S7) The Action Potential & Neuromuscular Junction

Question 1

What is an action potential?

Answer 1

An action potential is a change in voltage across the cell membrane

Question 2

What are the properties of action potentials?

Answer 2

Action potentials:

• Depend on ionic gradients and relative permeability
• Only occur if a threshold level is reached
• Are propagated without loss of amplitude

Question 3

Describe the sodium hypothesis of the action potential

Answer 3

1 - Threshold potential reached

2 - Na⁺ channels open

3 - Na⁺ channels inactivate & K⁺ channels open

Question 4

Explain the term threshold potential

Answer 4

• Threshold potential is the critical level to which a membrane potential must be depolarised to initiate an action potential
• Once this membrane potential is reached, a positive feedback occurs as Na⁺ channels begin to open

Question 5

Depolarisation is followed by repolarisation.

What happens during this process?

Answer 5

• Potassium channels open causing an efflux of K⁺
• Sodium channels inactivate stopping the influx of Na⁺

Question 6

What is the absolute refractory period?

Answer 6

The absolute refractory period is the time between the initial opening and subsequent closing of the Na⁺ channels (approx. ~1 millisecond in nerves)

Question 7

What is the relative refractory period?

Answer 7

The relative refractory period is the time needed for Na⁺ channels to recover back to their resting membrane potential so they can open again (approx. 4 milliseconds)

Question 8

Local anaesthetics act by binding to and block Na⁺ channels, thereby stopping action potential generation.

How is this achieved?

Answer 8

• Most local anaesthetics are weak bases and cross the membrane in their unionised form
• They block Na⁺ channels easily when the channel is open
• They also have a higher affinity for the inactivated state of the Na⁺ channel

Question 9

In what order do local anaesthetics block the conduction in nerve fibres?

Answer 9

• Small myelinated axons
• Non-myelinated axons
• Large myelinated axons

Question 10

What is electrical stimulation and how is it done?

Answer 10

- Electrical stimulation is used to stimulate an axon / group of axons to threshold potential, thus intiating an action potential

• Stimulation occurs under a cathode (negatively charged)

Question 11

How does one calculate conduction velocity?

Answer 11

Conduction velocity = distance / time

Question 12

How is an action potential conducted along an axon?

Answer 12

• A change in membrane potential in one part can spread to adjacent areas of the axon by local currents
• When local current spread causes depolarisation of part of the axon to threshold potential then an action potential is initiated in that location

Question 13

What are the properties of the axon that lead to a high conduction velocity?

Answer 13

• A high membrane resistance
• A low membrane capacitance
• A large axon diameter

Question 14

What is capacitance?

Answer 14

Capacitance is the ability of a lipid bilayer to store charge

Question 15

Why does a low capacitance lead to a faster conduction velocity?

Answer 15

• A high capacitance takes more current to charge
• It can cause a decrease in local current spread

Question 16

What is membrane resistance?

Answer 16

The membrane resistance is a function of the number of open ion channels

Question 17

Why does a low membrane resistance lead to a slower conduction velocity?

Answer 17

The lower the resistance, the more ion channels are open and the more loss of local current occurs across the membrane

Question 18

Local currents cause an action potential to propagate down an axon.

Why do action potentials not move backwards?

Answer 18

The action potential will not move backwards because the area of axon that has just fired an action potential is refractory and cannot fire another action potential until it has recovered from being refractory

Question 19

Explain how the myelin sheath affects conduct velocity

Answer 19

Myelin reduces the capacitance and increases the resistance of the axonal membrane, hence increasing the overall conduction velocity

Question 20

Identify the cells which form myelin and describe their actions

Answer 20

• Schwann cells myelinate peripheral axons
• Oligodendrocytes myelinate axons in the CNS

Question 21

Describe the process of saltatory conduction in myelinated nerve fibres

Answer 21

• Saltatory conduction is the propagation of action potentials along myelinated axons from one node of Ranvier to the next node
• This increases the conduction velocity of action potentials

Question 22

What is the relationship with fibre diameter and conduction velocity?

Answer 22

• Myelinated fibres: velocity proportional to diameter
• Unmyelinated fibres: velocity proportional diameter^1/2

Question 23

In terms of myelination & conduction velocity, describe what occurs in multiple sclerosis?

Answer 23

Multiple sclerosis is an autoimmune disease wherein the myelin is destroyed in certain areas of the CNS, leading to decreased conduction velocity and/or the complete block of action potentials