Action Potentials

Question 1

Define action potentials.

Answer 1

• Change in membrane potential associated with propagation down a neuron

Question 2

How are action potentials triggered in pressure-responsive perpheral neurons?

Answer 2

• Pressure sensitive ion channels open
• Influx of sodium ions
• Interior of cells becomes more positive - depolarisation
• Once threshold potential met, action potential generated

Question 3

What occurs upon meeting threshold potential?

Answer 3

• Opening of voltage gated sodium channels
• Further influx of sodium ions
• Membrane potential becomes positive relative to outside
• Further activation of voltage-gated sodium channels - allows action potential to propagate

Question 4

How does lidocaine work?

Answer 4

**- Block voltage gated sodium channels
- Prevents propagation of action potentials in pain sensing fibres

Question 5

Describe repolarisation.

Answer 5

• Voltage gated sodium channels remain open until membrane potential of +40 mV reached
• Voltage gated sodium channels close into a voltage insensitive state
• Opening of voltage gated potassium channels - efflux of potassium
• Membrane potential becomes progressively more negative

Question 6

Why does the undershoot occur?

Answer 6

• Voltage-gated potassium channels stay open longer than required
• Close and voltage gated sodium channels become voltage sensitive

Question 7

Describe the absolute refractory period.

Answer 7

• Sodium channels are in a refractory state
• Potassium channels open
• Further action potentials not generated, regardless of level of depolarisation

Question 8

Describe relative refractory period.

Answer 8

• Occurs during hyperpolarisation
• Potassium channels close/sodium channels become voltage-sensitive
• Action potential can be triggered - requires greater level of depolarisation than normal

Question 9

Describe how the refractory period allows monodirectional propagation of action potentials?

Answer 9

• Voltage gated sodium channels will open - influx of sodium - triggers further channel opening
• Once further channels opened, preceding channel enteres refractory state
• If not, action potential would stimulate opening of preceding channels. Action potential propagates back up axon.

Question 10

Using the Goldman-Hodgkin-Katz equation, what hapens in patients with hyperkalaemia?

Answer 10

• Decrease in resting membrane potential - brought closer to threshold potential
• Spontaenous propagation of action potentials
• Effect on cardiomyocytes - greater susceptiblity to fluctuations in potassium

Question 11

Describe saltatory conduction. PART 1

Answer 11

• Voltage-gated sodium channels exist at nodes of Ranvier
• Arrival of depolarising signal causes membrane depolarisation at node

Question 12

Describe saltatory conduction. PART 2

Answer 12

• Sodium diffuses down axon until reaches next node
• Cycle repeats

Question 13

What does the axon contain?

Answer 13

• AXOPLASM - containing mitochondria, microtubules and vesicles

Question 14

How does axon diameter influence velocity? PART 1

Answer 14

• SMALL DIAMETER - axoplasmic organelles are non-conductive. Occupy larger amount of space relative to axon size. Greater resistance to propagation

Question 15

How does axon diameter influence velocity? PART 2

Answer 15

• LARGE DIAMETER - space around axoplasmic organelles to conduct ions. Greater number of voltage-gated sodium channels - greater sodium conductance

Question 16

What is the difference between A- and C-fibres?

Answer 16

• A-fibres - heavily myelinated, large diameter, fast action potential conduction
• C-fibres - thin, unmyelinated, slow conduction velocities

Question 17

What are failed action potentials?

Answer 17

When depolarisation occurs but insufficient to reach threshold potential e.g during summation

Question 18

Define temporal summation.

Answer 18

Single synapse fires multiple times within a very short time space

Question 19

Define spatial summation.

Answer 19

• Multiple synapses fire simultaneously
• Large influx of sodium across the whole body of a neuronal soma

Question 20

What are the three types of receptors neurotransmitters bind to?

Answer 20

• Ligand gated cation channel - causes depolarisation
• Ligand-gated anion channel
• GPCRs - Gs/Gq = depolarisation || Gi = hyperpolarisation