Saturday, February 16, 2019
  Axon first again

This new research on neurons fits neatly with something I noticed last year.

This article focuses on the role of the terminal at the end of an axon. Most of the time this terminal is a lot like an output terminal in a circuit, sending electrochemical signals to nearby dendrite inputs that lead to other neurons.

Previously it was assumed that the dendrite inputs did the wandering and searching to find an axon that provided the chemicals they wanted.
The greatest surprises came from auditing the neuron's growth cones -- the outermost tips of the axonal tentacles, which develop into the signaling synapses. This portion contained much of the molecular machinery of an independent cell, including proteins involved in growth, metabolism, signaling and more.

This finding, Macklis says, challenges the dogma that the nucleus and cell body are the control centers of the neuron. Instead, it proposes a more intricate web of decision-making and the existence of semi-independent units far from central command.

"What our results suggest is that growth cones are capable of taking in information from the outside world, making signaling decisions locally, and functioning semi-autonomously without the cell body," he said. "It's a whole new way of thinking about neurons."
So the axon terminal is ALSO deciding which way to grow, which dendrites are suitable receptacles for its signals.



= = = = =

Last year I noticed:

= = = = = START REPRINT:


Linked via RCS, an interesting speculation about brain development.
In the developing brain, the subplate sits below the cortical plate, a precursor to the cortex. During some stages of development, the subplate is the largest layer of the brain—making its ultimate disappearance all the more confounding.

“The understanding about the subplate was that it expands and then the cells of the subplate just die out. But we hypothesized: What if these subplate cells are not dying? What if they’re just moving to a different level of the cortex—becoming part of the cortex?” says Brivanlou.
The researchers tried to duplicate the situation in vitro, and found that the subplate cells didn't die. They migrated outward after they had performed their main function of generating lots of other neurons.

For clarity: the idea of migration is astonishing, and NOT part of conventional wisdom, but it is well-known in serious neurology. This research isn't claiming to discover the migration, only trying to see what happens after the migration.



Here's the migration as currently understood. The subplate forms from the brainstem. It first builds the radial glia (the tracks) extending to the outer rim of the brain (the cortex). Then the subplate generates a few billion fresh neurons, each equipped to ride the tracks. These neurons ride to the frontier where they claim their homesteads.

Previously it was believed that the subplate simply faded out after building the rails and sending the trains. This new finding indicates that the subplate's cells hop on the tracks and join the migration. Do they still have a special leadership role in the new territory?

The article includes a little video of the (in vitro) subplate neurons migrating in open territory without rails. They wander in various directions, but all are traveling axon-first, perhaps showing that each neuron is seeking an appropriate destination for its signals rather than an appropriate source as you might assume. In other words, each neuron knows its PURPOSE, its intended JOB, rather than its desired CONSUMPTION.

= = = = = END REPRINT.


The current article shows that axons are purposeful seekers from a static neuron, like plant roots seeking nutrition. Last year's article showed axons as purposeful seekers on a moving neuron, like free-swimming animals seeking nutrition.

Both modes are amazing, and the combination is doubly amazing.

= = = = =

** Stupid footnote: Early paleontologists guessed that dinosaurs had butt brains. Extremely long spinal axons need many seconds to transfer signals, and dinos had an expanded area in their vertebrae around the hip. The two facts implied an independent sub-brain. Later paleontologists dismissed the idea. Now that we see the axon terminal as an independent decision center, the old idea turns out to be right after all. There wasn't necessarily a visible sub-brain, but the terminals of the axons were serving as a sub-brain.

*** Not-so-stupid footnote: When we cut off a dog's tail for the sake of fashion, we aren't simply eliminating "useless" length from the spinal axons. We are excising the reflector brains at the end of the axons.

LIFE IS PURPOSE. PURPOSE IS LIFE.

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