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First published online August 4, 2005
Journal of Experimental Biology 208, 3065-3073 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01752
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Spike firing allometry in avian intrapulmonary chemoreceptors: matching neural code to body size

S. C. Hempleman1,*, D. L. Kilgore, Jr2, C. Colby3, R. W. Bavis4 and F. L. Powell5

1 Department of Biological Sciences, Northern Arizona University, Flagstaff, AZ 86011-5640 USA
2 Division of Biological Sciences, The University of Montana, Missoula, MT 59812 USA
3 Department of Respiratory Care, Boise State University, Boise, ID 83725 USA
4 Department of Biology, Bates College, Lewiston, ME 04240 USA
5 Division of Physiology, Department of Medicine, University of California San Diego, La Jolla, CA 92093-0623 USA

* Author for correspondence (e-mail: steven.hempleman{at}nau.edu)

Accepted 14 June 2005

Biological rates in small animals are usually higher than those in large animals, yet the maximal rate of action potential (spike) generation in sensory neurons encoding rate functions is similar in all animals, due to the conserved genetics of voltage-gated ion channels. Therefore, sensory signals that vary at rates approaching maximal spike generation rate, as might occur in animals of diminished body size, may require specialized spike coding to convey this information. To test whether spike coding scales allometrically in sensory neurons monitoring signals that change frequency with body size, we recorded action potentials from 70 avian intrapulmonary chemoreceptors (IPC), respiratory neurons that detect lung CO2 changes during breathing, in five different avian species ranging in size from body mass Mb = 0.045 kg (lovebirds) to 5.23 kg (geese). Since breathing frequency scales approximately to Mb–1/4 (higher in small birds, lower in large birds), we reasoned that IPC discharge frequencies may also scale to maintain spike information transmission within each breath. We found that phasic action potential discharge pattern, as quantified by the peak discharge rate and the magnitude of spike frequency adaptation, scaled between Mb–0.22 and Mb–0.26, like breathing rate (P<0.05). Previously published values of peak discharge rate in IPC also fit this allometric relationship. We suggest that mass-dependent scaling of neural coding may be necessary for preserving information transmission with decreasing body size.

Key words: allometry, body size, bird, intrapulmonary chemoreceptors, neural coding






© The Company of Biologists Ltd 2005