Physiology Topics
Only neurons and muscle cells have voltage-gated ion channels, making them "excitable". Voltage-gated channels open transiently in response to changes in membrane potential. Such a change may be initiated by other types of ion channels. Ligand-gated channels open and close in response to a specific extracellular neurotransmitter. Signal-gated channels open and close in response to intracellular molecules.
When
a stimulus causes ion channels to open or close in an excitable cell's plasma
membrane, the cell produces a graded potential, a small deviation from
the resting membrane potential that makes the cell either more polarized (cytosol
is even more negative) or less polarized (cytosol is less negative). When the
result is a more negative potential, the graded potential is a hyperpolarizing
graded potential. When the result is a less negative potential, it is a
depolarizing graded potential.
In most cells, if a depolarizing graded potential causes the cell to depolarize to a threshold level (different for each kind of cell), voltage-gated Na+ channels will open. This will trigger an action potential, a sequence of rapidly occurring events that decrease and eventually reverse the membrane potential, the restores the potentials to its resting state.
Action potentials arise according to the all-or-none principle: only if depolarization reaches the threshold level will the Na+ voltage-gated channels open and an action potential that is always of the same magnitude will occur. The sum of several graded potentials can cause an action potential by either spatial or temporal summation. For an example of spatial summation, several presynaptic neurons conduct each a graded potential to one postsynaptic neuron. For an example of temporal summation, one presynaptic neuron conducts several graded potentials to one postsynaptic neuron in a short period of time.
Lets examine the neuronal action potential as an example. The figure below shows the system at the resting membrane potential.
If a depolarizing graded potential causes the membrane to depolarize to a threshold level of about -55 mV, the voltage-gated Na+ channels open. The resulting inrush of Na+ causes further depolarization, reaching +30 mV.
Voltage-gated K+ channels are also opened by the threshold depolarization, but they open more slowly than the Na+ channels. By the time the K+ channels open, the Na+ are already closed. This starts the repolarizing stage of the action potential.
While the voltage-gated K+ channels are still open, K+ outflow may be large enough to cause a brief hyperpolarization, drifting towards the K+ equilibrium potential (-90 mV). Eventually, as voltage-gated K+ channels close, the resting potential is restored.
Voltage-gated Na+ channels have two gates: the activation gate and the inactivation gate. According to the position of these gates, the channel may be in one of three states: resting, active or inactive. In the resting state (cell membrane resting potential), the inactivation gate is open but the activation gate is closed. At the threshold, the Na+ channel enters its active state with both the activation and inactivation gates open. Depolarization closes the inactivation gate a few milliseconds after the activation gate opens, leaving the Na+ in its inactive state.
The activity of the Na+ channels determines an excitable cell refractory period, a time during which the cell cannot generate another action potential. Inactive Na+ channels cannot open: they must first return to their resting state. During the absolute refractory period, Na+ channels are inactive and a second action potential cannot be initiated. During the relative refractory period, Na+ channels have returned to their resting state but voltage-gated K+ channels are still open, therefore a second action potential can be initiated only if there is a superthreshold stimulus.
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Advance Topics: Cell Excitability (Intro to Pharm & Tox)
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