Action Potentials and Neuromuscular Junctions

Question 1

What is an Action Potential

Answer 1

Change in voltage across the cell membrane

Question 2

What are the properties of an Action Potential?

Answer 2

• Depends on ionic gradients and relative permeability
• Only occur if a Threshold Level is reached
• Propagated without Loss of amplitude

Question 3

What is the Threshold Potential

Answer 3

The minimum membrane potential that must be reached for an Action Potential to be generated

Question 4

What happens during Repolarisation?

Answer 4

• Na channels inactivate, Na stops moving in
• K channels open, K moves out
• Na Pump is NOT involved

Question 5

Compare Absolute and Relative Refractory Period

Answer 5

Absolute: Nearly all Na channels inactivated so no AP can be generated

Relative: Na channels recovering from inactivation, More Na channels becoming activated, AP can be generated from a large enough stimulus

Question 6

Compare voltage gated Na and K channels.

Answer 6

Na:
- 1 Channel= 1 Alpha subunit= Central pore surrounded by 4 repeats. (Similar to Ca Channel)

K:
1 Channel= 4 Alpha subunits around a Central pore

Question 7

How do local anaesthetics bind and block Na channels?

Answer 7

• Most are weak bases and cross membrane in unionised form
• Block open Na channels easily
• Higher Affinity for inactivated Na Channels

Question 8

In what order do local anaesthetics block nerve fibre conduction?

Answer 8

• Small myelinated neurons
• Non myelinated neurons
• Large myelinated neurons

Question 9

How to calculate Conduction Velocity?

Answer 9

Distance travelled by impulse/ time taken

In m/s

Question 10

What axon properties lead to a high Conduction Velocity?

Answer 10

• Low Membrane Capacitance (Cm)
• High Membrane Resistance (Rm)
• Large Diameter ( Lower Axon Resistance)

Question 11

What is Capacitance, Cm?

How does it affect conduction velocity

Answer 11

• Ability of a Lipid bilayer to store charge

High Cm means more charge stored, so Decreased spread of Local Current, so Decreased Conduction Velocity

Question 12

What is Membrane Resistance, Rm?

How does it affect conduction velocity?

Answer 12

• A function of open ion channels

High Rm means less open ion channels, means less Local Current Loss ( Less leakage)

Question 13

Describe the Local Circuit Theory

Answer 13

Depolarisation of a small region of membrane, produced local currents which depolarise adjacent regions, caused voltage gated Na channels to open.

Question 14

What is the relationship between Diamter and Conduction Velocity for Myelinated and Non-Myelinated neurons?

Answer 14

Non-myelinated: Velocity is proportional to Diameter^0.5

Myelinated: Velocity is proportional to Diameter

Question 15

How does Saltatory Conduction work?

Answer 15

Action Potential jumps from one Node of Ranvier to another and so on

Question 16

Why don’t Action Potentials move backwards?

Answer 16

Channels that have been opened, are now in Refractory Period so are inactivated

Question 17

How does myelin sheath affect conduction velocity?

Answer 17

Reduced Cm, Increased Rm

= Increased Conduction Velocity

Question 18

What are some properties of voltage gated Ca Channels?

Answer 18

• Similar to Na Channel
• Various channel types
• Ca channels activate/ inactivate slower than Na Channels
• High intracellular [Ca] leads to channel inactivation

Question 19

Explain the process of Transmitter release

Answer 19

1. Ca enters cell
2. Binds to a protein, which brings vesicle to membrane
3. Snare complex makes fusion pore in vesicle
4. Transmitter is released through pore
5. Transmitter degraded

Question 20

How do Competitive and Depolarizing Neuromuscular Blockers work?

Give an example of Depolarizing

Answer 20

1. Competitive: Occupy receptors so transmitter can’t bind
2. Depolarising: Maintain depolarisation In Muscle end-plate, so inactivated Na channels do not activate, and no Na enters cell (E.g. Succylincholine)

Question 21

Where are Neuromuscular blockers used and why?

Answer 21

In surgery, causing temporary paralysis to relax muscles

Someone paralysed like this can’t move/ speak but WILL FEEL PAIN

Question 22

What is Myasthenia Gravis, how does it work?

Answer 22

• Autoimmune disease targeting nAChRs
• Antibodies target nAChRs on muscle end-plate
• Receptor degradation leads to loss of functioning nAChRs
• Reduced amplitude of Potentials at end-plate (threshold not reached)

Question 23

What are some symptoms of Myasthenia Gravis

Answer 23

• Muscle weakness and fatigue
• Weakness increases with exercise
• Double vision
• Drooping eyelids