LECTURE 2

Question 1

What forces move ions across membranes?

Answer 1

Chemical and electrical

Question 2

How do chemical forces move ions across membranes

Answer 2

Differences in concentration: diffusion from a region of high concentration to a region of low concentration

Question 3

How do electrical forces move ions across membranes

Answer 3

Interior of cell is negatively charged so positively charged cations are retained and negative ions will be expelled

Question 4

What is the electrochemical driving force

Answer 4

The electrochemical driving force is a combination of the chemical and electrical forces acting on any particular ion

Question 5

2 broad categories of ion channels

Answer 5

1. Channels that are gated and require a stimulus to open
   ligands, mechanical force or voltage
   specific to particular ion(s)
2. Channels that are always open and allow free movement of ions

Question 6

resting concentration Na+ and K+

Answer 6

Under resting conditions, the concentration of Na+ ions is ~ 10x higher outside the neuron compared to the concentration of Na+ ions inside

At the same time, levels of K+ ions are ~ 15x higher inside the neuron compared to the extracellular environment

Question 7

Potassium movement

Answer 7

There is a constant flow of K+ ions down their concentration gradient, from the inside of the neuron to the outside

This movement occurs via open (or leaky) K+ channels that are situated in the membrane of the neuron

Question 8

Na+/K+ ATPase pump

Answer 8

The ion gradient is maintained by the continuous operation of the Na+/K+ ATPase pump

It moves 3 Na+ ions from the inside of the neuron to the outside of the cell

At the same time, 2 K+ ions are moved from outside the neuron to the inside of the cell

At each cycle of the Na+/K+ ATPase pump, the cell loses one positively charged ion from the intracellular environment

Question 9

polarisation meaning

Answer 9

The difference in charge across the membrane of the neuron is referred to as polarisation

Question 10

Where is there more positive charge at rest

Answer 10

At rest, there is more positive charge outside the neuron compared to the inside of the neuron

Question 11

What is the resting membrane potential (meaning and number)

Answer 11

The difference in voltage across the plasma membrane when the neuron is at rest is called the resting membrane potential

For most neurons, the resting membrane potential is ~ -70mV

Question 12

Electrochemical gradients of sodium

Answer 12

When Na+ channels open:
chemical gradient drives ion movement into the cell

electrical force pulls + ions into the cell

both act in the same direction = Na+ will enter the cell

Question 13

Equilibrium of sodium movement

Answer 13

As Na+ moves into the neuron, the charge inside the cell starts to become positive and the electrical gradient decreases, along with the chemical gradient

Eventually, the chemical and electrical forces will be exactly in balance and there will be no net flow through any open channels

Question 14

What is the equilibrium potential

Answer 14

The equilibrium potential (E) is the membrane potential required to exactly counteract the chemical forces acting to move one particular ion across the membrane.

Question 15

Electrochemical gradient of potassium

Answer 15

When K+ channels open:
chemical gradient drives ion movement out of the cell

but electrical force pulls + ions into the cell

two forces act in opposite directions

chemical force > electrical force, so K+ moves out of the neuron

Question 16

Equilibrium of potassium movement

Answer 16

As K+ moves out of the neuron, the charge inside the cell starts to become even more negative, so the electrical gradient becomes stronger

Eventually, the chemical force that drives K+ out of the cell = the electrical force driving K+ back into the cell

At this point, there will be no net flow of K+ ions

Question 17

What equation is used to calculate the equilibrium potential

Answer 17

Nernst equation
E = 61 log Co
___ ___
z Ci

E= equilibrium potential in millivolts (mV)
z = charge of ion
Co = concentration of the ion outside the neuron
Ci = concentration of the ion inside the neuron

Question 18

How are action potentials triggered or made less likey

Answer 18

If the membrane potential is depolarised beyond a certain critical level (threshold potential = -55mV) then an action potential is triggered in the neuron

Other incoming signals can do the reverse and hyperpolarise the membrane (i.e. cause the membrane potential to decrease), so making an action potential less likely

Question 19

What are Voltage-gated ion channels

Answer 19

Embedded in the plasma membrane of the neuron are ion channels that are sensitive to the voltage of the cell

These channels open only when the voltage in the cell reaches a certain value

Question 20

Voltage-gated Na+ & K+ channels

Answer 20

Voltage-gated Na+ channels have both an activation gate and an inactivation gate. At rest, the activation gate is closed and the inactivation gate is open

Voltage-gated K+ channels have one activation gate, which opens to allow the flow of K+ ions through the channel and closes to stop the flow of K+ ions

Question 21

1. Initial stimulation of neuron

Answer 21

When the neuron receives an excitatory signal or stimulus, ligand-gated ion Na+ channels open

Small amounts of Na+ will move down their concentration gradient into the neuron and the resting potential will start to become more positive

Question 22

2. Depolarisation of neuron

Answer 22

Once the membrane potential reaches a critical threshold of -55 mV, voltage-gated activation gates in the Na+ channel open quickly, allowing Na+ to flood into the neuron

As a result of the large influx of positively charged Na+, the neuron quickly loses its negative charge and undergoes depolarisation

Question 23

Is the neuron positive or negative after it undergoes depolarisation

Answer 23

positive

Question 24

3. Inactivation of Na+ channels

Answer 24

When the inside of the neuron become highly positive, the pore of the voltage-gated Na+ channels is plugged by the inactivation gate and the flow of Na+ into the neuron stops

Question 25

4. Repolarisation

Answer 25

Eventually the intracellular environment of the neuron becomes sufficiently positive that voltage-gated K+ channels begin to open slowly

Opening of these channels allows K+ to flow down its concentration gradient out of the cell

This movement of K+ causes the inside of the neuron to quickly regain its negative charge in a process called repolarisation

Question 26

5. Hyperpolarisation

Answer 26

In response to the increasingly negative charge inside the neuron, the voltage-gated K+ channels close.

Because this process is slow, some K+ ions continue to move outside the cell while the channel is closing

This extra efflux of K+ causes the membrane potential to become more negative than the resting potential of -70 mV. This process is called hyperpolarisation

Question 27

6. Refractory period

Answer 27

During the period of hyperpolarization, the neuron will not be able to fire another action potential. This is termed the refractory period

Eventually, the action of the Na+/K+ ATPase pump will restore the resting membrane potential to -70mV and the neuron will be ready to fire another action potential