ICPP 8 - Action Potentials & The Neuromuscular Junction

Question 1

How can you experimentally show that Na+ influx is responsible for AP depolarisation?

Answer 1

• Remove EC Na+
• Electrochemical gradient for Na+ into the cell decreases
• Peak of AP is reduced

Question 2

What is conductance? (in terms of an AP)

Which channels are responsible for each phase in the AP?

Answer 2

• Conductance is the number of open ion channels for a particular ion
• Na+ responsible for upstroke/DP, K+ efflux responsible for repolarisation
• Na+ channels are activated then inactivated rapidly, K+ channels also activated by DP, but much slower and remain open longer allowing for repolarisation of membrane.

Question 3

What are the 2 recovery periods that occur after an AP has been triggered?

Answer 3

1) Absolute refractory period - where all Na+ channels are in inactive state and an AP cannot be triggered.
2) Relative refractory period - where Na+ channels are recovering from inactive period - only large stimuli can generate an AP here.

Question 4

Describe the basic structure of voltage-gated Na+, Ca2+ and K+ channels.

Answer 4

• Na+ and Ca2+ channels are structurally similar. Both only need 1 a subunit to be functional. Each alpha subunit consists of 4 units each with 6 TM domains. Within each TM domain is an S4 voltage sensor that allows for channel opening and an inactivation particle which plugs the pore when the channel is open.
• K+ channels require 4 x a-subunits, similarly have an S4 voltage sensor, but are lacking an inactivation particle.

Question 5

Which forms of anaesthetic can enter the cell?

How do they cause blockage of Na+ channels?

Answer 5

• Unprotonated forms (as they are uncharged and therefore lipophilic)
• Either by open channel block or by hydrophobic block pathway during inactivated state.

Question 6

How is conduction velocity of an AP measured?

Answer 6

• Electrodes placed over nerve cell
• Measure time it takes for AP to reach a certain point (X)
• Use x/time to work out velocity in meters per second.

Question 7

Explain the local current theory of membrane depolarisation.

Answer 7

Injection of current into an axon causes the resulting charge (depolarisation) to spread along the axon and cause an immediate local change in the membrane potential. Depolarisation causes AP to be triggered in adjacent areas of the axon in a wave like fashion.

Question 8

What 2 things does spread of local current in the membrane depend on?
What conditions would allow a local current to spread further along a membrane?

Answer 8

1) Membrane resistance = the number of ion channels open (lower resistance = more open)
2) Membrane capacitance = Ability to store charge (high capacitance = voltage spreads more slowly)

Therefore, a high resistance and a low capacitance will spread the local current further along the membrane.

Question 9

Why is conduction velocity of AP’s faster in myelinated axons?

Answer 9

• Na+ ion channels do not exist underneath strips of myelin on the axon, instead are concentrated at nodes of Ranvier.
• This increases the length constant, allowing local circuit currents to jump from node to node, allowing for. much faster conduction - known as saltatory conduction.

Question 10

Why does demyelination in disease slow conduction velocity?

Name a demyelination disease in the CNS and PNS.

Answer 10

• Regions of demyelination mean that the length constant is decreased so the next node is not DP’d. Leads to a failure to reach threshold.
• CNS = MS
• PNS = Charcot-Marie-Tooth disease

Question 11

What are the sequence of events at a nerve terminal that lead to depolarisation of the post-synaptic terminal.

Answer 11

• AP arrives
• Ca2+ channels open, increasing IC Ca2+
• Increase in IC Ca2+ causes release of NT (ACh) from vesicles in pre-synaptic terminal

Question 12

Describe the events that occur leading upto transmitter release from the pre-synaptic terminal.

Answer 12

• Ca2+ entry through Ca2+ channels
• Ca2+ binds to synaptotagmin
• Vesicle brought close to the membrane
• SNARE complex makes a fusion pore
• Transmitter released into cleft through the pore

Question 13

How is skeletal muscle depolarised after ACh is released from the synapse at the neuromuscular junction?

Answer 13

• ACh binds to nAChR
• This is a ligand gated ion channel with integral ion channel
• Binding opens its non-specific cation channel
• Influx of cations cause DP

Question 14

What does curare poison (d-CT) cause and why?

Answer 14

• Paralysis

- Blockage of Na+ channels to prevents DP and therefore transmission between nerve and muscle

Question 15

What are the 2 ways in which nAChR can be blocked - give an example of a drug that does each.

Answer 15

1) Competitive block - competes with ACh for binding to the receptor, e.g.: d-tubocurarine (d-TC) - this can be overcome by increasing the concentration of ACh.
2) Depolarising block - Continuous activation of receptors so a maintained DP occur (as not broken down by ACh esterase) - therefore Na+ channels became inactivated and stay this way, AP’s cannot fire. E.g.: succinylcholine

Question 16

What is myasthenia gravis and why does it cause significant muscle weakness?

Answer 16

• Autoimmune disease that targets nAChR’s
• Antibodies directed against these receptors leads to loss of functional nAChR by mediated lysis and receptor degradation.
• Leads to muscle weakness and fatigue that increases with exercise

Question 17

What is the edrophonium test for myasthenia gravis?

Answer 17

Edrophonium is a short acting ACh esterase, which alleviates facial weakness rapidly. If this occurs, it is a positive test for MG.