Lecture #6

Question 1

how do cations migrate?

Answer 1

positive ions migrating towards a negative charge

Question 2

how do anions migrate?

Answer 2

negative ions towards a positive charge

Question 3

what is the charge of potassium outside vs inside of a cell?

Answer 3

cytoplasm: 140 mM

outside: 2-4 mM

Question 4

what is the concentration of sodium outside vs inside of a cell?

Answer 4

cytoplasm: 6-10 mM

outside: 130-140 mM

Question 5

what is the concentration of chloride outside vs inside a cell?

Answer 5

cytoplasm: 13-14 mM

outside: 130-150 nM

Question 6

what is the concentration of calcium outside vs inside of a cell?

Answer 6

cytoplasm: 0.001 mM

outside: 1.5-2 mM

Question 7

why is the concentration of calcium so low?

Answer 7

almost all cells use calcium as a second messenger → when there is an influx of calcium inside the neurons, the Ca binding proteins are activated

Question 8

are organic ions?

Answer 8

several proteins that are clusters under the cell membrane and are fundamental of rte maintenance the gradient and the membrane potential

Question 9

what allows for the establishment of the membrane potential in the neuronal cell membrane?

Answer 9

there is a thin cloud of positive and negative ion located in a tiny virtual space which is in tight contact with the cell membrane

Question 10

what is the charge of a neuron?

Answer 10

neutral → the only position of the cell membrane having a net charge is the tiny space located in tight contact with the cell membrane

Question 11

what are the three important features of ion channels?

Answer 11

1. they recognize and select specific ions
2. they open and close in response to specific signals
3. they conic ions across the membrane

Question 12

what can GABA and Glycine ion channels flux?

Answer 12

Cl from the outside to the inside → since Cl is negatively charged, it contributes to the hyperpolarization of the membrane

membrane potential starts to become more negative when the chloride crosses the cm

Question 13

what do channels that respond to acetylcholine, glutamate, or serotonin flux?

Answer 13

both Na and Ca ions → since both are positively charged they contribute to the depolarization of the membrane

Question 14

in a cell not involved on the propagation of electrical impulses, what is the membrane potential related to?

Answer 14

the distribution and the abundance of specific ion channels the can cross only the potassium

Question 15

which ion is fundamental for the maintenance of the negative charge on the cell membrane?

Answer 15

K → both the chemical and electrical driving force

Question 16

how does K act as the chemical driving force?

Answer 16

we have a bigger concentration of K inside the cell than outside, therefor the K tends to diffuse across the membrane moving from inside to outside the membrane by simple diffusion, creating a positive outside

Question 17

how does K act as the electrical driving force?

Answer 17

because K is positive, it is also attracted by negative charges inside the cell

Question 18

in cells other than neurons, describe the K channels:

Answer 18

K+ channels are always open and therefore the K tends to exit due to the diffusion force but at the same time, tends to enter due to the electrostatic force

Question 19

at equilibrium, what is the membrane potential?

Answer 19

-65 mV

Question 20

for sodium, how are electrical and chemical forces oriented?

Answer 20

the two forces pull the ion inside of the cell

Question 21

for chloride, how are electrical and chemical forces oriented?

Answer 21

• chemical force pushes the Cl to enter since the concentration is higher on the outside
• electrostatic force pushes the Cl to exit because the environment outside the cell is negative

Question 22

what is the resting potential of neurons?

Answer 22

~ -70 mV → the cytoplasmic side is more negative than the outside of the clll due to the huge amount of inorganic ions

Question 23

what equation starts from the free energy deriving from the thermodynamic movement of ions (chemical force)?

Answer 23

Nernst equation →
ΔG= -RTln(ion o / ion i)

Question 24

what equation represents free energy variation?

Answer 24

ΔG= -EzF

Question 25

what is the state of the ion potential at equilibrium?

Answer 25

both equations are identical → the free energy variation deriving from a thermodynamic change is equal to the one deriving from a variation in electrical energy

Question 26

what does R stand for in ΔG= -RTln(ion o / ion i)?

Answer 26

the universal gas constant

R= 8.314472 J/K-mol

Question 27

wha does T stand for in ΔG= -RTln(ion o / ion i)?

Answer 27

absolute temperature in Kelvin

Question 28

what does F stand for in ΔG= -EzF?

Answer 28

Faradays constant

F= 9.64853399x10^4 C/mol

Question 29

what does Z stand for in ΔG= -EzF?

Answer 29

oxidation state of the ion

Question 30

describe the electrochemical driving force of neurons:

Answer 30

in normal conditions when Na channels start to open, since the electrochemical equilibrium of Na is far from the resting potential of a neuron, there is a strong influx of this ion inside of the neuron

Question 31

what happens if we have a K channel that starts to open?

Answer 31

in a resting condition the K will exit because its more negative than the equilibrium

the passive movement of K out of the cell balances the passive movement of Na into the cell

Question 32

what would happen if the leakage of K continued unopposed for a long period of time?

Answer 32

the K gradient would degrade and the cell would die

Question 33

what prevents the dissipation of the ionic gradient?

Answer 33

sodium-potassium pump

Question 34

how does the sodium-potassium pump work?

Answer 34

moves Na and K against their electrochemical gradients → extrudes Na from the cell while taking in K

requires energy that comes from ATP hydrolysis

Question 35

what does the Goldman, Hodgkin, and Katz equation represent?

Answer 35

permeability depends on the number and characteristics of the channel

the Na or Cl that can cross the membrane is linked to the concentration of the ion channels, but also their ability to open in a certain number, or to let a certain number of ions to cross the membrane

Question 36

what is the Ohm rule?

Answer 36

V=IR (current x resistance)

Question 37

despite the thinness of the membrane, because of the hydrophobic part, what does the membrane act as?

Answer 37

a capacitor

Question 38

what is a capacitor?

Answer 38

a devise able to store charges in different sites - they are useful because when you connect a resistance, the charges start to move and generate a current

Question 39

what equation represents the electrical potential deriving from the accumulation of charge from a capacitor?

Answer 39

V=Q/C

Q= charge in Coulomb and C=capacitance in Farads

Question 40

what are the ion channels equivalent to in an electric circuit?

Answer 40

resistors

Question 41

how is conductance measured?

Answer 41

y=1/c

the current passing through the K channel is the conductance x membrane potential

Question 42

how do we calculate the classical Ohms law a[plied to an ion channel?

Answer 42

iK=yK x Vm

Question 43

how do we calculate the Toal resistance considering the number of ion channels allowing the flux of a specific ion

Answer 43

g=N x y

N=number of channels and y= conductance of channel

Question 44

what is the final scheme in which we have the chemical driving force (E), the conductance of the channels (y), the pump, and the capacitance of the membrane all in parallel?

Answer 44

iK=yK x (Vm-Ek)

Question 45

what does the chemical gradient (E) depend on?

Answer 45

depends entirely on the difference in the concentration of the ion between the inside and outside of the membrane, so this equation is true only in a stationary state when these concentrations do not chang

Question 46

describe the excitatory postsynaptic potential (EPSP):

Answer 46

the release of neurotransmitters modulated by the opening of the Na and Ca channels allowing the ion influx which leads to a depolarization

excitatory because they are driving the membrane potential from the classic resting potential (-70mV) to something more positive

Question 47

describe the situation of inhibitory synaptic potential (ISP):

Answer 47

the opposite of the release of Na and Ca → the release of Cl and K can cause a drift in membrane potential from -70 mV to something more negative (hyperpolarization)

Question 48

what is the key part of the neuron for the generation of the action potential?

Answer 48

axonal hillock

Question 49

what is the axonal hillock?

Answer 49

part of the membrane at the base of the axon connecting the cell body to the axon → fundamental because it contains specific classes of ion channels that drive the generation of the action potential

there is a tight accumulation of K / Na voltage channels

Question 50

what is the critical value for a neuron?

Answer 50

20-30 mV above the resting potential so ~ -55mV

Question 51

what happens when a neuron reaches the critical value?

Answer 51

the generation of the action potential in the axonal hillock

Question 52

what happens when the critical value is reached?

Answer 52

all of the Na+ voltage-gated channels open and the action potential is conducted without decrement along the axon

Question 53

what happens directly after the action potential is created and conducted?

Answer 53

the refractory period: for a brief period after the action potential has been generated the neurons ability to fire is repressed

Question 54

what is the absolute refractory period?

Answer 54

the period in which a neuron cannot generate a new action potential in any way

Question 55

what is the relative refractory period?

Answer 55

a new action potential can be generated but only by a very large stimulus

Question 56

what two parts of the Na voltage-gated channels can cross the membrane?

Answer 56

the helix and the P region

Question 56

what two parts of the Na voltage-gated channels can cross the membrane?

Answer 56

the helix and the P region

Question 57

describe the helix of a Na voltage gated channel:

Answer 57

rich in Glycine amino acids which is positive in charge → constitutes the voltage sensor of the channel

when the threshold is reached, they helix moves and the movement is strong enough to make the channel change conformation and to open

Question 58

what does the P region function as in a Na voltage-gated channel?

Answer 58

the filter → these ion channels are extremely selective and only allow Na to flow through

Question 59

when do we have the absolute refractory period?

Answer 59

during the peak when the Na / K channels are opening and closing , because when the Na enters the cell in that region of the membrane all of the ions would be inside, so no more Na could enter, and only after the Na / K pump pushes the Na out again there could be another action potential

Question 60

when does the relative refractory period occur?

Answer 60

during the hyperpolarization phase - due to the more negative action potential from K we will need a greater depolarization to reach the -55 mV threshold

Question 61

how do the action potentials move across the axons?

Answer 61

instead of moving in a continuous way, they jump from node to node (due to the spaces called Nodes of Ranvier) and in this way it can travel much faster (120 m/s)