Action potential essay Flashcards
Why are action potentials needed?
So neurons can conduct electrical signals quickly over large distances in the body to coordinate behaviour
Who proposed the all-or-nothing principle of action potentials?
What was this based on?
Lucas and Adrian, start of the 20th century
Based on indirect observations of muscle fibres
Who proposed that action potential size was constant?
What was this based on?
Lucas and Adrian, 1920s
Based on direct observations during earthworm experiments
Who demonstrated that an axon could be electrically stimulated by electrodes?
Hodgkin and Huxley, 1939
Inserted electrodes filled with seawater into giant squid axons and measured the potential difference during stimulation
Found axon at rest had negative membrane potential (resting membrane potential) and stimulation caused depolarisation (inside positively charged relative to outside) followed by hyperpolarisation
This disproved Bernstein’s hypothesis
What did Bernstein propose?
- Membrane selectively permeable to only potassium ions
- Action potential is a transient breakdown of the membrane so predicted peak at 0mV
Who else showed that the action potential overshot 0mV?
What value did they obtain for the membrane potential during an action potential?
Curtis and Cole 1940
110mV - this was later challenged and 30mV proposed
Who proposed the sodium theory of the action potential?
What does this state?
Hodgkin and Katz 1949
States that depolarisation is caused by increase in sodium ion permeability of membrane
What did Hodgkin and Katz do to study involvement of sodium ions? (1949)
Meausured action potentials when squid axons were placed in different extracellular sodium solutions
When no sodium ions present, no action potentials produced
When low sodium ion concentrations, action potential reduced in height
When action potential peak height plotted against log of extracellular sodium concentration - showed straight line with a slope as predicted by Nernst equation
Who else studied involvement of sodium ions in action potentials?
Hodgkin and Stämpfli (1951) - in myelinated frog axons
Narahashi (1963) - in insect axons
Who used radioactive isotopes to study movement of ions across membranes?
Keynes (1951)
Looked at membranes of Sepia axons - found each impulse had a net movement of 3.7pmolcm^-2 of sodium in and 4.3pmolcm^-2 of potassium out
Suggests depolarisation caused by movement of sodium ions inward and repolarisation caused by movement of potassium ions outward
Who developed the voltage-clamp method? What did they investigate using this?
Cole and Marmont
Investigated positive feedback loop for sodium - increasing membrane depolarisation increases sodium ion permeability
What is the threshold of an action potential?
The point where outward currents (leak current and voltage-gated potassium ion current) can no longer balance inward currents (voltage-gated sodium channels)
(proposed by Hodgkin and Huxley using voltage-clamp studies)
What is the absolute refractory period?
Impossible to generate an action potential as voltage-gated sodium channels are inactivated
What is the relative refractory period?
Action potential possible but threshold higher than at rest as some sodium channels still inactivated
How can an action potential be explained?
- Resting membrane potential determined by leak conductance at rest
- Axon stimulated, membrane depolarised until threshold value reached, sodium ion permeability increases so more depolarisation
- AP reached peak (around +40) when sodium channels close and potassium channels open so potassium moves out of axon
- Membrane repolarised, as potassium ions continue to leave there is a period of hyperpolarisation where membrane more permeable to potassium than at rest (tends towards -75)
