Brain complexity: axons are designed for precise speed (Introduction)

by David Turell , Thursday, July 12, 2018, 00:48 (2327 days ago) @ David Turell

An axon must send a signal with just the right speed so as not to overwhelm a neuron and allow for the neuron's refractory period:

https://medicalxpress.com/news/2018-07-neuron-axons-spindly-theyre-optimizing.html

"So what is the role of the geometry of axons in information flow in the brain?

"The answer—that axons are designed and optimized to balance the speed that information flows into the neuron relative to the time it takes the neuron to process that information—seems intuitive, but has never been quantified until now.

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"The specific balance that biological neurons are designed to accommodate is called the refraction ratio: it's the ratio between the refractory period of a neuron—when the neuron is unable to process incoming signals since its ion channels are resetting after being flooded with sodium— and the signal latency of information traveling down the axon. When that ratio approaches one, there is perfect balance, and the neuron is operating as efficiently as possible.

"In the study conducted by first author Francesca Puppo, a postdoctoral researcher in Bioengineering Professor Gabriel Silva's lab at the Jacobs School of Engineering at UC San Diego, the median refraction ratio value of the nearly 12,000 axonal branches examined was 0.92, quite close to the theoretically predicted perfect balance.

"The study used a dataset from the NeuroMorpho database that looked at a type of neuron called basket cells. This data was from rats, but humans have basket cells too. Puppo used the 3-D morphological data to reconstruct a graph-based model of the neurons' axons and axon branches. Then she calculated the conduction velocity along the axons given the diameter at different points along the axonal arborizations, and estimated the refractory period along the axon from soma to synaptic terminals based on data in the literature. The conduction velocity and length of each axon branch were used to calculate the propagation delay, which she compared to the refraction period to calculate the refraction ratio.

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"Long, short, straight and curvy axons all had a refraction ratio approaching one. This means that when axons grow in a long and curved shape, it's designed that way by the neuron to slow down the action potential of signals in order to optimize the refraction ratio. When neurons aren't signaling at this ratio, there is a breakdown in information flow efficiency between cells.

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"Puppo added that understanding this baseline of optimal function in neurons will allow researchers to better understand how information flow is perturbed in a variety of ways, including other neurological disorders such as schizophrenia, for example, as well as better understanding how drugs affect neuron function, since pharmacological drugs impact the cell's activity and the way synaptic transmission occurs in networks of neurons.

"'It would be interesting to investigate how drug perturbation affects signaling efficiency through computation of the refraction ratio for pairs of neurons in in-vitro networks of neurons before and after exposure to different chemical compounds," Puppo said. "The detection of a change in the refraction ratio could be helpful in the determination of their neural rescue properties.'"

Comment: Designed to perfection. We think with this complex system, helped by some sort of interpretive mechanism.