7 - Action Potential Flashcards
Draw the graph for an action potential.
- All or nothing if threshhold reach
- Same aplitude if threshold reached
- Membrane potential trying to get to ENa
What are the differences in action potentials in the:
- axon
- skeletal muscle
- SAN
- Cardiac ventricle
How is an action potential generated?
- If ENa is altered by change in membrane permeability, peak of action potential will change
In an axon, how does a neurotransmitter cause an action potential?
What is conductance (g) of a membrane?
- Depends on the number of channels for an ion that are open, higher conductance means more permeable
- Increase in conductance for an ion, increase in no of channels open, leads to membrane potential moving closer to that ions Ek
How many sodium ions have to move in order to result in an action potential?
Very small amount
40 um!!!
Larger diameter, even smaller conc change needed
What is voltage clamp?
A technique used to measure the voltage current at a set membrane potential
Can see how ions flow through voltage gated K+ and Na+ channels at different membrane potentials
What are the conductance changes that occur for Na and K during an action potential?
Explain the positive feedback of an action potential.
Repolarisation is two stages, inactivation of Na and opening of K
What are refractory periods?
ARP - All Na channels inactivate. No A.P can be fired whatsoever. Time taken from initial opening and initial inactivation of Na channels (1ms)
RRP - Na channels recovering as K channels open. If strong enough stimulus an A.P can be initiated. (4ms)
What is the structure of an Na and K channel?
- Remember how many subunits to be functional
- Ca channel similar to Na
How do individual channels open?
- Open in random manner
- Once open they are prone to inactivation and inactivate at random times
How does a voltage sensor work?
- Depolarised membrane means more positive charge in
- +ve charge in cell repels +ve sensor
- Conformatonal change, opens pore
How do local anaesthetics work?
- Lidocaine can be protonated (hydrophilic) or unprotonated (lipophilic)
- In body mainly protonated due to pKa
What fibres do local anaesthetics block?
In order:
- Small myelinated axons
- Unmyelinated axons
- Large myelinated axons
What happens when you change the diameter of an axon?
- Increase diameter
- Increase conduction velocity
How do you measure conduction velocity?
- Take nerve fibre with several different sized axons
- Stimulate an action potential with electric field
- Measure time taken from stimulus to electrode
- Velocity = Distance from stimulus to electrode / Time taken
What is the local current theory?
- Current flows into axon causing a local change in membrane potential, which spreads along the whole axon but decrease in amplitude as it does so
What properties does an axon need to have a high conduction velocity?
Capacitance = ability to store charge
Resistance = no of ion channels open, more channels open lower resistance
High resistance = voltage spreads further along axon
High capaticance = voltage changes more slowly
Draw graphs to represent increasing Cm and Rm
Where does myelination occur and what does it do to the properties of axons?
- Large axons myelinated (motor) but small (sensory) not
- Decreases capacitance, increases resistance and therefore increases length constant
- Allows saltatory conduction and therefore increased conduction velocity
How does myelin allow saltatory conduction?
- Good insulator
- Allows local current to depolarise next node above threshold and initiate an action potential
- Jumps from node to node
Draw a graph to show relationship between fibre diameter and conduction velocity
What are some diseases caused by demyelination?
- No myelin, length constant reduced, threshold not reached so conductance is blocked
- Stops saltatory conduction
