Neuroscience 3 - Resting and Action Potentials

Question 1

Define flux

Answer 1

• The rate of transfer of molecules

- The number of molecules that cross a unit area per unit of time

Question 2

Define voltage.

Answer 2

• Potential difference
• Unit: Volts
• Generated by ions to produce a charge gradient (i.e. like a chemical battery)

Question 3

Define current.

Answer 3

• Unit: Amps

- Movement of ions due to a potential difference

Question 4

Define resistance.

Answer 4

• Unit: Ohms

- Barrier that prevents the movement of ions

Question 5

How is the membrane potential measured?

Answer 5

• A reference electrode is placed outside the cell. This is the zero volt level.
• Another electrode is placed inside the cell. It measures a voltage difference that is negative compared with the outside (i.e. reference).

Question 6

What is electrochemical equilibrium?

Answer 6

This is where the concentration gradient of ions is balanced by electrical forces, so there is no further diffusion across membranes. This generates a stable membrane potential.

Question 7

What is the equilibrium potential?

Answer 7

The potential at which electrochemical equilibrium has been reached. It is the potential that prevents diffusion of the ion down its concentration gradient

Question 8

What is the nernst equation used for?

Answer 8

Calculating the equilibrium potential.

Question 9

What is the Nernst equation and what does everything stand for?

Answer 9

E= (RT/zF)ln([x2]/[x1])

R = gas constant
T = Temp.  Kelvin
Z = charge on ion
-1 for Cl-, +2 for Ca2+
F = Faraday’s number
charge per mol of ion
ln = log to base e

Question 10

What is the composition of Na+ inside and outside the cell?

Answer 10

150mM outside and 10mM inside

Question 11

What is the composition of K+ inside and outside the cell?

Answer 11

5mM outside and 150mM inside

Question 12

What is the equilibrium potential for K+?

Answer 12

-90mV

Question 13

What is the equilibrium potential for Na+?

Answer 13

+72mV

Question 14

What is the Goldman-Hodgkin-Katz equation?

Answer 14

It is an equation used to determine the resting membrane potential.

Question 15

Describe graded potentials.

Answer 15

• They can differ by stimulus type, either excitation or inhibitory.
• The second is the stimulus strength,which may be strong or weak, causing a different change in membrane potential.
• Finally, the magnitude of the change in membrane potential will decrease with distance, so if measured 1mm from the stimulus site the depolarisation will be less.
• This is decremental spread

Question 16

Where do graded potentials occur and what is their function?

Answer 16

• They occur at synapses and sensory receptors

- The function is to contribute to initiating or preventing action potentials.

Question 17

What are the ion pumps responsible for?

Answer 17

• Concentration gradients of potassium and sodium

- Not the membrane potential - this is due to potassium diffusing out of cells.

Question 18

Why is depolarisation faster than repolarisation?

Answer 18

• Sodium ion channels open quickly, and voltage gated ion channels also open causing depolarisation.
• Voltage gated potassium ion channels open slowly, and the movement of potassium is less than the sodium entering.

Question 19

What causes depolarisation?

Answer 19

Movement towards the equilibrium potential of sodium.

Question 20

What causes repolarisation?

Answer 20

Movement towards the equilibrium potential of potassium.

Question 21

What is the absolute refractory period, and what causes it?

Answer 21

• This is the time where a new action potential cannot be triggered, even with a very strong stimulus.
• Caused by sodium channel inactivation

Question 22

What causes sodium channel inactivation?

Answer 22

• When depolarisation occurs, this is detected by sodium ion channels and an inactivation protein prevents entry of sodium into the cell.
• It does this by a ball and chain mechanism, which swings and blocks the site of entry until repolarisation occurs, when it swings off again.

Question 23

What is the relative refractory period, and what causes it?

Answer 23

• This is where, due to hyperpolarisation, a larger stimulus is required to trigger an action potential than normal.
• The inactivation gate is open.

Question 24

List the 5 stages of action potential.

Answer 24

1) Resting membrane potential
2) Depolarising stimulus
3) Upstroke (depolarisation)
4) Repolarisation
5) After hyperpolarisation

Question 25

What is the threshold?

Answer 25

-55mV. Once this potential is reached an action potential is triggered.

Question 26

What is the nature of action potentials?

Answer 26

All or nothing - once triggered, a full sized action potential occurs

Question 27

How is positive feedback behaviour present in action potentials?

Answer 27

• Opening of sodium channels causes more sodium channels to open due to change in voltage
• This cycle contiunues until voltage gated sodium ion channels inactivate

Question 28

To what extent are ion pumps involved during action potentials?

Answer 28

• Not directly, they exchange some ions but this is a slow process.
• Changes occur to small numbers of ions crossing the membrane and changing the membrane potential (less than 1%)

Question 29

How is electrochemical equilibrium restored following an action potential?

Answer 29

Potassium and sodium ions moving through non-voltage gated ion channels.

Question 30

What is passive propagation?

Answer 30

• This is where an impulse travels down the axon passively, as it is less than the threshold potential.
• Only the resting potassium channels are open

Question 31

What affects passive propagation?

Answer 31

• Membrane resistance and internal resistance alter propagation distance and velocity

Question 32

Describe why action potentials are regenerative

Answer 32

• The potential change induced by the opening of voltage gated channels decreases as you record further and further away from the Na+ channel.
• This depolarisation triggers the opening of neighboring Na+ voltage-dependent channels, which results in more Na+ entering the cell, and, in effect, regenerates the action potential.

Question 33

Describe the active propagation of action potentials

Answer 33

• The local current flow of ions depolarises the adjacent region towards the threshold.
• This stimulates sodium ion channels to open, and the action potential moves along the axon
• This continues down the axon, in one direction as the old active region is in the refractory period

Question 34

What is saltatory conduction?

Answer 34

This is where areas of myelin contain fewer voltage gated channels, so the action potential jumps between the nodes.

Question 35

Which factors increase conduction velocity?

Answer 35

• There is an increase with axon diameter, as the resistance to flow decreases.
• It is higher in myelinated neurones, as the action potentials only occur at the nodes of Ranvier.

Question 36

What causes conduction velocity to slow?

Answer 36

• Reduced axon diameter, for example caused by regrowth after injury
• Reduced myelination (caused by multiple sclerosis and diptheria)
• Cold, anoxia (lack of oxygen), and drugs (some anasthetics).