Action Potential Tip from the Hundred Acre Wood

Last month we explained some of the outlets available with HAPS for publication. This week we are bringing you a glimpse of a Teaching Tip. The analogy provided below is a portion of a Teaching Tip recently submitted by HAPS member Micah Meltzer and his student Megan Spears. To see the full tip, visit the HAPS website

The Curriculum and Instruction Committee welcomes tip submission in all content areas; however, they are currently especially interested in tips for the following areas, which could use more tips to support our HAPS outcome guidelines.

  • Muscular system: skeletal muscle metabolism, characteristics of muscle tissue types, principles and types of whole muscle contraction (twitch, motor unit or contraction types)
  • Nervous system: neurotransmitters and their role at the synapse, sensory and motor pathways in CNS, ANS functions, body system survey
  • General A&P introduction: body cavities/regions, directional terms in A&P

Undergraduate physiology students seem to relate well to A.A. Milne’s characters Tigger & Eeyore from the Winnie-the-Pooh stories. The different behaviors of the voltage-gated Na+ & K+ channels can be likened to the personalities of Tigger & Eeyore, respectively. Tigger has a bouncy, excitable personality which is similar to the behavior of the voltage-gated Na+ channels (VGNC) responsible for rapid depolarization. In contrast, Eeyore is a mopey, sluggish character who behaves more like the voltage-gated K+ channels (VGKC) responsible for repolarization & hyperpolarization. These character associations can help students remember the differences between the two different voltage-gated ion channels involved in the generation of the neuronal action potential, which is a fundamental concept of neurophysiology.

Tigger Channels

Tigger is known for being friendly, energetic, and more than a little rambunctious. Tigger can be seen in the Hundred Acre Wood bouncing around and engaging excitedly with the world. Tigger’s exuberant and enthusiastic qualities are analogous to the rapid-open/rapid-close properties of the VGNC (Voltage-Gated Na+ Channel).

Neuronal VGNCs each contain a voltage-dependent activation gate & a time-dependent inactivation gate. The activation gate is triggered to open once a certain membrane potential, the threshold voltage, is present across the local membrane. The activation gates open rapidly allowing a significant influx of Na+ ions, causing depolarization and the rapid upstroke of an action potential, much like Tigger is known to suddenly burst into short-lived activity.  After a brief period of time (1-2 ms following activation), the inactivation gate rapidly “plugs” up the ion pore from the inside of the cell. This event abruptly stops Na+ ion influx, ending depolarization and defining the peak of the upstroke. The inactivation gate can easily be remembered by likening it to Tigger’s tail getting in the way.

tigger-e1543429392815.jpg

Eeyore Channels

And then there is the gloomy Eeyore. Oh bother. In this mnemonic, his tail can be thought of as the sole activation gate swinging open and closed in response to changes in voltage. Eeyore is often seen moping around or moseying behind his friends around the Hundred Acre Wood. Eeyore’s slow and deliberate manner is analogous to the slow-to-open/slow-to-close nature of the VGKC (Voltage-Gated K+ Channel).

The VGKCs contain a voltage-dependent activation gate but, unlike VGNCs, do not contain an inactivation gate. The kinetics of the VGKC activation gate are slower, responding less quickly to changes in membrane potential when compared to the VGNC’s activation gate. The repolarization phase begins at the same time as the peak of the depolarization upstroke.  It takes that long to get most of the VGKCs opened allowing for significant K+ efflux. Once the membrane potential returns toward threshold voltage, the VGKCs begin to close, also slowly. If K+ continues to exit the cell after threshold voltage has been reached there will be a hyperpolarization phase.

eeyore-e1543429406892.jpg


 

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Micah Meltzer M.D. is an Assistant Professor of Biology at Contra Costa Community College (CA). He teaches Human Anatomy & Physiology, through a clinical lens, to students who are interested in (mainly) pursuing careers in the healthcare field.

 

Megan Headshot

 

Megan Spears is an Anatomy Teaching Assistant and student at Contra Costa College. She is on track to apply to medical school next Spring.

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