Membrane Potential and Action Potentials Flashcards
Ion Flux
The number of molecules that cross a unit area per unit of time ( number of particles )
Voltage ( potential difference )
Generated by ions to produce a charge gradient
Current ( amps)
Movement of ions due to a potential difference
Resistance
Barrier that prevents the movement of ions
What are changes in membrane potential caused by
Ion transporters
When is an electrochemical equilibrium achieved
When electrical force prevents further diffusion across the membrane . So when the concentration gradient exactly balances the electrical gradient .
What does the Nernst equation tell you
The equilibrium potential ( E)
What is R in the Ernst Equation
Gas constant ( 8.31)
What is T in the Ernst Equaton
Temperature in Kelvin ( 37 degrees or 310K)
What is z on the Ernst Equation
Charge on the ion ( eg. -1 for Cl - etc)
What is F in the Ernst Equation
Faradays number - charge per mol of ion
What do you do to the Ernst equation when calculating
Assume T = 37 degrees or 310 K
Convert natural log to common log
State E in mV
Male compartment 2 the inside and 1 outside
X 2 and X1 in Ernst Equation
X2 is the intracellular ion conc and X 1 is theextra cellular ion conc
Typical concentrations of sodium
150 mmol outside and 10 mmol inside
Typical concentrations of potassium
5 mmol outside and 150 mmol inside
The version of the Ernst equation we need
E = -61/z x log ( X inside /X outside )
Why do membrane potentials not rest at E k or Ena
Because there are always some channels open at all times
What does the GhK questions do
Describes the resting membrane potential ( look at the examples on OneNote )
What is the supposed membrane potential supposed to be
A typical membrane resting potential is -70 mmol and not -90 mmol which is Ek as some of the other ion channels are always open . As even at rest, the membrane has some finite permeability to some Na + and so the membrane potential is higher
Typical concentrations of Cl-
110 extra cellular and 5 intracellular
What does depolarisation mean
Membrane potential moves towards 0 mV
What does repolarisation mean
Membrane potential decreases towards the resting potential
Overshoot
When membrane potential becomes more positive
What does hyperpolarisation mean
When membrane potential decreases beyond resting potential
What are graded potentials
They have decrement all speed and strength and they occur at synapses and in sensory receptors , they also contribute to initiating or preventing action potentials
Action potentials and graded potentials
Actions potentials occur when a graded potentials reaches a threshold for the activation of Na + channels resulting an ‘all or nothing’ principal
Where do action potentials occur
Excitable cells ( mainly neurone and muscle cells but also in some endocrine tissues)
What does the permeability of ions depend on in action potentials
Permeability depends on conformational state of ion channels, they are opened by membrane depolarisation , inactivated by sustained depolarisation and closed by membrane hyperpolarisation / repolarisation
Changes in membrane potential during the action potential are due to
Ion channels and not pumps . The sodium potassium potential manga is the concentration gradient on which the movement of the ion depends on
Phase 1 of action potential
Resting membrane potential . Permeability for potassium ions are greater than sodium ions, the membrane potential is nearer the equilibrium potential for potassium ( -90mV) than that for sodium ( +72mV)
Phase 2 depolarising stimulus
The stimulus depolarises the membrane potential and moves it in the positive direction towards the threshold
Phase 3
Upstroke .starts at threshold potential, sodium channels open quickly and potassium channels open slowly , so potassium is moving out less than sodium is entering and membrane potential moves towards the sodium equilibrium potential
Phase 4 repolarisation
The sodium ion channels close and sodium entry stops and potassium ion channels opens and remains open so membrane potential moves towards the potassium equilibrium potential
At the start of repolarisation
Absolute refractory period , activation gate is open but inactivation gate is closed in sodium so new action potential cannot be triggered even with very strong stimulus
Later in repolarisation
Absolute refractory period continues and so the activation and inactivation gates are closed
After hyperpolarisation
At rest the potassium channels are still open and so potassium continues to leave the cell down the electrochemical gradient . Membrane potential moves closer to the potassium equilibrium - some voltage gated potassium channels then close . Membrane potential returns to resting potential
After hyperpolarisation what happens to sodium channels
Relative refractory period and so the inactivation gate is open and a stronger than normal stimulus is required to trigger an action potential
What affects how quickly the graded potential decays away from the site of initiation
Internal diameter of the axon
Internal resistance of axon
How well the axon is insulated
Where are voltage gated channels mostly located
Nodes of Ranvier
What affects the conduction velocity
Axon diameter and myelinated
Large diameter , myelinated axons travel very quickly while small diameter non myelinated axons travel slower.
Cold anoxia compression and drugs ( anaesthetics ) also decrease conduction velocity . Reduced myelination can be due to multiple sclerosis and diphtheria)
What are the Three main factors that influence the movement of ions across the membrane
Concentration of ion on both sides of the membrane, the change of the ion and the voltage across the membrane
Why is potassium ion equilibrium negative and sodium positive when they are both positive ions
There is more K+ inside the cell than outside to tend to flow out of the cell, while more Na+ outside the cell than in therefore tend to flow into the cell. A potential to -70mV is needed to attract K+ and stop net flow outwards while a positive charge of +40mV is needed to repel Na+ from entering the cell.
Which ion is imp for the upstroke and which is imp for the falling portion of the action potential ?
The upstroke is mediated by the sodium ions moving down their conc gradient into the cell . The falling portion of the action potential is by potassium ions going down their conc gradient and exiting the cell
What factors influence the speed of propagation of an action potential along the axon
Larger 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 the conduction velocity and myelin thickness
What causes a graded potential
Local changes in ionic conductance ( e.g. synaptic or sensory that produces a local current) that spreads along a stretch of membrane becoming smaller
