Membrane Potentials

Question 1

What is the membrane potential?

Answer 1

Voltage (difference in electrical charge) across the plasma membrane - can be measured using a voltmeter, reference electrode within the extracellular fluid and a recording microelectrode that spans the membrane and passes into intracellular fluid

Question 2

What is the resting potential?

Answer 2

Membrane potential of a cell not sending signals.

Question 3

What is the resting potential of neuronal and cardiac cells?

What is the [Na+] inside and outside the cell? What is the [K+] inside and outside the cell?

Answer 3

NEURONAL: -70 mV
CARDIAC: -90 mV

[Na+]
Inside cell: 15mM
Outside cell: 150mM

[K+]
Inside cell: 140mM
Outside cell: 5mM

Question 4

How are Na+ and K+ gradients maintained?

Answer 4

They are maintained by the sodium-potassium pump.
The sodium-potassium takes 3 Na+ out for every 2 K+ brought in.

Question 5

Why is there such a major voltage difference between the inside and outside of the cell, and how does it lead to the electrochemical gradient?

Answer 5

• Concentration of potassium greater in the cell than outside the cell
• Concentration of sodium greater outside the cell than inside the cell
• Concentration of chloride greater outside the cell than inside the cell
• K+ channels are always open at the resting potential (“potassium leak”). There’s a net outflow of K+ down its concentration gradient through K+ channels; this leads to a net negative charge inside the cell due to presence of anions - causing a CHARGE SEPARATION.
• There is a separation of charge across the membrane with a more negative charge INSIDE the cell
• We have a separation of positive and negatively charged ions – this leads to a POTENTIAL DIFFERENCE

Question 6

When can we measure the equilibrium potential (Eion)?

Answer 6

Measure it at equilibrium, when both the electrical and chemical gradients are balanced.

EQUILIBRIUM POTENTIAL
- the membrane potential at which the electrical and chemical gradient of a specific ion are balanced (Eion)

Question 7

How can we calculate the equilibrium potential?

Answer 7

It can be calculated using the Nernst Equation.
The Nernst Equation is:

Eion = 62 mV (log ([ion]outside/ [ion]inside))
-used to calculate the equilibrium potential of a specific ion in a cell

EK = Potassium Equilibrium Potential
ENa = Sodium Equilibrium Potential

ENa = +60mv
EK= -90mv

Question 8

Why is there a difference in the resting cell potential of neurones and cardiac cells?

Answer 8

Because, in the neurons, there are many more K+ channels than there are Na+ channels, so the resting potential is a slight compromise between EK and ENa.
In cardiac cells, the majority are K+ channels, so the resting potential is more towards EK due to greater permeability to K+ ions.

Question 9

How do we model the resting potential for membranes permeable to multiple ions?

Answer 9

Use the Goldman Equation

Question 10

Describe how hyperpolarisation comes about.

Answer 10

Hyperpolarisation is when the membrane potential is MORE NEGATIVE than the resting potential. When gated K+ channels open, too many K+ diffuses out, making the inside of the cell more negative. Graded hyperpolarisation produced by two stimuli that increase membrane permeability to K+. Na+/K+ pump reverts membrane potential to resting potential.

Question 11

What brings about an action potential?

Describe the generation of an action potential in NEURONES.

Answer 11

THE RESTING STATE
At resting potential, most voltage-gated sodium and potassium channels are closed.
DEPOLARISATION
When an action potential is generated, voltage-gated sodium channel open first and sodium flows out into the cell. The stimulus needs to cause sufficient
depolarisation to raise the voltage above the THRESHOLD
RISING PHASE OF THE ACTION POTENTIAL - DEPOLARISATION
During the rising phase, the threshold is crossed and the membrane potential increases. This is due to further opening of voltage-gated sodium ion channels, causing a further influx of sodium ions (positive feedback). The membrane potential does not reach ENa as channels INACTIVATE
FALLING PHASE OF THE ACTION POTENTIAL - REPOLARISATION
During the falling phase, voltage-gated sodium channels become inactivated; voltage-gated potassium channels open and potassium flows out of the cell (efflux of potassium ions).
BRIEF HYPERPOLARISATION
During the overshoot, membrane permeability to potassium is high, at first, more negative than at rest, then voltage-gated potassium channels close. Na+/K+ pump returns membrane potential to -70mV

Question 12

What is the refractory period after an action potential, and what causes it?

Answer 12

During the refractory period after an action potential, a second action potential cannot be initiated. The refractory period is a result of a temporary inactivation of the sodium channels.

Question 13

Describe how an action potential travels down the axon (ion-wise).

Answer 13

1) An action potential is generated as Na+ flows inwards across the membrane at one location on the axon.
2) The depolarisation of the action potential spreads to the neighbouring region of the membrane, reinitiating the action potential there. In the original position, the membrane is repolarising as K+ flows outwards.
3) The depolarisation process is repeated in the next region. Local currents of ions, therefore, propagate the action potential along the length of the axon.

Question 14

What is the myelin sheath, and what is its purpose?

Answer 14

It is a layer of cells insulating the axon. In the CNS, those cells are Oligodendrocytes, while in the PNS the cells are Schwann cells.
In vertebrates, axons are insulated by a myelin sheath; this causes an action potential’s speed to increase.

Question 15

Describe the generation of an action potential in CARDIAC CELLS.

Answer 15

DIASTOLE
At resting potential, most voltage-gated sodium and potassium channels are closed.
DEPOLARISATION
Sodium channels are open, allowing the flow of positive charge into the cell - the membrane depolarises.
RAPID NA+ INACTIVATION - REPOLARISATION
Na+ channels deactivate, and the K+ channels open.
PLATEAU PHASE
Potassium channels remain open, L-type calcium channels are activated.
Ca2+ flow inwards, activating ryanodine receptors on the SR to liberate more Ca2+ for muscle contraction. Ca2+ also activate chloride channels, allowing the flow of Cl- into the cell. The outflow of K+ is counteracted by the inflow of Ca2+ and Cl-, leading to the plateau phase.

Question 16

What is the Goldman equation used for?

Answer 16

Used to calculate the resting potential of a cell taking into account multiple ion permeabilities.

Question 17

TRUE OF FALSE - As axon diameter increases so does the speed of transmission of an action potential

Answer 17

TRUE

Question 18

What are the grey and white areas in the brain?

Answer 18

GREY AREAS - Cell bodies
WHITE AREAS - Axons

Question 19

Describe saltatory conduction

Answer 19

• Saltatory conduction occurs in myelinated neurones. Myelin sheath prevents ion exchange, except at nodes of Ranvier which is densley packed with voltage-gated sodium ion channels, therefore sodium ions diffuse (local currents) along axon until node of Ranvier, where the next action potential can be generated, so impulse jumps from node to node (faster).

Question 20

Describe the structure and functions of neurons

Answer 20

• Neurones transmit impulses as action potentials (rapid depolarisation of membrane).
• Neurones are very long as they must transmit action potentials over long distances.
• Plasma membranes contain many gated ion channels and sodium/potassium pumps (use ATP).
• Dendrites connect to other neurones that carry impulses towards the cell body.
• Neurones are surrounded by a fatty layer called the myelin sheath (made up of Schwann cells) that prevent electrical activity being passed to other nerve cells nearby.
• Myelin sheath prevents ions moving across neurones, so they can only move at the nodes of Ranvier (occuring every 1-3mm) which means the impulse/action potential jumps from node to node (fast).

Question 21

What occurs when sodium ion channels become mutated?

Answer 21

One type of mutation in Na+ channels can cause absence of channel function e.g needed for detection of pain
*Absence of pain

Question 22

What do the graphs for neuronal and cardiac neurotransmission look like?

Answer 22

See slides

When comparing, take into account
Resting membrane potential
Shape of the action potential
Action potential duration

Question 23

How does cardiac action potentials differ from neuronal action potentials?

Answer 23

FOR CARDIAC ACTION POTENTIALS:
-shorter depolarisation
-has a plateau phase
-Em is at -90mV

Question 24

What is meant by the all or nothing principle?

Answer 24

• All or nothing response, as an action potential is either generated (if membrane potential reaches threshold value) or it is not generated (if membrane potential does not reach threshold value).

Question 25

What do all action potentials have in common?

Answer 25

• All action potentials are the same intensity (+40mV), so a more intense stimulus produces a higher frequency of action potentials as opposed to higher intensity action potentials

Question 26

Describe how depolarisation occurs

Answer 26

Depolarisation is the membrane potential is MORE POSITIVE than the resting potential - Na+ in. When voltage-gated Na+ channels open, Na+ diffuses in, making the inside of the cell more positive/less negative. Graded depolarisations produced by two stimuli that increase membrane permeability to Na+.
* The sodium ions continue to move down the membrane to where their concentration is lower, generating more action potentials. Cannot move backwards as Na+ concentration still high.