Electrical Excitability and Action Potentials

Question 1

What is an action potential?

Answer 1

A change in voltage across a membrane

Question 2

Name 3 characteristics of an action potential

Answer 2

• Depends on ionic membrane and membrane permeability
• Only occurs if a threshold is reached
• All or nothing
• Propagated without loss of amplitude

Question 3

Which 2 types of voltage-gated channels are involved in action potentials?

Answer 3

K+ and Na+

Question 4

What do ion channels move towards in action potentials?

Answer 4

The equilibrium potential

Question 5

What effect does maintained depolarisation during an action potential have?

Answer 5

Inactivation of Na+ channels

Question 6

What are the 2 differences between Na+ and K+ equilibrium potentials?

Answer 6

• Na+ ion move into the cell and K+ ions move out of the cell
• ENa is positive and Ek is negative

Question 7

What proves that Na+ is responsible for action potential depolarisation?

Answer 7

• It is due to a large increase in permeability to Na+ ions

- Peak of the action potential changes in parallel with changes in ENa

Question 8

How can the voltage clamp technique measure action potentials?

Answer 8

Enables membrane currents to be measured over time at a set membrane potential

Question 9

What part of action potential activity is an example of positive feedback?

Answer 9

The opening of Na+ channels and influx to cause membrane depolarisation

Question 10

When do K+ channels open during an action potential and what effect does this have?

Answer 10

• When the Na+ channels inactivate

- Causes hyperpolarisation to depolarise the membrane

Question 11

What is an absolute refractory period?

Answer 11

• The time between the initial opening and initial activation of Na+
• Nearly all Na+ channels are inactivated
• No stimulus can initiate an action potential

Question 12

What is the relative refractory period?

Answer 12

• The time it takes to recover back to the resting state

- Na+ channels are reactivating and K+ channels close

Question 13

What process reactivates Na+ channels during an action potential?

Answer 13

Hyperpolarisation

Question 14

Describe 3 characteristics of voltage-gated ion channel structure

Answer 14

• 6 transmembrane spanning domains
• 1 transmembrane domain that senses the voltage field
• Pore region which is selective
• Individual channels open and close randomly

Question 15

What is the structural difference between Na+ and K+ voltage-gated ion channels?

Answer 15

Na+ - one alpha subunit with 4 sections

K+ - four individual alpha subunits

Question 16

What is the basic mechanism of local anaesthetic on action potentials?

Answer 16

• Unprotonated form permeates the membrane
• Act by binding and blocking Na+ channels
• Stops conduction of pain signals

Question 17

What is local current theory?

Answer 17

Current flow results in a local change in membrane potential along an axon

Question 18

Which 2 factors influence the spread of charge along an axon?

Answer 18

• Membrane resistance

- Membrane capacitance

Question 19

What is membrane capacitance?

Answer 19

• Ability to store charge

- + capacitance = + current

Question 20

What is membrane resistance?

Answer 20

• Depends on the number of open ion channels

- decreased resistance = increased open ion channels

Question 21

What characteristic of axon firing prevents action potentials going backwards?

Answer 21

They are refractory

Question 22

What effect does myelination have on capacitance and resistance?

Answer 22

• Reduces capacitance

- Increases resistance

Question 23

How does myelination improve conduction?

Answer 23

• Increases membrane resistance
• Decreases membrane capacitance
• Increases length constant
• Decreases time constant

Question 24

What does saltatory conduction refer to?

Answer 24

Nerve impulses jumping between nodes due to reduced capacitance

Question 25

What disease is related to demyelination?

Answer 25

Multiple Sclerosis