This semester, I’m finally taking a course in neurobiology. I have loved my general biology classes (seriously, the kidneys and the immune system are both beyond cool, and I would happily study either one if I weren’t already so emotionally invested in neuroscience), but I’m glad to finally get into the really interesting, specific stuff.
Really, the only problem with this class is that when I tell people about it, they blink at the word “neurobiology.” It’s a weirdly common reaction–and I try to take it as a reaction to the lack of neuroscientists in the general population rather than as a reflection on me, personally.
Anyway, the first few lectures have mostly been review, but I just thought I’d share something cool about neurons. First of all, neurons can be as small as your average cell, but they can also be over a meter long in humans — think the neuron that leaves your spinal cord and signals the muscles in your big toe to move — that’s all one long neuron, which averages about a meter in length in adults. Even in the smallest neurons, proteins and neurotransmitters must be synthesized in the cell body, and transported throughout the cell to where they are needed.
This video shows how kinesins move down microtubules, which extend all the way down from the cell body (the blobby part with the nucleus in your stereotyped neuron diagram) to the end of the axon (the long projection from the cell body that sends signals on to the next neuron or cell in the signaling pathway).
Kinesins move from the cell body (soma) to the end of the axon (nerve terminal). Dyneins, on the other hand, carry things from the axon terminal back up to the soma — so, for instance, worn-out parts that need to be degraded.
I love its cartoonishness. The two “feet” really do walk down the microtubules like that (of course, even in the largest neurons, this is all two tiny to see in such detail, and the colors are not so… well, colorful). The stalk on top of the feet carries cargoes of neurotransmitters and other proteins that were produced in the body of the cell, down to the nerve terminal, where they are used or released as needed. Since proteins can’t be synthesized in the nerve terminals, things like neurotransmitters and membrane proteins constantly need to be replenished as they are released or worn out.
What’s really important to remember is the microscopic scale on which this all takes place. The kinesin protein is about 4 or 5 centimeters tall on my computer screen; in real life, a kinesin protein measures about 50 nanometers — a nanometer (nm) is one billionth of a meter, or one millionth of a millimeter. That means a kinesin molecule has to travel approximately 20,000,000 times its own length to travel from the base of your spinal cord to your big toe. If (as Wikipedia says) the average American woman from 2003 to 2006 was 5 feet 4 inches tall, this would be the equivalent of walking 20,202.02 miles, which is nearly once around the equator.
Of course, this is based on sloppy cheap-calculator math, and is a pretty sketchy analogy anyway, because kinesin molecules don’t have legs, and “walking” cargo from one end of the cell to another is their sole function.
Still here are these unimaginably tiny yet incredibly complex structures walking down meters of microtubules in axons, just so you can move your big toe — or do anything at all, like, say, be a sentient being. Pretty neat.