Ionic mechanisms of AP generation and propogation

Question 1

Overview of AP Generation

Answer 1

• channels are shut when the the membrane potential is near the resting potential but rapidly being to open if the membrane potential increases to a specific voltage threshold, which depolarises the transmembrane potential
• Channels open —> Na+ comes in —> changes electrochemical gradient —> rise in membrane potential —> more channels open —> huge, explosive hyper-polarisation until all channels are open
• rapid influx of Na+ causes the polarity to reverse and the ion channels then quickly inactivate
• Na+ channels close —> can no longer enter the cell —> actively transported out of the membrane
• potassium channels are activated —> K+ goes out of the cell —> returns electrochemical gradient to resting state
• after AP has occurred, there is a momentary hyper-polarisation before returning to its normal state

Question 2

Two types of AP

Answer 2

• generated by voltage-gated Na+ channels
• don’t last very long, under one ms
• generated by voltage-gated Ca+ channels
• last longer, up to 100ms or longer
• slow spikes causes provide driving force for a long burst of sodium spikes

Question 3

Neurons are electrically polarized

Answer 3

• at resting state, they maintain a voltage difference across the cell’s plasma membrane
• this is due to a complex interplay between ion pumps and ion channels
• types of channels vary at different parts of the cell, leading variations in electrical properties

—> some parts of the membrane are excitable while others are not

Question 4

There are two levels of membrane potential

Answer 4

Resting potential: value of membrane potential is maintained as long as nothing perturbs the cell

Threshold potential: right before the firing

• axon hillock is at -70mV during rest, with a threshold of -55mV
• synaptic inputs cause changes in the membrane potential
• when enough depolarisation accumulates to reach the threshold, AP triggers —> then potential abruptly shoots upward and also abruptly shoots back downward

Question 5

Voltage-gated ion channels (properties and process)

Answer 5

3 Key properties

• can change between open and closed
• at least one state creates a channel so specific ions can get through
• changes in state are influenced by membrane potential

voltage-gated ion channels are capable of producing AP because they can give rise to positive feedback loops

• membrane potential controls state of ion channels and vice versa
• membrane potential rise can cause ion channels to open –> further increasing membrane potential

Process

• sodium channels open —> Na+ goes in cell causing hyperpolarization—> K+ channels open and K+ exit the cell causing hypopolarization and then returning to resting potential
• other ions do cross the membrane but are are returned again by the sodium-potassium pump and other ion transporters that maintain normal ratio of ion concentrations

Question 6

All or none principle

Answer 6

• amplitude of AP is independent of the amount of current that is produced
  —> larger currents do not create larger APs
• this means that AP are all or none as they either fully occur or the don’t

Question 7

Refractory Period

Answer 7

• each AP is followed by a refractory period in which it is impossible to evoke another AP
• this is followed by a relative refractory period in which a stimulus has to be stronger than usual to cause an AP
• refractory period causes by changes in the state of Na+ and K+ channel molecules

—> after AP, Na+ channels enter inactivated state in which they cannot open regardless of potential

—> absolute refractory period- over time, Na+ channels transition back to resting state, though some K+ channels may still be open, making it difficult to depolarize the membrane —> relative refractory period