Physiology of Excitable Cells - Membrane Biophysics

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Action Potential
rapid, all or nothing change in membrane potential, self propagating
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Passive conduction
components of cell membrane similar to passive elements of electric circuits. ion channels correspond to resistor and lipid bilayer acts as capacitor.
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Time constant
τ=R*C equals time taken for voltage to rise exponentially by 63% of the difference between its initial and final values
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Length constant
Distance over which the change in potential decreases to 1/e (37%) of its maximal value. increasing axonal diameter increases length constant, decreasing Rm increases leak conductance - decrease in length constant. Dependent on Rm and Ra = √(Rm/Ra)
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Electrotonic conduction
passive spread of voltage across membrane. greatest when Rm is high - high insulation, and when Ra is low -large membrane diameter this results in faster APs. passively propagated signals decrease in amplitude with distance -leaks out of membrane.
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Membrane resistance Ω
ability of charge to move from inside of cell to out of cell. Increasing Rm increases max response altitude and slows down rise time. decreasing Rm decreases max response altitude and rapid rise time
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Membrance capacitance F
ability of membrane to store charge. Altering Cm only affects time course, increasing Cm slows down response, decreasing Cm faster response
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Absolute refractory period
Na+ channels inactivated and cant be reopened until membrane repolarised
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Relative refractory period
Na+ channels recovering from inactivation, high open probability of K+ channels, stimulus stronger required to fire 2nd AP to recruit critical number of Na+ channels
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Factors that increase length constant
Low Ra, High Rm - enhance AP conduction velocity by increasing longitudinal flow of current - more current generated and further it can spread - faster AP propagated
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Factors that shorten time constant
Low Cm, Low Rm -enhance rate of depolarisation (speed of AP upstroke)
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Evidence for local circuit theory
Hodgkin - frog sciatic nerve, stimulated AP and placed block of cold metal - blocked active conduction. Measured small electrical signal at other end of block - decreased with distance = AP travel by passive conduction
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Myelination
increases conduction velocity of nerve axon because myelin increases Rm and lowers Cm. allows action potential to be conducted very rapidly from one node of Ranvier to next, making AP appear to jump from node to node in form of conduction = SALTATORY
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Evidence current only flows at nodes of Ranvier
microelectrodes applied depolarising current - easier to excite if stimulus applied close to node - easier to excite if stimulus applied close to node. can also only measure changes in potential close to node of ranvier
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Nernst equation
measure value where concentration gradient and electrical gradients balance each other = EA = (58/z)log10(([A]o/[A]i)) - theoretical assumes membrane perfectly selective for that ion
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Goldmann-Hodgkin-Katz equation
considers relative permeabilities for other ions - permeability of membrane to ions determines membrane potential (changes during AP)
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Permeability changes during AP
due to voltage sensitive ion channels opening and closing, these protein pores are inserted in membrane and they are selective for specific ions or groups of ions, they open and close responding to changes in membrane potential.
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Voltage clamp
technique of controlling membrane potential and measuring current to rapidly return membrane potential to chosen level, compensates (therefore measures) current. Clamps voltage and providing current equal and opposite - can measure current
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Conductance
measure of ability to pass ionic current (s) allows us to see voltage dependent behaviour independent of driving force
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During Action Potential
exceeds -55mV AP triggered depolarisation Na channels open, Na channels close at 140, K open at 140 repolarisation , Na channels open - afterhyperpolarisation
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Stochastic behaviour
cant be predicted precisely, independent behaviour of individual channels
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Na+ channels
Two voltage dependent gates ( activation and inactivation), resting - closed, depolarisation - open, 4 homologous domains, each domain six transmembrane segments
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K+ channels
closed and open state, tetramers -4 identical peptide units channels can co assemble with subunits - diff sub groups - increasing functional diversity.
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Ion channel states
selectivity filter in closed state. Open state - inner helices apart - aperture permitting ions to move through. Closed state - inner helices bundled - narrows pore preventing ions to pass through
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Other cards in this set

Card 2

Front

components of cell membrane similar to passive elements of electric circuits. ion channels correspond to resistor and lipid bilayer acts as capacitor.

Back

Passive conduction

Card 3

Front

τ=R*C equals time taken for voltage to rise exponentially by 63% of the difference between its initial and final values

Back

Preview of the back of card 3

Card 4

Front

Distance over which the change in potential decreases to 1/e (37%) of its maximal value. increasing axonal diameter increases length constant, decreasing Rm increases leak conductance - decrease in length constant. Dependent on Rm and Ra = √(Rm/Ra)

Back

Preview of the back of card 4

Card 5

Front

passive spread of voltage across membrane. greatest when Rm is high - high insulation, and when Ra is low -large membrane diameter this results in faster APs. passively propagated signals decrease in amplitude with distance -leaks out of membrane.

Back

Preview of the back of card 5
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