action/membrane potential Flashcards
normal flow of ions
high to low concentration
until equilibrium is reached
why is it useful
transport over short distances
Spontaneous
No energy input required
what do ions produce as tehy flow from high to low
equilibrium value
what is flux
The number of molecules that cross a unit area per unit of time (number of particles).
i.e. molecules.m−2.s−1
what is voltage also known as
potential differnce
measured in
volts
how is P.D. generated
and what does it produce
Generated by ions to produce a charge gradient
what is current
Movement of ions due to a potential difference
measurement of current
Amps
what is resistance
Barrier that prevents the movement of ions
measurement of resistance
Ohms
when is the electrochemical equilibrium
achieved when electrical force prevents further diffusion across the membrane
what is the Equilibrium potential
the potential at which electrochemical equilibrium has been reached. It is the potential that prevents diffusion of the ion down its concentration gradient
what does the The Nernst Equation calculate
equilibrium potential (E
what is the equation
E=(RT/zF)-ln(x2/x1)
what do the letters stand for
R = gas constant T = Temperature in Kelvin z = charge on ion (-1 for Cl-, +2 for Ca2+) F = Faraday’s number - charge per mol of ion ln = natural logarithm (log to base e) X2 = intracellular ion concentration X1 = extracellular ion concentration
what assumptions can be made
Assume T = 37 °C = 310 K
you must also convert natural log to what
common log
what is E measured in
mV
What accounts for the -20mV difference between the typical membrane potential and Ek
the membranes are permeable to other ions as well as the potassium like sodium which will make it more positive
each ion’s contribution to membrane potential is proportional what?
to how permeable the membrane is to the ion at any time
why does Goldman-Hodgkin-Katz (GHK) equation equation describes the membrane potential more accurate than The Nernst Equation?
the GHK takes into account permeability of the membrane to particular ions
what does a value of 1 for p describe
p is permeability or channel open probability
a value of 1 means the channel is open 100 percent of the time
what does a value of 0.5 for p show
the channel is open 50 percent of the time
what does Subscript on P indicate?
the ion
when the Electrochemical equilibrium is formed what 2 forces are balanced
electrical forces balance diffusional forces
when will the nernst equation work
when the membrane is uniquely selective to one particular ion
define Depolarisation
Membrane potential becomes more positive towards zero mV
define Repolarisation
Membrane potential decreases towards resting potential
Define Overshoot
Membrane potential becomes positive
Define Hyperpolarisation
Membrane potential decreases beyond resting potential
what causes a membrane potential to change
external stimulation or neurotransmitters
what is the change in membrane potential graded by
the type or strength of stimulation
what is the realtionship between distnace and potentail differnce
the p.d. will always decay from the site of the stimulus to father out
what 2 changes in membrane potential can a stimulus cause
either depolarisation or hyperpolarisation
why does the graded potential decay down the length of an axon
Charge ‘leaks’ from axon and the size of the potential change decreases along the axon
an action potential can only occur when a graded potential reaches a…
threshold
positives of nerve impulses
allow the transmission of information reliably and quickly over long distancesq
a major function of nerve impulses
in cell-to-cell communication and can be used to activate intracellular processes
what are changes in membrane potential is dependent on
opening or closing of ion channels
are changes in membrane potential during the action potential are due to ion pumps?
no
due to flow of ions through an ion channel
what are the 5 stages of action potential
Phase 1.
Resting membrane potential
Phase 2.
Depolarising stimulus
Phase 3.
Upstroke
Phase 4.
Repolarisation
Phase 5.
After-hyperpolarisation
during phase 1, is the permiability of sodium higher than potassium
no
the permability of potassium is greater than sodium
when does phase 3 start
at the threshold potential
why does the permability of sodium increase?
because voltage-gated Na+ channels open quickly [Na+ enters the cell down electrochemical gradient]
why doesn’t the membrane potential reach the sodium equilibrium potential
na channels close
why does the permeability of the potassium increase
the voltage-gated K+ channels start to open slowly [K+ leaves the cell down electrochemical gradient]
why does the entry of sodium stop in phase 4?
because the voltage-gated Na+ channels close
lower permeability for sodium
why does the membrane potential go towards the potassium equilibrium potential
as more voltage-gated K+ channels open & remain open
K+ leaves the cell down its electrochemical gradient
how is the na channel inactivated/closed
a portion of the sodium channel protien moves in to block the pore of the channel stopping sodium entering the cell
what is the period of time when a new action potential cannot be triggered called
Absolute refractory period
during that period, are the activation gates opened or closed
closed
why is the refractory period important
it limits the amount of signalling
are Na+ channels open during the relative refractory period
some gates are open
why do you need a stronger stimulus to produce an action potential during the relative refractory period
due to hyperpolarisation, you need a larger depolarisation to reach the threshold
what is the “all-or-none” principle
when a stimulus exceeds the threshold potential, the nerve will produce a complete response; otherwise, the response is graded and decays.
the rate of graded potential/action potential decay along an axon is dictated by what factors
internal diamter
intenal resistance
how well it is insulated
how does an axon being myelinated affect the time taken for the action potential to decay
takes longer
what factors increase conduction velocity
higher axon diameter and more myelination
what factors decrease the conduction velocity
with reduced axon diameter (i.e. re-growth after injury), reduced myelination (e.g. multiple sclerosis and diphtheria), cold, anoxia, compression and drugs (some anaesthetics)
What are the three main factors that influence the movement of ions across the membrane?
Concentration of the ion on both sides of the membrane, the charge on the ion and the voltage across the membrane.
Why is the K+ equilibrium potential negative (e.g. -70mV) and the Na+ equilibrium potential positive(e.g. +40mV) when both are positive ions?
More K+ inside the cell than outside so tend to flow out of the cell, while more Na+ outside the cell than in, therefore tend to flow into the cell. A potential of -70mV is needed to attract K+ and stop net outward flow, while a positive charge of +40mV is needed to repel Na+ from entering the cell.
Which ion is important for the upstroke (rising phase) and which is important for the falling phase of the action potential?
In which direction do these ions move?
The upstroke mediated largely by Na+ ions moving down their concentration gradient into the cell. The falling portion of the action potential dominated by K+ ions moving down their concentration gradient and therefore exiting the cell
What factors influence the speed of propagation of an action potential along an axon?
Larger diameter axons have lower resistance, so ions move faster – conduction velocity is proportional to the square root of the axon diameter.
There is a linear relationship between conduction velocity and myelin thickness
