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Why do axons differ in caliber?

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posted on 2023-06-08, 11:25 authored by János A Perge, Jeremy NivenJeremy Niven, Enrico Mugnaini, Vijay Balasubramanian, Peter Sterling
CNS axons differ in diameter (d) by nearly 100-fold (~0.1-10 µm); therefore, they differ in cross-sectional area (d(2)) and volume by nearly 10,000-fold. If, as found for optic nerve, mitochondrial volume fraction is constant with axon diameter, energy capacity would rise with axon volume, also as d(2). We asked, given constraints on space and energy, what functional requirements set an axon's diameter? Surveying 16 fiber groups spanning nearly the full range of diameters in five species (guinea pig, rat, monkey, locust, octopus), we found the following: (1) thin axons are most numerous; (2) mean firing frequencies, estimated for nine of the identified axon classes, are low for thin fibers and high for thick ones, ranging from ~1 to >100 Hz; (3) a tract's distribution of fiber diameters, whether narrow or broad, and whether symmetric or skewed, reflects heterogeneity of information rates conveyed by its individual fibers; and (4) mitochondrial volume/axon length rises =d(2). To explain the pressure toward thin diameters, we note an established law of diminishing returns: an axon, to double its information rate, must more than double its firing rate. Since diameter is apparently linear with firing rate, doubling information rate would more than quadruple an axon's volume and energy use. Thicker axons may be needed to encode features that cannot be efficiently decoded if their information is spread over several low-rate channels. Thus, information rate may be the main variable that sets axon caliber, with axons constrained to deliver information at the lowest acceptable rate.

History

Publication status

  • Published

File Version

  • Published version

Journal

Journal of Neuroscience

ISSN

1529-2401

Publisher

Society for Neuroscience

Issue

2

Volume

32

Page range

626-638

Department affiliated with

  • Neuroscience Publications

Full text available

  • Yes

Peer reviewed?

  • Yes

Legacy Posted Date

2012-05-03

First Open Access (FOA) Date

2016-03-22

First Compliant Deposit (FCD) Date

2016-08-17

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