gating current at intial depolarization

2. Inward current max when most Na channels open

3. Inactivation by h-gate

4. Capacitative and gating current repolarize

Tetradotoxin

Binds to outer Na voltage to block ion flow (no effect inside)

Saxitoxin

Equivalent to TTX

Tetraethylammonium (TEA)

Blocks voltage K channels

Do all channels open at the same time?

No, but there are areas where it’s more likely to

Local Anaesthetics

Decreases AP firing frequency

Threshold

Depolarization that opens enough Na channels to counter efflux of K through leak channels

A stimulud just at threshold will

Abort or make a local response

Temporal

Multiple signals from same neurons

Spatial

Many signals from different neurons

Increase in strength of stimulus causes

Increase in frequency of AP (200 - 300Hz)

Absolute Refractory Period

1-2 ms of no activity from inactivate Na channels

Relative Refractory Period

3-4 ms that requires strong stimulus and amplitude of AP may be smaller from inactivation and increased permeability in K

Conduction Velocity

time needed for electrical information to travel the length of neuron

Things that Impact Conduction Velocity

• Axon diameter (bigger = less resistance)

• Myelination (preserves current)

Skeletal AP

Briefer than contraction, acting a trigger using Na

Cardiac AP

Longer, sustained signals thoroughout contraction from Ca

Cardiac AP Process

1. Depolarization phase: Na and Ca flows inwards

2. Plateau: Ca continues in but balances out from K

3. Repolarization: Ca channels close but K out increases

A cardiac AP influences

Duration and strength of contraction