Action Potential

-70mV

-70mV is the Resting Membrane Potential of a resting neuron. The neuron charge (Membrane Potential) is held at -70mV unitl a stimulus occurs. -70mV means the inside of the neuron is 70mV more NEGATIVE than the charge outside the neuron.

Why is the Depolarization graph going up (more positive)?

Because when Membrane Potential reaches -55mV, voltage-gated Sodium channels swing open and positive Sodium ions rush into neuron making inside quickly more positive.

At the Resting Membrane Potential, is the inside of the neuron negative or positive compared to the outside?

Negative (-70mV) - see above diagrams

Resting Membrane Potential (RMP)

The charge inside a neuron compared to the charge outside the neuron when the neuron is not transmitting an impulse. Resting membrane potential is -70mV. This means the inside of the neuron is 70mV more NEGATIVE than the outside of the neuron.

What causes Hyperpolarization?

Voltage-gated Potassium channels that opened during Repolarization stay open just long enough to drop Membrane Potential below (more negative) than -70mV as K+ ions continue to leave neuron.

What happens when the Membrane Potential depolarizes and reaches +30mV?

Voltage-gated Sodium channels close, and voltage-gated Potassium channels open.

Sodium and Potassium concentrations in a Resting Neuron

When a neuron is not transmitting an Action Potential (impulse), there is more Sodium outside the neuron than inside and more Potassium inside the neuron than outside.

A resting neuron

A neuron that is not, at that moment, transmitting an Action Potential (impulse). (A neuron that is just sitting there waiting for a stimulus to start the next Action Potential.)

What happens when membrane potential becomes more positive and reaches THRESHOLD of -55mV?

At Threshold, voltage-gated Sodium channels open, Na enters neuron, and depolarization begins.

Why does Potassium rush OUT of the neuron when voltage-gated Potassium channels open?

Because there are more Potassium ions inside the neuron and K+ ions quickly DIFFUSE out of the neuron when the K+ channels open. (Diffusion = ions move from High to Low concentration).

Why does Sodium rush INTO neuron when voltage-gated Sodium channels open?

Because there are more Sodium ions outside the neuron and Na+ ions quickly DIFFUSE into the neuron when the Na+ channels open. (Diffusion = ions move from High to Low concentration).

Why is the Repolarization graph going down (more negative)?

Because when Membrane Potential reaches +30mV during Action Potential, voltage-gated Sodium channels close and Voltage-gated Potassium channels swing open letting + Potassium rush out of neuron leaving the neuron quickly more negative inside.

At Resting Membrane Potential of -70mV, what voltage-gated channels are closed/open?

Both Voltage-gated Sodium and Potassium channels are closed when Resting Membrane Potential is more negative (lower) than -55mV.

Voltage-gated channels

Different cell membrane channels (or gates) open or close at different charges to let ions into or out of cell. When a Membrane charge moves from -70mV to -55mV, this OPENS voltage-gated Sodium channels. Membrane charge of +30mV OPENS voltage-gated Potassium channels but CLOSES voltage-gated Sodium channels.

During Depolarization, what voltage-gated channels are open/closed?

During Depolarization, voltage-gated Na channels open (Na rushes into neuron) and voltage-gated K channels are closed.

When is the Membrane Potential more negative - during Repolarization or Hyperpolarization?

During Hyperpolarization - see diagrams above

Threshold

Membrane Potential reaches Threshold when the charge rises from -70mV to -55mV. Voltage-gated Sodium channels swing open at this voltage (charge) of -55mV.

During Repolarization, is the inside of the neuron becoming more negative or positive?

More negative (see downward direction of graph in diagrams.)

During Depolarization, is the charge inside the neuron becoming more negative or more positive?

More positive (upswing on diagrams above)

What is Na+ and K+?

Na = Sodium, K = Potassium, + means they are positively charged ions

What happens if a stimulus slightly raises the membrane potential but it does not reach Threshold at -55mV?

No Action Potential occurs because the voltage-gated Sodium channels don't open until Threshold of -55mV is reached.

Are some Action Potentials stronger than others?

No, all Action Potentials are of the same strength. A stronger stimulus produces more frequent Action Potentials but not stronger Action Potentials.

Are voltage-gate Sodium channels and voltage-gated Potassium channels open at the same time during an Action Potential?

No, during an Action Potential, voltage-gated Na channels open first and then close as voltage-gated K channels are opening. See diagram.

All-or-None Response

Once an Action Potential begins, it will travel all the way down the axon without stopping.

Sodium/Potassium Pump (Na+/K+ Pump)

Pumps located in neuron cell membrane that pump 3 Na+ ions OUT of neuron and pumps 2 K+ ions INTO neuron. Results in inside of neuron becoming more negative than outside because pump moves more + ions out of neuron than it brings in.

What returns Resting Membrane Potential to -70mV after an Action Potential?

Sodium/Potassium pump - see description of this pump's action above

What does "more positive" mean?

This means a negative charge is moving closer to zero or a positive charge is moving farther from zero. A charge that moves from -70 to -55 has become more positive even though it is still negative.

What does "more negative" mean?

This means a positive charge is moving closer to zero or a negative charge is moving farther from zero. A charge that moves from +30 to +10 has become more negative even though it is still positive.

Why does Hyperpolarization need to occur?

To make the neuron even more negative and slightly delay the next Action Potential. Hyperpolarziation moves Membrane Potential farther from Threshold very briefly.

Refractory

When an axon is refractory, it is unable to respond to a stimulus and an Action Potential cannot occur.

Hyperpolarization

When the charge inside a neuron becomes more negative than -70mV. See diagram. (Note that threshold should be marked at -55mV, not -65mV). No Action Potential can occur during Hyperpolarization (refractory).

Repolarization

When the charge inside a neuron is becoming more negative than +30mV. See diagram. (Note that Threshold should be marked at -55mV, not -65mV, on diagram).

Depolarization

When the charge inside a neuron is becoming more positive than -70mV. See diagram. (Note that Threshold should be marked at -55mV, not -65mV, on diagram).

Axon Hillock

Where cell body becomes the axon. Very high number of voltage-gated Sodium channels located here that fly open when Threshold -55mV is reached.