Membrane Potentials

Question 1

Describe the distribution of Na+ and K+ across the neuronal cell membrane and predict in which direction these ions will flow passively, under the influence of their respective electrochemical gradients

Answer 1

Na+ : outside = 141mM
inside= 10-20mM
Equilibrium = 60mV

K+ outside=3.3 mM
inside = 120-140mM
Equilibrium = -90mV

Ca+ equilibrium = 180mV

Cl- Equilibrium = -70mV

At resting potential, Na+ (and Ca+): INWARD, causing depolarization

K+: OUT, causing hyperpolarization

Cl-: inward (usually), causing hyperpolarization (usually)

Question 2

How do we calculate ion equilibrium potentials?

Answer 2

Nernst equation

The potential at which no net movement of K+ occurs is the equilibrium potential for K+( EK) and can be calculated by the Nernst equation.

Question 3

How do the ion channels have an influence on the MP?

Answer 3

The Na+ into cell will increase resting potential of -65mVand the K+ out of the cell will decrease the resting potential.

open Na+ channels drive the MP toward Ena+ of 60mV.

open K+ channels will drive the membrane towards Ek+ of -90mV

Question 4

What are the properties of ACTION POTENTIALS in neurones?

Answer 4

AP are brief electrical signals where polarity is reversed for about 2milliseconds.

Action potentials caused by Na and K (VOLTAGE-GATED) channels.

They are generated ONLY when a threshold is reached

They maintain a CONSTANT SIZE AND SPEED which allows travel over long distance.

Question 5

What is the the ionic basis of the action potential in neurones?

Answer 5

when the threshold of excitation is reached, voltage activated sodium channels in the membrane suddenly open and Na+ ions flow into the cell (by both principles of diffusion and electrostatic pressure.)

This results in a change of membrane potential from -70mV to +30 or 50mV (because of the positively charged sodium ions entering the cell).

However, at the peak of Na+ flow, voltage-activated potassium channels are triggered which drive K+ ions near the membrane out of the cell (by the same forces), which causes the inside of the cell to have a negative value again - or in other words the cell is ‘repolarised’.

Question 6

What does ABSOLUTE refractive period mean?

Answer 6

No matter how strong the stimulus, during this refractory period it is impossible for second AP as all the channels at this point are inactivated (not just closed)

Question 7

What does refractory period mean?

Answer 7

If depolarisation is maintained channels will become INACTIVATED WHERE IT NO LONGER CONDUCTS and then repolarises to become closed again.

Inactivation contributes to the repolarizing phase of AP and is responsible for the refractory period

There is a separate inactivation gate in the channel which closes (he activation gate will remain open)

Question 8

What is the process of impulse propagation in (un) myelinated nerve fibers?

Answer 8

The voltage-gated ion channels are known to be found within certain areas of the axonal membrane and initiate action potential, conduction, and synaptic transmission

Question 9

The different states of the Na+ channel

Answer 9

Closed state

Depolarisation causes open state

Maintained depolarisation cause inactivation

Repolarizaion(from inactivated state causes the closed state

Question 10

What causes Vm (membrane potential)?

Answer 10

Separation of opposite charges, ie cations (+) and anions (-) across the membrane.

In nerve cells on average Vm is -65mV

Question 11

How do you generate a MP?

Answer 11

1) Selective permeability of the membrane to certain ions and concentration gradients exist across the membrane which allow for diffusion of ions down their own gradients. This results in a electrical potential which is the resting membrane potential.

Question 12

In what ways does the membrane act as a capacitor?

Answer 12

It can compared o an elecrical device that stores charge; the net difference of the chare is shown through out the ICF (INSIDE) and ECF (OUTSIDE) beyond the membrane surfaces.

Question 13

What is the driving force of ion transport across the membrane?

Answer 13

Ions are driven across the membrane at a rate proportional to the difference between the membrane potential (Vm) and the equilibrium potential Eion [i.e. the driving force (Vm – Eion)]

Question 14

List the Equilibrium potentials for ions K, Na, Cl and Ca.

Answer 14

EK= 61 log10 [Ko]/ [Ki]

KNa = 61 log10 [Nao]/ [Nai]

KCl = -61 log10 [Clo]/ [Cli]

KCa = 30.5 log10 [Cao]/ [Cai]

Question 15

How do we predict the resting membrane potential (Vm) ?

Answer 15

Goldman-Hodgkin-Katz (GHK) equations

membrane potential (Vm) is dependent upon their concentration gradients and their relative permeabilities

Watch out for Cl-: Inverting concentrations so [Cl-i] is on top allows for negative valency (-1) correction. ie. In [A]/[B] = - In [B]/[A]

Question 16

In what ways can an ion be gated?

Answer 16

1) Membrane Voltage
2) Chemical substances like ligands
3) Physical stimuli like temperature.

Question 17

Name a known blocker of the Na voltage channel.

Answer 17

TTX (Tetrodotoxin)

Question 18

Name a known blocker of the K voltage channel.

Answer 18

TEA (Tetrathylammonium)

Question 19

How do the Na+ and K+ channels apply to the Ionic basis

Answer 19

Both channels are activated by depolarization but Na+ depolarise rapidly, and there is a delay in K+ channels opening.

This is because the Na+ channels have POSITIVE FEEDBACK where opened channels and an inward Na flow maintains depolarisation.

Whereas the K channel once opened by depolarisation will lead to repolarisation to resting potential instead.

Question 20

What does RELATIVE refractive period mean?

Answer 20

If a signal is stronger than normal, closed channel could be reopened giving a second action potential

Question 21

Explain how AP are able to conduct over large distance despite a LEAKY NERVE CELL MEMBRANE.

Answer 21

Leakyness means passive signals do not go far from orgin, however, AP amplify themselves.

Increase passive current spread and therefore action potential.

Question 22

What ways can we Increase passive current spread and therefore action potential.

Answer 22

Increasing the ratio of membrane resistance and axial resistance can increase the (length constant and ) length current will spread and therefore the AP VELOCITY.

So decrease axial resistance by INCREASING AXON DIAMETER,

And Increase membrane resistance by adding INSULATION (MYELIN); shown to increase axon conduction.

Question 23

Where is myelin produced?

Answer 23

Schwann cells in the PNS.

Oligodendrocyte in the CNS

Question 24

What is SALTATORY conduction?

Answer 24

In myelinated axons, the AP can JUMP from one node of Ranvier (gap in between myelin sheaths/bundles) to the next.

This happens at about 0.3-5 m/s.

At this point, where there is no myelin, action potentials can be generated. electrical currents produced at each node of Ranvier are conducted with little attenuation to the next node in line, where they remain strong enough to generate another action potential resulting in a propagation speed much faster than even the fastest unmyelinated axon can sustain.