The Action Potential

Transcript

0:00 – 0:30  [Basic Definition]  The action potential is a rapid increase and decrease in voltage which is caused by changes in ionic permeability.  In a typical action potential, the rapid increase in voltage occurs as voltage-gated Na+ channels open, then the rapid decrease in voltage occurs as voltage-gated Na+ channels inactivate and voltage-gated K+ channels open.  Importantly, an action potential propagates along the axon as depolarization in one segment of membrane triggers an action potential in adjacent locations.

0:30-2:30  

The action potential waveform is a rapid change in transmembrane voltage and is characterized by 4 distinct phases. The rising phase and overshoot approach the equilibrium potential for sodium, while the falling phase and undershoot approach the equilibrium potential for potassium.

In contrast to the resting membrane potential, the action potential is generated by voltage-gated channels. Voltage-gated channels have charged subunits that allow them to open up when the membrane potential becomes more positive. In reality, depolarizations from rest are due to synaptic currents, however we can also use electrodes to artificially depolarize neurons.

Voltage-gated channels selectively permeable to sodium are the first to respond to depolarizations from rest. Sodium current flows in and moves the membrane potential toward sodium’s equilibrium. Voltage-gated sodium channels have a blocking particle tethered near the pore, so once opened they rapidly inactivate, and are only reset by a repolarization to rest. This is important, since more depolarization opens more channels, providing more paths for current to flow or conductance, which opens more channels, etc. This positive feedback loop would keep neurons close to sodium’s equilibrium if they did not block.

After a short delay, voltage-gated channels selectively permeable to potassium respond to membrane depolarization. The driving force for potassium is high this close to sodium’s equilibrium, and so potassium current flows out, moving the membrane potential in a negative direction toward potassium’s equilibrium. This leads to a decrease in potassium conductance as voltage-gated potassium channels begin to close, functioning like a negative feedback loop. Thus, no blocking particle required. The repolarization also allows for voltage-gated sodium channels to become unblocked. For this reason, these potassium channels are known as ‘delayed rectifiers’.

Because voltage-gated sodium channels are responsible for the rising phase and overshoot, and delayed-rectifier potassium channels are responsible for the falling phase and undershoot, spike threshold is defined as the membrane potential at which sodium conductance exceeds potassium’s, setting off the rapid positive feedback loop. Similarly, a refractory period is created following a spike when insufficient numbers of voltage-gated sodium channels have repolarized and reset to cross threshold.

2:30-3:00  [Parallel Vocabulary] In introductory classes you learned about the action potential.  But Neuroscience is an interdisciplinary field, so there are many words for this term – For example, it might also be called a nerve impulse, neural impulse, or electrical impulse and this term may be shortened to just impulse.  You may also hear the term spike, and a series of action potentials referred to as a spike train.  So….“action potential”, “spike”, and “nerve impulse” may all refer to the same thing.

3:00-4:00  [Here’s a real world example]  The action potential is important in almost every aspect of neural communication including sensation, motor control, and cognition.  For example, did you know that many anesthetics reduce pain sensations by blocking the propagation of action potentials.  The pain-reducing solution that a dentist injects near the teeth contains chemicals that block the voltage-gated sodium channels and this technique, which is called local anesthesia, stops sensory receptors near your teeth from sending signals to the brain.  Recall that voltage-gated sodium channels are responsible for the rising phase of an action potential. When the voltage-gated sodium channels are blocked, somatosensory neurons near the teeth are still be depolarized by mechanical stimulation; however, no matter how large this depolarization is, action potentials cannot propagate when the voltage-gated sodium channels are blocked – this means sensory signals never reach the brain and therefore you feel no pain.

4:00-6:00  [Follow along with this example]

6:00-6:30  [Here are a few readings to help you review] 

1) Neuroscience Exploring the Brain (Bear)

• Chapter 4: “The Action Potential”

2) Principles of Human Physiology (Stanfield)

• Chapter 7: “Nerve Cells and Electrical Signaling”

Media attributions

Action potential schematic by Peter Duncan is licensed under a Creative Commons Attribution Sharealike 4.0 license (CC-by-SA 4.0).

Inactivation diagram by Clara fcn is licensed under a Creative Commons Attribution Sharealike 3.0 license (CC-by-SA 3.0).