4 - Action Potentials II Flashcards
What is a signal?
Change in voltage (deviation from RMP)
What are the 2 methods of transporting an electrical signal?
- passive
- active
How does passive transport of a signal work?
Diffusion of ions through cytoskeleton. Doesn’t need additional energy.
Why is passive ion diffusion unreliable for transferring signals between neurons?
Slow and breaks down over long distances
-> signal gets smaller as it propagates across distance because of K2p leak channels (K+ always leaking & pushing mempot back to RMP)
Are action potentials passive or active?
Active - require energy
Are APs reliable?
Yes - doesn’t break down over long distances
Why is the equilibrium pot for Na+ not reached during an AP?
Because K2p channels always leaking, Nav channels start closing and Kv start opening so overshoot doesn’t reach ENa+
What are the 6 steps in an AP?
- Depolarisation to threshold
- Nav channels open - rapid depolarisation
- Nav channels close - repolarisation
- Kv delayed rectifier K+ channels open - hyperpolarisation
- Nav channels inactivate - absolute refractory period
- Kv channels close - relative refractory period
- Depolarisation to threshold
AP begins when membrane depolarised past threshold, allowing Nav channels to open. Mempot rapidly heads towards ENa
What are causes of initial membrane depolarisation?
Physically-gated Na channels open or depolarisation may be ‘inherited’ from somewhere else in neuron
- Nav channels open - rapid depolarisation
Vm moves towards ENa and as membrane continues to depolarise, more Nav channels open (positive feedback)
- Nav channels close - repolarisation
Nav channels only open for 1ms and then close and inactivate.
As Nav channels close, return to PNa < Pk and Vm moves towards Ek
Can Vm still move towards EK if there are no Kv channels?
Yes - but if we do have Kv channels, hyperpolarisation can occur
- Kv delayed rectifier K+ channels open - hyperpolarisation
K2p channels stay open. Kv channels open at high voltages but they’re delayed. Mempot hyperpolarises below RMP as Vm approaches Ek
- Nav channels inactivate - absolute refractory period
Nav channels inactive until Vm hyperpolarised. Can’t have 2nd AP for at least 1ms.
- Kv channels close - relative refractory period
Delayed rectifier Kv channels take sometime to close — prolong hyperpolarisation and makes it more difficult to indicate further APs
Channel opening are stochastic
AP depends on thousands of channels working together. There’s variability between each Nav channel or each Kv.
T/F - opening of Nav channels is delayed relative to Kv channels
False - other way round
Does AP propagate away or towards spike initiation zone?
Away from spike initiation zone and towards axon terminal
What happens to Nav channels in adjacent part of axon if sufficient Na+ diffuses?
AP can be triggered by Na+ entry which causes depolarisation
What prevents back-propagation of an AP?
Inactivated Nav channels
Where are ion channels concentrated in an axon? (Hint talking about myelination)
Nodes of ranvier - only places where Nav and Kv channels are & only place where APs can occur
Where does passive diffusion occur in an axon?
Myelinated areas
What axons will have faster conduction velocity?
Thicker, more myelinated and at colder temp
How does lethal injection KCl work?
Large increase in extracellular K+ depolarises RMP of excitable cells like neurons and cardiac cells (stops heartbeat)
