Neurons & Action Potentials - TPR

[MDJerome]

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Do Neurons have Sodium Leak Channels? TPR says they do, although not to the same degree as Potassium Leak Channels (which is confirmed by the -70 RMP of the cell). However, other sources say that only Potassium Leak Channels exist. Which is true? I can see this being a choice in a multiple choice question on the MCAT and them asking us to pick which answer is false.

Second thing I'm uncertain about is this: TPR states that "axons are capable of propagating action potentials in either direction, even though this is not what they normally do." 99.9% of the time I've read that action potentials are unidirectional, so what the heck??????

How do I approach these MC questions when wierd exceptions like these exist.

 

[lorenzomicron]

the normal direction is known as orthodromic, away from the soma, towards some point.

but, antidromic is in the contrary direction (towards cell body).

orthodromic is considered to be normal and functioning. the antidromic propagation is a potential of the neuron, however it does not do this on its own. antidromic potential is usually stimulated by experimental methods.

 

[docelh]

There are no sodium leak channels. Leak channels are entirely based on passive movement, which is impossible for Na because of the relative concentrations of Na in the interstitital vs. inside the cell.

 

[ilovemcat]

There are no sodium leak channels. Leak channels are entirely based on passive movement, which is impossible for Na because of the relative concentrations of Na in the interstitital vs. inside the cell.

I think you have your perspective wrong. Passive movement just means that an ion is moving from a high to low concentration gradient. A cell with tons of Sodium Leak Channels would cause Sodium to diffuse down its concentration gradient (in this case that would mean INTO the cell since Sodium has a high extracellular to intracellular concentration). In reality, Sodium Leak Channels do exist but not to the same extent as Potassium Leak Channels. The ratio is something like 100:1.

There are exceptions however. Take for example the Sinoatrial (SA) Node of the Heart. The cells of the SA node have an unstable RMP due to the abundance of Sodium Leak Channels. These cells go through continuous cycles of depolarization and re-polarization and dictate the heart rate of the heart.

 

[OldMCATPro]

Neurons DO have Na+ leak channels. Always. This is not an exception or an anomaly. Yes there are fewer than K+ leak channels. But you should know they are there and understand their function.

Antidromic propagation in neurons is very rare that's true- and most often performed in the laboratory. So TPR's explanation is correct. But my feeling is that it is highly unlikely that you will be asked about it on the real MCAT. In 10+ years of teaching MCAT biology courses, I have not seen a question involving antidromic propagation in neurons appear on a real test. So bottom line- know that it is exists and is possible but rare- but don't worry too much about it.

 

[docelh]

In my mind, I categorize those as voltage-gated channels as opposed to leak channels, but your points are valid.