Voltage-Gated Sodium Channel
- It has three states:
- Closed - at resting membrane potential
- Open - when the membrane depolarizes
- Inactive - quickly stops flow of Na+ after depolarization
- There are many more voltage-gated Na+ channels in the axon than leaky K+ channels
- The equilibrium potential for Na+ (ENa+) is +70 mV
- This means that the more Na+ that can enter the cell, the more positive the membrane potential will become
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- These Na+ channels are targets for many naturally-occurring neurotoxins, local anesthetics and anti-epileptic medications
Voltage-Gated Potassium Channel
- Only open or closed
- Slow - doesn't open right away which allows Na+ to have a head-start. Without this slowness, there would be no AP at all!
- Helps with repolarization - K+ moves down its electrochemical gradient.
The Action Potential
- The way in which neurons propagate information: brief electrical impulses called action potentials
- All-or-nothing: if threshold is reached, there will be a full action potential
- Threshold potential is around -50 mV: corresponds to number of Na+ channels that opened
- Unidirectional (flows from the initial segment of the axon to the presynaptic terminals)
Steps of the Action Potential
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This can also be observed in this format:
- Voltage-gated sodium channels open at threshold
- At the peak, voltage-gated sodium channels become inactivated , and voltage-gated potassium channels open. The cell now repolarizes.
- While sodium channels are inactivated, the neuron is in the absolute refractory period (can’t fire another AP)
- The cell becomes hyperpolarized because voltage-gated potassium channels stay open
- At this point, voltage-gated sodium channels are ready to open again (absolute refractory ends)
- While hyperpolarized, the neuron is in the relative refractory period (harder to fire another AP)
- Cell is returned to RMP by resting machinery (i.e. Na + /K + ATPase pump)
Action Potential Propagation
Maintained by sodium diffusion in the neuron.
- The diffusion of Na+ in one area causes the voltage gated Na+ channels further down the axon to open.
- This continues all the way down the axon like a game of telephone until the signal reaches the axon terminal.
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- Action potential propagation is unidirectional due to the absolute refractory period (Na+ channels are inactivated)
Factors Affecting Speed of Propagation
- Diameter of the axon - larger axons propagate faster.
- Myelination - more insulation means less ion leakage, increases conduction velocity
- Saltatory conduction - the term used when AP can skip places on the axon that are myelinated.
- Myelin is formed by Schwann cells in the PNS and oligodendrocytes in the CNS
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You are bored in class and start drawing on your hand with a pen. You notice that the more pressure you apply to your arm, the more intense the sensation of the pen against your skin becomes. How do neurons communicate to your brain the differences in intensity of sensation, in terms of action potential patterns?
- Action potentials are all-or-none events. As long as the threshold potential is reached, the AP will fire.
- That means that when you press the pen against your skin, this does not increase the magnitude (size) of the action potential. What increases is the frequency of firing of action potentials.
- In this situation: more pressure ==> more frequent firing.