Potentials

Question 1

what is the resting membrane potential?

Answer 1

The resting membrane potential is the potential difference across a membrane when it is not under an action potential.

Question 2

what is resting membrane potential in nerve cells?

Answer 2

in nerve cells it is around -70mV

Question 3

what convention is used to measure the resting membrane potential?

Answer 3

the convention that the inside of the cell is more negative than that outside.

Question 4

how is the resting membrane potential created?

Answer 4

The resting membrane potential is created by currents (charged ions) flowing across the membrane.

Question 5

how does current travel across the membrane to generate the resting membrane potential?

Answer 5

Diffusion – the Na+ and K+ ions can diffuse down their concentration gradients via channels in the membrane

Active transport – pumps such as the ATPase, pump 3Na+ out and 2k+ in, which creates the concentration gradients for diffusion

Question 6

what is the diffusion potential?

Answer 6

the potential difference generated across the membrane when the ion moves down the concentration gradient – contributing to the resting membrane potential

Question 7

what factors does the diffusion potential depend on?

Answer 7

Polarity of Charge – the charge that each ion carries, in the case of the neuron, the main two ions have the same 1+ charge

Concentration Gradient – the concentrations of charged ions in the extracellular and intracellular fluid, on either side of the membrane

Permeability – the relative permeability of the membrane to each of these ions. Different ions have different permeabilities because of the different carrier proteins and channels that transport them across the membrane

Question 8

what 2 main ions show the diffusion potential, and contribute to the resting membrane potential?

Answer 8

Potassium (K+) – abundant in the ICF, as such they move down the concentration gradient from the inside, to the outside of the cell. The membrane is very permeable to K+

Sodium (Na+) – abundant in the ECF, as such they move down their concentration gradient from the outside, to the inside of the cell. The membrane is less permeable to NA+

Question 9

what is the equilibrium potential?

Answer 9

The potential when the diffusion potential of an ion is equal to the opposing electrical gradient

Question 10

what is the electrical gradient?

Answer 10

the electrical gradient causes ions to move to balance the charge across the membrane.
opposite to diffusion potential

Question 11

give the mechanism of how the chemical and electrical potentials interact for an ion

Answer 11

As potassium moves across the membrane, it will transfer positive charge to the ECF, making it more positive, and will leave negative ions behind in the ICF.
As such the ICF is more electronegative than the ECF, creating a potential difference (diffusion potential of potassium).
As the potential difference grows, the electrical gradient will create a force that tries to move potassium back into the ICF.

Question 12

is resting membrane potential closer to EK or ENA?

Answer 12

The membrane is more permeable to potassium because of the presence of pumps and leaky channels. As such the conductance of potassium is greater, and its contribution to the resting membrane potential is greater than that of sodium. This means that the resting membrane potential will be closer to the value of Ek than ENa.

Question 13

what does the Nernst equation show?

Answer 13

The Nernst Equation describes this relationship between the diffusion potential and concentration gradient under non-standard conditions.

Question 14

what is the Nernst equation?

Answer 14

EMF = +/- (61/Z) x log (conc inside/conc outside)

Question 15

what does the Goldman/constant field equation show?

Answer 15

shows the affects ions (Na+, K+ and Cl-) have on the potential difference. It shows that the permeability of an ion is proportional to its contribution to the resting potential membrane (if permeability was 0 the ion would have no impact because anything x0 = 0).

Question 16

what is the Goldman/constant field equation?

Answer 16

membrane pot. = (gas constant x temp/F) x ln (ion permeability x conc. out/ion

permeability x conc. in)

then add every subsequent ion to the ln brackets in the same fraction format

Question 17

what is an action potential?

Answer 17

a self-propagating electrical signal

Question 18

describe the mechanism of an action potential

Answer 18

Depolarisation – a stimulus depolarises the membrane, making the potential difference less negative. some voltage gated sodium channels are opened by the stimulus, which allows Na+ to diffuse into the cell and make the ICF more positive.

Reaching the threshold – if the stimulus is great enough, it will depolarise the membrane to the threshold of around -55mV. In order for the action potential to be generated this threshold must be reached.
This is because of the “all or none” phenomenon

Rapid depolarisation – once the threshold is reached, a positive feedback loop is formed as more voltage gated sodium channels open at -55mV
This increases the conduction of Na+ so more diffuse into the cell, further depolarising the membrane and opening more channels.

Reaching the peak – membrane potential reaches a peak of around +30mV. Because of the high Na+ permeability, this value is nearer ENa. At this point the sodium channels become inactivated, reducing Na+ conductance.

Repolarisation – with the sodium channels inactivated, the potassium channels slowly become activated (repolarisation is more gradual than depolarisation). This increases potassium conductance meaning more K+ ions move from the inside of the cell to the outside, down the concentration gradient.

Hyperpolarisation – eventually the resting membrane potential is reached, however the potassium channels are still activated and allowing K+ to diffuse through. As such the membrane continues to be polarised beyond the resting potential

Back to the resting membrane potential – eventually there is reduced activation of potassium channels (they don’t become inactivated) which reduces K+ conduction, depolarising the membrane back t -70mV.

Question 19

what are the key PD values to remember?

draw an acton potential showing these values

Answer 19

• 70mV = resting membrane potential
• 50mV = initial threshold

+30mV = action potential peak

Question 20

what is the “all or none” phenomenon?

Answer 20

the stimulus must depolarise the membrane to the threshold for an action potential to occur, and a depolarisation larger than the threshold does not change the action potential.

Question 21

How is an action potential self propagating?

Answer 21

local currents are set up when current flows from positive to negative on both sides of the membrane at the action potential peak.
positive charge is transferred to the adjacent part of the membrane, depolarising it, and causing another action potential.
This cycle continues along the axon.

Question 22

how is an action potential propagation unidirectional?

Answer 22

due to refractory periods

Question 23

describe the absolute refractory period

Answer 23

Absolute refractory period – occurs from the time the Na channels are activated to the time they are inactivated.
depolarisation relies on Na+ channels re-opening, which can only be done after they first become inactivated.
Inactivation only occurs after the membrane has been repolarised.
It is impossible to generate enough Na+ current for another action potential without channels, so the membrane must wait for repolarisation to complete

Question 24

describe the relative refractory period

Answer 24

Relative refractory period – occurs from when the potassium channels open until the membrane reaches -70mV.
During this period, only a larger stimulus can create another action potential as the threshold is higher.

This is because some sodium channels will still not be able to be activated and potassium channels are still open.

Question 25

what determines the speed of an action potential?

Answer 25

The axon diameter – larger diameter means less resistance as there are more pathways for internal currents to take. This increases conductance

Myelination – many neurons have a thick myelin sheath around them that acts as an electrical insulator and thus allows the current to jump between junctions in the sheath called nodes of Ranvier. As myelination increases so does speed.

Question 26

what are the clinical impacts of demyelination?

Answer 26

Multiple sclerosis - immune system attacks the myelin sheath or the cells that produce and maintain it

blindness in one eye
double vision
scarring