Resting Potential and Action Potential

37
Resting potential and Action Potential Chapter 2.2

Transcript of Resting Potential and Action Potential

Page 1: Resting Potential and Action Potential

Resting potential and Action Potential

Chapter 2.2

Page 2: Resting Potential and Action Potential

Review: parts of a neuron

Page 3: Resting Potential and Action Potential

This is the membrane of the entire neuron (soma, axon, dendrites, everything).

SemipermeableWater, oxygen, carbon dioxide move freely.

Na+, Ca2+, K+, Cl- move thru ion channels (protein channels).

Page 4: Resting Potential and Action Potential

Na+

K+

Cl-

A-

Na+

Na+ Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+Na+

Cl-

Cl-

Cl-Cl-

Cl-

Cl- Cl-

Cl-

Cl-

Cl-K+

K+

K+

K+K+

K+

K+

K+

K+

K+

A-

A-

A-

A-

A-

A-

A-A-

A-

This is a cell. For now, let’s think of it as a cross section of an axon.

There are ions inside and outside the cell.

These ions are not equally distributed K+

Na+

Cl-

Volts0-70

Page 5: Resting Potential and Action Potential

Na+

K+

Cl-

A-

Na+

Na+ Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+Na+

Cl-

Cl-

Cl-Cl-

Cl-

Cl- Cl-

Cl-

Cl-

Cl-K+

K+

K+

K+K+

K+

K+

K+

K+

K+

A-

A-

A-

A-

A-

A-

A-A-

A-

Because of unequal distribution of ions, the inside of the cell is negatively charged compared to outside the cell.

The cell’s membrane has a concentration gradient (different levels of ions inside versus outside) & an electrical gradient (different charge inside versus outside).

The cell’s membrane is “polarized”

Page 6: Resting Potential and Action Potential

Important Concept: Polarized

• Different inside vs. outside (-70 mV)

• As cell becomes less negative, it is “depolarized” (less polarized)

• As cell becomes more negative, it is “hyperpolarized” (more polarized)

Page 7: Resting Potential and Action Potential

-70 mV is the resting potential.

Less negative than -70 is depolarized.

More negative than -70 is hyperpolarized.

-70 mV

time

-50 mV

Volts0-70

K+

Na+

Cl-

How could we depolarize the cell?

How could we hyperpolarize the cell?

Page 8: Resting Potential and Action Potential

How is the resting potential maintained?

Selective Permeability of the membrane: some molecules pass more freely across the membrane than do other molecules; controlled by “channels”

At rest:• H2O, CO2, O2 pass freely • K+ and Cl- ions pass slowly

•No net change in these ions• Na+ channels are closed*• Na+/K+ pump (next slide)

*certain types of stimulation cause the Na+ channels to open

Page 9: Resting Potential and Action Potential

Acts to maintain proper concentrations of Na+ and K+ Needed for maintaining resting potential and for recovery from an action potential.3 Na+ out for every 2 K+ in, so more positive on outside

Sodium-Potassium Pump

Page 10: Resting Potential and Action Potential

(Midway) Summary

• Ions are unequally distributed across the membrane of a cell.

• The membrane is polarized.

• The membrane has a resting potential of -70 mV.

• The semi-permeable nature of the membrane, and the Na+/K+ pump, maintain the resting membrane potential.

Page 11: Resting Potential and Action Potential

The Action Potential

• Electrical impulse down an axon.

• Depends on sodium (Na+) and potassium (K+) moving thru channels in the axon’s membrane

Page 12: Resting Potential and Action Potential

What forces could move ions into and out of cells?

• Concentration gradient: ions flow from areas of high concentration to low concentration

• Electrical gradient: ions flow to areas of opposite charge – negative to positive, positive to negative

Page 13: Resting Potential and Action Potential

-70mV

K+

Na+ Cl-

If a potassium channel opens, what will the concentration gradient (diffusion) try to force potassium to do?

If a potassium channel opens, what will the electrical gradient try to force potassium to do?

If a sodium channel opens, what will the concentration gradient (diffusion) try to force sodium to do?

If a sodium channel opens, what will the electrical gradient try to force sodium to do?

Page 14: Resting Potential and Action Potential

-70mV

K+

Na+ Cl-

What happens if sodium rushes into cell?

A. Cell becomes depolarized (less negative)

B. Cell becomes hyperpolarized (more negative)

C.Nothing

Page 15: Resting Potential and Action Potential

-70mV

K+

Na+ Cl-

What happens if potassium rushes out of a cell?

A. Cell becomes depolarized (less negative)

B. Cell becomes hyperpolarized (more negative)

C.Nothing

Page 16: Resting Potential and Action Potential

So, why do we care about ion movement??

That is how action potentials work!

This ion movement is the action potential

Page 17: Resting Potential and Action Potential

Threshold is reached at the axon hillock

Voltage-gated Na+ channels open

Na+ enters the cell

What forces bring in Na+??

What happens to the voltage of the cell when Na+ enters??

Propagation of The Action Potential

Page 18: Resting Potential and Action Potential

Now, VGNaC’s close and VGKCs open

K+ leaves the cell

What forces push out K+??

What happens to the voltage of the cell when K+ leaves??

Propagation of The Action Potential

Page 19: Resting Potential and Action Potential

So much K+ exits the cell, that it becomes hyperpolarized!

When it reaches resting membrane potential again, it can conduct another action potential

That period of hyperpolarization is called the refractory period

Propagation of The Action Potential

Page 20: Resting Potential and Action Potential

• Sodium in

• Potassium out

• Refractory period (The neuron cannot fire another action potential until it is back up to resting potential) and repolarization

-70 mV

time

-50 mV

Refractory period

Sodium in

Potassium out

Page 21: Resting Potential and Action Potential

Threshold*The critical level of depolarization that must

be achieved to trigger an action potential is called the threshold.

-70 mV

time

-50 mV

Page 22: Resting Potential and Action Potential

When Na+ entered cell, it flowed down axon, depolarizing the neighboring membrane and thus opening other VGNaC’s What happened when those VGNaC’s opened?

Then what, then what, then what??

Propagation of The Action Potential

Page 23: Resting Potential and Action Potential

Action Potential

Transmitter-dependentNa+ channels open

Voltage-dependentNa+ channels open*

V-dependent Na+ channels close (cannot reopen until membrane is back to resting potential)

Voltage-dependent K+ channels open (less sensitive than V-D Na+ ch; K+ leaves b/c chemical & electrical gradients)

Voltage-dependentK+ channels close

Na/K pumps repolarize

Page 24: Resting Potential and Action Potential

Summary: The Action Potential• The Action Potential is the signal that conveys information over

distances in the nervous system.

• The action potential is a rapid reversal of the situation at rest --for an instant, the inside of the membrane becomes positively charged relative to the outside.

• Action potentials are “all or none”.

• The frequency and pattern of action potentials is the code used by neurons to transfer information from one location to another.

• All of this occurs at the nodes of ranvier, between the segments of myelin….

Page 25: Resting Potential and Action Potential

Propagation of the action potential depends on:

• Diameter of axon• Insulation (myelin)

Page 26: Resting Potential and Action Potential

Myelin and Saltatory Conduction

Voltage-gated Na+ channels are concentrated at the Nodes of Ranvier.

Page 27: Resting Potential and Action Potential

In Myelinated axons, action potential can “jump” down axon.Much faster. Allows long distance rapid communication

Passive conduction that diminishes but has enough strength at next node for another AP to occur

Action potential

Saltatory conduction

energy & speed

Page 28: Resting Potential and Action Potential

Fun fact

Anesthetics prevent APs

Local Anesthetics (novacaine, xylocaine)• attach to Na+ channels & prevent Na+ from entering cells

General Anesthetics (ether, chloroform)• decrease brain activity by opening K+ channels wider than usual

Page 29: Resting Potential and Action Potential

Study Questions• What ions are important in the action potential? When the cell is at rest, which ions

are most highly concentrated inside of the cell, and which ones are most highly concentrated outside of the cell?

• Understand the forces working on the ions (electrical gradient and concentration gradient).

• What is a voltage-gated channel? Where are they?• Understand the steps of the action potential, and how one leads to the next. How is

an action potential started and propagated? What ion enters first? Thru what type of channel does it enter? What forces drive it inside? Why does that channel close? What channel opens next? What ion moves thru that? What forces drive that ion? Etc etc.

• Terms to know and understand with regard to neurophysiology: polarized, depolatization, hyperpolarization, repolarization, resting potential, threshold

– With regard to those last 2 terms, What voltage is resting potential? What voltage is threshold? Where must threshold be reached for an action potential to occur?

• What is the sodium/potassium pump? What does it do? What purpose does it serve?• What is the purpose of myelin? What happens at the nodes of Ranvier? What is

saltatory conduction?• What does it mean that an action potential is “all or none”?

Page 30: Resting Potential and Action Potential

• The rest of the slides contain additional (non-testable) material, for inquiring minds

• Or additional slides on what we just covered, for clarification.

Page 31: Resting Potential and Action Potential

Multiple Sclerosis: A Demyelinating Disease

MS causes weakness, lack of coordination, impaired vision and speech.

It is also characterized by remissions and relapses that occur over many years.

MS attacks the myelin sheaths of axons in the brain, spinal cord, and optic nerves.

The name comes from the Greek word for “hardening” which describes the lesions that develop around bundles of axons; and the sclerosis is multiple because the disease attacks many sites at the same time.

The lesions can be viewed by MRI. However, neurologists have been able to diagnose MS by measuring changes in conduction velocity. One test involves visual stimulation with a checkerboard pattern and measuring the elapsed time until an electrical response occurs in scalp electrodes. The response is slowed in people with MS because of the slowing of the conduction velocity of the optic nerve.

Page 32: Resting Potential and Action Potential

The effects of toxins on the sodium channel.

Tetrodotoxin (TTX): This toxin clogs the pore by binding to a site on the outside of the channel. Originally isolated from the puffer fish.

Another channel blocking toxin is saxitoxin. This toxin is produced by dinoflagellates and can be concentrated in clams, mussels, and other shellfish that feed on the protozoa. A dinoflagellate bloom causes what is known as a “red tide”. Eating shellfish during a bloom can be fatal.

Other toxins, such as batrachotoxin (from the skin of a Colombian frog) causes the channels to open inappropriately (i.e. at more negative potentials) and stay open much longer than usual.

Other toxins with similar mechanisms come from lilies (veratridine) and buttercups (aconitine).

Some toxins, such as those from scorpions and sea anemones, disrupt channel inactivation.

Page 33: Resting Potential and Action Potential

Q’s from other class

• How fast does an action potential occur?– ~3 ½ msec, including

refractory period

• How does caffeine work?

• How exactly do voltage-gated Na+ (and K+) channels work?– Next slide

-70 mV

time

-50 mV

Page 34: Resting Potential and Action Potential

The sodium channel has a pore loop (b/t S5-S6) that functions as a selectivity filter making it 12X more selective for Na+ than K+. (Similar to the potassium channel)

Sodium Channel Structure

Created from a single long polypeptide

4 domains, I-IV, that form the pore

6 transmembrane alpha helices S1-S6

Voltage-gated Na+ channel

Page 35: Resting Potential and Action Potential

The channel is gated by a change in voltage across the membrane.

The voltage sensor resides in the S4 segment.

Positively charged amino acid residues are regularly spaced along the coils of the helix.

Depolarization pushes the S4 away from the inside of the membrane causing a conformational change that opens the gate.

Page 36: Resting Potential and Action Potential

Resting Potentials of Neurons

The Membrane of a Nerve Cell Maintains an Electrical Polarization

outside (+)

The cell has a concentration gradient: difference in distribution of ions between the inside and outside of a membrane

The cell is polarized: at rest, an electrical gradient is maintained across the plasma membrane (negative charge is greater inside the cell)

The cell has a resting potential: difference in voltage across the membrane of a cell (~ -70 mV) a rest.

inside (-)

Therefore…

Page 37: Resting Potential and Action Potential

How do the concentration gradient and electrical gradient affect ion movement?

ConcentrationGradient

Electricalgradient

C

E

Forces

Less Na+ inside

More negative inside

More K+ inside