Membrane & Action potentials Flashcards
Where do electrical signals start?
They start in the receptor cells
What three kinds of electrical potentials are there?
- Receptor Potential
- Synaptic potential
- Action potential
Which of the three types of electrical potential is the fastest?
Action potential is the fastest
Synaptic is the second and the third is Receptor potential
What are the requirements for electrical signaling between nerve cells?
- Must be fast
- Must cross a long distance
- Should not lose strength
What is the nature of the resting potential inside a cell, compared to the extracellular environment?
The nature of the charge is negative ( -65 mV )
What happens when you feed a positive current into a nerve cell? What happens if you increase the strength?
The positive current will depolarize the cell until the threshold is met. This translates to an action potential. A stronger positive current does not result in a stronger action potential but leads to multiple action potentials.
What would happen if only passive conduction was present?
The charges would diffuse across the cell - the signal becomes weaker the further it goes.
What is the benefit of active conduction compared to passive conduction?
The signal does not lose strength over time.
What two principles is the resting membrane potential based on?
- Diffusion of particles (from high concentration to low concentration)
- Electrical forces between electrical charges (positive and negative forces)
What are the four main ions for signal transduction in neurons?
K+ , Na+ , Cl- and Ca2+
What happens at the electrochemical equilibrium?
At the equilibrium, the diffusion forces will equal the electrical forces of the ions (K+)
No net movement of K+ ions on either side of the membrane.
What is the equilibrium potential?
The potential at the electrochemical equilibrium.
Why don’t ion concentrations change, if there are ions flowing in an out constantly, even at a resting membrane potential?
Because the amount of ions creating this flow is very small. About one millionth of the total concentration.
(Small, almost non-detectable shift)
What can you use the Nernst equation for?
Calculating the Equilibrium potential.
What is the reversal potential?
The reversal potential creates a flow that brings the potential back to the equilibrium if the concentrations deviate.
What is hyperpolarization and depolarization, regarding the resting potential?
Hyperpolarization goes under the resting potential (becomes more negative) and depolarization goes above the resting potential (becomes more positive)
What factors influence the resting potential?
Concentrations of K+, Na+ and Cl- inside and outside the cell, as well as the membrane permeability for those ions (Ca2+ is NOT found freely inside or outside the cell)
What ion channels are leaky on the membrane?
K+ ion channels are leaky.
Na+ channels are closed
What ion dominates the resting membrane potential mechanism?
K+ dominates the control of the resting membrane potential.
What happens if the extracellular concentration of K+ is increased?
The resting membrane potential is also increased (becomes more positive)
Why does K+ dominate the resting membrane potential?
Because the permeability for K+ is much higher than all other ions, at the resting membrane potential.
What happens to permeability during an action potential?
The membrane becomes much more permeable for Na+ ions than K+ ions, thus creating an ion flow.
What is the function of ion transporters?
It does active transport of ions against the gradient. This costs energy (ATP)
What is the function of ion channels?
It allows ions to diffuse down the concentration gradient. Those are selectively permeable for certain ions.
Roughly explain how the Na/K ATPase pump works
The enzyme starts with a high affinity for Na+. After 3 Na+ ions bind to the enzyme, ATP phosphorylates the enzyme, which changes configuration and opens the gates on the other side. The affinity changes and the Na+ is released. The enzyme then gets a high affinity for K+. After the K+ bings to the enzyme, it dephosphorylates and changes configuration again, releasing K+ on the other side and gaining a high affinity for Na+ again. (a transport cycle)
How much approximate energy does the Na/K ATPase use in the brain?
Around 20 - 40% of the energy consumption in the brain.
Why is the efflux of Na+ reduced when external K+ is removed?
Because relatively less K+ will bind to the Na/K ATPase
What happens when the ATPase activity is blocked?
The efflux of Na+ is greatly reduced, as the protein cannot get phosphorylated.
What is the energy source of Ion exchangers?
The ion gradient op the ions it transports.
What ion is (usually) used for Co-transport and anti-port
Na+ is usually used as an “exchange” for other ions.
What are the model requirements for studying electrical signaling in nerve cells?
- Nerve cells that produce action potentials
2. It is well accessible
Why were the first nerve experiments done on squids?
The squid had a giant axon which was easily accessible.
What two types of recordings were performed on the squid experiment in the 50s?
- Voltage clamp - Artificial changes in voltage, induced with current
- Current clamp - induce a current to see how the voltage changes over time
Voltage = amount of mV Current = flow of ions
What is the difference in duration of how long ion channels are open, for Na+ and K+ channels?
K+ channels are open for a longer amount of time than Na+ channels.
Why do Na+ channels remain closed after a brief moment of depolarisation?
The Na+ channels are affected by two mechanisms. The first mechanism is the opening of the channel after depolarisation. After it has been activated, a second mechanism is put in motion, that blocks ions from passing through. It is like a cork on a wine bottle.
The Na+ ion channel is open, but inactive and cannot transport Na+ inwards for a brief moment of time, until the “cork” is removed.
What are the two components of the Na+ ion channel? (two mechanisms)
- Fast activation gates - open during depolarisation
2. Slow inactivation gates - close during depolarisation
Why is it important that the inactivation gates in the Na+ ion channel are slower than the fast activation gates?
Because otherwise current will be completely impaired and no flow would happen. With this system, the flow is controlled, as only a small discharge is needed.
What is the refractory period?
The period after an action potential, in which no other action potential can happen again
Why is there a refractory period?
This has to do with the inactivation of the Na+ channels. After the opening of the channels, the slow inactivation gates cover up the channel and don’t let ions through. They keep it covered for some time - during which no flow is possible, even after another depolarisation.
What leads to the opening of K+ channels?
A change in the voltage
K+ channels have a (depolarized) voltage sensor
What types of ion channel classes are there?
- Voltage Gated
- Ligand gated (GTP)
- Thermosensitive
- Mechanosensitive
Describe Phase 1 of depolarization.
Na+ and K+ channels are closed, but there are some leaky K+ channels
Describe Phase 2 of depolarization.
When the membrane gets depolarized, Na+ channels start to open (Permeability for Na+ is increased)
Why does the cell become more depolarized during Phase 2 of depolarization?
Because the Na+ channels are starting to open, letting more positive ions inside the cells, thus depolarizing it
(positive feedback loop)
Describe Phase 3 of depolarization.
Na+ channels are open and a lot of ions flow inside the cells
Describe Phase 4 of depolarization.
In Phase 4, Na+ channels are closed by the inactivation gates, and K+ channels open, letting K+ ions flow outside the cell.
Describe Phase 5 of depolarization.
:(
Where does the initial action potential take place?
It happens at the soma (cell) and NEVER in the middle of the neuron
This is because the impulse must go only in one direction
What does myelin prevent in terms of ion concentration?
Myelin prevents ion leaks
This is how the ion concentration gradient remains constant, very little to none is lost over a distance.
Why is diffusion faster in myelinated axons?
The concentration difference is maintained and the ions will be more “willing” to move towards a low concentration gradient, as the differences remain high (hope this is understandable)
