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Motor Unit Recruitment Pattern in a Respiratory Muscle of Unanesthetized Chickens 1
266
H. MENGE
Edwards, H. M., Jr., and J. E. Marion, 1963. Influence of dietary menhaden oil on growth rate and tissue fatty acids of the chick. J. Nutrition, 8 1 : 123-130. Havey, W. R., 1960. Least squares analysis of data with unequal numbers. U.S.D.A., A.R.S. 20-28. Holman, R. T., 1960. The ratio of trienoic-tetraenoic acids in tissue lipids as a measure of essential fatty acid requirement. J. Nutrition, 70: 405-410.
Menge, H., C. C. Calvert and C. A. Denton, 1965. Influence of dietary oils on reproduction in the hen. J. Nutrition, 87 : 365-370. Miller, E. C , H. Menge and C. A. Denton, 1963. Effect on long-term feeding of a fat-free diet to laying hens. J. Nutrition, 80: 431-440. Mohrhauer, H., and R. T. Holman, 1963. The effect of dose level of essential fatty acids upon fatty acid composition of the rat liver. J. Lipid Research, 4 : 151-159.
R. R. TSCHORN AND M. R. FEDDE Neuromuscular Laboratory, Department of Physiological Sciences, Kansas State University, Manhattan, Kansas 66502 (Received for publication August 10, 1970)
T
HE amplitude of electrical activity of respiratory muscles of the chicken, as recorded with a needle electrode, seems to be related to the level of anesthesia. Fedde et al. (1964) have shown that motor unit potentials of large amplitude are absent when chickens are deeply anesthetized. Since many investigations of the pattern of motor unit recruitment have used anesthetized animals (Sears, 1964; Fedde et al., 1969), we attempted to determine the motor unit recruitment pattern in unanesthetized chickens. METHODS A biopolar, electromyographic electrode (Fig. 1) was implanted in the transversus abdominis muscle (TA) in six, lightly anesthetized (20 mg./kg. of body weight of sodium pentobarbital administered intravenously) Single Comb White Leghorn males, 1 Contribution No. 68, Department of Physiological Sciences, College of Veterinary Medicine, KSAES, K.S.U., Manhattan, Kansas 66502. Supported by Public Health Service Research Grant NB-05786 from the National Institute of Neurological Disease and Stroke.
12 to 16 weeks old. The skin was incised along the ventro-lateral surface of the abdominal wall; then the fibers of the external abdominal oblique and rectus abdominis muscles were separated by blunt dissection to expose the lateral surface of the TA. The electrode was placed between the rectus abdominis and the TA muscles so that the wires were imbedded in the belly of the TA. The overlying muscles and skin then were sutured separately. The electrode cable was secured to the skin of the abdominal wall and back so that the chicken could not dislodge the electrode from its internal position. To be certain that the electrode was placed properly, electrical activity was monitored before the bird had recovered from the anesthetic by connecting it to a differential amplifier (Model PS, Grass Instrument Co., Quincy, Mass.), the output of which was inserted in one channel of a multichannel oscilloscope (Model 565, Tektronix, Inc., Beaverton, Oregon). A strain gauge transducer was constructed to record respiratory movements in unrestrained chickens by filling twelve centimeters of flexible rubber tubing, 5
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on June 1, 2015
Motor Unit Recruitment Pattern in a Respiratory Muscle of Unanesthetized Chickens1
MOTOR UNIT RECRUITMENT
mm. in diameter, with a conducting gel (EKG sol, Beck-Lee Corp., Chicago, 111.) and plugging the ends with copper screws. Insulated wires, which connected the strain gauge to the remainder of the bridge circuit, were tied to the end of the strain gauge, which was then placed around the sternum, 3 cm. from the caudal end, stretched slightly, and tied over the back with the strings. The ends of the strain gauge were secured to the skin with skin clamps to prevent cranial-caudal displacement during respiratory movements. The tubing thus functioned as a variable resistor, the resistance increasing as the tubing stretched during inspiration and decreasing as it contracted during expiration. It constituted one arm of the bridge circuit,
shown in Fig. 2. The output of the bridge was connected to a second channel of the multichannel oscilloscope. Approximately 48 hours after implanting the electrode, we enclosed the chicken, in a standing position, in a plexiglass chamber 26 cm. high, 36 cm. long, and 18 cm. wide. A flow meter system (Burger and Lorenz, 1960) was used to regulate the carbon dioxide composition in the chamber gas at several levels (0, 2, 4, 6%) to determine the type of motor unit recruitment pattern in the TA during varying amplitudes of muscle contraction. The gas flow rate through the chamber was 6800 ml./min. The time required to attain a constant C0 2 composition after changing from one level to another was determined by taking gas samples from the chamber at three minute intervals following the change. The samples were then analyzed for PCOo (Model 123, Instrumentation Laboratory, Inc., Boston, Mass.). A steady state was reached within IS minutes. Therefore, at least 15 minutes was allowed before recording electromyographic and respiratory activity after each C0 2 change was made. Recordings from the oscilloscope were made with a recording camera (Model C4, Grass Instrument Co., Quincy, Mass.) in darkness, to minimize exciting the chickens from external stimuli.
FIG. 2. Schematic representation of apparatus used to measure respiratory motions in the unanesthetized chicken. The strain gauge constitutes one arm of a four-arm Wheatstone bridge ac'ivated by a small battery. The output of the bridge is connected to an oscilloscope. The bridge could be activated by closing the switch and the variable resistor was used to balance the bridge.
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on June 1, 2015
Fio. 1. Implantable electrode used to detect electromyographic activity in the transversus abdominis muscle of the unrestrained chicken. Two teflon-coated, stainless steel wires, 250u.. in diameter and 1 cm. long, were welded to a 32gauge, 2-conductor shielded cable, 15 cm. long, which had female pin connectors on the other end. The electrode wires and a short length of cable were imbedded in a flat sheet of "plasic" rubber (Woodhill Chemical Corporation, Cleveland, Ohio) 2 cm. in diameter and 2 mm. thick. Three millimeters of uninsulated wire was exposed. The tips of the wires were separated by 4 mm. The flat, circular shape assured continuous contact between the electrode tips and the muscle fibers and prevented a change in position during muscular activity.
267
268
R. R. TSCHORN AND M. R. FEDDE RESULTS AND DISCUSSION
FIG. 3. The effect of increasing CO2 levels in inspired gas on respiratory amplitude and motor unit recruitment patterns in the transversus abdominis muscle of the unanesthetized cock. The upper tracing in each recording is EMG activity in the transversus abdominis muscle; lower tracing is sternal movement, inspiration up and expiration down. A, 0% C0 2 in the inspired gas; B, 2% C0 2 ; C, 4% C0 2 ; D, 6% CO2. Horizontal bar represent one-half second; vertical bar represents 20u,.V. Spikes retouched.
c/
rri
0 2 4 6 1
"t 1 ' BPM.
Inspiration 1 % of cycle
Expiration 1 % of cycle
15.3+0.1 21.1+0.1 26.4 + 0.1 38.5+0.1
27.4±0.1 37.2+0.1 39.5+0.2 40.7 + 0.2
72.6 + 0.1 62.8 + 0.1 60.5 + 0.2 59.3+0.2
Mean + standard error of mean.
The pattern of motor unit recruitment during muscular contraction appeared to be identical with that present in anesthetized or decerebrate animals (Sears, 1964; Henneman et al., 196Sa,b; Olson et al., 1968; Burke, 1968; Fedde et al., 1969). However, the degree of sensitivity to the C0 2 stimulus appeared to be considerably increased in the unanesthetized bird. Maximal motor unit recruitment in the anesthetized bird (which had been unidirectionally, artificially ventilated) had not occurred until the pulmonary C0 2 level was at least 13% (Fedde et al, 1969). Although minute volume of the birds in this study was not measured, it is doubtful that the intrapulmonary C0 2 was that high when the chamber level was 6%, since the control system of the organism attempts to maintain a normal intrapulmonary C0 2 pressure by increasing ventilation as the inspired C0 2 pressure increases (Tenney and Lamb, 1965). The repiratory frequencies of unanesthetized chickens at four levels of C0 2 in the chamber are given in Table 1. Respiratory rate increased markedly as the C0 2 level was elevated; in contrast when the pulmonary C0 2 level was elevated in anesthetized birds (Richards and Sykes, 1967; Ray and Fedde, 1969) there was essentially no increase or, in some cases, a reduced respiratory rate. In this study respiratory amplitude increased markedly as respira-
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on June 1, 2015
When the chamber was flushed with gas containing no C0 2 , the birds had only a small amplitude of sternal movement, accompanied predominantly by motor unit potentials of small amplitude in the TA (Fig. 3A). As the respiratory amplitude increased, induced by increasing the respired C0 2 level, additional motor units producing larger potentials were selectively recruited. Maximal contraction was produced when the COo level in the chamber gas was 6%; higher levels induced symptoms of narcosis, the bird becoming prostrate.
TABLE 1.—Changes in respiratory movements in unanesthetized chickens, induced by increasing the level of inspired COi
MOTOR UNIT RECRUITMENT
SUMMARY
Electromyographic activity was recorded from an expiratory muscle (transversa abdominis) in the unrestrained, unanesthetized chicken during increased degrees of contraction induced by elevation of inspired carbon dioxide. The pattern of motor unit recruitment during muscle contraction was similar to that recorded from anesthetized or decerebrate animals; motor unit potentials of small amplitude appeared when contracting tensions were small, followed by sequentially larger potentials as tension increased. Pentobarbital anesthesia appears to decrease the sensitivity of the chicken to C0 2 but not to alter the sequence in which motor units are recruited during muscular contraction.
ACKNOWLEDGEMENTS
The authors are grateful to Dr. R. E. Burger for suggesting the design of the respiratory strain gauge transducer and for comments on the manuscript. REFERENCES Burger, R. E., and F. W. Lorenz, 1960. Artificial respiration in birds by unidirectional air flow. Poultry Sci. 39: 236-237. Burke, R. E., 1968. Firing patterns of gastrocnemius motor units in the decerebrate cat. J. Physiol., London, 196: 631-654. Fedde, M. R., R. E. Burger and R. L. Kitchell, 1964. Electromyographic studies of the effects of bodily position and anesthesia on the activity of the respiratory muscles of the domestic cock. Poultry Sci. 43 : 839-846. Fedde, M. R., P. D. deWet and R. L. Kitchell, 1969. Motor unit recruitment pattern and tonic activity in respiratory muscles of Gallns domesticus. J. Neurophysiol. 32 : 995-1004. Henneman, E., G. Somjen and D. O. Carpenter, 1965a. Functional significance of cell size in spinal motoneurons. J. Neurophysiol. 28: 560580. Henneman, E., G. Somjen and D. O. Carpenter, 1965b. Excitability and inhibitibility of motoneurons of different sizes. J. Neurophysiol. 28: 599-620. Henneman, E., and C. B. Olson, 1965. Relations between structure and function in the design of skeletal muscles. J. Neurophysiol. 28: 581-598. Olson, C. B., D. O. Carpenter and E. Henneman, 1968. Orderly recruitment of muscle action potentials. Arch. Neurol. 19 : 591-597. Ray, P. J., and M. R. Fedde, 1969. Responses to alterations in respiratory Po 2 and Poo2 in the chicken. Respir. Physiol. 6: 135-143. Richards, S. A., and A. H. Sykes, 1967. The effects of hypoxia, hypercapnia and asphyxia in the domestic fowl {Gallus domesticus). Comp. Biochem. Physiol. 2 1 : 691-701. Sears, T. A., 1964. Efferent discharges in alpha fusimotor fibres of intercostal nerves of the cat. J. Physiol., London, 174: 295-315. Tenney, S. M., and T. W. Lamb, 1965. Physiological consequences of hypoventilation and hyperventilation. In Handbook of Physiology, section 3 : Respiration, vol. II, ed. W. O. Fenn and H. Rahn. pp. 979-1010. American Physiological Society, Washington.
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on June 1, 2015
tory C0 2 increased (Fig. 3), in a similar manner to that found in the anesthetized bird (Richards and Sykes, 1967; Ray and Fedde, 1969). Hence, increases in both respiratory rate and amplitude appear to cause the increased minute volume of the unanesthetized bird. At atmospheric C0 2 levels, inspiration occupied a smaller percentage of the respiratory cycle than did expiration (Table 1). However, as the level of C0 2 in the chamber gas was increased, the proportion of the respiratory cycle occupied by expiration progressively decreased while that of inspiration increased. Continuous decreases in the relative percentages of the expiratory phase of respiration suggests that the TA increased its energy output to overcome the increased resistances to breathing resulting from more rapid movement of the body wall and movement of larger volumes of gas in and out of the respiratory system in a shorter time interval. This greater work output could be reflected in the selective recruitment of larger motor units as has been suggested in mammals (Henneman and Olson, 1965).
269
H. MENGE
Edwards, H. M., Jr., and J. E. Marion, 1963. Influence of dietary menhaden oil on growth rate and tissue fatty acids of the chick. J. Nutrition, 8 1 : 123-130. Havey, W. R., 1960. Least squares analysis of data with unequal numbers. U.S.D.A., A.R.S. 20-28. Holman, R. T., 1960. The ratio of trienoic-tetraenoic acids in tissue lipids as a measure of essential fatty acid requirement. J. Nutrition, 70: 405-410.
Menge, H., C. C. Calvert and C. A. Denton, 1965. Influence of dietary oils on reproduction in the hen. J. Nutrition, 87 : 365-370. Miller, E. C , H. Menge and C. A. Denton, 1963. Effect on long-term feeding of a fat-free diet to laying hens. J. Nutrition, 80: 431-440. Mohrhauer, H., and R. T. Holman, 1963. The effect of dose level of essential fatty acids upon fatty acid composition of the rat liver. J. Lipid Research, 4 : 151-159.
R. R. TSCHORN AND M. R. FEDDE Neuromuscular Laboratory, Department of Physiological Sciences, Kansas State University, Manhattan, Kansas 66502 (Received for publication August 10, 1970)
T
HE amplitude of electrical activity of respiratory muscles of the chicken, as recorded with a needle electrode, seems to be related to the level of anesthesia. Fedde et al. (1964) have shown that motor unit potentials of large amplitude are absent when chickens are deeply anesthetized. Since many investigations of the pattern of motor unit recruitment have used anesthetized animals (Sears, 1964; Fedde et al., 1969), we attempted to determine the motor unit recruitment pattern in unanesthetized chickens. METHODS A biopolar, electromyographic electrode (Fig. 1) was implanted in the transversus abdominis muscle (TA) in six, lightly anesthetized (20 mg./kg. of body weight of sodium pentobarbital administered intravenously) Single Comb White Leghorn males, 1 Contribution No. 68, Department of Physiological Sciences, College of Veterinary Medicine, KSAES, K.S.U., Manhattan, Kansas 66502. Supported by Public Health Service Research Grant NB-05786 from the National Institute of Neurological Disease and Stroke.
12 to 16 weeks old. The skin was incised along the ventro-lateral surface of the abdominal wall; then the fibers of the external abdominal oblique and rectus abdominis muscles were separated by blunt dissection to expose the lateral surface of the TA. The electrode was placed between the rectus abdominis and the TA muscles so that the wires were imbedded in the belly of the TA. The overlying muscles and skin then were sutured separately. The electrode cable was secured to the skin of the abdominal wall and back so that the chicken could not dislodge the electrode from its internal position. To be certain that the electrode was placed properly, electrical activity was monitored before the bird had recovered from the anesthetic by connecting it to a differential amplifier (Model PS, Grass Instrument Co., Quincy, Mass.), the output of which was inserted in one channel of a multichannel oscilloscope (Model 565, Tektronix, Inc., Beaverton, Oregon). A strain gauge transducer was constructed to record respiratory movements in unrestrained chickens by filling twelve centimeters of flexible rubber tubing, 5
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on June 1, 2015
Motor Unit Recruitment Pattern in a Respiratory Muscle of Unanesthetized Chickens1
MOTOR UNIT RECRUITMENT
mm. in diameter, with a conducting gel (EKG sol, Beck-Lee Corp., Chicago, 111.) and plugging the ends with copper screws. Insulated wires, which connected the strain gauge to the remainder of the bridge circuit, were tied to the end of the strain gauge, which was then placed around the sternum, 3 cm. from the caudal end, stretched slightly, and tied over the back with the strings. The ends of the strain gauge were secured to the skin with skin clamps to prevent cranial-caudal displacement during respiratory movements. The tubing thus functioned as a variable resistor, the resistance increasing as the tubing stretched during inspiration and decreasing as it contracted during expiration. It constituted one arm of the bridge circuit,
shown in Fig. 2. The output of the bridge was connected to a second channel of the multichannel oscilloscope. Approximately 48 hours after implanting the electrode, we enclosed the chicken, in a standing position, in a plexiglass chamber 26 cm. high, 36 cm. long, and 18 cm. wide. A flow meter system (Burger and Lorenz, 1960) was used to regulate the carbon dioxide composition in the chamber gas at several levels (0, 2, 4, 6%) to determine the type of motor unit recruitment pattern in the TA during varying amplitudes of muscle contraction. The gas flow rate through the chamber was 6800 ml./min. The time required to attain a constant C0 2 composition after changing from one level to another was determined by taking gas samples from the chamber at three minute intervals following the change. The samples were then analyzed for PCOo (Model 123, Instrumentation Laboratory, Inc., Boston, Mass.). A steady state was reached within IS minutes. Therefore, at least 15 minutes was allowed before recording electromyographic and respiratory activity after each C0 2 change was made. Recordings from the oscilloscope were made with a recording camera (Model C4, Grass Instrument Co., Quincy, Mass.) in darkness, to minimize exciting the chickens from external stimuli.
FIG. 2. Schematic representation of apparatus used to measure respiratory motions in the unanesthetized chicken. The strain gauge constitutes one arm of a four-arm Wheatstone bridge ac'ivated by a small battery. The output of the bridge is connected to an oscilloscope. The bridge could be activated by closing the switch and the variable resistor was used to balance the bridge.
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on June 1, 2015
Fio. 1. Implantable electrode used to detect electromyographic activity in the transversus abdominis muscle of the unrestrained chicken. Two teflon-coated, stainless steel wires, 250u.. in diameter and 1 cm. long, were welded to a 32gauge, 2-conductor shielded cable, 15 cm. long, which had female pin connectors on the other end. The electrode wires and a short length of cable were imbedded in a flat sheet of "plasic" rubber (Woodhill Chemical Corporation, Cleveland, Ohio) 2 cm. in diameter and 2 mm. thick. Three millimeters of uninsulated wire was exposed. The tips of the wires were separated by 4 mm. The flat, circular shape assured continuous contact between the electrode tips and the muscle fibers and prevented a change in position during muscular activity.
267
268
R. R. TSCHORN AND M. R. FEDDE RESULTS AND DISCUSSION
FIG. 3. The effect of increasing CO2 levels in inspired gas on respiratory amplitude and motor unit recruitment patterns in the transversus abdominis muscle of the unanesthetized cock. The upper tracing in each recording is EMG activity in the transversus abdominis muscle; lower tracing is sternal movement, inspiration up and expiration down. A, 0% C0 2 in the inspired gas; B, 2% C0 2 ; C, 4% C0 2 ; D, 6% CO2. Horizontal bar represent one-half second; vertical bar represents 20u,.V. Spikes retouched.
c/
rri
0 2 4 6 1
"t 1 ' BPM.
Inspiration 1 % of cycle
Expiration 1 % of cycle
15.3+0.1 21.1+0.1 26.4 + 0.1 38.5+0.1
27.4±0.1 37.2+0.1 39.5+0.2 40.7 + 0.2
72.6 + 0.1 62.8 + 0.1 60.5 + 0.2 59.3+0.2
Mean + standard error of mean.
The pattern of motor unit recruitment during muscular contraction appeared to be identical with that present in anesthetized or decerebrate animals (Sears, 1964; Henneman et al., 196Sa,b; Olson et al., 1968; Burke, 1968; Fedde et al., 1969). However, the degree of sensitivity to the C0 2 stimulus appeared to be considerably increased in the unanesthetized bird. Maximal motor unit recruitment in the anesthetized bird (which had been unidirectionally, artificially ventilated) had not occurred until the pulmonary C0 2 level was at least 13% (Fedde et al, 1969). Although minute volume of the birds in this study was not measured, it is doubtful that the intrapulmonary C0 2 was that high when the chamber level was 6%, since the control system of the organism attempts to maintain a normal intrapulmonary C0 2 pressure by increasing ventilation as the inspired C0 2 pressure increases (Tenney and Lamb, 1965). The repiratory frequencies of unanesthetized chickens at four levels of C0 2 in the chamber are given in Table 1. Respiratory rate increased markedly as the C0 2 level was elevated; in contrast when the pulmonary C0 2 level was elevated in anesthetized birds (Richards and Sykes, 1967; Ray and Fedde, 1969) there was essentially no increase or, in some cases, a reduced respiratory rate. In this study respiratory amplitude increased markedly as respira-
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on June 1, 2015
When the chamber was flushed with gas containing no C0 2 , the birds had only a small amplitude of sternal movement, accompanied predominantly by motor unit potentials of small amplitude in the TA (Fig. 3A). As the respiratory amplitude increased, induced by increasing the respired C0 2 level, additional motor units producing larger potentials were selectively recruited. Maximal contraction was produced when the COo level in the chamber gas was 6%; higher levels induced symptoms of narcosis, the bird becoming prostrate.
TABLE 1.—Changes in respiratory movements in unanesthetized chickens, induced by increasing the level of inspired COi
MOTOR UNIT RECRUITMENT
SUMMARY
Electromyographic activity was recorded from an expiratory muscle (transversa abdominis) in the unrestrained, unanesthetized chicken during increased degrees of contraction induced by elevation of inspired carbon dioxide. The pattern of motor unit recruitment during muscle contraction was similar to that recorded from anesthetized or decerebrate animals; motor unit potentials of small amplitude appeared when contracting tensions were small, followed by sequentially larger potentials as tension increased. Pentobarbital anesthesia appears to decrease the sensitivity of the chicken to C0 2 but not to alter the sequence in which motor units are recruited during muscular contraction.
ACKNOWLEDGEMENTS
The authors are grateful to Dr. R. E. Burger for suggesting the design of the respiratory strain gauge transducer and for comments on the manuscript. REFERENCES Burger, R. E., and F. W. Lorenz, 1960. Artificial respiration in birds by unidirectional air flow. Poultry Sci. 39: 236-237. Burke, R. E., 1968. Firing patterns of gastrocnemius motor units in the decerebrate cat. J. Physiol., London, 196: 631-654. Fedde, M. R., R. E. Burger and R. L. Kitchell, 1964. Electromyographic studies of the effects of bodily position and anesthesia on the activity of the respiratory muscles of the domestic cock. Poultry Sci. 43 : 839-846. Fedde, M. R., P. D. deWet and R. L. Kitchell, 1969. Motor unit recruitment pattern and tonic activity in respiratory muscles of Gallns domesticus. J. Neurophysiol. 32 : 995-1004. Henneman, E., G. Somjen and D. O. Carpenter, 1965a. Functional significance of cell size in spinal motoneurons. J. Neurophysiol. 28: 560580. Henneman, E., G. Somjen and D. O. Carpenter, 1965b. Excitability and inhibitibility of motoneurons of different sizes. J. Neurophysiol. 28: 599-620. Henneman, E., and C. B. Olson, 1965. Relations between structure and function in the design of skeletal muscles. J. Neurophysiol. 28: 581-598. Olson, C. B., D. O. Carpenter and E. Henneman, 1968. Orderly recruitment of muscle action potentials. Arch. Neurol. 19 : 591-597. Ray, P. J., and M. R. Fedde, 1969. Responses to alterations in respiratory Po 2 and Poo2 in the chicken. Respir. Physiol. 6: 135-143. Richards, S. A., and A. H. Sykes, 1967. The effects of hypoxia, hypercapnia and asphyxia in the domestic fowl {Gallus domesticus). Comp. Biochem. Physiol. 2 1 : 691-701. Sears, T. A., 1964. Efferent discharges in alpha fusimotor fibres of intercostal nerves of the cat. J. Physiol., London, 174: 295-315. Tenney, S. M., and T. W. Lamb, 1965. Physiological consequences of hypoventilation and hyperventilation. In Handbook of Physiology, section 3 : Respiration, vol. II, ed. W. O. Fenn and H. Rahn. pp. 979-1010. American Physiological Society, Washington.
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on June 1, 2015
tory C0 2 increased (Fig. 3), in a similar manner to that found in the anesthetized bird (Richards and Sykes, 1967; Ray and Fedde, 1969). Hence, increases in both respiratory rate and amplitude appear to cause the increased minute volume of the unanesthetized bird. At atmospheric C0 2 levels, inspiration occupied a smaller percentage of the respiratory cycle than did expiration (Table 1). However, as the level of C0 2 in the chamber gas was increased, the proportion of the respiratory cycle occupied by expiration progressively decreased while that of inspiration increased. Continuous decreases in the relative percentages of the expiratory phase of respiration suggests that the TA increased its energy output to overcome the increased resistances to breathing resulting from more rapid movement of the body wall and movement of larger volumes of gas in and out of the respiratory system in a shorter time interval. This greater work output could be reflected in the selective recruitment of larger motor units as has been suggested in mammals (Henneman and Olson, 1965).
269