Action potentials Flashcards
What had already been established in the early 20thc
Neurons respond to stimulation with ‘all-or-none’ spikes of electrical activity which self-propagate along axons
What did Bernstein propose as the cause of action potentials
Bernstein (1902)- due to transient loss of selective membrane permeability, which would lead to membrane potential to shoot to 0mV
Whydid the mechanisms underlying action potential generation remain obscure for a long time
It had not been possible to directly record the transmembrane potential in axons as they are so thin- electrophysiological studies on axons had relied on extracellular recordings
Who got a Nobel prize for the functions of neurons
1932- Sherrington and Adrian
Who got a Nobel prize for the function of single nerve fibres
1944- Erlanger and Gasser
Who got a Nobel prize for the mechanisms of axonal excitability
1963- Hodgkin and Huxley
Who got a Nobel prize for the study of single ion channels
1991- Neher and Sakmann
How were the first intracelllular recordings of the action potential carried out
Hodgkin and Huxley (1945)-used the giant axon of a squid as it is big enough to enable the insertion of microelectrodes into the axoplasm
What did the first intracellular recordings of action potential reveal
Hodgkin and Huxley (1945)- the membrane potential reversed during action potentials (overshot 0mV) then became transiently hyperpolarised below resting membrane potential following the action potential (after-hyperpolarisation)
What did the first intracellular recordings of action potential reveal about their cause
Hodgkin and Huxley (1945)- Could not be explanied by loss of selective membrane permeability, but rather changes in the permeability to specific ions
What is the sodium hypothesis
Suggested the upstroke of action potential could be explained by increased Na+ permeability- this could cause Na+ to diffuse into the cell, dragging the membrane potential towards the positive Nernst potential for Na+ ions (around 55mV)
Study investigating the sodium hypothesis
Nastuk and Hodgkin (1950)- plotting action potential peak against the log of extracellular Na+ conc reveals a straight line with a slope predicted by Nernst for Na+, though the membrane potential never quite reaches E Na due to leak channels
What increases the membrane permeability to Na+ in the action potential
Voltage-gated Na+ ion channels- if the membrane is depolarised the channels open and Na+ permeability increases, which increases membrane depolarisation, which increases Na+ permeability in a positive feedback loop
What 3 membrane conductances can be multiple phases of the action potential be explained by
Leak conductance, voltage-dependent Na+ conductance, voltage-dependent K+ conductance that all try to clamp the membrane potential at their Nernst potential
Which of the 3 membrae conductances determines the resting membrae potential
The leak conductance
Which of the 3 membrane conductances is recruited whe the membrane is depolarised
Voltage-dependent Na+ conductance- the membrane potential shoots towards E Na as the Na+ conductance increases in a positive feedback loop, dominating leak conductance
What happens to the Na+ conductance when the peak of the action potential is reached
Na+ conductance inactivates, and there is a delayed recruitement of voltage-dependent K+ conductance that drags the membrane potetial towards E K and rapidly repolarises the membrane
Which of the 3 membrane conductances causes the undershoot/after-hyperpolarisation
The K+ conductance takes time to turn off after repolarisation, causing the undershoot as the membrane is more permeable to K+ ions than at rest
When the K+ conductance turns off, the membrane potential is again set by leak conductance
What is it necessary to do in order to determine the properties of the ionic conductances underlying the action potential by manipulating membrane potential
Membrane potential affects membrane permeability which in turn affects membrane potential, so it is necessary to break this feedback loop in experiments
How can the membrane potential/permeability feedback loop be broken
Axial voltage-clamp- a wire is inserted along the length of an axon and connected to an electronic feedback circuit to clamp the axon’s membrane potential and prevent further depolarisation, while recording the current required to achieve this voltage clamp (Hodgkin and Huxley, 1952)
What does the axial voltage clamp allow the measurement of
Measurement of membrane currents as a function of time at any given voltage (Hodgkin and Huxley, 1952)
Hodgkin and Huxley (1952)- what was the effect of stepping the membrane potential to a depolarised level (0mV)
Reveals a transient inward current of Na+, followed by a sustained outward current of K+
Hodgkin and Huxley (1952)- how could the current for each ion be independently measured WITHOUT using pharmacological blockers
Removing Na+ from the extracellular fluid eliminates the inward current, leaving the delayed and sustained outward current mediated by the voltage-dependent K+ conductance.. subtraction can calculate the current mediated by the Na+ conductance
Hodgkin and Huxley (1952)- how could the current for each ion be independently measured using pharmacological blockers
Tetrodotoxin (TTX) blocks the Na+ conductance, while tetraethylammonium ions (TEA+) block the delayed K+ conductance