The accessory phrenic nerve in the rat

Summary Reinnervation studies of the diaphragm led us to reinvestigate the normal anatomy of the phrenic nerve of the rat. The phrenic nerve originates from the cervical nerve roots C₄ and C₅. In 16 out of 19 normal rats an accessory phrenic nerve was observed receiving its segmental fibres from C₆. The number of myelinated axons of the accessory phrenic nerve varied from 41 to 101 (mean: 64.3, i.e. about 15% of the average number of axons in the common phrenic nerve). The accessory phrenic nerve innervates the dorsal part of the costal and the lateral part of the crural region, whereas the remaining parts of the hemidiaphragm are supplied by the segments C₄ and C₅. There is no evidence for any additional contribution to the motor innervation of the diaphragm from intercostal nerves.Direktor: Prof. Dr. W. ZenkerThis study was supported by the Fonds zur Förderung der wissenschaftlichen Forschung in Österreich.     

Development of the rat phrenic nerve and the terminal distribution of phrenic afferents in the cervical cord

The development of the right phrenic nerve and the distribution of phrenic nerve afferents to the spinal cord have been examined with the aid of electron microscopy and carbocyanine dye retrograde diffusion along the phrenic nerve, respectively. The formation of fascicles in the right phrenic nerve commenced at E15, while Schwann cells penetrated the nerve from E17 and myelination began at P0. The total number of axons in the right phrenic nerve decreased from E15 (943, 965 in two animals) to E19 (539, 582), remained steady until P0 (564, 594) before rising to almost adult values by P7 (689, 934). The postnatal rise in number of axons appears to be due to a large influx of unmyelinated axons. Carbocyanine dye tracing revealed that at E13, neurons in dorsal root ganglia C₂ to C₆ contributed peripheral processes to the phrenic nerve. Phrenic afferents arrived in the spinal cord by E13 and penetrated the dorsal horn at E14. Three terminal fields for phrenic afferents became apparent by E17. These were:(1) in the central parts of laminae I to V, (2) medially in laminae V to VII or adjacent area X near the central canal, (3) in laminae VIII and IX, around the differentiating phrenic motoneurons. Around the time of birth, some phrenic afferents in the second group were distributed across the midline and could be seen to approach the ventromedial dendritic bundle of phrenic motoneurons on the contralateral side, but these were no longer seen by P4. Just before birth (E21), afferents in the third group divided into two further subsets, supplying the dorsolateral and ventromedial groups of phrenic motoneuron dendritic bundles, respectively. Our findings strongly suggest that phrenic afferent differentiation is largely complete by birth. Accepted: 31 May 1999     

大鼠迷走神经和膈神经的解剖相关性实验研究

目的 为大鼠迷走神经移位膈神经重建高位颈髓损伤大鼠的膈肌功能提供显微解剖学依据。 方法　10只健康雌性SD大鼠在10倍手术显微镜下解剖双侧膈神经、迷走神经及其分支。用数显卡尺测量迷走神经与膈神经在“膈神经主干起始平面”、“锁骨上平面”、“入膈肌平面”的相对距离,用读数显微镜测量各平面迷走神经和膈神经的直径。 结果　在颈部,迷走神经直径为（0.3284±0.0247）mm,膈神经直径为（0.2267±0.0164）mm,二者的相对距离很接近,无论是在“膈神经主干起始平面”还是“锁骨上平面”,平均都不超过2.5 mm;在“入膈肌平面”平面,迷走神经直径为（0.2912±0.0326）mm,膈神经直径为（0.2794±0.0282）mm,二者的相对距离较颈部远,左侧为（8.71±0.804）mm,右侧为（6.203±0.952） mm。 结论　(1)在颈部,迷走神经与膈神经的直径相差不大,相对距离很接近,二者可直接无张力缝合。(2)在入膈肌平面,迷走神经与膈神经的直径大致相同,相对距离稍远,但将膈神经和迷走神经向上游离一段距离后仍可实现二者的直接无张力缝合。     

胸腔镜下切取膈神经的应用解剖

目的对胸段膈神经的解剖关系进行了研究。为电视胸腔镜下切取全长膈神经进行移位治疗臂丛损伤奠定解剖学基础。方法选用新鲜尸体6具12侧,对膈神经及其周围器官进行解剖观察。结果胸腔内覆盖膈神经的胸膜较疏松,易于分离;膈神经从锁骨下缘到入肌点的长度为右侧(16.50±2.17)cm,左侧(23.00±2.50)cm;膈神经的血供主要来源于心包膈动脉,心包膈动脉的直径为(1.30±0.15)mm;在胸廓入口处膈神经、心包膈动脉及胸廓内动脉之间的解剖关系不恒定。结论膈神经在胸腔内的解剖特点适合进行电视胸腔镜下的全长游离。但在胸廓入口处,膈神经、胸廓内动脉及由胸廓内动脉发出的心包膈动脉解剖位置不恒定。在临床操作时,该区域的处理是整个手术的关键。     

Morphological study of the ansa cervicalis and the phrenic nerve

Morphological features of ansa cervicalis and phrenic nerve were studied in 106 cadavers. Ansa cervicalis was located medial to the internal jugular vein in 63% (medial type) and lateral to the vein in 33.7% (lateral type). Ansa cervicalis was derived from a combination of C1-C4 spinal segments, with C1-C3 being the most frequent pattern (87.5%). In >60% the ansa was bilaterally symmetrical. The distribution of medial and lateral types was equal on left and right sides of the body. The segmental composition of the inferior root was higher in the medial type and also on the left side of the body. In the lateral type the branches that formed the inferior root frequently (75%) formed a common trunk before joining the superior root, but in 74.8% of the medial type they joined the superior root independently. The phrenic nerve was derived from C4 and C5 in 52%. The C4 segment was present in the phrenic nerve in all cases except one. Additional phrenic components that pass anterior to the subclavian vein were defined as accessory phrenic nerves and found in 28.7%, while those passing posterior to the same vein were defined as secondary phrenic nerves (19.8%). Most of the accessory phrenic nerves contained a C5 segment and the nerve to subclavius was the commonest source. Various relationships between the ansa cervicalis and the phrenic nerve are investigated and, based on these findings, two separate classifications for the two nerves are suggested.     

The ultrastructure and synaptic architecture of phrenic motor neurons in the spinal cord of the adult rat

Summary Although light microscopic studies have analysed phrenic motor neurons in several different species, there has never been an ultrastructural investigation of identified phrenic motor neurons. In addition, electrophysiological studies have raised questions relating to the function of phrenic motor neurons which may be answered only by direct electron microscopic investigation. Thus, the present study was carried out to provide a detailed ultrastructural analysis of identified phrenic motor neurons. Phrenic motor neurons in the spinal cord of the rat were labelled by retrogradely transported horseradish peroxidase (HRP) after transecting the phrenic nerve in the neck and applying the enzyme directly to the central stump of the transected nerve. The results showed that the general ultrastructural characteristics of phrenic motor neurons were similar to those previously reported for other spinal motor neurons. However, phrenic primary dendrites appeared to be isolated from all other dendritic profiles in the neuropil. Primary dendrites were not fasciculated. Fasciculation occurred only among the more distal secondary and tertiary phrenic dendritic branches. Direct dendrodendritic or dendrosomatic apposition was rarely seen; gap junctions between directly apposing phrenic neuronal membranes were not observed. The membranes of adjacent phrenic neuronal profiles were most frequently separated by intervening sheaths of astroglial processes. Myelinated phrenic axons and a phrenic axon collateral were identified. The initial portion of the phrenic axon collateral was cone-shaped, lacked myelin, and thus resembled a miniature axon hillock. In one instance, a large accumulation of polyribosomes was observed within the hillock-like structure of a phrenic axon collateral. Eight morphological types of synaptic boutons, M, P, NF_s, S, NF_f, F, G and C were classified according to criteria used by previous investigators. Most of these endings (M, NF_s, NF_f, S and F) made synaptic contact with profiles of labelled phrenic somata and dendrites. F, NF_f, and S boutons also terminated on phrenic axon hillocks. C and G boutons contacted exclusively phrenic somata and small calibre dendrites, respectively. P boutons established axo-axonic synaptic contacts with the M and NF_s bouton. The morphological findings of the present study provide new data that may be related to phrenic synchronized output and presynaptic inhibition of primary afferents terminating on phrenic motor neurons.     

Left phrenic nerve anatomy relative to the coronary venous system: Implications for phrenic nerve stimulation during cardiac resynchronization therapy

The objective of this study was to quantitatively characterize anatomy of the human phrenic nerve in relation to the coronary venous system, to reduce undesired phrenic nerve stimulation during left‐sided lead implantations. We obtained CT scans while injecting contrast into coronary veins of 15 perfusion‐fixed human heart‐lung blocs. A radiopaque wire was glued to the phrenic nerve under CT, then we created three‐dimensional models of anatomy and measured anatomical parameters. The left phrenic nerve typically coursed over the basal region of the anterior interventricular vein, mid region of left marginal veins, and apical region of inferior and middle cardiac veins. There was large variation associated with the average angle between nerve and veins. Average angle across all coronary sinus tributaries was fairly consistent (101.3°–111.1°). The phrenic nerve coursed closest to the middle cardiac vein and left marginal veins. The phrenic nerve overlapped a left marginal vein in >50% of specimens. Clin. Anat. 28:621–626, 2015. © 2015 Wiley Periodicals, Inc.     

Human phrenic nerve: analysis of nerve fibre types on the base of their acetylcholinesterase activity (author's transl)

As marked differences in the Acetylcholinesterase (ACHE)-activity of myelinated nerve fibres of ventral and dorsal spinal roots can be found also in human post mortem material (ZENKER et al. 1978), the Karnovsky-method for histochemical demonstration of ACHE-activity has been used for differentiation of motor and sensory fibres in the human phrenic nerve. In cross sections of the phrenic nerve 1--2 cm above its entrance into the diaphragm, after an incubation period of 24 hours, 86% of the medullated nerve fibres displayed a high enzyme activity and therefore were classified as motoric. The histogram of these stained fibres revealed a large group of fibres with a peak at 9--10 micron in diameter which were interpreted as A-alpha fibres and a small group of fibres with a peak at 2--3 micron which were classified as A-gamma fibres. The mean diameter of the A-alpha group fibres is smaller than the mean diameter in a "typical" muscle nerve. Furthermore, the number of A-gamma fibres in the phrenic nerve, as compared with a "typical" muscle nerve is strikingly small. This seems to be in accordance with the small number of unstained fibres (14% only) which were interpreted as sensoric. In this group no fibre was found larger than 9 micrometers in diameter. This could mean a complete lack of primary afferents within the human phrenic nerve.     

Ventilation estimated from efferent phrenic nerve activity in the paralysed cat

The purpose of this study is to develop a reliable method for obtaining information about "spontaneous respiration" in paralysed cats. Therefore action potentials from one of the phrenic nerves are recorded. In a spontaneously breathing animal, a CO2 rebreathing experiment is performed in order to obtain a relationship between phrenic nerve activity and tidal volume. This phrenic nerve activity is corrected for the noise measured during expiration and quantified proportional to the square root of the mean impulse rate of the whole nerve bundle. Thus, high correlation coefficients (0.95 or more) between phrenic nerve activity and tidal volume can be obtained. After paralysing the cat this relationship can be used to estimate "spontaneous tidal volume" from the phrenic nerve activity. It appears to be necessary to perform unilateral phrenicotomy on the nerve from which recordings are taken, because there is a condiserable amount of afferent signals in the phrenic nerve which is dependent on the stroke volume of the respirator, on the alveolar PCO2 and somewhat on the alveolar PO2. It is concluded that after vagotomy and phrenicotomy and if suitably quantified, the electrical activity in the phrenic nerve gives accurate information on "spontaneous ventilation" in a paralysed cat.     

Neurophysiological and technical considerations for the design of an implantable phrenic nerve stimulator

Sequential stimulation during one muscle contraction of several compartments of a motor nerve, using multiple-electrodes, allows individual nerve-muscle compartments to be stimulated at fairly low frequencies. This provides time for recovery even during muscle contraction. However, the whole muscle is stimulated at near to its optimum fusion frequency, which provides smooth muscle contraction. This stimulation system imitates the natural activation of skeletal muscle. The new phrenic nerve stimulator described utilises the principle of sequential motor nerve stimulation. It also incorporates a sigh function. The sigh current recruits additional axons at certain intervals and thus creates and keeps available a reserve of conditioned muscle. Clinical advantages result: the conditioning phase after the beginning of long-term phrenic nerve stimulation for electroventilation is shortened and muscle fatigue is delayed. A need of increase of gas exchange can be answered by increasing tidal volume instead of respiration rate alone.     

5-Hydroxytryptamine in the phrenic nerve diaphragm: evidence for its existence and release

Evidence is presented for the existence of 5-hydroxytryptamine (5-HT) within the phrenic nerve of the rat and its release following electrical stimulation. Contents of 5-HT and its metabolite, 5-hydroxyindoleacetic acid (5-HIAA) in the phrenic nerve and the indoleamine released into the bathing fluid were estimated fluorimetrically after isolation on Sephadex G-10 and/or solvent-solvent extraction. Bioassays of 5-HT were done on rat fundus strip. The phrenic nerve and the end-plate zone contains high levels of 5-HT (1.9 micrograms/g wet weight) and 5-HIAA (1.5 micrograms/g wet weight). The resting release of around 1 ng 5-HT/diaphragm/min was enhanced by 50% (1.5 ng 5-HT/diaphragm/min) upon supramaximal (2-4 V) electrical stimulation of 5 Hz. Phrenic nerve diaphragm prepared from the denervated and p-chlorophenylalanine (300 mg/kg/day i.p. for 3 days) treated rats failed to release 5-HT confirming the neuronal origin and the identity of the indoleamine respectively. Furthermore, methysergide, an antagonist of 5-HT in rat fundus strip, blocked the response obtained by the sample on it. A modulatory role of 5-HT in the phrenic nerve diaphragm of the rat is envisaged from the present study.     

Diaphragmatic effects of selective resection of the upper phrenic nerve root in dogs

The aim of this study was to evaluate the effects on the diaphragm of upper phrenic nerve root resections in dogs. During laryngeal reinnervation, selective resections of the upper phrenic nerve root (C5) were performed unilaterally (right side, n=7; Group A) and bilaterally (n=6; Group B) and compared to non denervated animals (n=5). After 8 months, a diaphragmatic evaluation was performed: X-ray, EMG, transdiaphragmatic pressure (Pdi) after ipsi- and bilateral tetanic stimulation of the phrenic nerves and a bilateral histological study of five hemidiaphragmatic regions. EMG alterations were significantly more severe in Group B than in Group A, for the left (p<0.05) and right hemidiaphragms (p<0.01). No differences in the X-rays were noted between the three groups. The Pdi of the three groups after occlusion and phrenic nerve stimulations (unilateral and bilateral) were not statistically different. Histological data demonstrated that there were no differences in fibre irregularity, predominant fibre type or fibrosis between the three groups. Macroscopic and microscopic atrophy, which was mainly present on the anterior regions of the hemidiaphragms, was significantly higher in Group B than in Group A and undenervated dogs (p<0.05). In conclusion, resection of the upper phrenic nerve root of one phrenic nerve (right side) have limited effect on the diaphragm in dogs. However, resection of the upper phrenic nerve root on both sides resulted in a significant effect on the EMGs and histology of the entire diaphragm without any significant consequences on transdiaphragmatic pressure.     

Beta-adrenergic drugs do not affect phrenic nerve afterdischarge

Summary In anesthetized cats, a carotid sinus nerve was stimulated electrically. After this stimulation the time course of the afterdischarge in the phrenic nerve activity was studied in the control situation, during infusion of isoprenalin and after administration of metoprolol. The time courses were identical in all situations.It is concluded that in spite of the fact that the beta-adrenergic drugs change the steady state phrenic nerve activity, the afterdischarge is unchanged and therefore probably mediated by a separate mechanism. A comparison is made with analogous findings in patients with a hyperventilation syndrome.     

Studies on the cellular localization of spinal cord substance P receptors

Substance P-immunoreactivity and specific substance P binding sites are present in the spinal cord. Receptor autoradiography showed the discrete localization of substance P binding sites in both sensory and motor regions of the spinal cord and functional studies suggested an important role for substance P receptor activation in autonomic outflow, nociception, respiration and somatic motor function. In the current studies, we investigated the cellular localization of substance P binding sites in rat spinal cord using light microscopic autoradiography combined with several lesioning techniques. Unilateral injections of the suicide transport agent, ricin, into the superior cervical ganglion reduced substance P binding and cholinesterase-stained preganglionic sympathetic neurons in the intermediolateral cell column. However, unilateral electrolytic lesions of ventral medullary substance P neurons which project to the intermediolateral cell column did not alter the density of substance P binding in the intermediolateral cell column. Likewise, 6-hydroxydopamine and 5,7-dihydroxytryptamine, which destroy noradrenergic and serotonergic nerve terminals, did not reduce the substance P binding in the intermediolateral cell column. It appears, therefore, that the substance P binding sites are located postsynaptically on preganglionic sympathetic neurons rather than presynaptically on substance P-immunoreactive processes (i.e. as autoreceptors) or on monoamine nerve terminals. Unilateral injections of ricin into the phrenic nerve resulted in the unilateral destruction of phrenic motor neurons in the cervical spinal cord and caused a marked reduction in the substance P binding in the nucleus. Likewise, sciatic nerve injections of ricin caused a loss of associated motor neurons in the lateral portion of the ventral horn of the lumbar spinal cord and a reduction in the substance P binding. Sciatic nerve injections of ricin also destroyed afferent nerves of the associated dorsal root ganglia and increased the density of substance P binding in the dorsal horn. Capsaicin, which destroys small diameter primary sensory neurons, similarly increased the substance P binding in the dorsal horn. These studies show that the cellular localization of substance P binding sites can be determined by analysis of changes in substance P binding to discrete regions of spinal cord after selective lesions of specific groups of neurons. The data show the presence of substance P binding sites on preganglionic sympathetic neurons in the intermediolateral cell column and on somatic motor neurons in the ventral horn, including the phrenic motor nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)     

Episodic phrenic-inhibitory vagus nerve stimulation paradoxically induces phrenic long-term facilitation in rats

All respiratory long-term facilitation (LTF) is induced by inspiratory-excitatory stimulation, suggesting that LTF needs inspiratory augmentation and is the result of a Hebbian mechanism (coincident pre- and post-synaptic activity strengthens synapses). The present study examined the long-term effects of episodic inspiratory-inhibitory vagus nerve stimulation (VNS) on phrenic nerve activity. We hypothesized that episodic VNS would induce phrenic long-term depression. The results are compared with those obtained following serotonin receptor antagonism or episodic carotid sinus nerve stimulation (CSNS). Integrated phrenic neurograms were measured before, during and after three episodes of 5 min VNS (50 Hz, 0.1 ms), each separated by a 5 min interval, at a low (˜50 μA), medium (˜200 μA) or high (˜500 μA) stimulus intensity in anaesthetized, vagotomized, neuromuscularly blocked and artificially ventilated rats. Medium- and high-intensity VNS eliminated rhythmic phrenic activity during VNS, while low-intensity VNS only reduced phrenic burst frequency. At 60 min post-VNS, phrenic amplitude was higher than baseline (35 ± 5 % above baseline, mean ± S.E.M., P < 0.05) in the high-intensity group but not in the low- (-4 ± 4 %) or medium-intensity groups (-10 ± 15 %), or in the high-intensity with methysergide group (4 mg kg⁻¹, I.P.) (-11 ± 5 %). These data, which are inconsistent with our hypothesis, indicate that phrenic-inhibitory VNS induces a serotonin-dependent phrenic LTF similar to that induced by phrenic-excitatory CSNS (33 ± 7 %) and may require activation of high-threshold afferent fibres. These data also suggest that the synapses on phrenic motoneurons do not use the Hebbian mechanism in this LTF, as these motoneurons were suppressed during VNS.     

Identification of the neural pathway underlying spontaneous crossed phrenic activity in neonatal rats

Cervical spinal cord hemisection at C2 leads to paralysis of the ipsilateral hemidiaphragm in rats. Respiratory function of the paralyzed hemidiaphragm can be restored by activating a latent respiratory motor pathway in adult rats. This pathway is called the crossed phrenic pathway and the restored activity in the paralyzed hemidiaphragm is referred to as crossed phrenic activity. The latent neural pathway is not latent in neonatal rats as shown by the spontaneous expression of crossed phrenic activity. However, the anatomy of the pathway in neonatal rats is still unknown. In the present study, we hypothesized that the crossed phrenic pathway may be different anatomically in neonatal and adult rats. To delineate this neural pathway in neonates, we injected wheat germ agglutinin conjugated to horseradish peroxidase (WGA–HRP), a retrograde transsynaptic tracer, into the phrenic nerve ipsilateral to hemisection. We also injected cholera toxin subunit B–horseradish peroxidase (BHRP) into the ipsilateral hemidiaphragm following hemisection in other animals to determine if there are midline-crossing phrenic dendrites involved in the crossed phrenic pathway in neonatal rats. The WGA–HRP labeling was observed only in the ipsilateral phrenic nucleus and ipsilateral rostral ventral respiratory group (rVRG) in the postnatal day (P) 2, P7, and P28 hemisected rats. Bilateral labeling of rVRG neurons was shown in P35 rats. The BHRP study showed that many phrenic dendrites cross the midline in P2 neonatal rats at both rostral and caudal parts of the phrenic nucleus. There was a marked reduction of crossing dendrites observed in P7 and P28 animals and no crossing dendrites observed in P35 rats. The present results suggest that the crossed phrenic pathway in neonatal rats involves the parent axons from ipsilateral rVRG premotor neurons that cross at the level of obex as well as decussating axon collaterals that cross over the spinal cord midline to innervate ipsilateral phrenic motoneurons following C2 hemisection. In addition, midline-crossing dendrites of the ipsilateral phrenic motoneurons may also contribute to the crossed phrenic pathway in neonates.     

Uncoupling of rhythmic hypoglossal from phrenic activity in the rat

During eupnoea, rhythmic motor activities of the hypoglossal, vagal and phrenic nerves are linked temporally. The inspiratory discharges of the hypoglossal and vagus motor neurones commence before the onset of the phrenic burst. The vagus nerve also discharges in expiration. Upon exposure to hypocapnia or hypothermia, the hypoglossal discharge became uncoupled from that of the phrenic nerve. This uncoupling was evidenced by variable times of onset of hypoglossal discharge before or after the onset of phrenic discharge, extra bursts of hypoglossal activity in neural expiration, or complete absence of any hypoglossal discharge during a respiratory cycle. No such changes were found for vagal discharge, which remained linked to the phrenic bursts. Intracellular recordings in the hypoglossal nucleus revealed that all changes in hypoglossal discharge were due to neuronal depolarization. These results add support to the conclusion that the brainstem control of respiratory-modulated hypoglossal activity differs from control of phrenic and vagal activity. These findings have implications for any studies in which activity of the hypoglossal nerve is used as the sole index of neural inspiration. Indeed, our results establish that hypoglossal discharge alone is an equivocal index of the pattern of overall ventilatory activity and that this is accentuated by hypercapnia and hypothermia.     

Calcitonin gene-related peptide enhances spontaneous acetylcholine release from the rat motor nerve terminal

The effect of synthetic rat calcitonin gene-related peptide (rCGRP) on neuromuscular transmission was examined in a superfused rat phrenic nerve-diaphragm preparation using an intracellular microelectrode technique. The superfusion with rCGRP (10(-8) to 2 x 10(-7) M) caused significant increases in the frequency of miniature endplate potentials (MEPPs). It, however, had no effect on the resting membrane potential, amplitude of MEPPs, or acetylcholine quantum size and content. Enhancement of spontaneous acetylcholine release from the motor nerve terminal by rCGRP was demonstrated.     

Cross-innervation of fast and slow-twitch muscles by motor axons of the sural nerve in the mouse

The extensor digitorum longus (EDL) or soleus muscles of adult mice were cross-innervated by the sural nerve (SN) and deprived of their original innervation. The number and sizes of motor units and the location of endplates in these muscles were studied 1.5 to 16 months later. In the EDL muscle, the SN cross-innervated the original endplates. Very few ectopic endplates were seen, even when the nerve was implanted well outside of the original endplate area. Only 3% of the fibres were polyneuronally innervated. In the soleus muscle, however, the SN formed large numbers of ectopic endplates whether the nerve was implanted in the original endplate zone or outside of it. In addition, 20% of the muscle fibres were polyneuronally innervated. The SN cross-innervated both EDL and soleus muscles completely. There was no preference for a particular group of the SN motoneurones since all the cross-innervated muscles were innervated by all SN motor axons and the motor unit sizes of the SN were similar in the cross-innervated EDL and soleus muscles. It is concluded that intrinsic properties of a muscle determine the ability to form ectopic synapses. The distribution of the motor unit sizes is determined by the particular pool of motoneurones which innervates the muscle.     

Blockade of brain stem gap junctions increases phrenic burst frequency and reduces phrenic burst synchronization in adult rat

Recent investigations have examined the influence of gap junctional communication on generation and modulation of respiratory rhythm and inspiratory motoneuron synchronization in vitro using transverse medullary slice and en bloc brain stem-spinal cord preparations obtained from neonatal (1-5 days postnatal) mice. Gap junction proteins, however, have been identified in both neurons and glia in brain stem regions implicated in respiratory control in both neonatal and adult rodents. Here, we used an in vitro arterially perfused rat preparation to examine the role of gap junctional communication on generation and modulation of respiratory rhythm and inspiratory motoneuron synchronization in adult rodents. We recorded rhythmic inspiratory motor activity from one or both phrenic nerves before and during pharmacological blockade (i.e., uncoupling) of brain stem gap junctions using carbenoxolone (100 microM), 18alpha-glycyrrhetinic acid (25-100 microM), 18beta-glycyrrhetinic acid (25-100 microM), octanol (200-300 microM), or heptanol (200 microM). During perfusion with a gap junction uncoupling agent, we observed an increase in the frequency of phrenic bursts (~95% above baseline frequency; P < 0.001) and a decrease in peak amplitude of integrated phrenic nerve discharge (P < 0.001). The increase in frequency of phrenic bursts resulted from a decrease in both T(I) (P < 0.01) and T(E) (P < 0.01). In addition, the pattern of phrenic nerve discharge shifted from an augmenting discharge pattern to a "bell-shaped" or square-wave discharge pattern in most experiments. Spectral analyses using a fast Fourier transform (FFT) algorithm revealed a reduction in the peak power of both the 40- to 50-Hz peak (corresponding to the MFO) and 90- to 110-Hz peak (corresponding to the HFO) although spurious higher frequency activity (> or =130 Hz) was observed, suggesting an overall loss or reduction in inspiratory-phase synchronization. Although additional experiments are required to identify the specific brain stem regions and cell types (i.e., neurons, glia) mediating the observed modulations in phrenic motor output, these findings suggest that gap junction communication modulates generation of respiratory rhythm and inspiratory motoneuron synchronization in adult rodents in vitro.