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Refractory period (physiology)
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Refractory period (physiology)
In physiology, a refractory period is a
period of time during which an organ or cell
is incapable of repeating a particular action,
or (more precisely) the amount of time it
takes for an excitable membrane to be ready
for a second stimulus once it returns to its
resting state following an excitation. It most
commonly refers to electrically excitable
muscle cells or neurons.
Electrochemical usage
See also: Action potential
After initiation of an action potential, the
refractory period is defined two ways:
• The absolute refractory period is the
interval during which a second action
potential absolutely cannot be initiated,
no matter how large a stimulus is applied.
Schematic of an electrophysiological recording of an action potential showing the
various phases which occur as the wave passes a point on a cell membrane.
• The relative refractory period is the
interval immediately following during which initiation of a second action potential is inhibited but not impossible.
The absolute refractory period coincides with nearly the entire duration of the action potential. In neurons, it is
caused by the inactivation of the Na+ channels that originally opened to depolarize the membrane. These channels
remain inactivated until the membrane hyperpolarizes. The channels then close, de-inactivate, and regain their ability
to open in response to stimulus.
The relative refractory period immediately follows the absolute. As voltage-gated potassium channels open to
terminate the action potential by repolarizing the membrane, the potassium conductance of the membrane increases
dramatically. K+ ions moving out of the cell bring the membrane potential closer to the equilibrium potential for
potassium. This causes brief hyperpolarization of the membrane, that is, the membrane potential becomes transiently
more negative than the normal resting potential. Until the potassium conductance returns to the resting value, a
greater stimulus will be required to reach the initiation threshold for a second depolarization. The return to the
equilibrium resting potential marks the end of the relative refractory period.
Refractory period (physiology)
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Cardiac refractory period
The refractory period in cardiac physiology is related to the ion
currents which, in cardiac cells as in nerve cells, flow into and out of
the cell freely. The flow of ions translates into a change in the voltage
of the inside of the cell relative to the extracellular space. As in nerve
cells, this characteristic change in voltage is referred to as an action
potential. Unlike nerve cells, the cardiac action potential duration is
closer to 100 ms (with variations depending on cell type, autonomic
tone, etc.). After an action potential initiates, the cardiac cell is unable
to initiate another action potential for some duration of time (which is
slightly shorter than the "true" action potential duration). This period of
time is referred to as the refractory period.
Effective Refractory Period
Classically, the cardiac refractory period is separated into an absolute
refractory period and a relative refractory period. During the absolute refractory period, a new action potential cannot
be elicited. During the relative refractory period, a new action potential can be elicited under the correct
circumstances.
Neuronal refractory period
The refractory period in a neuron occurs after an action potential and generally lasts one millisecond. An action
potential consists of three phases.
Phase one is depolarization. During depolarization, voltage-gated sodium ion channels open, increasing the neuron's
membrane conductance for sodium ions and depolarizing the cell's membrane potential (from typ. -70mV towards a
positive potential). In other words, the membrane is made less negative. After the potential reaches the activation
threshold (-55mV) the depolarization is actively driven by the neuron and overshoots the equilibrium potential of an
activated membrane (+40mV).
Phase two is repolarization. During repolarization, voltage-gated sodium ion channels inactivate (different than the
close state) due to the now depolarized membrane, and voltage-gated potassium channels activate (open). Both the
sodium ion channels closing and the potassium ion channels opening act to repolarize the cell's membrane potential
back to its resting membrane potential.
When the cell's membrane voltage overshoots its resting membrane potential (near -60mV), the cell enters a phase of
hyperpolarization. This is due to a larger than resting potassium conductance across the cell membrane. Eventually
this potassium conductance drops and the cell returns to its resting membrane potential.
The refractory periods are due to the inactivation property of voltage-gated sodium channels and the lag of potassium
channels in closing. Voltage-gated sodium channels have two gating mechanisms, the activation mechanism that
opens the channel with depolarization and the inactivation mechanism that closes the channel with repolarization.
While the channel is in the inactive state it will not open in response to depolarization. The period when the majority
of sodium channels remain in the inactive state is the absolute refractory period. After this period there are enough
voltage-activated sodium channels in the closed (active) state to respond to depolarization. However, voltage gated
potassium channels that opened in response to depolarization don't close as quickly as voltage gated sodium channels
return to the active closed state. During this time the extra potassium conductance means that the membrane is at a
higher threshold and will require a greater stimulus to cause action potentials to fire. This period is the relative
refractory period.
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