Document Detail

Why do axons differ in caliber?
MedLine Citation:
PMID:  22238098     Owner:  NLM     Status:  MEDLINE    
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.
János A Perge; Jeremy E Niven; Enrico Mugnaini; Vijay Balasubramanian; Peter Sterling
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Publication Detail:
Type:  Comparative Study; Journal Article; Research Support, N.I.H., Extramural; Research Support, U.S. Gov't, Non-P.H.S.    
Journal Detail:
Title:  The Journal of neuroscience : the official journal of the Society for Neuroscience     Volume:  32     ISSN:  1529-2401     ISO Abbreviation:  J. Neurosci.     Publication Date:  2012 Jan 
Date Detail:
Created Date:  2012-01-12     Completed Date:  2012-03-06     Revised Date:  2014-09-20    
Medline Journal Info:
Nlm Unique ID:  8102140     Medline TA:  J Neurosci     Country:  United States    
Other Details:
Languages:  eng     Pagination:  626-38     Citation Subset:  IM    
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MeSH Terms
Anatomy, Comparative / methods*
Cerebellum / cytology,  physiology
Cochlear Nerve / cytology,  physiology
Ganglia, Invertebrate / cytology,  physiology
Guinea Pigs
Macaca mulatta
Nerve Fibers, Myelinated / classification*,  physiology*
Nerve Fibers, Unmyelinated / classification*,  physiology*
Optic Nerve / cytology,  physiology
Rats, Sprague-Dawley
Species Specificity
Grant Support
EY08124/EY/NEI NIH HHS; R01 EY008124-11/EY/NEI NIH HHS; R01 NS 09904/NS/NINDS NIH HHS; R01 NS009904/NS/NINDS NIH HHS; R01 NS009904-39/NS/NINDS NIH HHS

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