Channels: Action Potentials review

Sodium channel inactivation?

(Inactivation of Na+ channels and activation of K+ channels) It coincides with the opening of voltage-gated potassium channels.

What is the amount of millivolts in a typical neuron?

-70mV.

What is the resting potential of a typical neuron?

-90mV.

Changes in the transmembrane potential at one location during the generation of an action potential?

1.A graded depolarization brings an area of excitable membrane to threshold. (-60mV). 2.Voltage-gated sodium channels open and sodium ions move into the cell. The transmembrane potential rises to +30mV. 3.Na+ channels close, voltage-gated potassium channels open, and K+ ions move out of the cell. Repolarization begins. 4.K+ channels close, and both sodium and potassium channels, return to their normal states.

Steps of an action potential?

1.Depolarization to threshold. 2.Activation of sodium channels and rapid deploarization. 3.Inactivation of sodium channels and activation of potassium channels. 4.Potassium channels close. Begins and ends with resting potential.

Hyperpolarization?

A shift in the transmembrane potential past resting levels.

What does graded potentials depend on?

Chemically gated channels.

When does the degree of depolarization decrease?

Decrease with the distance away from the stimulation site, primarily because the ions are entering only in one location but spreading in all directions.

Relative refractory period?

During this period, the membrane can respond only to a larger-than-normal stimulus.

Absolute refractory period?

During this period, the membrane cannot respond to further stimulation.

What happens as the plasma membrane depolarizes?

Extracellular sodium ions move toward the open channels, replacing ions that have entered the cell.

How many channels are in a sodium channels?

Have two gates that function independently:an activation gate that opens on stimulation, letting sodium ions into the cell, and an inactivation gate that closes to stop the entry of sodium ions.

What determines the number of open sodium channels?

Impact on the transmembrane potential is proportional to the size of the stimulus, because the intensity of the stimulus determines the number of open sodium channels.

Where are mechanically gated channels important?

Important in sensory receptors that respond to touch, pressure, or vibration.

What happens when gated channels open?

Increases the rate of ion movement across the plasma membrane and this changes the transmembrane potential.

What does resting potential depend on?

Leak channels.

What can graded change in membrane potential involve?

May involve depolarization or hyperpolarization.

Where are chemically gated channels most abundant?

Most abundant on the dendrites and cell body of a neuron, the areas where most synaptic communication occurs.

What happens when the gated channels close?

Most gated channels are closed at the resting potential.

What is the consequence of the electrochemical gradient and the membrane permeability of sodium ions?

Na+ has only a small effect on the normal resting potential, making it just slightly less negative than it would be otherwise.

When do voltage gated channels open and close?

Open and close in response to changes in the transmembrane potential.

Mechanically gated channels?

Open in response to physical distortion of the membrane surface.

Chemically Gated Channels?

Open when they bind to specific chemicals.

When ACh is applied?

Opens chemically gated sodium ion channels, leads to membrane depolarization.

Why is the millivoltage of a resting potential important?

Plasma membrane is highly permeable to potassium ions, the resting potential is fairly close to -90mV, the equilibrium potential for K+.

What happens after sodium channel inactivation?

Positively charged K+ ions move out of the cytosol, shifting the transmembrane potential back toward resting levels, repolarization now begins.

How does a neuron receive information?

Receives information is the form of graded potentials on its dendrites and cell body, and graded potentials at the synaptic terminals trigger the release of neurotransmitters.

What does the inrush of sodium do? (Gated channels for sodium ions.)

Reduces the transmembrane potential in this area.

What does the potassium pump serve as?

Serves to stabilizes the resting potentioal when the ratio of Na+ entry to K+ loss through passive channels is 3.2.

An example of a chemically gated channel?

The receptors that bind ACh at the neuromuscular junction are an example.

Depolarization? (Gated channels for sodium ions.)

Any shift from the resting potential toward a more positive value.

Graded potentials?

Are changes in the transmembrane potential that cannot spread far from the site of stimulation.

Voltage gated channels?

Are characteristic of areas of excitable membrane, a membrane capable of generating and conducting an action potential.

What are the most important voltage gated channels?

Are voltage-gated sodium channels, potassium, and calcium channels.

Where is the transmembrane most affected?

At the site of stimulation and the effect decreases with distance, effects spread passively through local currents.

Threshold?

The opening of the channels occurs at a transmembrane potential.

Why do the sodium ions spread out?

The sodium ions inside the cell now spread out, attracted by the negative charges along the inner surface of the membrane. Depolarization spreads out too.

What does action potentials depend on?

Voltage-gated channels.

Activation of sodium channels and rapid depolarization?

When the Na+ gates open, the plasma membrane becomes much more permeable to Na+. Driven by the large electrochemical gradient, and sodium ions rush into the cytoplasm, and rapid depolarization occurs.

Repolarization?

When the chemical stimulus is removed, the membrane returns to its normal resting potential as the excess sodium ions are transported out of the cytoplasm.

When do gated sodium channels open? (Gated channels for sodium ions.)

When the membrane is exposed to a chemical to a chemical that opens the chemically gated sodium channels, sodium ions enter the cell.

Action potentials?

Propagated changes in the transmembrane potential that, once initiated, affect an entire excitable membrane.

Inactivation of sodium channels and activation of potassium channels?

As the transmembrane approaches +30 mV, the inactivation gates of the voltage-gated sodium channels close.

Resting potential after the action potential?

As the voltage-gated K+ channels close, the transmembrane potential returns to normal resting levels. The action potential is now over and the membrane is once again at the resting potential.

What happens after the voltage gated channels remain inactivated?

At this time they regain their normal status:closed but capable of opening. The voltage-gated K+ begin closing as the membrane reaches the normal resting potential (-70mV). Until all of these K+ channels have closed, K+ ions continue to leave the cell. This produces a brief hyper polarization.

All-or-none principle?

Because a given stimulus either triggers a typical action potential, or it does trigger one at all.

What happens at -60mV?

Both gates open. (Channel open)

How does the millivoltage change in depolarization? (Gated channels for sodium ions.)

Changes in potential from -70mV to smaller negative values (toward 0mV) as well to membrane potentials above 0mV.

What happens at -70mV?

The activation gate closes and the inactivation gate opens. (Channels closed)

What happens at +30mV?

The activation gate is open and the inactivation gate is closed. (Channel inactivated)

What is the electrochemical gradient and membrane permeability like for sodium ions?

The electrochemical gradient is very large and the membrane's permeability to these ions is very low.

What does the sodium potassium pump eject?

The exchange pump ejects 3 Na+ ions for every 2 K+ ions that brings into the cell.

What happens to the inner membrane in step two?

The inner membrane surface now contains more + ions than negative ones, and the transmembrane potential has changed from -60mV to a positive value.

Local current?

This movement of positive charges parallel to the inner and outer surfaces of the membrane.

Depolarization to threshold?

The stimulus that initiates an action potential is a graded depolarization large enough to open voltage-gated sodium channels.

What determines a strong stimulus?

The stronger the stimulus, the greater is the change in the transmembrane potential, and the larger is the area affected.

Gated channels?

The transmembrane potential then changes, primarily because the plasma membrane contains gated channels that open or close in response to specific stimuli.

Potassium channels close?

The voltage-gated channels remain inactivated until the membrane has repolarized to near threshold levels.