An electron microscopic analysis of the left phrenic nerve in the rat

Proprioceptors of the human pericardium

In the human organism, all functions are regulated and, therefore, require a feedback mechanism. This control involves a perception of the spatial tensile state of cardiac tissues. The presence and distribution of respective proprioceptive corpuscles have not been considered so far. Therefore, a comprehensive study of the entire human fibrous pericardium was conducted to describe the presence of proprioceptors, their density, and distribution patterns. Eight human pericardial specimens gained from our body donation program were used to create a three-dimensional map of proprioceptors in the pericardium based on their histological and immunohistochemical identification. The 3D map was generated as a volume-rendered 3D model based on magnetic resonance imaging of the pericardium, to which all identified receptors were mapped. To discover a systematic pattern in receptor distribution, statistical cluster analysis was conducted using the Scikit-learn library in Python. Ruffini-like corpuscles (RLCs) were found in all pericardia and assigned to three histological receptor localizations depending on the fibrous pericardium's layering, with no other corpuscular proprioceptors identified. Cluster analysis revealed that RLCs exhibit a specific topographical arrangement. The highest receptor concentrations occur at the ventricular bulges, where their size reaches its maximum in terms of diameter, and at the perivascular pericardial turn-up. The findings suggest that the pericardium is subject to proprioceptive control. RLCs record lateral shearing between the pericardial sublayers, and their distribution pattern enables the detection of distinct dilatation of the heart. Therefore, the pericardium might have an undiscovered function as a sensor with the RLCs as its anatomical correlate.

Aging affects the number and morphological heterogeneity of rat phrenic motor neurons and phrenic motor axons

Diaphragm muscle (DIAm) motor units comprise a phrenic motor neuron (PhMN), the phrenic nerve and the muscle fibers innervated, with the size of PhMNs and axons characteristic of motor unit type. Smaller PhMNs and their axons comprise slow (type S) and fatigue-resistant (type FR) DIAm motor units, while larger PhMNs and their axons comprise more fatigable (type FF) motor units. With aging, we have shown a loss of larger PhMNs, consistent with selective atrophy of type IIx/IIb DIAm fibers and reduced maximum DIAm force. In the present study, we hypothesized that with aging there is a loss of larger myelinated phrenic α motor axons. Female and male young (6 months) and old (24 months) Fischer 344 rats were studied. PhMNs were retrogradely labeled by intrapleural injection of 488-conjugated CTB. The phrenic nerves were excised ~1 cm from the DIAm insertion and mounted in resin, and phrenic α motor axons were delineated based on size (i.e., >4 μm diameters). In older rats, the number of larger PhMNs and larger phrenic α motor axons were reduced. There were no differences in non-α axons. In addition, there was evidence of demyelination of larger phrenic α motor axons in older rats. Together, these findings are consistent with the selective age-related vulnerability of larger PhMNs and denervation of type FF motor units, which may underlie DIAm sarcopenia.

Targeting drug or gene delivery to the phrenic motoneuron pool

Phrenic motoneurons (PhrMNs) innervate diaphragm myofibers. Located in the ventral gray matter (lamina IX), PhrMNs form a column extending from approximately the third to sixth cervical spinal segment. Phrenic motor output and diaphragm activation are impaired in many neuromuscular diseases, and targeted delivery of drugs and/or genetic material to PhrMNs may have therapeutic application. Studies of phrenic motor control and/or neuroplasticity mechanisms also typically require targeting of PhrMNs with drugs, viral vectors, or tracers. The location of the phrenic motoneuron pool, however, poses a challenge. Selective PhrMN targeting is possible with molecules that move retrogradely upon uptake into phrenic axons subsequent to diaphragm or phrenic nerve delivery. However, nonspecific approaches that use intrathecal or intravenous delivery have considerably advanced the understanding of PhrMN control. New opportunities for targeted PhrMN gene expression may be possible with intersectional genetic methods. This article provides an overview of methods for targeting the phrenic motoneuron pool for studies of PhrMNs in health and disease.

The phrenic neuromuscular system

The phrenic neuromuscular system consists of the phrenic motor nucleus in the mid-cervical spinal cord, the phrenic nerve, and the diaphragm muscle. This motor system helps sustain breathing throughout life, while also contributing to posture, coughing, swallowing, and speaking. The phrenic nerve contains primarily efferent phrenic axons and afferent axons from diaphragm sensory receptors but is also a conduit for autonomic fibers. On a breath-by-breath basis, rhythmic (inspiratory) depolarization of phrenic motoneurons occurs due to excitatory bulbospinal synaptic pathways. Further, a complex propriospinal network innervates phrenic motoneurons and may serve to coordinate postural, locomotor, and respiratory movements. The phrenic neuromuscular system is impacted in a wide range of neuromuscular diseases and injuries. Contemporary research is focused on understanding how neuromuscular plasticity occurs in the phrenic neuromuscular system and using this information to optimize treatments and rehabilitation strategies to improve breathing and related behaviors.

Phrenic afferent activation modulates cardiorespiratory output in the adult rat

Phrenic afferents project to brainstem areas responsible for cardiorespiratory control and the mid-cervical spinal cord containing the phrenic motor nucleus. Our purpose was to quantify the impact of small- and large-diameter phrenic afferent activation on phrenic motor output. Anesthetized and ventilated rats received unilateral phrenic nerve stimulation while contralateral phrenic motor output and blood pressure were recorded. Twelve currents of 40-Hz inspiratory-triggered stimulation were delivered (20 s on, 5 min off) to establish current response curves. Stimulation pulse width was varied to preferentially activate large-diameter phrenic afferents (narrow pulse width) and recruit small-diameter fibers (wide pulse width). Contralateral phrenic amplitude was elevated immediately poststimulation at currents above 35 µA for wide and 70 µA for narrow pulse stimulation when compared with animals not receiving stimulation (time controls). Wide pulse width stimulation also increased phrenic burst frequency at currents ≥35 µA, caused a transient decrease in mean arterial blood pressure at currents ≥50 µA, and resulted in a small change in heart rate at 300 µA. Unilateral dorsal rhizotomy attenuated stimulation-induced cardiorespiratory responses indicating that phrenic afferent activation is required. Additional analyses compared phrenic motor amplitude with output before stimulation and showed that episodic activation of phrenic afferents with narrow pulse stimulation can induce short-term plasticity. We conclude that the activation of phrenic afferents 1) enhances contralateral phrenic motor amplitude when large-diameter afferents are activated, and 2) when small-diameter fibers are recruited, the amplitude response is associated with changes in burst frequency and cardiovascular parameters.NEW & NOTEWORTHY Acute, inspiratory-triggered stimulation of phrenic afferents increases contralateral phrenic motor amplitude in adult rats. When small-diameter afferents are recruited, the amplitude response is accompanied by an increase in phrenic burst frequency, a transient decrease in mean arterial blood pressure, and a slight increase in heart rate. Repeated episodes of large-diameter phrenic afferent activation may also be capable of inducing short-term plasticity.

The human phrenic nerve serves as a morphological conduit for autonomic nerves and innervates the caval body of the diaphragm

Communicating fibres between the phrenic nerve and sympathetic nervous system may exist, but have not been characterized histologically and immunohistochemically, even though increased sympathetic activity due to phrenic nerve stimulation for central sleep apnoea may entail morbidity and mortality. We, therefore, conducted a histological study of the phrenic nerve to establish the presence of catecholaminergic fibres throughout their course. The entire phrenic nerves of 35 formalin-fixed human cadavers were analysed morphometrically and immunohistochemically. Furthermore, the right abdominal phrenic nerve was serially sectioned and reconstructed. The phrenic nerve contained 3 ± 2 fascicles in the neck that merged to form a single fascicle in the thorax and split again into 3 ± 3 fascicles above the diaphragm. All phrenic nerves contained catecholaminergic fibres, which were distributed homogenously or present as distinct areas within a fascicle or as separate fascicles. The phrenicoabdominal branch of the right phrenic nerve is a branch of the celiac plexus and, therefore, better termed the “phrenic branch of the celiac plexus”. The wall of the inferior caval vein in the diaphragm contained longitudinal strands of myocardium and atrial natriuretic peptide-positive paraganglia (“caval bodies”) that where innervated by the right phrenic nerve.

Anatomy and physiology of phrenic afferent neurons

Large-diameter myelinated phrenic afferents discharge in phase with diaphragm contraction, and smaller diameter fibers discharge across the respiratory cycle. In this article, we review the phrenic afferent literature and highlight areas in need of further study. We conclude that 1) activation of both myelinated and nonmyelinated phrenic sensory afferents can influence respiratory motor output on a breath-by-breath basis; 2) the relative impact of phrenic afferents substantially increases with diaphragm work and fatigue; 3) activation of phrenic afferents has a powerful impact on sympathetic motor outflow, and 4) phrenic afferents contribute to diaphragm somatosensation and the conscious perception of breathing. Much remains to be learned regarding the spinal and supraspinal distribution and synaptic contacts of myelinated and nonmyelinated phrenic afferents. Similarly, very little is known regarding the potential role of phrenic afferent neurons in triggering or modulating expression of respiratory neuroplasticity.

Histological identification of phrenic afferent projections to the spinal cord

Limited data are available regarding the spinal projections of afferent fibers in the phrenic nerve. We describe a method that robustly labels phrenic afferent spinal projections in adult rats. The proximal end of the cut phrenic nerve was secured in a microtube filled with a transganglionic tracer (cholera toxin β-subunit, CT-β, or Cascade Blue) and tissues harvested 96-h later. Robust CT-β labeling occurred in C3-C5 dorsal root ganglia cell bodies and phrenic afferent projections were identified in the mid-cervical dorsal horn (laminae I-III), intermediate grey matter (laminae IV, VII) and near the central canal (laminae X). Afferent fiber labeling was reduced or absent when CT-β was delivered to the intrapleural space or directly to the hemidiaphragm. Soaking the phrenic nerve with Cascade Blue also produced robust labeling of mid-cervical dorsal root ganglia cells bodies, and primary afferent fibers were observed in spinal grey matter and dorsal white matter. Our results show that the 'nerve soak' method effectively labels both phrenic motoneurons and phrenic afferent projections, and show that primary afferents project throughout the ipsilateral mid-cervical gray matter.

Sensory neuropathy in progressive motor neuronopathy (pmn) mice is associated with defects in microtubule polymerization and axonal transport

Motor neuron diseases such as amyotrophic lateral sclerosis (ALS) are now recognized as multi-system disorders also involving various non-motor neuronal cell types. The precise extent and mechanistic basis of non-motor neuron damage in human ALS and ALS animal models remain however unclear. To address this, we here studied progressive motor neuronopathy (pmn) mice carrying a missense loss-of-function mutation in tubulin binding cofactor E (TBCE). These mice manifest a particularly aggressive form of motor axon dying back and display a microtubule loss, similar to that induced by human ALS-linked TUBA4A mutations. Using whole nerve confocal imaging of pmn × thy1.2-YFP16 fluorescent reporter mice and electron microscopy, we demonstrate axonal discontinuities, bead-like spheroids and ovoids in pmn suralis nerves indicating prominent sensory neuropathy. The axonal alterations qualitatively resemble those in phrenic motor nerves but do not culminate in the loss of myelinated fibers. We further show that the pmn mutation decreases the level of TBCE, impedes microtubule polymerization in dorsal root ganglion (DRG) neurons and causes progressive loss of microtubules in large and small caliber suralis axons. Live imaging of axonal transport using GFP-tagged tetanus toxin C-fragment (GFP-TTC) demonstrates defects in microtubule-based transport in pmn DRG neurons, providing a potential explanation for the axonal alterations in sensory nerves. This study unravels sensory neuropathy as a pathological feature of mouse pmn, and discusses the potential contribution of cytoskeletal defects to sensory neuropathy in human motor neuron disease.

An insulin based model to explain changes and interactions in human breath-holding

Until now oxygen was thought to be the leading factor of hypoxic conditions. Whereas now it appears that insulin is the key regulator of hypoxic conditions. Insulin seems to regulate the redox state of the organism and to determine the breakpoint of human breath-holding. This new hypoxia-insulin hypotheses might have major clinical relevance. Besides the clinical relevance, this hypothesis could explain, for the first time, why the training of the diaphragm, among other factors, results in an increase in breath-holding performance. Elite freedivers/apnea divers are able to reach static breath-holding times to over 6 min. Untrained persons exhibit an unpleasant feeling after more or less a minute. Breath-holding is stopped at the breakpoint. The partial oxygen pressure as well as the carbon dioxide pressure failed to directly influence the breakpoint in earlier studies. The factors that contribute to the breakpoint are still under debate. Under hypoxic conditions the organism needs more glucose, because it changes from the oxygen consuming pentose phosphate (36 ATP/glucose molecule) to the anaerobic glycolytic pathway (2ATP/glucose molecule). Hence insulin, as it promotes the absorption of glucose, is set in the center of interest regarding hypoxic conditions. This paper provides an insulin based model that could explain the changes and interactions in human breath-holding. The correlation between hypoxia and reactive oxygen species (ROS) and their influence on the sympathetic nerve system and hypoxia-inducible factor 1 alpha (HIF-1α) is dealt with. It reviews as well the direct interrelation of HIF-1α and insulin. The depression of insulin secretion through the vagus nerve activation via inspiration is discussed. Furthermore the paper describes the action of insulin on the carotid bodies and the diaphragm and therefore a possible role in respiration pattern. Freedivers that go over the breakpoint of breath-holding could exhibit seizures and thus the effect of insulin, blood glucose levels and corticosteroids in hippocampal seizures is highlighted.

Impact of unilateral denervation on transdiaphragmatic pressure

The diaphragm muscle (DIAm) has a large reserve capacity for force generation such that in rats, the transdiaphragmatic pressure (Pdi) generated during ventilatory behaviors is less than 50% of maximal Pdi (Pd(imax)) elicited by bilateral phrenic nerve stimulation. Accordingly, we hypothesized that following unilateral denervation (DNV), the ability of the contralateral DIAm to generate sufficient Pdi to accomplish ventilatory behaviors will not be compromised and normal ventilation (as determined by arterial blood gas measurements) will not be impacted, although neural drive to the DIAm increases. In contrast, we hypothesized that higher force, non-ventilatory behaviors requiring Pdi generation greater than 50% of Pd(imax) will be compromised following DIAm hemiparalysis, i.e., increased neural drive cannot fully compensate for lack of force generating capacity. Pdi generated during ventilatory behaviors (eupnea and hypoxia (10% O2)-hypercapnia (5% CO2)) did not change after DNV and arterial blood gases were unaffected by DNV. However, neural drive to the contralateral DIAm, assessed by the rate of rise of root mean squared (RMS) EMG at 75 ms after onset of inspiratory activity (RMS75), increased after DNV (p<0.05). In contrast, Pdi generated during higher force, non-ventilatory behaviors was significantly reduced after DNV (p < 0.01), while RMS75 was unchanged. These findings support our hypothesis that only non-ventilatory behaviors requiring Pdi generation greater than 50% of Pd(imax) are impacted after DNV. Clinically, these results indicate that an evaluation of DIAm weakness requires examination of Pdi across multiple motor behaviors, not just ventilation.

Functional and histopathological identification of the respiratory failure in a DMSXL transgenic mouse model of myotonic dystrophy

Acute and chronic respiratory failure is one of the major and potentially life-threatening features in individuals with myotonic dystrophy type 1 (DM1). Despite several clinical demonstrations showing respiratory problems in DM1 patients, the mechanisms are still not completely understood. This study was designed to investigate whether the DMSXL transgenic mouse model for DM1 exhibits respiratory disorders and, if so, to identify the pathological changes underlying these respiratory problems. Using pressure plethysmography, we assessed the breathing function in control mice and DMSXL mice generated after large expansions of the CTG repeat in successive generations of DM1 transgenic mice. Statistical analysis of breathing function measurements revealed a significant decrease in the most relevant respiratory parameters in DMSXL mice, indicating impaired respiratory function. Histological and morphometric analysis showed pathological changes in diaphragmatic muscle of DMSXL mice, characterized by an increase in the percentage of type I muscle fibers, the presence of central nuclei, partial denervation of end-plates (EPs) and a significant reduction in their size, shape complexity and density of acetylcholine receptors, all of which reflect a possible breakdown in communication between the diaphragmatic muscles fibers and the nerve terminals. Diaphragm muscle abnormalities were accompanied by an accumulation of mutant DMPK RNA foci in muscle fiber nuclei. Moreover, in DMSXL mice, the unmyelinated phrenic afferents are significantly lower. Also in these mice, significant neuronopathy was not detected in either cervical phrenic motor neurons or brainstem respiratory neurons. Because EPs are involved in the transmission of action potentials and the unmyelinated phrenic afferents exert a modulating influence on the respiratory drive, the pathological alterations affecting these structures might underlie the respiratory impairment detected in DMSXL mice. Understanding mechanisms of respiratory deficiency should guide pharmaceutical and clinical research towards better therapy for the respiratory deficits associated with DM1.

Contribution of the spontaneous crossed-phrenic phenomenon to inspiratory tidal volume in spontaneously breathing rats

Spinal cord hemisection at C2 (C2HS) severs bulbospinal inputs to ipsilateral phrenic motoneurons causing transient hemidiaphragm paralysis. The spontaneous crossed-phrenic phenomenon (sCPP) describes the spontaneous recovery of ipsilateral phrenic bursting following C2HS. We reasoned that the immediate (next breath) changes in tidal volume (V(T)) induced by ipsilateral phrenicotomy during spontaneous breathing would provide a quantitative measure of the contribution of the sCPP to postinjury V(T). Using this approach, we tested the hypothesis that the sCPP makes more substantial contributions to V(T) when respiratory drive is increased. Pneumotachography was used to measure V(T) in anesthetized, spontaneously breathing adult male rats at intervals following C2HS. A progressive increase in V(T) (ml/breath) occurred over an 8 wk period following C2HS during both poikilocapnic baseline breathing and hypercapnic respiratory challenge (7% inspired CO(2)). The sCPP did not impact baseline breathing at 1-3 days postinjury since V(T) was unchanged after ipsilateral phrenicotomy. However, by 2 wk post-C2HS, baseline phrenicotomy caused a 16 ± 2% decline in V(T); a comparable 16 ± 4% decline occurred at 8 wk. Contrary to our hypothesis, the phrenicotomy-induced declines in V(T) (%) during hypercapnic respiratory stimulation did not differ from the baseline response at any postinjury time point (all P > 0.11). We conclude that by 2 wk post-C2HS the sCPP makes a meaningful contribution to V(T) that is similar across different levels of respiratory drive.

Anti-GD1a antibodies activate complement and calpain to injure distal motor nodes of Ranvier in mice

The motor axonal variant of Guillain-Barré syndrome is associated with anti-GD1a immunoglobulin antibodies, which are believed to be the pathogenic factor. In previous studies we have demonstrated the motor terminal to be a vulnerable site. Here we show both in vivo and ex vivo, that nodes of Ranvier in intramuscular motor nerve bundles are also targeted by anti-GD1a antibody in a gradient-dependent manner, with greatest vulnerability at distal nodes. Complement deposition is associated with prominent nodal injury as monitored with electrophysiological recordings and fluorescence microscopy. Complete loss of nodal protein staining, including voltage-gated sodium channels and ankyrin G, occurs and is completely protected by both complement and calpain inhibition, although the latter provides no protection against electrophysiological dysfunction. In ex vivo motor and sensory nerve trunk preparations, antibody deposits are only observed in experimentally desheathed nerves, which are thereby rendered susceptible to complement-dependent morphological disruption, nodal protein loss and reduced electrical activity of the axon. These studies provide a detailed mechanism by which loss of axonal conduction can occur in a distal dominant pattern as observed in a proportion of patients with motor axonal Guillain-Barré syndrome, and also provide an explanation for the occurrence of rapid recovery from complete paralysis and electrophysiological in-excitability. The study also identifies therapeutic approaches in which nodal architecture can be preserved.

Phrenicotomy alters phrenic long-term facilitation following intermittent hypoxia in anesthetized rats

Intermittent hypoxia (IH) can induce a persistent increase in neural drive to the respiratory muscles known as long-term facilitation (LTF). LTF of phrenic inspiratory activity is often studied in anesthetized animals after phrenicotomy (PhrX), with subsequent recordings being made from the proximal stump of the phrenic nerve. However, severing afferent and efferent axons in the phrenic nerve has the potential to alter the excitability of phrenic motoneurons, which has been hypothesized to be an important determinant of phrenic LTF. Here we test the hypothesis that acute PhrX influences immediate and long-term phrenic motor responses to hypoxia. Phrenic neurograms were recorded in anesthetized, ventilated, and vagotomized adult male rats with intact phrenic nerves or bilateral PhrX. Data were obtained before (i.e., baseline), during, and after three 5-min bouts of isocapnic hypoxia. Inspiratory burst amplitude during hypoxia (%baseline) was greater in PhrX than in phrenic nerve-intact rats (P < 0.001). Similarly, burst amplitude 55 min after IH was greater in PhrX than in phrenic nerve-intact rats (175 + or - 9 vs. 126 + or - 8% baseline, P < 0.001). In separate experiments, phrenic bursting was recorded before and after PhrX in the same animal. Afferent bursting that was clearly observable in phase with lung deflation was immediately abolished by PhrX. The PhrX procedure also induced a form of facilitation as inspiratory burst amplitude was increased at 30 min post-PhrX (P = 0.01 vs. pre-PhrX). We conclude that, after PhrX, axotomy of phrenic motoneurons and, possibly, removal of phrenic afferents result in increased phrenic motoneuron excitability and enhanced LTF following IH.

Phrenic nerve diabetic neuropathy in rats: unmyelinated fibers morphometry

We have demonstrated that phrenic nerves' large myelinated fibers in streptozotocin (STZ)-induced diabetic rats show axonal atrophy, which is reversed by insulin treatment. However, studies on structural abnormalities of the small myelinated and the unmyelinated fibers in the STZ-model of neuropathy are limited. Also, structural changes in the endoneural vasculature are not clearly described in this model and require detailed study. We have undertaken morphometric studies of the phrenic nerve in insulin-treated and untreated STZ-diabetic rats and non-diabetic control animals over a 12-week period. The presence of neuropathy was assessed by means of transmission electron microscopy, and morphometry of the unmyelinated fibers was performed. The most striking finding was the morphological evidence of small myelinated fiber neuropathy due to the STZ injection, which was not protected or reversed by conventional insulin treatment. This neuropathy was clearly associated with severe damage of the endoneural vessels present on both STZ groups, besides the insulin treatment. The STZ-diabetes model is widely used to investigate experimental diabetic neuropathies, but few studies have performed a detailed assessment of either unmyelinated fibers or capillary morphology in this animal model. The present study adds useful information for further investigations on the ultrastructural basis of nerve function in diabetes.

Comparison of the motor discharge to the crural and costal diaphragm in the rat

We compared the efferent innervation of the crural and costal regions of the mammalian diaphragm with regard to axonal motor discharge patterns and conduction speeds. Recordings were obtained from single crural (233) and costal (133) phrenic motoneurones. Median conduction speeds, calculated by spike triggered averaging (13.7 ms(-1) crural and 11.8 ms(-1) costal), and frequency histograms of conduction speed were not statistically significantly different between the two populations (p=0.27: Mann-Whitney test and p=0.9: Kolmogorov-Smirnov test, respectively). There was no difference in the proportions of inspiratory, post-inspiratory or non-respiratory units encountered in the crural and costal phrenic branches. Units that lacked respiratory rhythm did not express cardiac rhythm and were insensitive to ganglion blockade. In conclusion, there were few differences noted between the two motor pools and this may be related to the fact that the rat does not differentially regulate its diaphragm during swallowing and is not an emetic species.

Progressive motor neuronopathy: a critical role of the tubulin chaperone TBCE in axonal tubulin routing from the Golgi apparatus

Axonal degeneration represents one of the earliest pathological features in motor neuron diseases. We here studied the underlying molecular mechanisms in progressive motor neuronopathy (pmn) mice mutated in the tubulin-specific chaperone TBCE. We demonstrate that TBCE is a peripheral membrane-associated protein that accumulates at the Golgi apparatus. In pmn mice, TBCE is destabilized and disappears from the Golgi apparatus of motor neurons, and microtubules are lost in distal axons. The axonal microtubule loss proceeds retrogradely in parallel with the axonal dying back process. These degenerative changes are inhibited in a dose-dependent manner by transgenic TBCE complementation that restores TBCE expression at the Golgi apparatus. In cultured motor neurons, the pmn mutation, interference RNA-mediated TBCE depletion, and brefeldin A-mediated Golgi disruption all compromise axonal tubulin routing. We conclude that motor axons critically depend on axonal tubulin routing from the Golgi apparatus, a process that involves TBCE and possibly other tubulin chaperones.

Breath-holding and its breakpoint

This article reviews the basic properties of breath-holding in humans and the possible causes of the breath at breakpoint. The simplest objective measure of breath-holding is its duration, but even this is highly variable. Breath-holding is a voluntary act, but normal subjects appear unable to breath-hold to unconsciousness. A powerful involuntary mechanism normally overrides voluntary breath-holding and causes the breath that defines the breakpoint. The occurrence of the breakpoint breath does not appear to be caused solely by a mechanism involving lung or chest shrinkage, partial pressures of blood gases or the carotid arterial chemoreceptors. This is despite the well-known properties of breath-hold duration being prolonged by large lung inflations, hyperoxia and hypocapnia and being shortened by the converse manoeuvres and by increased metabolic rate. Breath-holding has, however, two much less well-known but important properties. First, the central respiratory rhythm appears to continue throughout breath-holding. Humans cannot therefore stop their central respiratory rhythm voluntarily. Instead, they merely suppress expression of their central respiratory rhythm and voluntarily 'hold' the chest at a chosen volume, possibly assisted by some tonic diaphragm activity. Second, breath-hold duration is prolonged by bilateral paralysis of the phrenic or vagus nerves. Possibly the contribution to the breakpoint from stimulation of diaphragm muscle chemoreceptors is greater than has previously been considered. At present there is no simple explanation for the breakpoint that encompasses all these properties.

Phrenic nerve afferents elicited cord dorsum potential in the cat cervical spinal cord

BACKGROUND

The diaphragm has sensory innervation from mechanoreceptors with myelinated axons entering the spinal cord via the phrenic nerve that project to the thalamus and somatosensory cortex. It was hypothesized that phrenic nerve afferent (PnA) projection to the central nervous system is via the spinal dorsal column pathway.

RESULTS

A single N1 peak of the CDP was found in the C4 and C7 spinal segments. Three peaks (N1, N2, and N3) were found in the C5 and C6 segments. No CDP was recorded at C8 dorsal spinal cord surface in cats.

CONCLUSION

These results demonstrate PnA activation of neurons in the cervical spinal cord. Three populations of myelinated PnA (Group I, Group II, and Group III) enter the cat's cervical spinal segments that supply the phrenic nerve.

Activation of thalamic ventroposteriolateral neurons by phrenic nerve afferents in cats and rats

It has been demonstrated that phrenic nerve afferents project to somatosensory cortex, yet the sensory pathways are still poorly understood. This study investigated the neural responses in the thalamic ventroposteriolateral (VPL) nucleus after phrenic afferent stimulation in cats and rats. Activation of VPL neurons was observed after electrical stimulation of the contralateral phrenic nerve. Direct mechanical stimulation of the diaphragm also elicited increased activity in the same VPL neurons that were activated by electrical stimulation of the phrenic nerve. Some VPL neurons responded to both phrenic afferent stimulation and shoulder probing. In rats, VPL neurons activated by inspiratory occlusion also responded to stimulation on phrenic afferents. These results demonstrate that phrenic afferents can reach the VPL thalamus under physiological conditions and support the hypothesis that the thalamic VPL nucleus functions as a relay for the conduction of proprioceptive information from the diaphragm to the contralateral somatosensory cortex.

The diaphragm: two physiological muscles in one

To the respiratory physiologist or anatomist the diaphragm muscle is of course the prime mover of tidal air. However, gastrointestinal physiologists are becoming increasingly aware of the value of this muscle in helping to stop gastric contents from refluxing into the oesophagus. The diaphragm should be viewed as two distinct muscles, crural and costal, which act in synchrony throughout respiration. However, the activities of these two muscular regions can diverge during certain events such as swallowing and emesis. In addition, transient crural muscle relaxations herald the onset of spontaneous acid reflux episodes. Studying the motor control of this muscular barrier may help elucidate the mechanism of these episodes. In the rat, the phrenic nerve divides into three branches before entering the diaphragm, and it is possible to sample single neuronal activity from the crural and costal branches. This review will discuss our recent findings with regard to the type of motor axons running in the phrenic nerve of the rat. In addition, we will outline our ongoing search for homologous structures in basal vertebrate groups. In particular, the pipid frogs (e.g. the African clawed frog, Xenopus laevis) possess a muscular band around the oesophagus that appears to be homologous to the mammalian crural diaphragm. This structure does not appear to interact directly with the respiratory apparatus, and could suggest a role for this region of the diaphragm, which was not originally respiratory.

Quantitative differences between kinetic properties of Na(+) currents in postganglionic sympathetic neurones projecting to muscular and cutaneous effectors

The activity of muscular and cutaneous sympathetic neurones has been shown to be differentially regulated. The differences may partially stem from the different ionic channel expression and current kinetics in these neurones, particularly that of Na(+) channels, which play a critical role in action potential generation and modulation of neuronal excitability. The whole cell patch-clamp technique was used to compare the kinetic properties of Na(+) currents in two groups of sympathetic neurones identified by the fluorescent tracer Fast Blue: putative muscular sympathetic neurones (PMSN) and putative cutaneous sympathetic neurones (PSSN). The tracer was injected into the muscular part of the diaphragm (to mark PMSN) and into the skin of the ear (to mark PSSN). Both kinds of neurones expressed fast activating, fast inactivating, voltage dependent and TTX sensitive Na(+) currents. However, the electrical characteristics of the cells were markedly different: (1) The capacitance of PMSN (21.7 pF) was larger than PSSN (12.7 pF). Maximum current in PMSN (3.1 nA) was also larger than in PSSN (2.0 nA). Calculated current density was smaller in PMSN (148.0 pA/pF) than in PSSN (181.1 pA/pF). Slope conductance was larger in PMSN compared to PSSN (102.7 nS and 73.6 nS respectively). (2) V(1/2) of activation for PMSN (-20.9 mV) was more negative than the potential recorded for PSSN (-16.7 mV); the slope factors were not different. (3) V(1/2) for inactivation was more negative for PMSN than for PSSN (-66.3 vs. -60.8 mV); again, the slope factors for inactivation were not different. (4) The rate of recovery from inactivation could be described by the sum of two exponential functions. In PMSN the fast and slow recovery exponential factors tau(f) and tau(s) were 12.6 (66%) and 83.9 (34%) ms, while in PSSN they were shorter and equalled 8.2 (62%) and 41.9 (38%) ms, respectively. We conclude that the Na(+) currents of PMSN and PSSN have different kinetic properties.

Powerful respiratory stimulation by thin muscle afferents

The ventilatory responses to electrical stimulation of phrenic afferents were examined in spontaneously breathing dogs at different levels of sodium pentobarbital anesthesia. High intensity stimulation (activation of all the afferents, including thin fibers) increased ventilation (V(E)). The increase in V(E) was comparable to that of breathing 10% CO2 and was inversely related to anesthesia level. Under light anesthesia, V(E) increased to 282+/-36% of the control value when the phrenic nerve was stimulated at 130 times the twitch threshold (n = 15; P < 0.01). The increase in V(E) was due to increases in breathing rate (193+/-19%) and tidal volume (V(T)) (143+/-8%). On the other hand, inspiratory time (T(I)) decreased. Thus, average airflow rate (V(T)/T(I)) increased to 204+/-23%. After administration of 20 and 40% of the initial dose of pentobarbital, V(E) response was attenuated to 157+/-21 and 121+/-4%, respectively. We conclude that thin muscle afferents are capable of eliciting pronounced ventilatory stimulation. The small responses observed earlier were likely due to depth of anesthesia.

Reflex inhibition of canine inspiratory intercostals by diaphragmatic tension receptors

1. Electrical stimulation of phrenic afferent fibres in the dog elicits a reflex inhibition of efferent activity to the inspiratory intercostal muscles. However, electrical stimulation has a poor selectivity, so the sensory receptors responsible for this inhibition were not identified. 2. In the present studies, cranial forces were applied during spontaneous inspiration to the abdominal surface of the central, tendinous portion of the canine diaphragm to activate tension mechanoreceptors in the muscle. Vagal afferent inputs were eliminated by vagotomy. 3. The application of force to the central tendon caused a graded, reflex reduction in inspiratory intercostal activity, especially in external intercostal activity. This reduction was commonly associated with a decrease in inspiratory duration and was invariably attenuated after section of the cervical dorsal roots. 4. In contrast, no change in inspiratory intercostal activity was seen when high frequency mechanical vibration was applied to the central tendon to stimulate diaphragmatic muscle spindles. 5. These observations provide strong evidence that tension receptors in the diaphragm, but not muscle spindles, induce reflex inhibition of inspiratory intercostal activity. The expression of this reflex probably involves supraspinal structures.

The canine phrenic-to-intercostal reflex

1. Paralysis of the diaphragm in the dog causes a non-vagal, non-chemical increase in the activity of the inspiratory intercostal muscles. In the present studies, the hypothesis was tested that phrenic afferent fibres may elicit a reflex inhibition of inspiratory intercostal activity. 2. The electrical activity of the three groups of inspiratory intercostal muscles (parasternal intercostals, external intercostals, levator costae) was recorded in twenty vagotomized, spontaneously breathing dogs, and the proximal end of one or both C5 phrenic nerve roots was stimulated during inspiration. 3. Stimulation of the ipsilateral and contralateral C5 phrenic roots caused an immediate reduction in inspiratory intercostal activity. This reduction was abolished when phrenic stimulation was repeated after section of the C5 dorsal roots. 4. The reduction in external intercostal and levator costae activity during bilateral C5 afferent stimulation appeared when the stimulus strength was 3 times the motor threshold and it increased in magnitude when stimulus intensity was increased further. In contrast, the reduction in parasternal intercostal activity occurred only when the stimulus strength was 12 times the motor threshold. 5. These observations confirm the hypothesis that diaphragmatic receptors may reflexly inhibit efferent activity to the inspiratory intercostal muscles, in particular the external intercostals and levator costae. This inhibition appears to be primarily mediated by small myelinated fibres.

Gene therapy of murine motor neuron disease using adenoviral vectors for neurotrophic factors

Motor neuron diseases such as amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy cause progressive paralysis, often leading to premature death. Neurotrophic factors have been suggested as therapeutic agents for motor neuron diseases, but their clinical use as injected recombinant protein was limited by toxicity and/or poor bioavailability. We demonstrate here that adenovirus-mediated gene transfer of neurotrophin-3 (NT-3) can produce substantial therapeutic effects in the mouse mutant pmn (progressive motor neuronopathy). After intramuscular injection of the NT-3 adenoviral vector, pmn mice showed a 50% increase in life span, reduced loss of motor axons and improved neuromuscular function as assessed by electromyography. These results were further improved by coinjecting an adenoviral vector coding for ciliary neurotrophic factor. Therefore, adenovirus-mediated gene transfer of neurotrophic factors offers new prospects for the treatment of motor neuron diseases.

Phrenic afferent input excites C1-C2 spinal neurons in rats

Effects of electrical stimulation of the ipsilateral phrenic nerve above the heart were determined on cells in the C1-C2 spinal cord segments of 27 rats anesthetized with pentobarbital. Forty-five cells that responded to this stimulus were included in this study. These cells then were examined at the same stimulus parameters for effects of stimulating the ipsilateral phrenic nerve below the heart, the contralateral phrenic nerve above the heart, and/or the left (ipsilateral) cervical vagus nerve. Ipsilateral phrenic nerve stimulation below the heart had no effect on 20 of 20 cells tested. Seven of 16 cells tested for effects of contralateral phrenic nerve stimulation above the heart were excited and activity of 9 cells was unaffected. Activity changes and activation latencies were not significantly different in the 7 cells excited by both ipsilateral and contralateral phrenic nerve stimulation. Thirty-seven of 45 cells excited by ipsilateral phrenic nerve stimulation also were excited by stimulation of the left cervical (ipsilateral) vagus nerve. The mean increase in cell activity was significantly greater following vagal nerve stimulation compared to phrenic nerve stimulation, and mean activation latency was significantly longer for vagal stimulation. Excitatory receptive somatic fields were classified for 35 cells. Somatic field locations for most cells (80%) included the ipsilateral neck and jaw. Activity of 26 cells was increased by both noxious pinch and brushing the hair, activity of 6 cells was increased only by noxious pinch, and activity of 3 cells was increased only by brushing the hair. Results of this study indicate that there are afferent fibers in the phrenic nerve above the heart, but not below the heart, that excite cells in the C1-C2 segments of the spinal cord. Most cells also were excited by noxious stimuli applied to their somatic receptive fields. Thus, the phrenic nerve may provide a pathway for referral of pain to the neck and jaw from thoracic structures.

Functional classification of afferent phrenic nerve fibres and diaphragmatic receptors in cats

1. Single afferent fibres with receptive fields in the diaphragm (272 units) dissected from the right phrenic nerve were classified according to the following properties: reaction to contraction of the diaphragm, resting activity, conduction velocity, location and properties of receptive fields, and reaction to injection of bradykinin and lactic acid into the internal thoracic artery. Nine additional fibres dissected from the phrenic nerve had receptive fields outside the diaphragm. The experiments were performed on chloralose-anaesthetized cats. 2. Ninety-six fibres (36%) had high resting activity when unloaded by contraction of the diaphragm, had low-threshold receptive fields in the muscle and were mostly group II and III fibres. They probably innervated muscle spindles. 3. Eighty-eight fibres (32%) were vigorously activated by contraction of the diaphragm. They had low-threshold receptive fields located in the musculotendinous border and central tendon. Their conduction velocity was in the range for group II and III fibres. We infer that they may innervate tendon organs. 4. Eighty-eight fibres (32%) were slightly affected or not affected by diaphragmatic contraction. They had low- and high-threshold receptive fields located mostly in the muscular part of the diaphragm, and negligible resting activity. Most of them were group III and IV afferent fibres and were activated when bradykinin and lactic acid were applied to their receptive fields. Possibly these low- and high-threshold receptors innervated diaphragmatic ergo- and nociceptors, respectively. 5. Sensory outflow from the diaphragm was found to be somatotopically organized, so that fibres with receptive fields in the sternocostal portion were predominantly located in the upper phrenic nerve root, and those with lumbar receptive fields were in the lower root. 6. It is concluded that the phrenic nerve contains fibres from several distinct classes of sensory receptors: muscle spindles, tendon organs, ergoceptors and nociceptors. The sensory diaphragmatic outflow to the spinal cord is somatotopically organized.

Shoulder region of the rat: anatomy and fiber composition of some suprascapular nerve branches

BACKGROUND

The pathophysiology of chronic supraspinatus tendinitis is not fully understood. This may be due to the scarcity of experimental studies on this issue.

METHODS

In search for a system suitable for experimental analysis, the present study describes the relevant gross anatomy of the rat shoulder region (dissection), and examines the fiber composition of relevant supra-scapular nerve branches (electron microscopy, selective denervations).

RESULTS

The rat shoulder region is similar to the human shoulder in terms of gross anatomy. The average suprascapular nerve (SSC) is derived mainly from the spinal cord segment C5 and contains 3,435 axons, 74% of which are unmyelinated. The supraspinatus branch (SSP) contains 627 fibers. Of the SSP fibers, 52% are myelinated, including 32% motor and 20% sensory axons. Of the C-fibers in the SSP 16% are sympathetic efferents and 32% are sensory. Many of the latter disappear after neonatal capsaicin treatment. The SSC emits a subacromial articular branch (ART), with some 260 axons, about 90% of which are unmyelinated. The myelinated ART fibers are sensory, and of the unmyelinated ones about 24% are sympathetic efferents and 66% are afferents. The latter resist neonatal capsaicin treatment.

CONCLUSIONS

In view of the anatomy of the supraspinatus muscle, of the subacromial space, and of relevant nerves, the rat shoulder should be appropriate for experimental studies on inflammatory conditions in the subacromial space.

Thin-fiber phrenic afferents mediate the ventilatory response to diaphragmatic ischemia

We assessed the role of groups III and IV phrenic afferents in the ventilatory response to diaphragmatic ischemia in mechanically ventilated, chloralose-anaesthetized dogs using the in-situ isolated and innervated left hemidiaphragm preparation. The inspiratory motor drive to the right (Rt Edi) and left (Lt Edi) diaphragms, parasternal (Eps), and alae nasi (Ean) muscles was measured from the peak integrated EMG activities. When left diaphragmatic ischemia was produced in the control group (n = 6) by occluding the left phrenic artery for 20 min, LtEdi increased to 158%, RtEdi to 160%, Eps to 150% and Ean to 135% of baseline values. Left diaphragmatic tension, however, remained unchanged during the ischemia period. In the capsaicin-treated group (n = 6), we injected repeated doses of capsaicin, a selective stimulant of groups III and IV afferents, into the left phrenic artery to eliminate inputs from these afferents. Repeated injections of capsaicin are known to induce prolonged periods of afferent dysfunction. The first two injections of capsaicin (1 mg each) produced transient activation of the inspiratory muscles and higher breathing frequencies. Subsequent injections, however, failed to elicit any ventilatory changes. When diaphragmatic ischemia was induced after the last injection of capsaicin, no changes in the Right Edi, Eps and Ean were observed, whereas Left Edi and left diaphragmatic tension declined significantly. We conclude that increased inspiratory motor drive during selective diaphragmatic ischemia is mediated through the activation of groups III and IV phrenic afferents.

Characteristics of sympathetic reflexes evoked by electrical stimulation of phrenic nerve afferents

In chloralose-anaesthetized cats, sympathetic reflex responses were recorded in left cardiac and renal nerve during stimulation of afferent fibres in the ipsilateral phrenic nerve. In cardiac nerve, a late reflex potential with a mean onset latency of 75.6 +/- 13.8 ms was regularly recorded which, in 20% of the experiments, was preceded by an early, very small reflex component (latency between 35 and 52 ms). In contrast, in renal nerve only a single reflex component after a mean latency of 122.1 +/- 13.1 ms was observed. Bilateral microinjections of the GABA-agonist muscimol into the rostral ventrolateral medulla oblongata resulted in a nearly complete abolition of sympathetic background activity and in an 88% reduction of the late reflex amplitude with only small effects on the latency of the evoked potentials. Under this condition, an early reflex component was never observed to appear. After subsequent high cervical spinalization, the residual small potentials which persisted after bilateral muscimol injections were completely abolished and in cardiac nerve an early reflex potential with a mean latency of 45 +/- 10 ms was observed in all but one experiment. The early reflex was therefore referred to as a spinal reflex component which, however, is suppressed in most animals with an intact neuraxis. In the renal nerve a spinal response was only observed in one experiment after spinalization. The results suggest that sympathetic reflexes evoked by stimulation of phrenic nerve afferent fibres possess similar spinal and supraspinal pathways as previously described for somato-sympathetic and viscero-sympathetic reflexes.(ABSTRACT TRUNCATED AT 250 WORDS)

Postnatal maturation of phrenic nerve in children

The phrenic nerve at the pericardial level was examined postmortem in 17 children, ages 3 days to 8 years. Detailed macroscopic and histologic examination of the central nervous system in all patients disclosed no abnormalities. Quantitative developmental studies demonstrated that myelinated axons doubled in number from birth to age 1 year when a plateau was reached. The main period of growth in diameter of myelinated axons also corresponded to the first year when median diameters increased from 1.75 microns at 3 days of age to 3.0 microns by 8 months of age. Unmyelinated axons also grew significantly in the first 11 months when median diameters reached 1.4 microns. There was no significant increase in axonal diameter at later ages. The slope of the regression line for the number of myelin lamellae on axonal diameters increased with age until 6 months of age, whereas the dispersion around the regression lines decreased in the same period. This finding suggests a direct relationship between myelination and axonal growth. Significant maturation of the phrenic nerve occurs during the first year of life.

Myelin-axon relationships in the rat phrenic nerve: longitudinal variation and lateral asymmetry

It is known that the myelin sheath thickness-axon perimeter relationship varies between peripheral nerves. This study examines the possibility that that relationship may vary between levels along a given nerve or between corresponding levels of the right and left examples of the same nerve. The relationship is examined for large and small fibre classes at well separated upper and lower intrathoracic levels in the rat phrenic nerve. The study shows that the myelin-axon relationship differs between levels along the same nerve bundle in the same (intrathoracic) environment. Thus, for a given increase in the perimeter of large axons, sheath thickness increases significantly more at lower than at upper levels. In addition, myelin sheath thickness shows a statistically significant lateral asymmetry in favour of the left side for the large fibre class at the upper thoracic level. The setting of the myelin sheath thickness-axon perimeter relationship also differs between the large and small fibre classes at each level examined. Large fibres have proportionately thicker sheaths than small fibres and this difference is reflected in the significantly smaller g-ratio of the former. Systematic differences in the setting of the myelin sheath thickness-axon perimeter relationship between large and small fibre classes may be a widely occurring phenomenon. It may be concluded that the myelin-axon relationship varies significantly both within and between nerves and also between fibre classes. Accordingly, morphometric studies of normal or pathological nerves should take into account possible consistent longitudinal variation or lateral asymmetry in fibre parameters and myelin-axon relationships within a given nerve bundle or fibre class, in order to avoid introducing systematic bias and to minimize variance between samples.

Properties of postganglionic sympathetic neurons with axons in phrenic nerve

The aim of the study was to test the reflex and resting properties of postganglionic sympathetic neurons with axons located in the right phrenic nerve. The experiments have been performed on chloralose-anesthetized cats with both vago-aortic nerves cut. The somata or the postganglionic sympathetic neurons were located in the stellate ganglion. Axons of these neurons passed through the upper and lower phrenic nerve roots and through the phrenic nerve itself. The presence of cardiac and respiratory rhythmicities was detected in the activity of the phrenic postganglionic sympathetic neurons. Hyperventilation, which abolished burst discharges of the phrenic nerve, decreased the sympathetic activity by 14%. Systemic hypoxia (ventilating the animals for 2 min with 8% O2 in N2) increased the sympathetic activity threefold. The results of our experiments suggest that axons of the sympathetic neurons located in the right phrenic nerve could possibly be diaphragmatic muscle vasoconstrictors.

Phrenic motoneuron morphology in the neonatal rat

The morphology of neonatal rat phrenic motoneurons was studied following retrograde labeling with horseradish peroxidase, which resulted in Golgi-like fills of phrenic motoneuron somata and dendrites. At birth, these neurons have well-developed dendritic trees with many characteristics described for phrenic motoneurons in the adult rat. The dendrites form tightly fasciculated bundles that emerge from the phrenic nucleus primarily along four axes: ventromedial, ventrolateral, dorsolateral, and rostral/caudal, with smaller and more variable projections directly lateral and ventral. Although sparse, some dendritic appendages were also present, and in a few animals, somata clustering was apparent. The most significant difference between adult and neonatal rat phrenic motoneurons is in the extent to which medially and laterally projecting dendrites extend beyond the borders of the ipsilateral gray matter. In the neonate, unlike the adult, these dendrites project extensively past the gray/white border to the edge of the hemicord. Ventromedial dendrites occasionally cross to the contralateral ventral horn in the ventral white commissure and laterally projecting dendrites could be seen reaching the edge of the cord, turning and traveling rostrally or caudally for up to 100 microns. Phrenic motoneurons are not unique in having long dendrites at birth. A brief comparative study showed that neonatal cervical, thoracic, and lumbar motoneurons also have long dendrites that project to the medial and lateral borders of the hemicord.

The cat cervical dorsal root ganglia: general cell-size characteristics and comparative study of neck muscle, neck cutaneous and phrenic afferents

Sizes of neuronal somata in the cat cervical dorsal root ganglia were determined at different levels (C1-C8). The average value and class distribution of mean cell diameter were analyzed. The ganglia from C1 to C5 could be clearly distinguished from those at levels of brachial plexus afferents (C6-C8) with respect to cell size range, distribution and average. The size distribution, most often limited to 70 microns from C1 to C5, skewed to more than 90 microns from C6 to C8. Cells in the 35-50 microns range of diameter constituted the main portion of the cell population (49-52%) at the C1-C5 levels, whereas from C6 to C8 51-77% of the ganglion cell bodies were more than 50 microns in diameter. The cell size distribution of afferents projecting from C1 to C5 and supplying different muscle or cutaneous targets was studied following retrograde labeling with horseradish peroxidase conjugated to wheatgerm agglutinin. Sizes of cell bodies of biventer cervicis (postural muscle), phrenic (purely respiratory muscle) and cutaneous afferents were all similar. The labelled cell bodies were in the majority (51-64%) less than 35 microns in diameter and ranged towards smaller diameters than counterstained cells in the corresponding ganglia. In spite of similarities in cell size distribution it was estimated from the fiber caliber spectra of the labelled afferents that both unmyelinated and myelinated cutaneous afferents originate from larger cell bodies than muscle afferents in the same diameter range.

Localization of substance P immunoreactivity in phrenic primary afferent neurons

Retrograde tracing with a fluorescent dye (Fast Blue) combined with immunohistochemistry was used to localize the putative neurotransmitter, substance P, in phrenic primary afferent neurons. Fast Blue was injected into the diaphragm and was found to label phrenic primary afferent neurons in sections from the fifth and sixth cervical dorsal root ganglia. The same sections were then treated with antiserum to substance P. A total of 11.4% of labelled phrenic primary afferent neurons contained substance P immunoreactivity. The diameters of the neurons ranged between 17 to 45 microns with a mean size of 29.7 +/- 0.7 microns (N = 81). The results suggest that substance P could be involved in mediating the transmission of sensory information from the diaphragm to the CNS.

Analysis of reflex activity in cardiac sympathetic nerve induced by myelinated phrenic nerve afferents

Electrical stimulation of the phrenic nerve afferents evoked excitatory responses in the right inferior cardiac sympathetic nerve in chloralose-anaesthetized cats. The reflex was recorded in intact and spinal cats. The latency and threshold of the volley recorded from the phrenic nerve as well as of the cord dorsum potentials evoked by electrical stimulation of the phrenic nerve indicated that group III afferents were responsible for this reflex. The phrenicocardiac sympathetic reflex recorded in intact cats was followed by a silent period. The maximum amplitude of the reflex discharges was 800 microV, the latency was 83 ms and the central transmission time 53 ms. Duration of the silent period lasted up to 0.83 s. In spinal cats the reflex was recorded 5.5-8 h after spinalization. The maximum amplitude of the spinal reflex discharges ranged from 22 to 91 microV and the latency from 36 to 66 ms.

Characteristic variations of relative myelin sheath thickness in 11 nerves of the rat

Computer-assisted measurements of relative myelin sheath thickness (the g ratio) were made in 11 peripheral nerves of the rat. The scatter diagrams showed nerve-specific variations in the distribution of relative myelin sheath thickness. Myelinated fibers of less than 3.5 microns axon diameter had relatively thin myelin sheaths, particularly in the splanchnic, vagus and glossopharyngeal nerves. The oculomotor nerve had two fiber populations clearly set apart in terms of relative myelin sheath thickness. Thickly myelinated fibers were found in facial and hypoglossal nerves. No single functional modality was evident for the thinly myelinated fibers.

Origin and distribution of phrenic primary afferent nerve fibers in the spinal cord of the adult rat

Previous studies from this laboratory have localized and morphologically characterized phrenic motor neurons in the rat spinal cord at light and electron microscopic levels. The present investigation used a modification of the TMB method for the retrograde transport of horseradish peroxidase (HRP) to describe at light microscope levels the origin and distribution of phrenic primary afferent axons in the adult rat spinal cord. Dry HRP crystals were applied to the central stump of the transected phrenic nerve in the neck to label spinal ganglion cell bodies and thus determine the levels of origin of afferent axons in the phrenic nerve. Camera lucida drawings were then made from serial sections through the appropriate spinal cord levels to determine the specific distribution of transganglionically labeled phrenic central axonal processes within the spinal cord. HRP-labeled phrenic primary neurons were observed in the C3 to C7 spinal ganglia. The camera lucida studies indicated that the transganglionically labeled central processes of phrenic primary afferent axons distributed into the dorsal horn at the C4 and C5 levels of the spinal cord. Furthermore, central processes distributing to C5 were more numerous than those that distributed to C4. Afferent axons were never seen in the dorsal horn at C3, C6, or C7. As spinal ganglion cells were labeled at C3 above and C6 and C7 below, it follows that central processes of phrenic afferent fibers descend and ascend in the dorsal columns of the spinal cord before distributing into the dorsal horn. Specifically, the labeled primary afferent axons and their collateral branches were found in the fasciculus cuneatus, and in laminae I, II, III, and IV of the dorsomedial aspect of the dorsal horn. The function of these central axonal processes is unknown, but based on a comparison of our morphologic data with previous physiological and anatomical studies, we suggest that phrenic afferent fibers may arise from proprioceptors (muscle spindles and Golgi tendon organs), nociceptors, or rapidly adapting mechanoreceptors (Pacinian corpuscles) within the diaphragm.

Unmyelinated axons in a muscle nerve. Electron microscopic morphometry of the sternomastoid nerve in normal and sympathectomized rats

In continuation of earlier studies on the innervation of the sternomastoid muscle of the rat, a detailed morphometric analysis was performed on the unmyelinated axons of the nerve, in normal rats and after extensive cervical sympathectomy. In 4 normal rats an average of 314 myelinated and 319 unmyelinated axons were present. 42 days after surgery, the 3 animals showed Horner's syndrome and a highly significant 40% loss of unmyelinated axons. We therefore suggest that 40% of the C-fibers in this nerve are postganglionic sympathetic efferents and that the remaining 60% are type IV fibers, i.e., unmyelinated afferents. Our counts also indicate that part of the Remak bundles of the Schwann cells contain only sympathetic axons, whereas others contain mixed groups of sympathetic and afferent axons. Myelinated nerve fibers were not lost due to sympathectomy. Unexpectedly, the 3 animals analyzed 7-13 days after surgery showed Horner's syndrome but only a 16% loss of unmyelinated axons, which was not even statistically significant. Morphological signs of degeneration and sprouting did not provide any clue, but a possible explanation would be that a transitory sprouting of the remaining afferent C-fibers or Schwann cells occurred.

Degeneration patterns of postganglionic fibers following sympathectomy

In cats the time course of degeneration following lumbal sympathectomy was studied in the ramus communicans griseus (rcg) and in the nerves to the triceps surae muscle using light and electron microscopic methods. The left lumbar sympathetic trunk including its rami communicantes was removed from L2 to S1 using a lateral approach. The animals were sacrificed between 2 and 48 days after the sympathectomy. Tissue samples were taken (a) one cm proximal to the entrance of the rcg into the spinal nerve, and (b) one cm proximal to the entrance of the nerve into the muscle belly. In the rcg signs of degeneration can already be recognized in the myelinated as well as in the unmyelinated axons 48 h after sympathectomy. The degenerative processes in the axons reach their peak activity at about 4 days p.o. They end a week later. Signs of the reactions of the Schwann cells and of the endoneural cells can first be seen 2 days p.o. They are most pronounced around the 8th day p.o., and last at least up to the third week. Thereafter the cicatrization processes settled to a rather steady state (total observation period 7 weeks). In the muscle nerves the first signs of an axonal degeneration of the sympathetic fibers can be recognized 4 days after surgery. The signs of axonal degeneration are most striking about 8 days p.o. They have more or less disappeared another week later. The reactions of the Schwann cells also start on the fourth day but outlast the degenerative processes by some 8 days. Thus the degenerative and reactive processes in the rcg precede those in the muscle nerves by 2 days early after surgery and by 6 days 3 weeks later. Seven weeks after surgery, fragments of folded basement lamella and Remak bundles with condensed cytoplasm and numerous flat processes are persisting signs of the degeneration. In addition to the differences in time course between the proximal and the distal site of observation, it was also noted that both the axonal degeneration and the reactions of the Schwann cells are more pronounced in the rcg than in the muscle nerve. For example there was abundant mitotic activity in the central endoneural and Schwann cells whereas we could not detect such activity in the periphery.(ABSTRACT TRUNCATED AT 400 WORDS)

Relations between axon length and axon caliber. "Is maximum conduction velocity the factor controlling the evolution of nerve structure"?

A search was made for any existent relationship between the length of a nerve fiber and the caliber of its axon. This was done in the hope of defining morphological parameters useful for assessing conduction time. Four fiber populations were examined: (1) phrenic fibers in rat and rabbit during different phases of body growth; (2) phrenic fibers of mature animals of greatly different body size including mouse and cow; (3) rat intercostal nerves which vary in length by a factor exceeding 5 due to the funnel-shape of the thorax; and (4) ventral root fibers of the cow. In all of these fiber populations, there was no evidence for a direct relationship between the length of a fiber and its caliber. Rather, a tendency was noted for fiber caliber to approach certain ceilings independent of length. These data, seen in conjunction with other information on fiber structure, cast serious doubt on the widely accepted concept that maximum conduction velocity is the factor controlling nerve structure. A much more likely factor controlling the structure of myelinated nerve fibers is the capacity to modulate information by frequency coding of impulses.

Unmyelinated axon ratios in cat motor, cutaneous and articular nerves

The goal of this electron microscopic study is to determine ratios of unmyelinated to myelinated axons and afferent unmyelinated to postganglionic autonomic (sympathetic) efferent axons in representative cat motor, cutaneous and articular nerves. Of the total axons in the tibial, medial gastrocnemius, sural and medial and posterior articular nerves, 70% or more are unmyelinated. The afferent unmyelinated axon components range from 49% and 45% in the medial and posterior articular nerves, respectively, to 58% in the medial gastrocnemius nerve and 61% in the sural nerve.

Afferent and efferent axons in the medial and posterior articular nerves of the cat

The goal of this study is to determine the average numbers of afferent axons and postganglionic autonomic (sympathetic) efferent axons supplying the cat knee joint through the medial and posterior articular nerves. Interestingly, both nerves are composed primarily of unmyelinated axons. Only 20% of the axons in the medial articular nerve are myelinated, with the overwhelming majority, 80%, being unmyelinated. The posterior articular nerve has 78% unmyelinated and 22% myelinated axons. Neither nerve contains ventral root efferent axons. The sympathetic chain, in both nerves, contributes no myelinated and only 50% of the unmyelinated axons. The medial and posterior articular nerves are therefore predominantly afferent, since all myelinated and the remaining 50% of the unmyelinated axons arise from the dorsal root ganglion cell. The ratio of afferent unmyelinated to myelinated axons is 2:1. The roles of these afferent unmyelinated axons must now be considered in regard to joint kinesthetics and pain.