Basis of neuronal excitability Flashcards
What is the voltage at the peak of depolarisation?
+40mV
What is called when the line goes down after the peak?
repolarisation
What is it called in the graph when the line goes below -70mV?
hyperpolarisation
How long does an action potential take?
1-2msec
What is depolarisation caused by?
influx of Na+ (increasing PNA)
What is repolarisation caused by?
- K+ influx (increasing PK by opening more channel)
- terminating activity of extra Na+ channels (decrease in PNa)
What is gK?
the conductance of potassium
What does the membrane act like?
an electrical resistor (R)
What is the equation for conductance
g=1/R
What happens to voltage gated channels during depolarisation and repolarisation?
- depolarisation=they open
- repolarisation=they close
What causes the initial depolarisation that opens the odium voltage gated channels?
synapses and generator potentials
Explain how an action potential occurs
- Initial stimulus (depolarisation) causes:
- Opening of V-gated Na+ channels (increase in gNa)
- Na+ entry causes further depolarisation increasing gNa further
- Em approaches ENa (PNa (gNa) has increased 1000x)
- Na+ channels inactivate (decrease gNa)
- Na+ influx stops despite maintained stimulus
- Opening of V-gated K+ channels (increase in gK)
- K+ exit => repolarisation (but gK increases after Em returns to rest)
- Em approaches Ek (hyperpolarisation)
- V-gates K+ channels close (decrease in gK)
- Em returns to resting value (leak channels)-NOT PUMPS
What has to happen for depolarisation to occur?
- Na+ influx (leak and voltage-gated) > K+ efflux (leak)
- therefore threshold has to be reached
In the absolute refractory period can an action potential be established?
no further AP by any stimulus because Na+ V-gated channels are mostly inactivated and the K+ V-gated channels are open
Can an action potential occur in the relative refractory period?
yes but you need a stronger stimulus to open sufficient Na+ channels to increase gK (threshold higher) this happens because Na+ v-gated channels are recovering from inactivation and some of the K+ channels are still open
Why can’t action potentials move backwards?
because the membrane is refractory
How does propagation of an action potential occur in a myelinated neutron?
- saltatory conduction
- ions flow from node of ranvier to another node of ranvier
What is myelin composed of?
glial cells
How does the current clamp experiment work?
1) current injected by electrode
2. current injection causes depolarisation and the stimulus to open channels
3. Record the change in Em in neurone during action potential
In the current clamp experiment, what does a brief injection cause?
a single action potential
What does a long injection cause?
multiple action potentials
