Lecture #6 Flashcards
how do cations migrate?
positive ions migrating towards a negative charge
how do anions migrate?
negative ions towards a positive charge
what is the charge of potassium outside vs inside of a cell?
cytoplasm: 140 mM
outside: 2-4 mM
what is the concentration of sodium outside vs inside of a cell?
cytoplasm: 6-10 mM
outside: 130-140 mM
what is the concentration of chloride outside vs inside a cell?
cytoplasm: 13-14 mM
outside: 130-150 nM
what is the concentration of calcium outside vs inside of a cell?
cytoplasm: 0.001 mM
outside: 1.5-2 mM
why is the concentration of calcium so low?
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
are organic ions?
several proteins that are clusters under the cell membrane and are fundamental of rte maintenance the gradient and the membrane potential
what allows for the establishment of the membrane potential in the neuronal cell membrane?
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
what is the charge of a neuron?
neutral → the only position of the cell membrane having a net charge is the tiny space located in tight contact with the cell membrane
what are the three important features of ion channels?
- they recognize and select specific ions
- they open and close in response to specific signals
- they conic ions across the membrane
what can GABA and Glycine ion channels flux?
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
what do channels that respond to acetylcholine, glutamate, or serotonin flux?
both Na and Ca ions → since both are positively charged they contribute to the depolarization of the membrane
in a cell not involved on the propagation of electrical impulses, what is the membrane potential related to?
the distribution and the abundance of specific ion channels the can cross only the potassium
which ion is fundamental for the maintenance of the negative charge on the cell membrane?
K → both the chemical and electrical driving force
how does K act as the chemical driving force?
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
how does K act as the electrical driving force?
because K is positive, it is also attracted by negative charges inside the cell
in cells other than neurons, describe the K channels:
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
at equilibrium, what is the membrane potential?
-65 mV
for sodium, how are electrical and chemical forces oriented?
the two forces pull the ion inside of the cell
for chloride, how are electrical and chemical forces oriented?
- 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
what is the resting potential of neurons?
~ -70 mV → the cytoplasmic side is more negative than the outside of the clll due to the huge amount of inorganic ions
what equation starts from the free energy deriving from the thermodynamic movement of ions (chemical force)?
Nernst equation →
ΔG= -RTln(ion o / ion i)
what equation represents free energy variation?
ΔG= -EzF
what is the state of the ion potential at equilibrium?
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
what does R stand for in ΔG= -RTln(ion o / ion i)?
the universal gas constant
R= 8.314472 J/K-mol
wha does T stand for in ΔG= -RTln(ion o / ion i)?
absolute temperature in Kelvin
what does F stand for in ΔG= -EzF?
Faradays constant
F= 9.64853399x10^4 C/mol
what does Z stand for in ΔG= -EzF?
oxidation state of the ion
describe the electrochemical driving force of neurons:
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
what happens if we have a K channel that starts to open?
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
what would happen if the leakage of K continued unopposed for a long period of time?
the K gradient would degrade and the cell would die
what prevents the dissipation of the ionic gradient?
sodium-potassium pump
how does the sodium-potassium pump work?
moves Na and K against their electrochemical gradients → extrudes Na from the cell while taking in K
requires energy that comes from ATP hydrolysis
what does the Goldman, Hodgkin, and Katz equation represent?
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
what is the Ohm rule?
V=IR (current x resistance)
despite the thinness of the membrane, because of the hydrophobic part, what does the membrane act as?
a capacitor
what is a capacitor?
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
what equation represents the electrical potential deriving from the accumulation of charge from a capacitor?
V=Q/C
Q= charge in Coulomb and C=capacitance in Farads
what are the ion channels equivalent to in an electric circuit?
resistors
how is conductance measured?
y=1/c
the current passing through the K channel is the conductance x membrane potential
how do we calculate the classical Ohms law a[plied to an ion channel?
iK=yK x Vm
how do we calculate the Toal resistance considering the number of ion channels allowing the flux of a specific ion
g=N x y
N=number of channels and y= conductance of channel
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?
iK=yK x (Vm-Ek)
what does the chemical gradient (E) depend on?
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
describe the excitatory postsynaptic potential (EPSP):
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
describe the situation of inhibitory synaptic potential (ISP):
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)
what is the key part of the neuron for the generation of the action potential?
axonal hillock
what is the axonal hillock?
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
what is the critical value for a neuron?
20-30 mV above the resting potential so ~ -55mV
what happens when a neuron reaches the critical value?
the generation of the action potential in the axonal hillock
what happens when the critical value is reached?
all of the Na+ voltage-gated channels open and the action potential is conducted without decrement along the axon
what happens directly after the action potential is created and conducted?
the refractory period: for a brief period after the action potential has been generated the neurons ability to fire is repressed
what is the absolute refractory period?
the period in which a neuron cannot generate a new action potential in any way
what is the relative refractory period?
a new action potential can be generated but only by a very large stimulus
what two parts of the Na voltage-gated channels can cross the membrane?
the helix and the P region
what two parts of the Na voltage-gated channels can cross the membrane?
the helix and the P region
describe the helix of a Na voltage gated channel:
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
what does the P region function as in a Na voltage-gated channel?
the filter → these ion channels are extremely selective and only allow Na to flow through
when do we have the absolute refractory period?
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
when does the relative refractory period occur?
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
how do the action potentials move across the axons?
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)
