Membranes + Action Potentials Flashcards
What is voltage?
• Potential difference
• Unit = Volts
• Generated by ions to produce a charge gradient
(i.e. like a chemical battery)
What is current?
- Unit = Amps
* Movement of ions due to a potential difference
What is resistance?
- Unit = Ohms
* Barrier that prevents the movement of ion
How is membrane potential measured?
- A reference electrode is placed outside the cell. This is the zero-volt level
- Another electrode is placed inside the cell to measure a voltage difference that is negative compared with the outside
- All cells have a membrane potential.
What are ion channels?
Permeable pores in the membrane (ion channels) open and close depending on transmembrane voltage, presence of activating ligands or mechanical forces
Why are ion channels necessary?
- Lipid (hydrophobic) cell membrane is a barrier to ion movement and separates ionic environment
- The cell membrane can selectively change its permeability to specific ions
How can equilibrium potential be calculated?
Using the Nernst equation
What is the Nernst equation?
Too complicated, google it
What is the composition of ions like, intracellular vs. extracellular?
Intracellular: → low Na → high K → high Ca2+ → low Cl- Opposite for extracellular
Why do membrane potentials not rest at Ek or ENa?
membranes have mixed K+ and Na+ permeability (but at rest K+»_space; Na+)
What ions contribute to the value of real membrane potential?
Potassium, Chloride + sodium
What is the permeability of the membrane to each significant ion proportional to?
The size of each ion’s contribution
What is the purpose of the GHK equation?
More accurate way to describe membrane potential
What is the GHK equation?
Check notes
What happens when you increase the permeability of a membrane to a particular type of ion?
Shifts the membrane potential toward the reversal potential for that ion
What happens to membrane potential when Na channels open?
Shifts membrane potential towards sodium reversal potential → usually around 100 mv → therefore in positive direction
What happens to membrane potential when K channels open?
shifts the membrane potential toward about –90 mV (shifts the membrane potential in a negative direction, except when the membrane is hyperpolarised to a value more negative than the K+ reversal potential)
What happens to membrane potential when Cl channels open?
shifts the membrane potential toward about –70 mV (shifts toward the resting potential)
What is pertsynaptic potential?
temporary change in membrane potential produced by activation of a synapse by a single graded or action potential
What is equilibrium potential (E)?
Potential at which electrochemical equilibrium is reached
Why are Action Potentials important?
• Play a central role in cell-to-cell communication
• can activate intracellular processes
- in muscle cells, an AP is the first of a series of events leading to contraction
- in beta cells of the pancreas and AP stimulates insulin release
What does membrane permeability to ions depend on?
Conformational state of ion channels
How is a resting membrane potential generated?
Uneven ion distribution due to impermeability of membrane to these ions = difference in ion concentrations = potential difference across membrane
What is resting membrane potential for neuronal cells?
Negative change inside compared to outside = - 40 to-90 mv
How is an action potential generated?
Through membrane depolarisation - opening of VGSC causes Na influx → generates impulse
Why do VGSCs open before VGKCs?
Faster kinetics
How is the membrane repolarised?
VGKSs open slowly as VGSCs start to open causing K+ efflux
Why does hyper polarisation occur?
VGKCs are still open so membrane potential moves closer to K+ equilibrium
What is an absolute refractory period?
Period of time where it’s impossible to evoke another action potential due to inactive state of VGSCs
What is an relative refractory period?
Very strong stimulus (stronger than normal) is needed to evoke an action potential - some of the VGSCs have recovered
What is the role of Na+/K+ ATPase during resting membrane potential?
Maintains resting potential - in its resting configuration pumps Na out of cell
What is the role of Na+/K+ ATPase after an action potential?
Helps return membrane potential to resting value - in its active configuration na is removed from cell + K is moved into cell
How does the action potential spread down the axon?
Through saltatory conduction.
What is the purpose of myelin sheath?
High resistance + low capacitance prevents spread of AP - forces AP to jump
What are the Nodes of Ranvier?
Small gaps between myelin intermittently placed along axon - AP has to jump from node to node
What happens to the AP at the axon terminal?
Can no longer continue
Why is an action potential an all or nothing event?
Once threshold potential is reached, a full sized AP is triggered
What mainly restores the electrochemical equilibrium after an action potential?
Non-voltage gated ion channels
Why does the action potential propagate down the length of the axon?
→ While one area of the axon is at peak AP, local current flow depolarizes adjacent area towards threshold
→ slight depolarization spreads and more VGSCs open + more N a rush in
→ old area is still returning to resting potential so depolarisation starts to happen at new adjacent area.
What is continuous conduction?
Propagation along an unmyelinated axon
What is saltatory conduction?
Propagation along an myelinated axon
Why is continuous conduction slower than saltatory?
No jumping + more time spent waiting for all voltage-gated sodium channels to open, etc.
What 2 factors affect conduction velocity?
→ axon diameter
→ myelination
When are ion channels opened?
Membrane depolarisation
What inactivates ion channels?
Sustained depolarisation
What closes ion channels?
Hyperpolarisation or repolarisation
What is passive propagation?
Only resting potassium channels open?
What increases conduction velocity?
Lange diameter + myelinated axons
What decreases conduction velocity?
reduced axon diameter (i.e. re-growth after injury), reduced myelination (e.g.
multiple sclerosis and diphtheria), cold, anoxia, compression and drugs (some anaesthetics)
What are the 3 main factors that influence the movement of ions across the membrane?
- Concentration of ion on both sides of the membrane,
- the charge on the ion
- the voltage across the membrane.
Why is the K+ equilibrium potential negative (e.g. -70mV) and the Na+ equilibrium potential positive(e.g. +40mV) when both are positive ions?
more K+ inside the cell than outside so tend to flow out of the cell, while more Na+ outside the cell than in, therefore tend to flow into the cell. A potential of -70mV is needed to attract K+ and stop net outward flow, while a positive charge of +40mV is needed to repel Na+ from entering the cell.
Which ion is important for the upstroke (rising portion) and which is important for the falling portion of the action potential? In which direction do these ions move?
The upstroke mediated largely by Na+ ions moving down their concentration gradient into the cell. The falling portion of the action potential dominated by K+ ions going down their concentration gradient and therefore exiting the cell
What factors influence the speed of propagation of an action potential along an axon?
Larger axons have lower resistance, so ions move faster – conduction velocity is proportional to the square root of the axon diameter. There is a linear relationship between conduction velocity and myelin thickness
