TEMA 8

Question 1

What is a membrane potential?

Answer 1

Difference in electrical potential (voltage) across the membrane (imbalance), the interior of the cell is more electronegative than the EC space

Some cell types are excitable, respond to physical or chemical stimuli by modifying their membrane potential, generating electrical impulses that are transmitted along their membranes.

Question 2

Memb potential of:
Skeletal muscle cells
Smooth muscle cells
Astrocytes
Neurons
Erythrocytes

Answer 2

-90mV
-50mV
-80/-90mV
-70mV
-12mV

Question 3

What is a chemical gradient?

Answer 3

Differences in chemical composition between extracellular and intracellular liquids.

Question 4

Ionic bases of the memb. potential

Answer 4

The lipid bilayer is impermeable to the passage of ions from the inside of the cell to the outside and vice versa.
* The ions can pass through the membrane through ion channels.

Transmembrane potential increases or decreases:
A. In response to temporary changes in membrane permeability.
B. It results from the opening or closing of specific channels in the membrane
-The permeability of the membrane to Na+ and K+ determines the transmembrane potential.
-Sodium and potassium transport can be passive and active.

Question 5

PASSIVE CHANNELS

Answer 5

Also called “leak” channels.
-The permeability changes with the conditions.
-They are responsible for the fact that permeability for K+ is greater than for Na+ and Cl- : PK» PNa and PCl
* They alternate randomly between the open and closed positions.
* They participate in the maintenance of the membrane
potential: many more passive channels for K+ than for Na+

Question 6

What is a diffusion potential?

Answer 6

The diffusion potential of an ion is the difference in potential that it generates in the membrane when diffusing in favor of a concentration gradient.

Question 7

Amount of K at a resting memb potential

Answer 7

Even though K+ diffuses out of the cell, the concentration does not change
Because it moves so little that it cannot be detected chemically

In this region of the cell, the movement of very small amounts of ions causes a large change in voltage. Until obtaining - 94 mV the concentration of K+ would decrease to 139.9999 mM. This 0.00001 mM variation is chemically undetectable and does not significantly affect the calculated Nernst potential.

Question 8

Electrochemical gradient of K+ and Na+

Answer 8

In the normal resting potential, the electric gradient (in) opposes the chemical gradient (out) of potassium (K+).

The net electrochemical gradient pushes the ions out of the cell

The electrical gradient and the chemical gradient of sodium (Na+) cooperate to introduce sodium into the cell.

Question 9

Equilibrium potential

Answer 9

1) The intracellular K+ concentration is 35 times higher than the extracellular K+.
2) The membrane is permeable to K+, but not to the phosphates (of the ATP) nor to the intracellular proteins (Prot-).
3) The leakage of K+ leaves an excess of negative charges on the inner side of the membrane.
4) The diffusion of K+ceases when a potential of -94 mV is reached. External positive load brakes the output of K+.

 The membrane potential opposes the tendency of diffusion to follow the difference in concentration.
 In electrochemical equilibrium, the chemical and electrical driving forces acting on an ion are equal and opposite and there is no diffusion.

Question 10

Na+ / K+ pump

Answer 10

3 Na to ECL // 2K to ICL
It is an ATPase

Question 11

Graded potential

Answer 11

Resting membrane potential with closed ligand-activated Na+ channels
Membrane is exposed to the ligand that opens the Na+ channels.
Entry of Na+ into the cell producing LOCAL CURRENTS THAT DEPOLARISE ADJACENT PORTIONS OF THE CELL MEMB

Question 12

Difference between graded and action potential

Answer 12

graded potentials are the variable-strength signals that can be transmitted over short distances whereas action potentials are large depolarizations that can be transmitted over long distances

Question 13

nerst equation

Answer 13

Membrane potential that would exactly balance the diffusion gradient and impede the net movement of a particular ion

Ex= Equilibirum potential for ion X in milivolts (mV) Xe= concentration of ion X outside the cell
Xi= concentration of ion X inside the cell
z= valence of ion X (+1 for Na+ or K+)

Question 14

Nerst equation of K, Na and Cl

Answer 14

-94mV
+66mV
-86Mv

Question 15

Goldman equation

Answer 15

When a membrane is permeable to several different ions, the joint diffusion potential can be calculated taking into account several factors

1.Concentrations of each ion inside and outside the membrane.
2.Permeability of the membrane to each ion.
2. Polarity (sign) of the electric charge of each ion.

Em= Resting membrane potential (mV)
gK+, etc = Conductance for K+, etc
gT = Total conductance
EK= Equilibrium potential for K+ (mV)

Question 16

how is the resting potential of a nerve fibre maintained and what is its value?

Answer 16

Nerve fibers at rest have a membrane potential of -90 mV
It is generated by

a) Na+-K+ pump: Consumes energy (ATP) and introduces in each cycle 2 K+ and extracts 3 Na+.
- It is a electrogenic pump: it generates accumulation of exterior positive charge
- It generates chemical gradients of Na+and K+

b)Diffusion channels for Na+ and K+
- The chemical gradients leads to the escape through selective channels.
Exit permeability for K+ is 100 times larger than the permeability for Na+ to enter

Question 17

Interaction between Na+ and K+ diffusions

Answer 17

Using Goldman equation:
-86mV
(net diffusion potential)

The contribution of Na+ diffusion to the resting potential is low, due to the low permeability for this ion

Question 18

HOW DO WE REACH -90mV AS THE RESTING POTENTIAL

Answer 18

the net output of positive charges caused by the Na+/K+ pump generates a potential of -4 mV, which added to the net diffusion potential (-86 mV) brings the resting potential to -90 mV.