15 + 16 Flashcards
Describe the diffusion of ions
- The permeability of membranes is very small for ions
How can we describe passive diffusion of ions?
By using the gradient of the electrochemical potential instead of the chemical potential
→ Also, the migration of charged particles is influenced by the electric field
(This equation gives the flux density of a charged particles maintained by the concentration gradient and electric potential gradient → Both of these are constant in time under steady-state condition)
Why do we use electrochemical potential gradient for passive diffusion of ions?
Because the migration of charged particles is influenced by electric field as well
The role of Na+ - K+ Pump
An antiporter main the conditions in which
- The extracellular concentration of na+ exceed its intracellular concentration
- [K+] is larger within the cell than in the extracellular space
→ The pump is coupled to hydrolysis of ATP
→ Both ions are translocated against electrochemical potential gradient
→ 2 K+ ions are pumped for every 3 Na+ ions
What is resting potential?
Potential difference between the inner and outer surface of the cytoplasmic membrane
Interpret the resting membrane potential by the Goldman-Hodgkin-Katz (GHK) equation
- The resting state of the membrane as a result of a steady state electrodiffusion of the permanent ions with a constant flow between 2 sides of membrane
=> This process is maintained by the electrical potential difference across the membrane
Is the resting membrane potential constant in time? What are the consequences?
Yes
→ The total electric charge and particle flux densities originating from the mobile ions must be zero
What is Donnan equilibrium?
It is the concentration difference of ions between 2 sides at equilibrium and obeying the principle of electroneutrality if one compartments on the 2 sides of membrane contains an impermeable polyelectrolyte
This model below characterize Donna equilibrium.
→ Describe
- Placing polyanion in “intracellular compartment”
- Place KCl solution in the extracellular compartment
→ Cl- ions stary diffusing from extracellular compartment to intracellular one (due to concentration difference between for Cl-) → Because the membrane is impermeable to polyanion, its concentration cannot be balanced
→ The electric potential difference generated by Cl- evoke K+ into intracellular commpartment
→ K+ current will ensure the electro-neutrality of the bulk solutions
→ Thermodynamic equilibrium keep these ionic currents of opposite directions in balance
(- there is an eletrochemical equilibrium in resting cells
- the cell membrane is permeable only for K+ ions)
What does electro-neutrality mean?
The sum of opposite charge separation on the both sides of the membrane
What does the electrochemical gradient require in order to be constant?
- The concentration gradients of permeant ions across membrane are kept constant
What are Local changes in membrane potential?
transient changes of membrane potential limited in space and time
Local changes of membrane potential
Characteristics of membrane potential in response to square pulses
a) Hyper-polarization
b) Depolarization due to stimulus below threshold
c) Depolarization due to stimulus exceeding the threshold
Local changes of membrane potential
How can we control the resting potential?
By altering the direction of the current, we can control the resting potential, either by increasing or decreasing it
Local changes of membrane potential
Describe the membrane potential response to a series of square-wave current pulses
- Below a certain stimulus threshold → There responses as the changes in the resting membrane potential; are proportional to the stimulating current irrespective pf oystering direction
- When exceeding a threshold level → the size of response will NOT depend on stimulus strength (→ characteristics for excited state of the cellular membrane)
Describe the membrane-potential curve for current pulse not exceeding the depolarization threshold
- This figure shows a stimulating current pulse (square-wave pulse) and the consequent membrane potential
- Compared to the step-function of the stimulating current pulse, the membrane potential changes with a characteristics delay
3 factors that characterize the cellular membrane
- Permeability constants
- Specific conductivities for each ion
- Capacitive properties
Is cellular membrane a good electric insulator? Why?
- Yes → because charges can accumulate on both side
Does the membrane has the same conductivity for different ions?
No, the membrane has different conductivities for different ions
The relationship of the capacity of membrane, time and membrane potential in resting state
The capacity of membrane is ….
- Constant in time
- Does not depend on membrane potential
→ its conductivity is constant in resting state
The relationship of the capacity of membrane, time and membrane potential in excited state
Its conductivity goes through characteristic membrane potential and time-dependent changes
Factors that determine the migration of individual ion
- The difference in the membrane potential and equilibrium potential
Formula for the conductive current
The conductive current
- Proportional to the difference of actual membrane potential and equilibrium potential of a given ion
What is time constant?
the time needed for the change in membane potential
- to reach 63% of its maximum value
- to drop to the e-th of its value at the end of stimulation
→ Time constant of the exponential process is given by the product of the resistance and the capacitance
What does time constant characterize in this graph?
it characterizes the steepness of the change in membrane potential
What is space constant?
the distance where the amplitude of stimulus is the e-th of its initial value
→i.e. the distance in which the maximal value (100%) of the induced local potential change (by a square pulse) is decreased to its e-th level (37%)
What does the space constant depend on?
Electric properties of the membrane
→ Transmembrane and intracellular resistances
What is SUMMATION in space and Time?
summation of local potential changes
(- excitatory potential - depolarization
- inhibitory potential - hyperpolarization)
What is this? Describe?
Threshold potential
→ critical membrane potential level at which an action potential can occur
Characteristic of amplitude of action potential
facultative
→ “All-or-none” amplitude conducted with constant amplitude
The role of RC circuit model in electric model of the membrane
It enables us to interpret measured response curves
Taking capacitive current and conductive current into account
→ What is the condition for the transport model?
- Ik is the total conductive currents for ions
- Ic is capacitive current
We have to take the stimulating current into account in such a way that conditions of the transport model of the resting state
→ According to convention, describe Istimulation
Istimulation is negative if positive charges enter the cell and vice versa
Based on electric model of the cell membrane, which elements that characterize the membrane
- Transmembrane resistances
- Capacity
- Electromotive forces
The role of electric model of the cell membrane
For the understanding the time- and distance-dependence of membrane potential changes within the resting state, induced by electric pulses
The role of electric model of the cell membrane
For the understanding the time- and distance-dependence of membrane potential changes within the resting state, induced by electric pulses
Describe the structure of electric model of cell membrane
- Ri - length of intracellular space
- Re the length of ECM
- Im - transmembrane current
- Ii - cytosolic current
Describe time-dependence of membrane potential changes
- We can determine the change of membrane potential as a function of time
- The difference in saturation value of membrane potential (Us) and resting potential before stimulation is directly proportional to the amplitude of the stimulating pulse
Describe distance-dependence of membrane potential changes
A local change of the membrane potential, generated at position x0 , in the vicinity of that point along the membrane (from x0, along the x axis, which is in the plane of the membrane surface)
→ The change in membrane potential exponentially decreases with the distance measured from the x0 point
What are temporal and spatial summation?
- Temporal summation refers to the sensory summation that involves the addition of single stimuli over a short period of time
- Spatial summation refers to the sensory summation that involves stimulation of several spatially separated neurons at the same time.
