3.2: Action Potential

Action Potential (AP)

a brief, pulse-like change in the membrane potential.
- moves along the surace of excitable cells (nerve and muscle)
- fastest way to convey a signal in the body.

Dendrites

collect information from neurons upstream

Soma

integrates signals to determine whether the motor neuron will initiate an action potential

Axon

conveys the action potential to nerve terminals

Myelin

insulates the axon and greatly increases the conduction velocity of the action potential

A negative/inward current ________ the membrane

depolarizes

a positive/outward current _______ the membrane

hyper polarizes

The electrical behavior is determined by the ________ of the axon, which depend on:
They determine the___

cable properties;
- membrane resistance
- membrane capacitance
- cytosol resistance

These properties determine the time constant and the length constant.

Anatomy of an action potential: Latency

Reaching above the threshold requires the movement of the sufficient charge

Anatomy of an action potential: Threshold

Voltage at which an AP results 50% of the time.

Anatomy of an action potential: Refractory Periods

Absolute and Relative (greater stimulus)

Voltage-gated Na+ channels have 4 key properties:

1. Selectivity filter for Na+ (and H+)
2. Voltage-dependent activation gate
3. Time/voltage-dependent inactivation gate
4. Blocked with selective agents
- tetrodotoxin (from puffer fish)
- saxitoxin (from Alaskan butter clam)

Voltage-gates K+ channel has .....

1. an activation gate
2. a selectivity filter
3. blocked by tetraethylammonium (TEA)

Voltage-gated Na+ and K+ channel overview

1. Na+ and K+ channels are in resting state (Em ~ 70 mV)

2. Depolarizing signal hits. Na+ activation gate opens, Na+ begins to enter.

3. Em heads towards ENa. The K+ activation gate opens, and K+ begins to move out of the cell.

4. Depolarization to -50mV, Na+ inactivation gate closes, K+ continues to exit.

5. Hyperpolarization (-80mV) as K+ conductance dominates and Em approaches Ek. Na+ activation gate closes.

6. K+ activation gate closes, Na+ inactivation gate opens -> returns to normal.

Absolute Refractory Period (ARP)

Inactivation gates of Na+ channels must be reset before another action potential can be fired.

Relative Refractory Period (RRP)

A greater stimulus is required to reach the threshold because of membrane hyper polarization (opening of K+ channels)

Ensemble current

The current a population of channels would give rise to.

Shape of the Action Potential: First Stage

A stimulus from a sensory cell or another neuron causes the target cell to depolarize toward the threshold potential.

Shape of the Action Potential: Threshold of Excitation

If the threshold of excitation is reached, Na+ channels open and the membrane depolarizes

Shape of the Action Potential: Peak Action Potential

At the peak action potential, K+ channels open and the K+ begins to leave the cell. At the same time, Na+ channels are inactivated and close.

Shape of the Action Potential: Hyperpolarization

The membrane becomes hyper polarized as K+ ions continue to leave the cell. The hyper polarized membrane is in a refractory period and cannot easily generate another AP.

Shape of the Action Potential: Final Stage

The K+ channels close and the NA+/K+ pumps restore the resting potential

In the absolute refractory period, it is ______ to initiate a second action potential. In the relative refractory period, it is more _______ to initiate a second action potential.

impossible; likely

Voltage-gated Na+ and K+ channels are involved in the action potential. Describe the order of opening and how long the channels are open, in relative terms.

Na+ channels open first and more transiently than K+ channels.

When Na+ channels open for a neuron at rest, Na+ flows _______ the cell causing the membrane potential to become _______

into ; positive

The __________ potential is the electric potential that triggers an action potential 50% of the time.

threshold

The time required to charge the membrane and bring up its potential leads to the:

stimulus-response latency