Nerves/synapse 2 Flashcards
what does permeable mean?
There are “holes” in the cell membrane that allow for the intake of an ion (or output, depending on the position of the ion).
When does this stop? (permeability of K+)
The accumulation of unpaired negative ions inside the cell creates an electrical gradient that pulls K+ ions back into the cell until the electrical and chemical gradient are equal
What is equilibrium in neuron?
The point at which the chemical and electrical gradients across the membrane are equal, as described by the Nernst equation.
- The concentration force pulling K+ out of the cell and the electrical force pulling K+ into the cell reach equal amounts
- K+ will both enter and exit the cell, hence it being at equilibrium
- Equilibrium potential for the K+ is the main factor determining neuron resting membrane potential
What would the voltage be if the neuron was exclusively permeable to K+?
The voltage difference across the membrane would be -90 mV, but its actually closer to -70 mV. This is b/c there is a small inward leak of Na+, which pushes the equilibrium closer to the Ena.
How is the cell membrane permeable to K+?
- the resting permeability of K+ is caused by leak channels ( a type of ion channel)
- Leak channels are proteins that from K+ selective pores through membrane
- They are open at the resting membrane potential ( it serves as a hole, not a gate)
how to reach equilibrium in Na+, and Cl-?
To reach equilibrium Na+ will want to get in the cell unit the cell reaches +70 mV, Cl- will want to get into the cell until the cell reaches -80 mV
How is the membrane potential determined?
it is determined by the concentration gradients and relative permeabilities of membrane to different physiological ions.
the concentration gradient do not change but the permeabilities can change rapidly and dramatically
Who makes the greatest contribution to the membrane potential?
the dominant permeability makes the greatest contribution to the membrane potential.
At rest the dominant permeability is to potassium, so the membrane potential is close to Ek
why is the resting membrane slightly more positive than Ek?
Because there is a small inward leak of Na+ which pushes the membrane slightly toward Ena
What happens if there is no mechanism that can maintain the negative resting membrane potential?
The ions will flow in and out of the cell until E_rest = 0 mV
What is the sodium-potassium pump?
The sodium potassium gradients are maintained by the sodium potassium pump which uses the energy produced by ATP hydrolysis to pump sodium OUT and potassium IN against their concentration gradient .
Very important to maintain gradients.
What is an axon potential?
Axons propagate info from one region of the nervous system to another by brief electrical impulses called action potentials.
How do AP’s work?
Action potentials start at the initial segment of the axon and the propagate down the length of the axon to the presynaptic terminals
AP’s are transient depolarizing spike that moves down the axon.
At the AP peak the membrane potential approaches E_na.
When is the action potential initiated?
When the membrane potential depolarizes to a threshold level which is determined by the properties of ion channels in the axon membrane, especially a class of channels called voltage-gated sodium channels.
What is the normal threshold potential?
it is normally -50 mV, which is more positive than the resting potential.
What do the word depolarization, Repolarization and hyperpolarization?
Depolarization: membrane is getting more positive
Repolarization: membrane going back to its original resting level
Hyperpolarization: membrane going from its resting level to an even more negative level
Can the cell generate action potential without reaching threshold?
NO, HAS to reach threshold to be able to generate action potential
IT IS AN ALL OR NOTHING EVENT
describe the voltage-gated sodium channels
The rising (depolarizing) phase of the action potential is caused by sodium ions flowing into the cell through voltage-gated sodium channels
concentrated in axons
what are the 3 critical properties of sodium channels?
1) they are closed at the resting membrane potential but open when the membrane depolarizes
2) they are selective for Na+
3) the open channel rapidly inactivates, stopping the flow of Na+ ions
What is the dominant permeability during action potential?
during AP the dominant permeability shifts to potassium ions.
Once the sodium channels inactivate (btw this does not mean close), the dominant permeability shifts back to potassium ions.
describe the positive feedback
only 10% of sodium channels open at the initial segment, then the number increases to 60 and 100% as the membrane reaches peak of AP like an explosive event.
more sodium enters cell= more sodium channels open umtil it reaches the maximum creating a positive feedback. Very rapidly depolarize
What happens when inactivation of sodium channels happen
sodium influx stops
What happens when potassium is leaving the cell?
Since the cell is permeable to potassium and the cell membrane potential is positive, potassium ions will leave the cell to replenish the resting membrane potential.
what is the importance of AP being very short?
so that neurons can send alot of AP’s over a short period of time
does Na+ goes all inside and K+ all outside?
NO this is not what happens!
the concentrations of those ions change only a lil bit . There is only a tiny imbalance compared to total ions during AP’s.
