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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. Hempleman^1,*, 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

View larger version (19K): [in a new window]   Fig. 1. The analysis method for determining phasic peak discharge rate and magnitude of spike frequency adaptation of individual intrapulmonary chemoreceptors. (A) The CO2 stimulus waveform delivered to the lungs in the ventilatory gas. (B) Raster plot of spike occurrence times as vertical hash marks (this example was recorded from a lovebird IPC). (C) A schematic representation of spike discharge frequency vs time – a cycle triggered stimulus histogram. Definitions of peak frequency and magnitude of spike frequency adaptation are indicated (both measurements are in units of frequency: s–1).

View larger version (31K): [in a new window]   Fig. 2. IPC discharge rate (s–1; means ± S.E.M.) vs time for each species. Smaller birds had larger phasic chemoreceptor responses to the CO2 stimulus down-step. Mean peak discharge frequencies and the mean magnitude of spike frequency adaptation were calculated from these data as described in Fig. 1, and shown in Figs 3 and 4.

View larger version (20K): [in a new window]   Fig. 3. Allometric plot of peak chemoreceptor discharge rate vs body mass (Table 2). The heavy black regression line was calculated without phylogenetic correction (dotted line indicates 95% confidence interval); the heavy gray regression line was calculated with phylogenetic correction (see text). Regression parameters are summarized in Table 3. Mean values measured in this study are shown in red (lovebird, quail, pigeon, and goose). Mean values from the literature are shown in green (chicken, muscovy duck, emu; see text). The red + green square is the mean value for Anas platyrhynchos (represented by mallard and Pekin ducks) combined from the literature and this study (Table 2). Silhouettes identify specific data points and are not scaled to actual bird size.

View larger version (20K): [in a new window]   Fig. 4. Allometric plot of chemoreceptor spike frequency adaptation vs body mass (Table 1). The heavy black regression line was calculated without phylogenetic correction (dotted line indicates 95% confidence interval); the heavy gray regression line was calculated with phylogenetic correction (see text). Regression parameters are summarized in Table 3. Values shown are all from this study: lovebird, quail, pigeon, Pekin duck and goose (see text). Silhouettes identify specific data points and are not scaled to actual bird size.

View larger version (30K): [in a new window]   Fig. 5. Examples of partially adapting (A) and tonic (B) quail IPC responding to an abrupt inspired CO2 down-step. Measured spike trains are shown in black. The heavy gray line shows chemoreceptor discharge rate from cycle triggered stimulus histogram averaged over 0.25 s intervals. We found examples of tonic and partially adapting chemoreceptors in all species studied, but IPC with large magnitude adaptation and higher peak discharge rates were more common in small birds, and IPC with lower peak discharge rates and smaller magnitude adaptation were more common in large birds (see text).

View larger version (35K): [in a new window]   Fig. 6. The effect of CO2 oscillation frequency on IPC action potential discharge in a 1.9 kg domestic chicken (modified from Stoll et al., 1971). This IPC tracked 20 min–1 CO2 oscillations that are close to its normal breathing rate more faithfully than faster oscillations (see text).

View larger version (25K): [in a new window]   Fig. 7. Breathing frequency in birds scales approximately to Mb–1/4, and breath duration scales approximately to Mb1/4 (Frappell et al., 2001; Lindstedt and Calder, 1981). Mb1/4 scaling of breath duration is indicated schematically by the heavy gray `V' lines on the diagram: larger birds have relatively longer breaths, and smaller birds have relatively shorter breaths. Representative spike recordings of IPC from five species responding to a CO2 downstep at t=0 (heavy black line) are overlayed on the figure, and are spaced vertically according to body mass of birds in which they were measured. The composite figure shows the general scaling relationship between spike discharge and breath duration. Note that as relative breath duration decreases with decreased body mass, peak chemoreceptor discharge rate and the magnitude of spike frequency adaptation increase. Note also that some IPC were silenced by 6% CO2; this was idiosyncratic of individual IPC and not species or mass related. On average, larger birds had lower peak chemoreceptor discharge rates, but their longer breaths give IPC ample time to transmit spike information about lung CO2 changes.