Lecture 4 - Action potentials Flashcards
Conductance of an ion across the membrane depends on …
Permeability (ion channels)
Equilibrium potential (driving force)
Conductance is the inverse of electrical resistance. If the conductance of the membrane to a particular ion is low, then the resistance to movement of that ion across the membrane is high.
Vm
Membrane potential - at the top of the action potential the membrane potential is reaching the equilibrium potential for sodium
GNa+
Na+ conductance - at rest, there is very little conductance of sodium, as the action potential starts the conduction of sodium goes up remarkably and then comes back down to just about zero - the sodium will depolarise the cell as it comes in
GK+
Potassium conductance - potassium conductance also rises more slowly and not to such a great extent but also lasts much longer, potassium we repolarise and hyper polarise the cell as it goes out
Voltage gated Na+ channels
Activation gate (voltage sensor) - the ‘on’ switch
Selectivity filter - specific for Na+
Inactivation gate - actively closed - the ‘off’ switch
Blocked by tetrodotoxin - from the puffer fish (binds irreversibly to the channel pore and block it)
When the sodium can flow through, it is flowing down its electrochemical gradient from outside to inside
Made up of two beta subunits and four alpha subunits
Subunits of Na+ channels
2 beta subunits and 4 alpha subunits
Inactivation gate
Inactivation gate - ‘ball and chain’ model
The inactivation gate is part of an intracellular loop
Not activated Na+ voltage gated channel
Not activated = pore closed - positive charge in the way therefore deflects the positive charge on the sodium (note: not activated is not the same as inactivated)
Activated Na+ voltage gated channel
Activated = pore open - positive charge nearby for selectivity - Something causes the voltage to become depolarised sufficiently to reach threshold and this voltage change moves the positive charges and opens the gate which allows for sodium to come through down its electrochemical gradient into the cell allowing for a larger depolarisation which is known as an action potential
Inactivated Na+ voltage gated channel
pore still open - but ball on the end of the chain blocks it
Later in the action potential the inactivation gates close - pore still open but the ball on the end of the chain has essentially plugged the sodium channel from the inside
The ball and chain is activated by a voltage change as well - the ball and chain responds more slowing to the charge than the activation gate so the not activated state opens more quickly and the inactivated state closes more slowly
Not activated is closed, inactivated is open but locked from the inside but in both cases the sodium cannot enter the cell
Voltage-gated K+ channels
Activation gate
Selectivity filter - specific for K+
Inactivation gate - slower than for voltage-gated Na+ channel
Blocked by tetraethylammonium
Voltage gated Na+ channels are blocked by
tetrodotoxin - from the puffer fish (binds irreversibly to the channel pore and block it)
Voltage gated K+ channels are blocked by
tetraethylammonium
Absolute refractory period
No action potential is possible
Relative refractory period
Action potential is possible but more difficult to initiate