Electric Potential of Cells Flashcards
Potential (Potential difference)
The voltage difference between two points due to separated electrical charges of opposite sign; these separated charges have the potential to do work
Membrane potential
The voltage difference between the inside and outside of a cell
Equilibrium potential
The voltage difference across a membrane that produces a flux of a given ion species that is equal but opposite to the flux due to the concentration gradient of that same ion
- where the net flow through any
open channel is 0
- indicates that the chemical and electrical
forces for that ion are in balance
Resting membrane potential (Vm)
The steady potential of an unstimulated cell
- RMP of ALL cells is NEGATIVE; -50 to -90mV in excitable cells and -5 to -15mV for non-excitable cells
Graded potential
A potential change of variable amplitude and duration that is conducted decrementally (further away from stimulus); has no threshold or refractory period
Can be:
- depolarizing OR hyperpolarizing
- vary in size d/t stimulus strength
- decay with distance from site of initial stimulus
Action potential
A brief all-or-none depolarization of the membrane, which reverses polarity in neurons; has a threshold and refractory period and is conducted without decrement
Synaptic potential
A graded potential change produced in the postsynaptic neuron in response to the release of a neurotransmitter by a presynaptic terminal; may be depolarizing (an excitatory postsynaptic potential or EPSP) or hyperpolarizing (an inhibitory postsynaptic potential or
IPSP)
Receptor potential
A graded potential produced at the peripheral endings of afferent neurons (or in separate receptor cells) in response to a stimulus
Pacemaker potential
A spontaneously occurring graded potential change that occurs in certain specialized cells
Threshold potential
The membrane potential at which an action potential is initiated
Energy
the ability of an object to do work on another object
Kinetic Energy
energy in motion
Potential energy
energy at rest
Effects of quantity and distance on separated charges
Force:
- INCREASES when charges are closer together
- INCREASES when there are MORE charges being separated
Ohm’s Law
V=IR
I=V/R
R = Resistance due to presence of the plasma membrane (measured in Ohms)
V = Difference in electric potential energy on either side of the membrane (measured in volts)
I = Flow of charged particles through “conductors” in the membrane (measured in Amperes
Capacitor
a device that can store energy by accumulating electric charges on two closely spaced surfaces that are insulated from one another
Insulator
structure that separates opposing surfaces so that the contents of one surface do not bleed into the other surface
Conductance (G)
a measure of how easily an ion moves through a membrane; reciprocal of resistance
- HIGH conductance= LOW resistance
- ion channels are conductors when open
Resistance (R)
a measure of how DIFFICULT an ion moves through a membrane; reciprocal of conductance
- HIGH resistance= LOW conductance
- ion channels are mini-resistors when closed
Determinant of resting membrane potential
- differences in [ion] in the ICF and ECF
- difference in membrane permeabilities to the ion
Nernst equation
describes the equilibrium potential of an ion
equation: 61/Z x log ([out]/[in])
E (Na+)
+60 mV
E (K+)
-90 mV
Net driving force
must consider both concentration and
electrical potential differences so can be calculated:
Vm-Eion
ex: E(Na+)= +60mV –> -70mV-60mV= -130mV which means driving force favors Na+ ENTERING the cell
Ionic current
refers to ion movement when AND membrane has conductance to that ion; can be calculated:
Ix = gx(Vm –Ex)
Goldman-Hodgkin-Katz Equation
determines RMP due to the influences of the 3 major ions on RMP
Vm= 61 log (Pk[Kout]+PNa[Naout]+PCl[Clin]/PK[Kin]+PNa[Nain]+PCl[Clout])
= -70mV
Permeability coefficient of K
1 (most permeable to membrane)
Permeability coefficient of Na
0.04
Permeability coefficient of Cl
0.45
Contribution of K to RMP
membrane is MOST permeable to K+, with the most leak channels which is why RMP is close to E(K)
Contribution of Na/K ATPase pump to RMP
3Na out and 2K which generates a small electric potential, making RMP negative
Contribution of Cl to RMP
leak channels for Cl- exist, but there is no pump to move the ions so not a big contribution
Contribution of Na to RMP
membrane is not as permeable to Na+, with fewer leak channels which is why RMP is close to E(K) and NOT E(Na)
Depolarization
potential moving from RMP to less
negative (closer to zero) values
Overshoot
a reversal of
membrane polarity, when the
inside of the cell becomes more
positive than the outside.
What ion channel is/are responsible for the OVERSHOOT?
Na+ activation gates slow to close and K+ channel slow to open
Repolarization
potential returning to the RMP
from a depolarized state
Hyperpolarization (Undershoot)
potential becoming more negative
than the RMP
What ion channel is/are responsible for the UNDERSHOOT?
K+ leak channels
Summation
addition of graded potentials from several stimuli that occur in rapid succession before each graded potential has died out
- allows the graded potentials to be added up to generate enough current to bring cell past threshold and fire an action potential
Time constant (τ)
the time that it takes for the voltage to reach 63% of its final value following injection of a current OR the time it takes voltage to drop to 37% of its initial value; depending on membrane resistance and membrane capacitance; is what allows for temporal summation
equation: τ= RmCm
Main contributor to the time constant
membrane resistance
= if the membrane has high resistance OR if channel is CLOSED
Temporal summation
the processing of multiple subthreshold potentials over a set period of time
Length constant (λ)
thee distance from the site of current
injection where the potential has fallen by 63% of the original value; indicates how far a depolarizing current will spread along a
nerve; the longer the length constant the further the current spreads down the neuron
- important determinant of
synaptic efficacy
λ= (rm/r/i)^1/2
When is the length constant the greatest?
- LOW internal resistance
- HIGH membrane resistance
- LARGE diameter of a neuron (makes for LOW internal resistance)
