Lecture 27. Action potential Flashcards
Hyperpolarisation
When the potential becomes more negative
(from -70 to -75)
the potential inside the cell moves closer to Ek( equilibrium potential of K+) and away from Ena( equilibrium potential of Na+)
Depolarization
When the potential becomes less negative( eg from -70 to -60 mV)
the potential inside the cell moves away from Ek and closer to Ena
The action potential
• A brief fluctuation in membrane potential caused by a transient opening of
voltage-gated ion channels which spreads, like a wave, along an axon.
• Action potentials occur after the membrane potential reaches a certain
voltage called the threshold (~ -55 mV). Needs to be at or above that
significance of action potentials
- The frequency of action potentials encodes information (a language by
which neurons communicate) - Action potentials are a key element of signal transmission along (often
very long) axons.
three stages of action potential
- Stimulus- membrane potential reaches threshold followed by fast depolarisation to ~ +30 mV (‘overshoot’)
- Repolarisation. K+ channel opens and the charge inside the cell decreases as K+ leaves.
- After-hyperpolarisation(AHP)
1+2= absolute refractory period. Even if there is another stimulus it cannot react. voltage-gated Na+ channels are inactivated. already open 3= relative refractory period. If a larger stimulus is present there is a possibility for another action potential
Ionic mechanism associated with each stage of AP
- When MP reaches the threshold there is a sudden activation (opening) of voltage-gated Na+ channels (PNa increase). At this moment PK/PNa is 1: 20 (before it was 40:1), therefore MP shifts towards the ENa+ towards +60 mV = overshoot.
- Opening of voltage-gated Na+ channels is short-lasting, as these channels inactivate quickly. This is followed by the transient opening of voltage-gated K+ channels, leading to repolarisation and
- After-hyperpolarisation AHP. Membrane potential shifts towards EK+ since PK/PNa becomes ~ 100:1
Voltage gated Na+ channels in AP
When the voltage threshold is reached, sodium channels open and Na+ ions
move into the cell along both the concentration and electrical gradient.
The influx of Na+ slows down and stops when:
1) The inside potential becomes positive (moves towards ENa) and thus attracts
Na+ less.
2) Na+ channels inactivate
Amplitude of AP
~100 mV and does not depend on the ‘stimulus’ intensity as long as the stimulus is ‘suprathreshold’.
How can action potential be evoked?
• Current follows the path of least resistance!
• Two main paths:
1) Outside the axon from + to –
2) Across the membrane and inside the axon. Only path 2 can change RMP!
When a current is generated as a circuit by an outside source and flows through the cell membrane from outside to inside = local hyperpolarisation at the anode (the MP becomes more negative).
When it flows from inside to outside = local depolarization (the MP becomes less negative)
where are AP generated first?
• APs are first generated in the axon initial segment (‘axon hillock’) which has the
lowest threshold, and thus serves as the ‘trigger zone’ for APs.
How are APs generated ‘physiologically’ in
CNS neurons?
- Depolarisation to the threshold is evoked by excitatory postsynaptic potentials (EPSPs) which spread mainly passively from dendrites. These potentials are graded and vary in intensity. When it reaches the threshold= -55mV it triggers the opening of V-gated ion channels-> AP.
- Once generated, APs are transmitted actively along the axon, away from the cell
body) .
- AP is generated in the axon initial segment
what happens if the inactivation gate is unable to close?
increased flow of Na+ into the cell
prolonged refractory period
paralysis- no more AP possible
what channels are responsible for generating AP?
voltage-gated Na+ channels
