Resting Potential

Question 1

Electric voltage across the cell membrane

Answer 1

-the membrane potential results from a separation of positive (cations) and negative (anions) charges across the cell membrane
-the charge separation gives rise to a membrane potential: V= Q/C, voltage= charge/ capacitance( amount of electrical energy separated for a given electrial potential)
reduction in charge separation-> less negative membrane potential -> Depolarization
-increase in charge separation-> more negative membrane potential -> hyperpolarization

Question 2

Ionic gradients

Answer 2

• two major players generate membrane potentials: ion pumps -> ion gradients; ion channels -> ion movement to reach electrochemical equilibrium
• ion pumps are active transporters that establish ionic gradients
• cytoplasm- K+ and Cl- and impermeable anions
• extracellular fluid- Na+ and Cl-
• ion gradients play a major role in the generation of resting potential (K+ primarily, and Na+ to a degree) and are involved in transmitter release (Ca++)

Question 3

Generation of the resting pitential

Answer 3

-electrical potential result from movement of ions down their concentration gradients, through ion channels charging the membrane capacitance
-ion channels are membrane proteins that can be selectively permeable to specific ions (for example Na+, K+, Ca++, Cl-0
-the direction of current flow is defined as the direction of net movement of positive charge
Selective dating of different types of ionic channels generates: action potentials, synaptic potentials (usually chemical communication between neurons), receptor potentials (photoreceptors, touch receptors, baroreceptors), by convention current flow is in the direction of positive ions

Question 4

Diffusion potential

Answer 4

• is the potential difference generated across a membrane when a charged solute (an ion) diffuses down its concentration gradient.
• a diffusion potential is caused by diffusion of ions

Question 5

Equilibrium potential

Answer 5

• simply an extension of the concept of diffusion potential
• if there is a concentration difference for an ion across a membrane and the membrane is permeable to that ion, a potential difference (diffusion potential) is created

Question 6

Resting potential in cells with only K channels

Answer 6

-chemical (concentration difference) force= electrical driving force
-ionic movement K+ from 1 to 2= ionic movement of K+ from 2 to 1
-the potential across the membrane is calculated using the Nernst equation= RT/zF ln (X2/X1) = 58/z log (X2/X1)
-if the concentration differences are 10/1:
58log 1/10 = 58log0.1 = -58 mV
-glial cells are virtually permeable to only potassium as they contain only minute numbers of channels permeable to sodium
Z=1 for Na and K; Z=2 for Ca and Mg; Z=-1 for Cl-

Question 7

Resting potential in cells with K+ and Na+ channels

Answer 7

• more realistic cell model
• resting potential is established by various kinds of ion channels that are permeable to K+, to Na+ and Cl-
• first analyze the diffusion of K+ and Na+ without considering Cl-
• you should be aware that most neurons do not have Cl- pumps so that Cl- is passively distributed and does not contribute to generation of the membrane potential

Question 8

Goldman equation

Answer 8

• in a neuron resting potential is established primarily by ion channels that are permeable to K+ and other channels that are permeable to Na+. The membrane potential is thus not equal to the equilibrium potential (Nernst potential) for K+
• the goldman equation accounts well for the contributions of K+ and Na+ ions to the resting potential
• Vm= 58log(Pk(K+)o +Pna(Na+)o/ Pk(K+)i+PNa(Na+)i)
• the greater the permeability of an ion that is not passively distributed the greater its influence in determining the membrane potential
• Pk- permeability to K ions at rest is 25X greater than sodium

Question 9

Ratio of ions in cells

Answer 9

• Most cell the ratio is Pk: PNa: PCl
• at resting potential 1.0: 0.4: .45
• at the peak of the action potential it is 1.0: 20: .45

Question 10

Resting potential summary

Answer 10

• membrane of resting neurons is more permeable to K+ than to any other ion
• in a neuron resting potential is established primarily by ion channels that are permeable to Na+ and for some cells Cl-. The membrane potential is thus not equal to the equilibrium potential (Nernst Potential) for K+ but is given by the Goldman equation
• in such case, the resting potential (Vm) is determined by the concentration gradients of the ions and by the ease with which each ion crosses the membrane (permeability)