Passive Potentials and the Action Potential

Question 1

What is a passive local potential?

Answer 1

Local changes in the membrane potential caused by the opening/closing of ion channels in response to events external to the membrane.
Either excitatory or inhibitory.

Question 2

When do passive local potentials occur?

Answer 2

Whenever information is moving into an excitable cell e.g. sensory receptors and synapses on dendrites

Question 3

Why are local potentials important?

Answer 3

All information processing in the nervous system is based on graded potentials.

Question 4

What is the size of the stimulus directly proportional to?

Answer 4

The bigger the stimulus = the more ion channels open
More channels = more inflow of ions
More ions = more depolarization
Therefore the size of the stimulus is directly proportional to the size of the potential

Question 5

Name the properties of a passive potential.

Answer 5

Outlast stimulus
Size of potential is reflected by size of stimulus
Decremental spread

Question 6

Describe outlast stimulus.

Answer 6

The neuron experiences an electrical stimulus and there is a delay in change in potential. This is due to the capacitance of the membrane - the inside of the membrane is negatively charged and the outside is positive.
Stimulus occurs and Na ions move into the cell, however some are trapped by the negative membrane. This results in a delayed depolarization.
The same occurs for K ions in repolarization.

Question 7

Describe how the size of the potential is reflected by the stimulus size.

Answer 7

Bigger stimulus = more channels open = bigger depolarization

Question 8

Describe decremental spread.

Answer 8

Potential at one part of the neuron will be different to another part of the neuron. This is because as the potential spreads from proximal to distal, it decreases in size due to capacitance and leakage. Ions are taken up by the membrane and leak out of the membrane.
Therefore potential decreases with distance.

Question 9

Why are graded potentials good for information processing?

Answer 9

They can spread
They outlast their stimulus
They can summate
Their size reflects the size and locality of the stimulus

Question 10

Why are graded potentials not good for carrying information over long distances?

Answer 10

Potentials get smaller as they spread and will eventually fade away.
Transmission is achieved by action potential

Question 11

Define temporal summation.

Answer 11

Local potentials arrive one after the other and are added together

Question 12

Define spatial summation.

Answer 12

Local potentials from different parts of the dendrite are added together.

Question 13

When do action potentials occur?

Answer 13

Occur when passive potentials arrive at membranes that have voltage gated channels

Question 14

Which channels in nerve axons give rise to an AP?

Answer 14

Na voltage gated channels

Question 15

State the properties of Na voltage gated channels.

Answer 15

Depolarization increases the open probability = action potential
After opening, they will eventually enter an “inactivated state” = unable to summate
Repolarization reactivates them to their original closed state = absolute refractory period

Question 16

Name the 5 stages of action potential generation.

Answer 16

Pre-potential
Threshold
Depolarization
Repolarization 
After hyperpolarization

Question 17

Describe what happens in the pre-potential phase.

Answer 17

Passive potential from sensory ending/synapse travels across dendrite into axon as the ions diffuse across

Question 18

Describe what happens in threshold.

Answer 18

Ions from the passive potential cause voltage gated Na channels to open
Leakage of Na ions which offsets further depolarization, but this reaches a saturated state = threshold.
Any additional ions will have a large effect on membrane potential triggering the action potential.
As more ions enter the axon, more Na channels open, more Na entering, more depolarization and therefore more channels opening (regenerative)

Question 19

What is an important thing to remember about action potentials?

Answer 19

They are all or nothing events.

Question 20

State what happens in the depolarization phase.

Answer 20

More Na enters = increasing depolarization = increasing probability of opening the remaining voltage gated Na channels
Causes more depolarization in a positive feedback loop

Question 21

What happens in the repolarization phase?

Answer 21

Na comes into the cell and is making it more positive
Therefore an electrical gradient develops that forces Na to stop entering and forces K out
AP reaches ENa = peak
Na is in equilibrium and K leaves the cells as channels open (repolarization)
This results in a more negative membrane potential.
Absolute refractory period occurs.

Question 22

Describe the absolute refractory period.

Answer 22

ENa is reached and Na channels are inactivated. Cannot evoke a superimposed AP no matter how strong the stimulus is.

Question 23

Describe what happens in the after hyperpolarization phase.

Answer 23

Voltage gated K channels stay open longer.
K:Na permeability ratio is higher than normal
Membrane potential goes closer to Ek than normal
Relative refractory period occurs.

Question 24

Describe relative refractory period.

Answer 24

Possible to evoke an AP, but is difficult due to high Na permeability.