Graded Potentials and Action Potentials

Neurons are _____

excitable

How does the signal get from the synapse to the nerve terminal? (3)

1. Excitable cells have gated channels that cause ion shifts (eg influx of Na+)

2. Change in the membrane potential

3. Generates an electrical signal (graded or action) that sends information from one area of the cell to another

Graded potential

Long or short?

Where?

Short; Dendrite

Action potential

Long or short?

Where?

Long; axons

Generation of a graded potential in a dendrite (3)

1. Ligand (neurotransmitter or chemical stimulus) binds to a receptor on a dendrite

2. Opens a Na+ channel (one example) (Ligand-gated channel)

3. A graded potential is generated

Will the graded potential generated from the opening of a ligand-gated Na+ channel cause a depolarization or hyperpolarization?

A. Depolarization

B. Hyperpolarization

A. Depolarization

Steps of a graded potential

1. Channel opens and ion (eg Na+) fluxs in

2. Channel closes and K+ leak and NaK-ATPase brings it back to RMP

3. Repolarization is passive (another channel does not open)

Graded potential is measured where/ when?

Graded potential measured at the stimulus site/ peak

What is the stimulus for a graded potential?

oftentimes the stimulus is a chemical stimulus (ligand), or a mechanical stimulus

If a stronger stimulus causes more ligand gated Na+ channels to open, what happens to the amplitude of the graded potential that is produced?

A. Larger

B. Smaller

C. No change, graded potentials are caused by opening of voltage-gated Na+ channels

A. Larger

KEY: Amplitude increases with increasing stimulus strength***

Decrementally

Amplitude decreases with distance from the origin

What happens if a second stimulus occurs before the membrane has repolarized from a first stimulus?

We have two stimulis causing more Na+ ligand channels to open

Summation

Not all graded potentials are Depolarizations. Some are

Some are hyperpolarizations

Depends on the neurotransmitters involved, the channel involved, and the ion flux

Fun facts about action potentials (2)

• Typically involve large changes in membrane potential

• Typically involve voltage gated Na+ and K+ channels

A graded potential is generated by opening of ___ and spreads from ___

by opening of ligand-gated Na+ channels and spreads from

the dendrite to the cell body to the axon hillock

Many times to generate an action potential you need:

• A stronger stimulus for the graded potential to reach the axon hillock

• Summation

If the graded potentials reach the axon hillock which contains voltage gated Na+ and K+ channels

An action potential can be generated

Identify the phases of an action potential

Voltage gated Na+ channel has

1. Activation gate

2. Inactivation gate

Channels when membrane is at rest

Na+ Inactivation gate is open, activation gate is closed

K+ channels closed

Channels when membrane is depolarized

Na+ channels open, Na+ Influx

K+ channels closed

Channels when membrane is repolarized

Na+ inactivation gate is closed

K+ Channels open, K+ Efflux

Channels at after-hyperpolarization

Na+ inactivation gate is closed

K+ Channels open, K+ Efflux

What causes all of the changes (i.e. opening and closing of the gates) in the Na+ and K+ channels?

A. Ligand changes

B. Voltage change

C. Time changes

B. Voltage change

What is the purpose of the inactivation gate on the Na+ channel?

A. It’s a backup in case the activation gate

doesn’t close

B. It’s a backup in case the activation gate

doesn’t open

C. To prevent the net efflux of Na+ during

repolarization

D. To prevent another action potential from

occurring during repolarization

D. To prevent another action potential from occurring during repolarization

Can a second stimulus activate more Na+ channels during the depolarization phase of the AP?

No because it is an all or nothing phenomenon

Can a second stimulus activate Na+ channels during the repolarization phase of the AP?

No because the inactivation gate is closed

Absolute refractory period

Contains depolarization and repolarization. At this point, another stimulus cannot change it.

Can a second stimulus activate Na+ channels during the after hyperpolarization phase of the AP?

Yes. the 2nd stimulus need to be stronger than a ‘normal’ stimulus because farther away from the threshold

Relative refractory period

Happens during After-hyperpolarization. Need a stronger stimulus to activate an action potential but can have a second stimulus.

Is the amplitude of an action potential higher

with a stronger stimulus – for example a loud

noise vs a whisper, or something that is

mildly painful vs very painful?

No. Irrespective of the strength of the stimulus, the amplitude of the AP is the same. AP does not have summation.

So how does the brain know that it is a

loud noise and not a whisper?

By the frequency of the action potentials

What is the physiological purpose of the refractory period?

Enables individual electrical signals to propagate down the axon. It is key in determining the direction of action potential propagation

Why is the propagation of the action potential important?

The first action potential is exactly the same as the last action potential. No decrement so it can travel long distances.

Propagation of an action potential down an unmyelinated axon pt 1 (3)

1. Membrane depolarizes Na channels activate

   1. Inside positive, outside negative

2. Local current starts to depolarize the adjacent membrane

Propagation of an action potential down an unmyelinated axon pt 2 (4)

1. First part starts to repolarize as Na channels deactivate and K channels open

   1. Inside negative, outside positive

   2. Cannot generate another AP

2. The second part starts to depolarize and only move in one direction

Propagation of an action potential down an unmyelinated axon is relatively slow. How can this be sped up?

Myelin sheath/ insulating parts of the axon.

Saltatory conduction

The electrical impulse “jumps” down the axon

Propagation of an action potential down a myelinated axon

A neuron in a petri dish is stimulated at the

point indicated by the triangle. If this

generates an action potential at that point,

which way does it propagate?

A. Both directions

B. Neither direction

C. Toward the axon terminals

D. Towards the cell body

A. Both directions

Traveling both ways. Typically goes one way because refractory period but no refractory period if placed in middle of axon.

A 6-year-old boy is brought to his pediatrician

after his parents noticed that he had difficulty

moving his arms after a soccer game. About 10

minutes after leaving the field, the boy became

so weak that he could not stand for about 30

minutes. Questioning revealed that he

complained of muscle spasms and weakness

after eating bananas. He was diagnosed with

hyperkalemic periodic paralysis. What do you

think is hyperkalemia?

A. Low intracellular K+

B. Low blood levels of K+

C. High intracellular K+

D. High blood levels of K+

D. High blood levels of K+

Caused by a mutation in the gene for the voltage gated Na+ channel

When the boy became hyperkalemic, this most likely caused his neurons and muscle cells to:

D. Depolarize

E. Hyperpolarize

D. Depolarize

How might the hyperkalemia affect the Na+

channels on his neurons and muscle cell?

A. The prolonged hyperpolarization will cause

the Na+ channels to open at a faster rate

causing the paralysis or muscle weakness

B. The prolonged hyperpolarization will cause

the Na+ channels to inactivate causing the

paralysis or muscle weakness

C. The prolonged depolarization will cause the

Na+ channels to open at a faster rate

causing the paralysis or muscle weakness

D. The prolonged depolarization will cause the

Na+ channels to inactivate causing the

paralysis or muscle weakness

D. The prolonged depolarization will cause the

Na+ channels to inactivate causing the

paralysis or muscle weakness