Action Potential 1

Na+ Channel

• Inward current

• Membrane depolarization results in the channel opening

• Voltage-regulated and time-dependent gates

• Fast Response

K+ Channel current, response time, and why it opens/closes

• Outward current (K+ leaves the cell)

• Both opening and closing depends on membrane potential

• Slow Response

Ball and chain mechanism

Na+ channel inactivation

• when closed, the balls float freely

• When open, one of the balls will enter and block the pore

Conductance of Na+ and K+ Channels

When an axonal membrane is depolarized more, more voltage gated Na+ and K+ channels open

• changes for the two channels are very similar

Voltage gated Na+ Channels opening process

Open immediately following depolarization, but then inactivate after less than a millisecond

Voltage gated K+ channels opening process

open after a delay, and do not inactivate

Voltage-gated Na+ Channel states

1. Deactivated: closed but capable of opening

2. Activated: open

3. Inactivated: closed and not capable of opening, inactivation is removed by repolarizeation around -70mV

Voltage-gated K+ Channel states

1. Deactivated

2. Activated

Stage 1 of action potential

Cell at resting membrane potential, both voltage-gates are closed (Na+ time dependent gate is open)

Stage 2 of Action Potential

Membrane is brought to threshold voltage by depolarizing stimulus, Na+ voltage-dependent gate opens

Stage 3 of Action Potential

Na+ flows through open gates, cell rapidly depolarizes the membrane, causing more Na+ channels to open

Stage 4

Inactivation of time-dependent Na+ gate and delayed opening of voltage-gated K+ channel halts membrane depolarization

Stage 5

Open voltage-gated K+ channesl continue to repolarize the membrane

Stage 6

Slowly closing voltage-gated K+ channels hyperpolarize membrane towards Ek; Na+ channels return to closed resting stage

Stage 7

Closure of voltage-gated K+ channels returns the membrane potential to its resting value

The Squid Giant Axon

Action potentials were recorded in a squid giant axon bathed in normal sea water and then in solutions containing lower concetnrations of Na+

What will lowering external Na+ concentrations do to the driving force on sodium ions?

Lower driving force for Na+ to go into cell

Positive feedback in action potentials

Na+ entry causes depolarization, which opens more voltage gated Na+ channels, causing even more depolarization

Negative feedback in action potentials

voltage gated K+ channels bring the action potential to an end and induce their own closing

Rate coding

neural communication where the intensity or strength of a stimulus is represented by the rate of action potentials a neuron fires

Population Coding

Concept where the brain represents sensory, motor, or cognitive information through the collective, simultaneous activity of a group of neurons

Distributed coding

group of neurons fire to encode a sense, rather than relying on just a single neuron