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Supraspinal involvement in the phrenic-to-phrenic inhibitory reflex
Brain Research, 414 (1987) 169-172 Elsevier
169
BRE 22329
Supraspinal involvement in the phrenic-to-phrenic inhibitory reflex Dexter F. Speck University of Kentucky Medical Center, Department of Physiology, Lexington, KY 40536-0084 (U.S.A.) (Accepted 17 March 1987) Key words: Brainstem; Intercostal-to-phrenic reflex; Phrenic afferent; Respiration; Spinal cord
Phrenic afferents are capable of attenuating the phrenic motor response elicited by the intercostal-to-phrenic excitatory reflex in decerebrate, paralyzed cats. High spinal transection eliminates the attenuating effect of the bilateral phrenic-to-phrenic inhibitory reflex. These results indicate that although phrenic nerve afferents do exert an inhibitory influence in the cervical spinal cord, some of the inhibitory effects are likely to involve supraspinal mechanisms.
Several investigators have reported the existence of a transient inhibitory phrenic-to-phrenic reflex4'7'8. A recent investigation has demonstrated that two different short latency inhibitory reflexes can be initiated by phrenic nerve afferents 8. The most prevalent reflex involves slowly conducting myelinated fibers which elicit a transient bilateral inhibition of the phrenic motor discharge. A second ipsilateral reflex can be initiated by stimulation of the large myelinated afferents in the whole phrenic nerve. Because of the relatively short latencies for onset of both inhibitory reflexes, it is likely that these responses are mediated through a paucisynaptic reflex pathway confined to the cervical spinal cord 7. Using intracellular recordings from phrenic motoneurons, Gill and Kuno 6 concluded that large phrenic afferent fibers produce inhibitory actions involving mechanisms at the cervical segmental level only, since spinal transection did not alter the effect. The existence of segmental phrenic afferent inhibition is supported by indirect evidence from our recent experiments 8'9. In those studies we observed that 25% of the respiratory modulated neurons in the dorsal respiratory group were excited by phrenic nerve stimulation 9. Since most dorsal respiratory group neurons are reported to be premotor neurons, one would expect this excitatory effect to be transmitted
tO the phrenic motoneurons. However, the phrenic motor discharge is inhibited by phrenic nerve stimulation. A possible explanation for these observations is that a spinal inhibitory reflex may prevent the supraspinal premotor excitation from being expressed in the phrenic motor output. Experiments were designed to test the hypothesis that phrenic afferent inhibition is mediated by a spinal reflex. Since rhythmic phrenic motor discharge is abolished after high cervical spinal transection, we examined the inhibitory effects of phrenic nerve afferents on an excitatory spinal reflex, i.e. the intercostal-to-phrenic reflex 1. Ten cats were anesthetized with sodium thiopental (45 mg/kg, i.p.). Animals were tracheotomized and the femoral artery and vein were cannulated for monitoring of arterial blood pressure and intravenous infusion of drugs. The external carotid arteries were ligated bilaterally for subsequent decerebration procedures 1°. Cats were placed in a rigid headholder, paralyzed with gaUamine triethiodide (4 mg/kg induction followed by 4 mg/kg/h), thoracotomized and artificially ventilated with 100% 0 2. After decerebration 1°, the preparation was left to stabilize for at least 15 rain before proceeding with additional surgery. Muscles overlying the first and second cervicai vertebrae were removed and a dorsal laminectomy was
Correspondence: D.F. Speck, University of Kentucky, Medical Center, Dept. of Physiology, Lexington, KY 40536-0084, U.S.A. 0006-8993/87/$03.50 © 1987 Elsevier Science Publishers B.V. (Biomedical Division)
170 performed. The dura was carefully opened and the spinal cord was covered with gauze soaked in warm mineral oil. Subsequently, the cervical vagus nerves were sectioned bilaterally. The cervical phrenic nerve (CPN) C 5 spinal roots were isolated bilaterally by a dorsal approach. The left cervical phrenic nerve was cut and desheathed but the right C5 spinal root remained intact. Both roots were positioned across a pair of electrodes. In 3 animals the right thoracic phrenic nerve (TPN) was isolated close to the diaphragm and prepared for stimulation. The right tenth internal intercostal nerve (T10) was dissected free, cut and positioned across a pair of stimulating electrodes in order to activate the excitatory intercostalto-phrenic reflex. Single shocks (0.2 ms duration) were applied to the Tl0 intercostal nerve while monitoring the evoked response in the contralateral phrenic nerve. Data were recorded on analog tape for later playback and analysis. The threshold currents for eliciting an excitatory phrenic motor response during either inspiration or expiration were determined in each experiment. Subsequently the threshold current required for eliciting the contralateral phrenic nerve inhibitory response to right cervical phrenic nerve stimulation was determined. Simultaneous stimulation of both nerves at their threshold currents was then performed. To eliminate the descending excitatory inspiratory drive, cats were then hyperventilated to apnea. The responses to stimulation of the Tt0 intercostal nerve, the phrenic nerve, and simultaneous activation of both nerves were again examined. After returning the end-tidal CO 2 concentration to eupneic values, the spinal cord was transected at the C 2 level. After cutting the spinal cord with a scalpel, complete transection was verified by lifting up a rigid wire which had been passed under the spinal cord. The responses to the 3 stimulation paradigms (Tl0 intercostal nerve, cervical phrenic nerve, and both) were examined at 30 min, 1 h and 2 h post-spinalization. In all 10 intact preparations, threshold stimulation of the cervical phrenic nerve during inspiration produced the characteristic transient inhibition of the contralateral phrenic motor discharge (see ref. 8). Stimulation during expiration did not elicit any response in the phrenic neurogram (Fig. 1, top trace). In contrast, stimulation of the T~0 intercostal nerve caused a marked excitatory response with an onset
latency of 11-15 ms and a duration of 5-10 ms (Fig. 1, middle trace). This response was strongest when the stimuli were delivered during inspiration, but it could be evoked throughout expiration by increasing the intensity of the stimulus. After hyperventilation to apnea the response to T10 intercostal nerve stimulation persisted, but cervical phrenic nerve stimuli did not elicit any response. Stimulation of the caudal intercostal nerves is known to elicit a marked excitation of phrenic nerve discharge 1-3. This intercostalto-phrenic excitatory reflex does not involve supraspinal structures e. In 9 out of 10 cats, simultaneous stimulation of the cervical phrenic nerve and the Tl0 intercostal nerve caused a marked reduction in the response elicited by only T10 stimulation (Fig. 1). This reduction was observed when the stimuli were applied during either inspiration or expiration, but was most clearly seen during expiration when there was no spontaneous phrenic nerve activity. In 3 animals the intercostalto-phrenic excitatory response could be totally abolished, while in the other 6 experiments it could be markedly attenuated to the degree shown in Fig. 1. In all 9 cases the decreased response was obvious
DECEREBRATE
C2 SPINALIZED
Nerve Sflmulal~on
CPN
~
Fig. 1. Representative tracings of contralateral phrenic nerve responses to stimulation of the cervical phrenic nerve (CPN) and the tenth intercostal nerve (Tl0). Left panels: in a decerebrate cat, a single shock of the CPN during expiration does not elicit any response in the contralateral phrenic nerve. Tlo stimulation produces an evoked phrenic response. Synchronous stimulation of both Tl0 and CPN (Both) attenuates the normal Tl0 response. Right panels: 1 h after C 2 transection of the spinal cord, the response to synchronous stimulation of Tt0 and CPN does not attenuate the intercostal-to-phrenic excitatory reflex. Each trace contains 10 superimposed traces triggered by stimulus pulses (0.2 ms duration, 2 Hz). Stimulus currents were 100 /~A for T10 and 300 # A for CPN stimulation in all conditions.
17t in the oscilloscope traces. Hyperventilation to apnea did not alter the qualitative response to the 3 stimulation paradigms. The demonstration that phrenic nerve stimulation is capable of attenuating an excitatory reflex effect on phrenic motoneurons suggests that the phrenic nerve afferents are ultimately responsible for eliciting an inhibitory action on phrenic motoneurons. Simple removal of an excitatory input (i.e. disfacilitation) at the level of the phrenic motor nucleus would be unlikely to produce the observed antagonism of an excitatory reflex. While it may be argued that the inhibition occurs on interneurons, there is only sparse evidence to suggest that interneurons exist within the phrenic motor nucleus (for a further discussion, see ref. 5). In 3 experiments involving thoracic phrenic nerve stimulation, the responses elicited in the ipsilateral and contralateral phrenic nerves were studied. Since the contralateral phrenic nerve response to thoracic phrenic nerve stimulation is delayed by approximately 10 ms (due to the increased conduction distance), the experimental protocol for activation of both nerves was performed in two ways: (1) by synchronous pulses delivered to both nerves; and (2) by delivery of an initial pulse to the thoracic phrenic nerve 10 ms before stimulation of Tt0. The synchronous pulses were only effective in inhibiting the ipsilateral phrenic nerve response to T~0 activation. However, T10 intercostal nerve and thoracic phrenic nerve stimuli separated by a 10 ms delay attenuated both the ipsilateral and contralateral phrenic nerve responses to T10 stimulation. The inhibitory effects were greatest on the ipsilateral nerve (Fig. 2), thereby suggesting that both types of inhibition could be acting on the ipsilateral phrenic motor nucleus. These results are consistent with the data which demonstrate that the ipsilateral phrenic inhibition has a short onset latency (<5 ms) and a long duration (20-50 ms) 8. In contrast the contralateral phrenic nerve response to thoracic phrenic nerve stimulation has a longer onset latency (20 ms) and is initiated by slower conducting afferent fibers 8. Therefore, with synchronous stimulation of the T10 intercostal nerve and the thoracic phrenic nerve, the afferent activity of the contralateral inhibitory reflex would arrive in the spinal cord after the excitatory intercostal-tophrenic reflex had begun. However, introduction of the 10 ms delay would enable an interaction between
DECEREBRATE
C2 SPINALIZED
Nerve Stlm ulation T10
TPN
T10
TPN
T10
lo
msec
Fig. 2. Ipsilateral phrenic nerve responses to stimulation of the thoracic phrenic nerve (TPN) and the tenth internal intercostal nerve (T10). Arrows indicate TPN and T10 stimulation. Left panels: Tl0 stimuli elicit an excitatory response in the ipsilateral phrenic nerve (top) which can be attenuated by prior stimulation of the TPN. Right panel: 2 h after C2transection of the spinal cord, the response to T10 stimulation is enhanced. Prior stimulation of the ipsilateral TPN still attenuates the normal response to Tl0 stimulation. Each trace contains ten superimposed traces triggered by a timing pulse (top) or by the TPN stimulus (bottom). Stimulus currents were 50 ktA for Ta0 and 400/~A for TPN stimulation in all conditions.
the excitatory and inhibitory reflexes. After spinal cord transection between the C1 and C2 dorsal rootlets, there was still no phrenic nerve response to stimulation of the cervical phrenic nerve. However, the threshold for eliciting the response to Tt0 intercostal nerve stimulation was greatly reduced and the response to a given suprathreshold stimulus was enhanced. Simultaneous stimulation of both nerves did not decrease the response when compared to T10 stimulation alone (Fig. 1). To ensure that the lack of response was not simply due to alterations in threshold, the Tl0 intercostal nerve was stimulated with the minimal current necessary to elicit any observable response while the cervical phrenic nerve was stimulated with 15 mA (the maximal intensity obtainable from the stimulator). Even with these stimulation conditions, simultaneous activation of the cervical phrenic nerve and T10 intercostal nerve did not attenuate the response elicited by only T10 stimuli. Since these results persisted beyond 2 h after the spinalization, it is unlikely that they represent a transient phenomenon associated with spinal shock. Similarly, stimulation of the thoracic phrenic nerve and Ta0 intercostal nerve after spinal cord transection did not diminish the contralateral phrenic nerve re-
172 motoneurons, it must be emphasized that there are alternate explanations. For example, there might be a descending input which plays a permissive role in modulating a spinal segmental phrenic-to-phrenic reflex. In such a system, a segmental reflex could be abolished by eliminating a tonic excitatory descending drive (i.e. spinalization), Further experiments are necessary to conclusively determine whether the phrenic-to-phrenic reflex pathway actually passes through supraspinal structures or if the brainstem only exerts a powerful influence on the expression of this reflex.
sponse to T10 activation. However, the ipsilateral phrenic nerve response was still attenuated (Fig. 2). This result indicates that the two phrenic-to-phrenic inhibitory reflexes are mediated by different central mechanisms. The rapid, short latency ipsilateral response is due to spinal cord circuits (which remained intact after the C2 transection), while the bilateral phrenic-to-phrenic inhibition presumably involves supraspinal connections. These experiments suggest a role for supraspinal involvement in the production of the inhibitory phrenic-to-phrenic reflex, since the phrenic nerve afferent bilateral reflex effects were abolished after C 2 spinal cord transection. Although these results are likely to be explained by a supraspinal projection of phrenic nerve afferents to a brainstem interneuron which has a descending inhibitory effect on phrenic
The advice and criticism of the respiratory group at the University of Kentucky is greatly appreciated. This investigation was sponsored by a grant from the Public Health Service (HL 34568).
1 Decima, E.E., von Euler, C. and Thoden, U., Intercostalto-phrenic reflexes in the spinal cat, Acta Physiol. Scand., 75 (1969) 568-579. 2 Decima, E.E. and von Euler, C., Excitability of phrenic motoneurones to afferent input from lower intercostal nerves in the spinal cat, Acta Physiol. Scand., 75 (1969) 580-591. 3 Downman, C.R.B., Skeletal muscle reflexes of splanchnic and intercostal nerve origins in acute spinal and decerebrate cats, J. Neurophysiol., 18 (1955) 217-235. 4 Duron, B., Jung-Caillol, M.C. and Marlot, D., Reflexe inhibiteur phrenico-phrenique. In B. Duron, (Ed.), Respiratory Centres and Afferent Systems, Vol. 59, I.N.S.E.R.M., Paris, 1976, pp. 193-197. 5 Feldman, J.L., Loewy, A.D. and Speck, D.F., Projections from the ventral respiratory group to phrenic and intercos-
tal motoneurons in cat: an autoradiographic study, J. Neurosci., 5 (1985) 1993-2000. 6 Gill, P.K. and Kuno, M., Excitatory and inhibitory actions on phrenic motoneurones, J. Physiol. (London), 168 (1963) 271-289. 7 Rijlant, P., Contribution a l'etude du controle reflex de la respiration, Bull. Acad. R. Med. Belg., 7 (1942) 58-107. 8 Speck, D.F. and Revelene, W.R., Attenuation of phrenic motor discharge by phrenic nerve afferents, J. Appl. Physiol., 62 (1987) 941-945. 9 Speck, D.F. and Revelette, W.R., Excitation of neurons in the dorsal and ventral respiratory groups by phrenic nerve stimulation, J. Appl. Physiol., 62 (1987) 946-951. 10 Speck, D.F. and Webber, C.L., Jr., Time course of intercostal afferent termination of the inspiratory process, Respir. Physiol., 43 (1981) 133-145.
169
BRE 22329
Supraspinal involvement in the phrenic-to-phrenic inhibitory reflex Dexter F. Speck University of Kentucky Medical Center, Department of Physiology, Lexington, KY 40536-0084 (U.S.A.) (Accepted 17 March 1987) Key words: Brainstem; Intercostal-to-phrenic reflex; Phrenic afferent; Respiration; Spinal cord
Phrenic afferents are capable of attenuating the phrenic motor response elicited by the intercostal-to-phrenic excitatory reflex in decerebrate, paralyzed cats. High spinal transection eliminates the attenuating effect of the bilateral phrenic-to-phrenic inhibitory reflex. These results indicate that although phrenic nerve afferents do exert an inhibitory influence in the cervical spinal cord, some of the inhibitory effects are likely to involve supraspinal mechanisms.
Several investigators have reported the existence of a transient inhibitory phrenic-to-phrenic reflex4'7'8. A recent investigation has demonstrated that two different short latency inhibitory reflexes can be initiated by phrenic nerve afferents 8. The most prevalent reflex involves slowly conducting myelinated fibers which elicit a transient bilateral inhibition of the phrenic motor discharge. A second ipsilateral reflex can be initiated by stimulation of the large myelinated afferents in the whole phrenic nerve. Because of the relatively short latencies for onset of both inhibitory reflexes, it is likely that these responses are mediated through a paucisynaptic reflex pathway confined to the cervical spinal cord 7. Using intracellular recordings from phrenic motoneurons, Gill and Kuno 6 concluded that large phrenic afferent fibers produce inhibitory actions involving mechanisms at the cervical segmental level only, since spinal transection did not alter the effect. The existence of segmental phrenic afferent inhibition is supported by indirect evidence from our recent experiments 8'9. In those studies we observed that 25% of the respiratory modulated neurons in the dorsal respiratory group were excited by phrenic nerve stimulation 9. Since most dorsal respiratory group neurons are reported to be premotor neurons, one would expect this excitatory effect to be transmitted
tO the phrenic motoneurons. However, the phrenic motor discharge is inhibited by phrenic nerve stimulation. A possible explanation for these observations is that a spinal inhibitory reflex may prevent the supraspinal premotor excitation from being expressed in the phrenic motor output. Experiments were designed to test the hypothesis that phrenic afferent inhibition is mediated by a spinal reflex. Since rhythmic phrenic motor discharge is abolished after high cervical spinal transection, we examined the inhibitory effects of phrenic nerve afferents on an excitatory spinal reflex, i.e. the intercostal-to-phrenic reflex 1. Ten cats were anesthetized with sodium thiopental (45 mg/kg, i.p.). Animals were tracheotomized and the femoral artery and vein were cannulated for monitoring of arterial blood pressure and intravenous infusion of drugs. The external carotid arteries were ligated bilaterally for subsequent decerebration procedures 1°. Cats were placed in a rigid headholder, paralyzed with gaUamine triethiodide (4 mg/kg induction followed by 4 mg/kg/h), thoracotomized and artificially ventilated with 100% 0 2. After decerebration 1°, the preparation was left to stabilize for at least 15 rain before proceeding with additional surgery. Muscles overlying the first and second cervicai vertebrae were removed and a dorsal laminectomy was
Correspondence: D.F. Speck, University of Kentucky, Medical Center, Dept. of Physiology, Lexington, KY 40536-0084, U.S.A. 0006-8993/87/$03.50 © 1987 Elsevier Science Publishers B.V. (Biomedical Division)
170 performed. The dura was carefully opened and the spinal cord was covered with gauze soaked in warm mineral oil. Subsequently, the cervical vagus nerves were sectioned bilaterally. The cervical phrenic nerve (CPN) C 5 spinal roots were isolated bilaterally by a dorsal approach. The left cervical phrenic nerve was cut and desheathed but the right C5 spinal root remained intact. Both roots were positioned across a pair of electrodes. In 3 animals the right thoracic phrenic nerve (TPN) was isolated close to the diaphragm and prepared for stimulation. The right tenth internal intercostal nerve (T10) was dissected free, cut and positioned across a pair of stimulating electrodes in order to activate the excitatory intercostalto-phrenic reflex. Single shocks (0.2 ms duration) were applied to the Tl0 intercostal nerve while monitoring the evoked response in the contralateral phrenic nerve. Data were recorded on analog tape for later playback and analysis. The threshold currents for eliciting an excitatory phrenic motor response during either inspiration or expiration were determined in each experiment. Subsequently the threshold current required for eliciting the contralateral phrenic nerve inhibitory response to right cervical phrenic nerve stimulation was determined. Simultaneous stimulation of both nerves at their threshold currents was then performed. To eliminate the descending excitatory inspiratory drive, cats were then hyperventilated to apnea. The responses to stimulation of the Tt0 intercostal nerve, the phrenic nerve, and simultaneous activation of both nerves were again examined. After returning the end-tidal CO 2 concentration to eupneic values, the spinal cord was transected at the C 2 level. After cutting the spinal cord with a scalpel, complete transection was verified by lifting up a rigid wire which had been passed under the spinal cord. The responses to the 3 stimulation paradigms (Tl0 intercostal nerve, cervical phrenic nerve, and both) were examined at 30 min, 1 h and 2 h post-spinalization. In all 10 intact preparations, threshold stimulation of the cervical phrenic nerve during inspiration produced the characteristic transient inhibition of the contralateral phrenic motor discharge (see ref. 8). Stimulation during expiration did not elicit any response in the phrenic neurogram (Fig. 1, top trace). In contrast, stimulation of the T~0 intercostal nerve caused a marked excitatory response with an onset
latency of 11-15 ms and a duration of 5-10 ms (Fig. 1, middle trace). This response was strongest when the stimuli were delivered during inspiration, but it could be evoked throughout expiration by increasing the intensity of the stimulus. After hyperventilation to apnea the response to T10 intercostal nerve stimulation persisted, but cervical phrenic nerve stimuli did not elicit any response. Stimulation of the caudal intercostal nerves is known to elicit a marked excitation of phrenic nerve discharge 1-3. This intercostalto-phrenic excitatory reflex does not involve supraspinal structures e. In 9 out of 10 cats, simultaneous stimulation of the cervical phrenic nerve and the Tl0 intercostal nerve caused a marked reduction in the response elicited by only T10 stimulation (Fig. 1). This reduction was observed when the stimuli were applied during either inspiration or expiration, but was most clearly seen during expiration when there was no spontaneous phrenic nerve activity. In 3 animals the intercostalto-phrenic excitatory response could be totally abolished, while in the other 6 experiments it could be markedly attenuated to the degree shown in Fig. 1. In all 9 cases the decreased response was obvious
DECEREBRATE
C2 SPINALIZED
Nerve Sflmulal~on
CPN
~
Fig. 1. Representative tracings of contralateral phrenic nerve responses to stimulation of the cervical phrenic nerve (CPN) and the tenth intercostal nerve (Tl0). Left panels: in a decerebrate cat, a single shock of the CPN during expiration does not elicit any response in the contralateral phrenic nerve. Tlo stimulation produces an evoked phrenic response. Synchronous stimulation of both Tl0 and CPN (Both) attenuates the normal Tl0 response. Right panels: 1 h after C 2 transection of the spinal cord, the response to synchronous stimulation of Tt0 and CPN does not attenuate the intercostal-to-phrenic excitatory reflex. Each trace contains 10 superimposed traces triggered by stimulus pulses (0.2 ms duration, 2 Hz). Stimulus currents were 100 /~A for T10 and 300 # A for CPN stimulation in all conditions.
17t in the oscilloscope traces. Hyperventilation to apnea did not alter the qualitative response to the 3 stimulation paradigms. The demonstration that phrenic nerve stimulation is capable of attenuating an excitatory reflex effect on phrenic motoneurons suggests that the phrenic nerve afferents are ultimately responsible for eliciting an inhibitory action on phrenic motoneurons. Simple removal of an excitatory input (i.e. disfacilitation) at the level of the phrenic motor nucleus would be unlikely to produce the observed antagonism of an excitatory reflex. While it may be argued that the inhibition occurs on interneurons, there is only sparse evidence to suggest that interneurons exist within the phrenic motor nucleus (for a further discussion, see ref. 5). In 3 experiments involving thoracic phrenic nerve stimulation, the responses elicited in the ipsilateral and contralateral phrenic nerves were studied. Since the contralateral phrenic nerve response to thoracic phrenic nerve stimulation is delayed by approximately 10 ms (due to the increased conduction distance), the experimental protocol for activation of both nerves was performed in two ways: (1) by synchronous pulses delivered to both nerves; and (2) by delivery of an initial pulse to the thoracic phrenic nerve 10 ms before stimulation of Tt0. The synchronous pulses were only effective in inhibiting the ipsilateral phrenic nerve response to T~0 activation. However, T10 intercostal nerve and thoracic phrenic nerve stimuli separated by a 10 ms delay attenuated both the ipsilateral and contralateral phrenic nerve responses to T10 stimulation. The inhibitory effects were greatest on the ipsilateral nerve (Fig. 2), thereby suggesting that both types of inhibition could be acting on the ipsilateral phrenic motor nucleus. These results are consistent with the data which demonstrate that the ipsilateral phrenic inhibition has a short onset latency (<5 ms) and a long duration (20-50 ms) 8. In contrast the contralateral phrenic nerve response to thoracic phrenic nerve stimulation has a longer onset latency (20 ms) and is initiated by slower conducting afferent fibers 8. Therefore, with synchronous stimulation of the T10 intercostal nerve and the thoracic phrenic nerve, the afferent activity of the contralateral inhibitory reflex would arrive in the spinal cord after the excitatory intercostal-tophrenic reflex had begun. However, introduction of the 10 ms delay would enable an interaction between
DECEREBRATE
C2 SPINALIZED
Nerve Stlm ulation T10
TPN
T10
TPN
T10
lo
msec
Fig. 2. Ipsilateral phrenic nerve responses to stimulation of the thoracic phrenic nerve (TPN) and the tenth internal intercostal nerve (T10). Arrows indicate TPN and T10 stimulation. Left panels: Tl0 stimuli elicit an excitatory response in the ipsilateral phrenic nerve (top) which can be attenuated by prior stimulation of the TPN. Right panel: 2 h after C2transection of the spinal cord, the response to T10 stimulation is enhanced. Prior stimulation of the ipsilateral TPN still attenuates the normal response to Tl0 stimulation. Each trace contains ten superimposed traces triggered by a timing pulse (top) or by the TPN stimulus (bottom). Stimulus currents were 50 ktA for Ta0 and 400/~A for TPN stimulation in all conditions.
the excitatory and inhibitory reflexes. After spinal cord transection between the C1 and C2 dorsal rootlets, there was still no phrenic nerve response to stimulation of the cervical phrenic nerve. However, the threshold for eliciting the response to Tt0 intercostal nerve stimulation was greatly reduced and the response to a given suprathreshold stimulus was enhanced. Simultaneous stimulation of both nerves did not decrease the response when compared to T10 stimulation alone (Fig. 1). To ensure that the lack of response was not simply due to alterations in threshold, the Tl0 intercostal nerve was stimulated with the minimal current necessary to elicit any observable response while the cervical phrenic nerve was stimulated with 15 mA (the maximal intensity obtainable from the stimulator). Even with these stimulation conditions, simultaneous activation of the cervical phrenic nerve and T10 intercostal nerve did not attenuate the response elicited by only T10 stimuli. Since these results persisted beyond 2 h after the spinalization, it is unlikely that they represent a transient phenomenon associated with spinal shock. Similarly, stimulation of the thoracic phrenic nerve and Ta0 intercostal nerve after spinal cord transection did not diminish the contralateral phrenic nerve re-
172 motoneurons, it must be emphasized that there are alternate explanations. For example, there might be a descending input which plays a permissive role in modulating a spinal segmental phrenic-to-phrenic reflex. In such a system, a segmental reflex could be abolished by eliminating a tonic excitatory descending drive (i.e. spinalization), Further experiments are necessary to conclusively determine whether the phrenic-to-phrenic reflex pathway actually passes through supraspinal structures or if the brainstem only exerts a powerful influence on the expression of this reflex.
sponse to T10 activation. However, the ipsilateral phrenic nerve response was still attenuated (Fig. 2). This result indicates that the two phrenic-to-phrenic inhibitory reflexes are mediated by different central mechanisms. The rapid, short latency ipsilateral response is due to spinal cord circuits (which remained intact after the C2 transection), while the bilateral phrenic-to-phrenic inhibition presumably involves supraspinal connections. These experiments suggest a role for supraspinal involvement in the production of the inhibitory phrenic-to-phrenic reflex, since the phrenic nerve afferent bilateral reflex effects were abolished after C 2 spinal cord transection. Although these results are likely to be explained by a supraspinal projection of phrenic nerve afferents to a brainstem interneuron which has a descending inhibitory effect on phrenic
The advice and criticism of the respiratory group at the University of Kentucky is greatly appreciated. This investigation was sponsored by a grant from the Public Health Service (HL 34568).
1 Decima, E.E., von Euler, C. and Thoden, U., Intercostalto-phrenic reflexes in the spinal cat, Acta Physiol. Scand., 75 (1969) 568-579. 2 Decima, E.E. and von Euler, C., Excitability of phrenic motoneurones to afferent input from lower intercostal nerves in the spinal cat, Acta Physiol. Scand., 75 (1969) 580-591. 3 Downman, C.R.B., Skeletal muscle reflexes of splanchnic and intercostal nerve origins in acute spinal and decerebrate cats, J. Neurophysiol., 18 (1955) 217-235. 4 Duron, B., Jung-Caillol, M.C. and Marlot, D., Reflexe inhibiteur phrenico-phrenique. In B. Duron, (Ed.), Respiratory Centres and Afferent Systems, Vol. 59, I.N.S.E.R.M., Paris, 1976, pp. 193-197. 5 Feldman, J.L., Loewy, A.D. and Speck, D.F., Projections from the ventral respiratory group to phrenic and intercos-
tal motoneurons in cat: an autoradiographic study, J. Neurosci., 5 (1985) 1993-2000. 6 Gill, P.K. and Kuno, M., Excitatory and inhibitory actions on phrenic motoneurones, J. Physiol. (London), 168 (1963) 271-289. 7 Rijlant, P., Contribution a l'etude du controle reflex de la respiration, Bull. Acad. R. Med. Belg., 7 (1942) 58-107. 8 Speck, D.F. and Revelene, W.R., Attenuation of phrenic motor discharge by phrenic nerve afferents, J. Appl. Physiol., 62 (1987) 941-945. 9 Speck, D.F. and Revelette, W.R., Excitation of neurons in the dorsal and ventral respiratory groups by phrenic nerve stimulation, J. Appl. Physiol., 62 (1987) 946-951. 10 Speck, D.F. and Webber, C.L., Jr., Time course of intercostal afferent termination of the inspiratory process, Respir. Physiol., 43 (1981) 133-145.