action/membrane potential Flashcards by J A

normal flow of ions

high to low concentration

until equilibrium is reached

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why is it useful

transport over short distances
Spontaneous
No energy input required

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what do ions produce as tehy flow from high to low

equilibrium value

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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

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what is voltage also known as

potential differnce

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measured in

volts

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how is P.D. generated

and what does it produce

Generated by ions to produce a charge gradient

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what is current

Movement of ions due to a potential difference

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measurement of current

Amps

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what is resistance

Barrier that prevents the movement of ions

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measurement of resistance

Ohms

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when is the electrochemical equilibrium

achieved when electrical force prevents further diffusion across the membrane

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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

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what does the The Nernst Equation calculate

equilibrium potential (E

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what is the equation

E=(RT/zF)-ln(x2/x1)

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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

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what assumptions can be made

Assume T = 37 °C = 310 K

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you must also convert natural log to what

common log

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what is E measured in

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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

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each ion’s contribution to membrane potential is proportional what?

to how permeable the membrane is to the ion at any time

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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

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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

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what does a value of 0.5 for p show

the channel is open 50 percent of the time

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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