Nerves/synapse 2

Question 1

what does permeable mean?

Answer 1

There are “holes” in the cell membrane that allow for the intake of an ion (or output, depending on the position of the ion).

Question 2

When does this stop? (permeability of K+)

Answer 2

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

Question 3

What is equilibrium in neuron?

Answer 3

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

Question 4

What would the voltage be if the neuron was exclusively permeable to K+?

Answer 4

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.

Question 5

How is the cell membrane permeable to K+?

Answer 5

• 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)

Question 6

how to reach equilibrium in Na+, and Cl-?

Answer 6

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

Question 7

How is the membrane potential determined?

Answer 7

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

Question 8

Who makes the greatest contribution to the membrane potential?

Answer 8

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

Question 9

why is the resting membrane slightly more positive than Ek?

Answer 9

Because there is a small inward leak of Na+ which pushes the membrane slightly toward Ena

Question 10

What happens if there is no mechanism that can maintain the negative resting membrane potential?

Answer 10

The ions will flow in and out of the cell until E_rest = 0 mV

Question 11

What is the sodium-potassium pump?

Answer 11

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.

Question 12

What is an axon potential?

Answer 12

Axons propagate info from one region of the nervous system to another by brief electrical impulses called action potentials.

Question 13

How do AP’s work?

Answer 13

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.

Question 14

When is the action potential initiated?

Answer 14

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.

Question 15

What is the normal threshold potential?

Answer 15

it is normally -50 mV, which is more positive than the resting potential.

Question 16

What do the word depolarization, Repolarization and hyperpolarization?

Answer 16

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

Question 17

Can the cell generate action potential without reaching threshold?

Answer 17

NO, HAS to reach threshold to be able to generate action potential

IT IS AN ALL OR NOTHING EVENT

Question 18

describe the voltage-gated sodium channels

Answer 18

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

Question 19

what are the 3 critical properties of sodium channels?

Answer 19

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

Question 20

What is the dominant permeability during action potential?

Answer 20

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.

Question 21

describe the positive feedback

Answer 21

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

Question 22

What happens when inactivation of sodium channels happen

Answer 22

sodium influx stops

Question 23

What happens when potassium is leaving the cell?

Answer 23

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.

Question 24

what is the importance of AP being very short?

Answer 24

so that neurons can send alot of AP’s over a short period of time

Question 25

does Na+ goes all inside and K+ all outside?

Answer 25

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.