Electrophysiology 3: The Hodgkin- Huxley model of the AP Flashcards
What is the Hodgkin and Huxley model of the action potential?
The mathematical model that explains how action potentials are generated and propagated.
What was known by the 1950s?
1) The negative resting potential
2)High internal [K+]
3)Na+ needed for nerve and muscle excitability
In a steady state. what is the membrane potential? and why?
0mv. This is because there is no net inward or outward current (Voltage between intracellular and extracellular means the net ionic current flow is zero)
What does Vm (at rest) depend on?
Ek (gk relatively high) with small influences of ENa (because low gNa)
what is the E in ENa and EK?
The equilibrium potential
What is the g in gNa and gK?
The sodium conductance and potassium conductance/ the permeability of the membrane, representing how easily sodium and potassium ions can pass through the membrane.
Explain the no net current flow.
The relatively high K+ conductance allows an outward k+ current but equally there is an inward Na+ current through the small Na+ conductors= large driving force over a small conductance but K+ at -65mv means there is a smaller difference from K+ when vm is given at -75mv so with K+ you have a small driving force across a larger conductance so the net result = inflow of Na+ and outflow of k+ = no net current flow
What is the ionic permeability hypothesis and what does this mean in terms of an action potential? as stated by Hodgkin in 1951
“The permeability changes during the action potential probably consist of a rapid but transient increase in permeability to sodium and a delayed increase in the permeability to potassium. It is suggested that both permeability changes vary with membrane potential in a graded but reversible manner.” (Hodgkin 1951)
what is the capacitive current?
The current that flows across the membrane capacitor when the membrane potential changes over time.
Define Capacitive current (Ic) in more detail.
The current that results from the change in charge stored on the membrane’s capacitor as the membrane potential (Vm) changes over time.
What is the equation for Ic?
Ic= Cm dVm/dt
Cm= membrane capacitance ( how much voltage the membrane can store for a given voltage difference)
dvm/dt = rate of change of membrane potential over time
What is the voltage clamp? and who was it developed by and when?
1) Developed by Cole, Hodgkin, Huxley, Katz and others between 1940-1950.
2) Reasoned that if Vm could be held (‘clamped’) at any desired test voltage, then Iion could be measured, and hence, gion determined for the voltage
Iion = gion (Vm-Eion)
what is the voltage clamp technique used for?
The voltage clamp is an experimental technique used to measure ionic current flowing across a cells membrane while maintaining a constant membrane potential (voltage), allowing us to understand how specific ion channels behave at different membrane potentials.
Describe the voltage clamp technique
1) You would clamp the membrane potential to the desired value by the command voltage
2)Voltage clamp amplifier compares the membrane potential of the desired (command) potential
3) Any difference between the two and the amplifier injects current into the axon via a second electrode to keep the difference minimal
What does a normal action potential look like?
Look at image in notes
What does an action potential recorded under a voltage clamp look like?
Look at image in notes
Describe what an action potential under a voltage clamp looks like in terms of its inwards and outwards current and what do these currents mean?
1) Inward current = negative
2)Outward current = positive
Inward current is what drives the depolarising phase, which is due to inward current flow (due to sodium ions) whereas, repolarising phase is inward dominated by the outward current flow of K+. At peak of action potential (change from + to -) there will be no current flow
What is the difference between voltage dependant activation and voltage dependent inactivation?
Voltage-dependant activation = the process by which voltage-gated Na+ channels open in response to depolarisation of the cell membrane
Voltage-dependent inactivation is the process by which the voltage-gated Na⁺ channels close after opening, even if the depolarization persists.
