Lecture 4 - Action Potentials Flashcards
what is GNA+?
the conductance of NA+ across the membrane `
what is GK+?
the conductance of K+ across the membrane
what is EK+?
the equilibrium potential for K+
what is ENa+?
the equilibrium potential for Na+
what is Vm?
the membrane potential
what 2 things does the conductance of an ion across the membrane depend on?
depends on permeability (ion channels) and equilibrium potential (driving force)
what determines permeability?
ion channels
what determines equilibrium potential?
driving force
what is the driving force?
the difference between the actual membrane potential and ions equilibrium potential (indicates how far an ion is from its equilibrium)
what is the relationship between a driving force and an ion that is not at equilibrium?
when an ion is not at its equilibrium, an electrochemical driving force acts on the ion, causing the net movement of an ion across the membrane down its electrochemical (conc) gradient
what is the effect on polarisation when K+ exits the cell?
K+ hyperpolarises the cell as it exits
what is hyperpolarisation?
makes the cell membrane potential more negative
what is the effect on polarisation when Na+ enters the cell?
Na+ depolarises. the cell as it enters
what is depolarisation?
a sudden change in cell membrane potential that makes the cell more positive (resulting in a less negative charge)
what are voltage-gated Na+ channels composed of? (in terms of activation gates)
composed of an activation gate with a voltage sensor that is an ‘on’ switch only specific for Na+ and an inactivation gate that is an ‘off’ switch that is actively closed
why is the inactivation gate of Na+ channels actively closed?
at the peak of an AP when enough Na+ has entered the neuron and MP is high enough the channels inactive themselves by closing their inactivation gates
what are voltage-gated Na+ channels blocked by?
blocked by tetrodotoxin from pufferfish
what is the effect of tetrodotoxin binding to Na+ channels?
binds irreversibly permanently blocking the Na+ channel
what are voltage-gated Na+ channels composed of?
composed of 2 beta-subunits and 4 alpha-subunits
what model is an inactivation gate of a Na+ channel?
a ball and chain model
what is a not-activated voltage-gated Na+ channel?
the pore is closed because it is blocked by a positive charge, closing the Na+ channel
- ball is not blocking the channel
what is an activated voltage-gated Na+ channel?
the pore is open as the positive charge is no longer blocking the channel but remains nearby for selectivity
what is an inactivated voltage-gated Na+ channel?
the pore is still open but the ball on the end of the chain is blocking it from activating acting as the inactivating particle
what are voltage-gated K+ channels composed of?
composed of an activation gate that is specific for K+ that selectively filters and an inactivation gate that is slower than a voltage-gated Na+ inactivation gate
what are voltage-gated K+ channels blocked by?
blocked by tetraethylammonium that binds irreversibly
what is the absolute refractory period?
the time period where it is impossible to trigger another AP
why does the absolute refractory period occur?
because voltage-gated Na+ channels become inactivated by the inactivation gate and remain inactivated until hyperpolarisation occurs
what is the relative refractory period?
the time period when another AP is possible to be triggered but it is more difficult to initiate
why does the relative refractory period occur?
because after hyperpolarisation occurs the MP is further away from the threshold than for an AP at rest, resulting in a larger depolarization required to reach the threshold triggering the next AP, the Na+ inactivation gate is open but K+ channels are still open
how is the speed of conduction increased?
to increase the speed of conduction you must decrease internal axial resistance (Rin) and membrane potential (Rm)
how is axon diameter increased to increase the speed of conduction?
axon diameter is increased by decreasing internal axial resistance (Rin) to reduce resistance to ion flow
how does myelination increase the speed of conduction?
myelination insulates the axon to increase the membrane potential (Rm) to reduce the leakage of ions
how do nodes of Ranvier increase the speed of conduction?
nodes of Ranvier allow saltatory conduction through a high density of Na+ channels that are not present in the internodes allowing the signal to rapidly jump from node to node
