T7

Question 1

Cell membrane: phospholipidic bilayer

Answer 1

the wall

Question 2

Proteins channels

Answer 2

the gates

Question 3

The intracellular and extracellular fluids of a neuron are filled with various ions, including:

Answer 3

Cations (positively charged)
Na+(sodium) ions
K+(potassium) ions

Anions (negatively charged):
Cl-(chloride) ions
Numerous negatively charged protein molecules (A -).

Question 4

Three factors influence the movement of anions and cations into and out of cells:

Answer 4

Diffusion
Concentration gradient
Charge

Question 5

Diffusion (ion movement)

Answer 5

Diffusion: movement of ions from an area of higher concentration to an area of lower concentration through random motion (intrinsic kinetic movement). Temperature dependent.

Question 6

Concentration gradient (ion movement)

Answer 6

Concentration gradient: differences in concentration of a substance among two regions of a container →that allows the motion of ions between the region with higher concentration through the region with low concentration

Question 7

Voltage gradient (ion movement)

Answer 7

Voltage gradient: The difference in charge between two regions that allows a flow of current if the two regions are connected.
Current: Flow of electrons
Voltage: difference between anode and cathode (V)
Conductance: ability to move from one side to the other (num channels).

Question 8

Three aspects of the semipermeable cell membrane contribute to this resting potential:

Answer 8

1. Large negatively charged protein molecules remain inside the cell.
2. Gates keep out positively charged Na+ ions, and channels allow K+ and Cl Ions to pass more freely.
3. Na+–K-pumps extrude Na+ from the intracellular fluid.

Question 9

Each ion has its own …

Answer 9

equilibrium potential

Question 10

Nernst equation

Answer 10

calculate the Ion equilibrium potential

Question 11

What is the Goldman equation used for?

Answer 11

membrane potential

Question 12

True or False. Any change in ion concentration can change the membrane potential, that varies between Na+ and K+ equilibrium.

Answer 12

True

Question 13

Graded potentials

Answer 13

Graded potentials: small voltage fluctuations that are restricted to the vicinity on the axon where ion concentrations change. -> see action potential

Question 14

Depolarization

Answer 14

Depolarization: When a –ion go out of the cell / or + go in (intracell space more positive) small decrease in electrical charge across a membrane, usually due to the inward flow of sodium (Na+) ions.

Question 15

Hyperpolarization

Answer 15

Hyperpolarization: When a -ion go into the cell / or + go out (intracell space more negative) small increase in electrical charge across a membrane, usually due to the inward flow of chloride (Cl-) or the outward flow of potassium (K+) ions.
-70 mVto -73 mV
-70 mVto -65 mV
Any change in ion concentration can change the membrane potential, that varies between Na+ and K+ equilibrium
This occurs when the permeability of the membrane to a specific ion change: change in conductance.

Question 16

Information processing in the brain

Answer 16

The ability to generate signals is based on the ability of nerve cells to rapidly change their voltage difference between the interior and the exterior. In other words, they are based on a rapid change in the Resting Potential
The changes require:
ion channels and
permeability changes of cell membranes by opening and closing these ion channels

Question 17

Recapituation electricity

Answer 17

The movement of electric charge: Current “I” [amp]
The force acting on a charged particle (equivalent to the difference in charge between anode and cathode) is called electrical potential or voltage; “U” [volt]. In the car: 12 V, in the socket: 220 V; high power current: 380 V
The relative ability of a charged particle to move from one point to another is the electrical conductance „g” Siemens [S].
Electrical resistance “R”, measured in ohms “Ω”: R = 1 / g
A simple relationship between current, voltage and resistance appear: Ohm’s law: U = R * I or alternatively: I = g * U or R = U / I.

Question 18

Intracellular recording of a neuron

Answer 18

If one punctures a very sharp glass electrode (diameter ~ 0.3μm, filled with saline solution) into a nerve cell (without permanently injuring the cell membrane) and at the same time a reference electrode is present in the extracellular medium, a voltage difference of - 40 to -90 mV (more negative inside than outside) can be measured
Electric charge is distributed differently along the nerve cell membrane (inside vs. outside).
This voltage difference was first measured ~ 1940 and is called resting membrane potential.

Question 19

Background information about electrical signals in the nervous system

Answer 19

In a first approximation, the cell is a cavity filled with an aqueous solution of proteins and ions
Intra- and extracellular environments have the same ionic strength (osmotic pressure) but the different ionic composition
The cytoplasm is separated from the extracellular space by a cell membrane
The lipid bilayer is impermeable to ions unless there are ion channels
If channels are present, ions can migrate along the membrane between inside and outside

Question 20

Ion channels

Answer 20

Transmembrane proteins with an aqueous pore
Up to 100,000,000 ions can travel through a single channel per second
This migration of charged particles (ions) through the cell membrane causes a current flow and a change of the resting membrane potential
Ion channels are often selective for ions, i.e. they let only certain ions pass - so either Na+, K+ Cl- or Ca2+.

Question 21

Passive channels (ion channels)

Answer 21

Passive channels with a certain conductivity in resting state (quiet membrane channels). These channels are open, remain open and are not significantly affected by the environment or other factors. They are responsible for the resting membrane potential.

Question 22

Gated ion channels (ion channels)

Answer 22

Gated ion channels open and close in response to external stimuli (ligands, voltage difference, etc.). During the resting membrane potential, these channels are usually closed (low probability of opening).

Question 23

Na/K ATPase

Answer 23

Low Na+ High K+ intracellular
Pumps out of the cell for each hydrolysed ATP 2 K+ ions into and 3 Na+ ions out of the cell
Electrogenic transport since per hydrolysed ATP a loss of 1 positive charge from the cell interior
Very low current flow
Consumes ~ 50% of total ATP in the brain

Question 24

Emergence of the resting potential

Answer 24

The K+ transport of the Na/K ATPase of a cell is approximately 100 cycles /sec. There are about 1000 pumps per nerve cell (very rough estimation!),
Which makes approx. 200,000 K+ ions / sec. pumped into the cell.
A K channel can let about 1 million K+ ions / sec pass through and exists in about 1000 copies per nerve cell - so about 1 billion K+ ions / sec. diffuses out of the cell.

Question 25

Consequences of ATPase and K+ channels for a “model cell”

Answer 25

A: no difference between both sides of the cell (inside and outside)
B: ATPase is incorporated into membrane - high K+ ion concentration within the cell

K+ ions diffuse through the resting channel until an equilibrium is reached and there is no net flow of ions
The electrochemical equilibrium sets in when the resting membrane potential is reached.

Question 26

The resting membrane potential is built along the cell membrane

Answer 26

The net difference of the electric charge arises at the inner and the outer surface of the membrane
Since the cell membrane is very thin, the positive and negative charges on both sides interact electrostatically together
The different amount of positive and negative charges on both sides of the membrane is only caused by a tiny part of all inside and outside ions present - so there are no osmotic effects - not even when the resting membrane potential would rise to +70 mV.
The flow of only a few ions along the membrane can have a big impact to the membrane potential.

Question 27

The number of ions that build up the resting membrane potential is

Answer 27

low

Question 28

The total number of ions inside and outside remains

Answer 28

virtually constant. There are no osmotic forces.

Question 29

The Nernst equation

Answer 29

Allows the calculation of the equilibrium potential at given ion concentrations inside and outside. Predicts the voltage at which the equilibrium potential is reached
For a cell in rest only permeable to K+ ions (e.g. glial cell)

Question 30

The Goldman equation

Answer 30

Membranes are not only permeable to K+ ions but to a lesser extent also to Na+ and Cl- ions.
Relative membrane permeabilities (P) in addition to the concentrations also important.
Permeability (P) depends on the difference in concentration and the difference between membrane potential - equilibrium potential

Question 31

Why do nerve cells have a resting membrane potential?

Answer 31

Membrane potential is a universal source of energy that can be used for all non-spontaneous reactions.
Energy is “pumped” into the cell to create an “unstable” balance that can respond very quickly to an external stimulus
All cells have a resting membrane potential!
Secondary active transport (electrochemical gradient) is used to transport other ions through the membrane against their own electrochemical gradient; Na-cotransport, Na-Ca exchanger (Symporter)!

Question 32

Do all cells have a resting membrane potential?

Answer 32

Yes