Review
,Relative refractory period
• There are two reasons why a larger than normal depolarizing graded potential is
needed to initiate another action potential during the relative refractory period:
• After hyperpolarization: during the after hyperpolarization phase the membrane
potential dips toward a more negative value. Instead of a depolarizing graded
potential needing to take the membrane potential from -70mv to -55mV (+15 mV
change) to initiate the action potential, the graded potential may need to take
the membrane potential from a more negative -80mV to -55mV (+25mV change
i.e. a more depolarizing graded potential) to initiate another action potential.
• During the relative refractory period Na+ channels have almost returned to their
resting state (they are still partially inactivated) and in this state it is theorized
that a more depolarizing graded potential than normal is need to open them up.
You can think of this as a change in threshold. Instead of -55mV opening the
voltage gated Na+ channel (like at rest) you need to reach -45mV to open the
partially inactive voltage gated Na+ channel.
• Both of these combined dictates the relative refractory period.
, Depolarizing graded potential
,Relative refractory period
• There are two reasons why a larger than normal depolarizing graded potential is
needed to initiate another action potential during the relative refractory period:
• After hyperpolarization: during the after hyperpolarization phase the membrane
potential dips toward a more negative value. Instead of a depolarizing graded
potential needing to take the membrane potential from -70mv to -55mV (+15 mV
change) to initiate the action potential, the graded potential may need to take
the membrane potential from a more negative -80mV to -55mV (+25mV change
i.e. a more depolarizing graded potential) to initiate another action potential.
• During the relative refractory period Na+ channels have almost returned to their
resting state (they are still partially inactivated) and in this state it is theorized
that a more depolarizing graded potential than normal is need to open them up.
You can think of this as a change in threshold. Instead of -55mV opening the
voltage gated Na+ channel (like at rest) you need to reach -45mV to open the
partially inactive voltage gated Na+ channel.
• Both of these combined dictates the relative refractory period.
, Depolarizing graded potential
