S7) The Action Potential & Neuromuscular Junction Flashcards
What is an action potential?
An action potential is a change in voltage across the cell membrane
What are the properties of action potentials?
Action potentials:
- Depend on ionic gradients and relative permeability
- Only occur if a threshold level is reached
- Are propagated without loss of amplitude
Describe the sodium hypothesis of the action potential
1 - Threshold potential reached
2 - Na+ channels open
3 - Na+ channels inactivate & K+ channels open
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Explain the term threshold potential
- 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
Depolarisation is followed by repolarisation.
What happens during this process?
- Potassium channels open causing an efflux of K+
- Sodium channels inactivate stopping the influx of Na+
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What is the absolute refractory period?
The absolute refractory period is the time between the initial opening and subsequent closing of the Na+ channels (approx. ~1 millisecond in nerves)
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What is the relative refractory period?
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)
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Local anaesthetics act by binding to and block Na+ channels, thereby stopping action potential generation.
How is this achieved?
- 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
In what order do local anaesthetics block the conduction in nerve fibres?
- Small myelinated axons
- Non-myelinated axons
- Large myelinated axons
What is electrical stimulation and how is it done?
- 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)
How does one calculate conduction velocity?
Conduction velocity = distance / time
How is an action potential conducted along an axon?
- 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
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What are the properties of the axon that lead to a high conduction velocity?
- A high membrane resistance
- A low membrane capacitance
- A large axon diameter
What is capacitance?
Capacitance is the ability of a lipid bilayer to store charge
Why does a low capacitance lead to a faster conduction velocity?
- A high capacitance takes more current to charge
- It can cause a decrease in local current spread