The identification of brainstem neurones projecting to thoracic respiratory motoneurones in the cat as demonstrated by retrograde transport of HRP

Brainstem neurones which project to the immediate vicinity of the spinal motoneurones which supply the intercostal and abdominal respiratory muscles were identified by means of the retrograde transport of horseradish peroxidase (HRP). A combined electrophysiological and histological technique was used in which recording of phasic inspiratory or expiratory motoneurone activity within upper (T3-T4) or lower (T8-T9) thoracic segments was followed by the ion-tophoretic injection of HRP at these recording sites. HRP labelled cells were concentrated in those brainstem regions known to contain phasic respiratory neurones, namely the ventrolateral nucleus of the solitary tract (vl-NTS) or dorsal respiratory group (DRG), the ambiguus complex or ventral respiratory group (VRG) and the parabrachial pontine (PB) nuclei. In 18 cats, 248 cells were labelled in these three respiratory regions of the brainstem while 668 were much more diffusely distributed in other regions of the medulla and pons. The ipsilateral and contralateral contributions within the respiratory regions were respectively; 23%:77% (DRG), 33%:67% (VRG), 95%:5% (PB). These results are considered in the general context of previous electrophysiological and histological findings, but also with particular reference to a related study of the projections from brainstem neurones to the phrenic nucleus [32].     

Brainstem projections to the phrenic nucleus: An anterograde and retrograde HRP study in the rabbit

Brainstem projections to the phrenic nucleus were studied in rabbits using horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP) as a retrograde and anterograde neuronal tracer. Injections of 1% WGA-HRP were centered in the phrenic nucleus in the C4-C5 ventral horn in 4 rabbits to identify pontomedullary nuclear groups that contain neurons projecting to the midcervical spinal cord. Regions of the rabbit brainstem that are homologous to the ventral respiratory group (VRG), dorsal respiratory group (DRG), B?tzinger Complex (B?tC) and K?lliker-Fuse nucleus in the cat and rat were shown to provide the major pontomedullary projections to the phrenic nucleus. Injections of WGA-HRP into physiologically identified locations within DRG, VRG and B?tC anterogradely labelled bulbospinal axons of these groups. These injections produced presumptive terminal labelling in the C4-C5 ventral horn in the region containing the phrenic cell column and the transverse phrenic motoneuron dendrite bundles as defined by WGA-HRP labelling of phrenic motoneurons. These results indicate: 1) The presumptive excitatory (DRG, VRG) and inhibitory (B?tC) bulbospinal control of phrenic motoneurons arise from the same medullary respiratory groups in the rabbit as in the cat and rat. 2) The bulbospinal control of phrenic motoneurons is primarily via direct projections to the phrenic motor nucleus, and not through segmental propriospinal interneurons. 3) As in the rat, the bulbospinal contribution of the DRG is less pronounced in the rabbit than in the cat. 4) The rabbit and rat have a slight ipsilateral predominance in their bulbospinal projections to phrenic nucleus; whereas these projections have a contralateral predominance in the cat.     

The source of the respiratory drive to nasolabialis motoneurones in the rabbit; a HRP study

Lower brainstem projections to the motoneurones of the nasolabialis muscles, which show rhythmic respiratory-phased activity were studied in the rabbit using the horseradish peroxidase (HRP) technique. The nasolabial motoneurone pool was first identified by the retrograde transport of HRP injected intramuscularly, and by antidromic stimulation and microelectrode recording techniques. The results from subsequent iontophoretic injection of HRP into the lateral division of the facial nucleus (the nasolabial pool) produced significant ipsilateral labelling in the nucleus ambiguus-retroambigualis (NA-NRA) complex. Labelled cells, predominantly ipsilateral, were also consistently observed in the parvocellular reticular nucleus. Smaller numbers of labelled cells were identified in the ventral, dorsal and gigantocellular nuclei of the reticular formation on both sides of the medulla. A large proportion of HRP-labelled cells of the NRA was located in the caudal medulla where the presence of propriobulbar and bulbospinal respiratory neurones has been well documented. These results suggest that some neurones of the NA-NRA complex may serve as upper respiratory motoneurones to the nasolabial musculature.     

The source of the respiratory drive to nasolabialis motoneurons in the rabbit; a HRP study

Lower brainstem projections to the motoneurons of the nasolabialis muscles, which show rhythmic respiratory-phased activity were studied in the rabbit using the horseradish peroxidase (HRP) technique.The nasolabial motoneurone pool was first identified by the retrograde transport of HRP injected intramuscularly, and by antidromic stimulation and microelectrode recording techniques. The results from subsequent iontophoretic injection of HRP into the lateral division of the facial nucleus (the nasolabial pool) produced significant ipsilateral labelling in the nucleus ambiguus-retroambigualis (NA-NRA) complex. Labelled cells, predominantly ipsilateral, were also consistently observed in the parvocellular reticular nucleus. Smaller numbers of labelled cells were identified in the ventral, dorsal and gigantocellular nuclei of the reticular formation on both sides of the medulla. A large proportion of HRP-labelled cells of the NRA was located in the caudal medulla where the presence of propriobulbar and bulbospinal respiratory neurones has been well documented.These results suggest that some neurones of the NA-NRA complex may serve as upper respiratory motoneurones to the nasolabial musculature.     

Efferent projections of inspiratory neurons of the ventral respiratory group. A dual labeling study in the rat

The efferent projections of the medullary respiratory neurons of the rat were studied using an anterograde tracer, Phaseolus vulgaris leucoagglutinin (PHA-L). In Nembutal-anesthetized rats, PHA-L was iontophoretically applied to (1) the area of inspiratory neurons of the ventral respiratory group (VRG) around the nucleus ambiguus, or (2) the area ventrolateral to the solitary tract. In addition, a fluorescence retrograde tracer, Fast blue (FB), was injected into the cervical phrenic nerve several days after the PHA-L injection. When PHA-L was injected into the area of predominantly inspiratory neurons of VRG, dense PHA-L-labeled axons were observed bilaterally in the spinal cord: the ipsilateral projections were noticeably denser than the contralateral ones. Fine axonal branches were distributed around a column of the phrenic motoneurons and boutons were observed on the somata of the FB-labeled motoneurons, suggesting monosynaptic connections between VRG inspiratory neurons and phrenic motoneurons. On the other hand, when PHA-L was injected into the area ventrolateral to the solitary tract, only a few descending axons to the spinal cord were seen bilaterally. No contacts between the PHA-L-labeled axons and the FB-labeled phrenic motoneurons were observed. The brainstem projections of the VRG were found bilaterally in the nuclei ambigui, Cajal's interstitial nuclei of the solitary nucleus, the solitary nuclei, the hypoglossal nuclei, the K?lliker-Fuse's nuclei, and the subcoeruleus areas.     

Serotonin immunoreactive boutons form close appositions with respiratory neurons of the dorsal respiratory group in the cat

The aim of this study was to examine the location of serotonin immunoreactive boutons on both the soma and dendrites of neurons in the dorsal respiratory group by using a combination of intracellular recording and labelling and immunohistochemistry. Inspiratory neurons in the ventrolateral nucleus of the solitary tract (vl-NTS) were intracellularly labelled with horseradish peroxidase (HRP) in anaesthetised adult cats. The morphology of 23 neurons, all antidromically activated from the contralateral C3 spinal segment, was examined. Six neurons displayed pronounced dendritic arborizations outside the vl-NTS, with prominent dorsal and/or medial projections. The dendrites of the remaining neurons were almost entirely confined to the vl-NTS. Intramedullary axon collaterals were detected in four of nineteen examined axons. Serotoninergic fibres were immunohistochemically demonstrated in the NTS, and the apposition of immunoreactive boutons to the HRP-filled neurons examined at the light microscopic level. Boutons were identified in close apposition with the somata, proximal and distal dendrites of these neurons. However, the density of contacts was found to be substantially less than in a previous study of phrenic motoneurons (Lipski et al: Soc. Neurosci. Abst. 14:379, '88; Pilowsky et al: J. Neurosci. in press, '90). The relative paucity of contacts of serotonin immunoreactive boutons with premotor inspiratory neurons of the dorsal respiratory group indicates that the serotoninergic system affects respiratory pathways mainly at the level of respiratory motoneurons or at brainstem sites outside the vl-NTS.     

Cells of origin of corticospinal projections to phrenic and thoracic respiratory motoneurones in the cat as shown by retrograde transport of HRP

A combined electrophysiological and histological approach was employed to identify neurones within the motor cortex which project to the vicinity of spinal respiratory motoneurones, and which may be involved in the alteration of the pattern of breathing under certain conditions. Recording of respiratory phased activity from phrenic, or from thoracic motoneurones within either the upper (T3-4) or lower (T8-9) segments, was followed by the iontophoretic injection of HRP at these recording sites. After injections within the cervical or thoracic ventral horn, 219 cells were retrogradely labelled in 14 experiments. The majority of these cells (88%) were labelled contralateral to the injection site. Following the injection of HRP into the phrenic nucleus, labelling was observed at two major sites within the anterior sigmoid gyrus (ASG), one along the anterolateral edge of the cruciate sulcus, and the other along the ventrolateral border of the ASG. In contrast, cells labelled after injections into the thoracic ventral grey matter were located more medially within the ASG and the posterior sigmoid gyrus (PSG). The populations of cells labelled following phrenic and thoracic injections overlapped, primarily at the lateral edge of the cruciate sulcus. The somas of labelled cells were pyramidal, round or oval. The mean diameters of cortical cells labelled after injections into the lower or upper thoracic segments were 30.5 +/- 6.2 and 31.5 +/- 5.6 respectively, which were not significantly different in size. However, they were significantly larger than the mean diameter of the cells labelled from injections into the phrenic nucleus (22.7 +/- 4.2 micron).(ABSTRACT TRUNCATED AT 250 WORDS)     

Respiratory neurons in the medulla of the rabbit: distribution, discharge patterns and spinal projections

To determine distribution, discharge patterns and the spinal projections of medullary respiratory neurons (RNs), a systematic mapping of 806 RNs was made in the medulla of anesthetized rabbits. In disagreement with previous reports that there are no discrete medullary respiratory neuronal groups in rabbits, two neuronal groups were identified: (1) dorsal respiratory group (DRG), associated with the nucleus tractus solitarius; and (2) ventral respiratory group (VRG), associated with the nucleus ambiguus compact formation. The density of RNs in the DRG was much lower than that in the VRG. In the VRG, 3 subdivisions of RN populations were found: predominantly expiratory neurons in the caudal and the rostral parts, and mainly inspiratory neurons in the intermediate region. Nine distinct types of RNs were classified on the basis of firing patterns. Nearly all types were found in both the DRG and each VRG subdivision. Antidromic mapping of 64 VRG neurons revealed that 67% projected to the spinal cord. Expiratory bulbospinal neurons in the rostral subdivision of the VRG projected only to the cervical cord (mainly ipsilaterally). Most neurons of the intermediate and caudal subdivisions of the VRG (74%) appeared to project either contralaterally or ipsilaterally below T. The axonal conduction velocity was 40-50 m/s by two-point determinations. We conclude that respiratory neuronal groups in the medulla of the rabbit are generally similar to those of the cat. Nearly equal proportions of bulbospinal RNs projected to the ipsilateral vs contralateral spinal cord.     

Morphology of inspiratory neurons located in the ventrolateral nucleus of the tractus solitarius of the cat

The morphology of 11 dorsal respiratory group (DRG) inspiratory neurons located in the ventrolateral nucleus of the solitary tract (vl-NTS) was studied using the technique of intracellular labeling with the enzyme horseradish peroxidase (HRP). Six of these cells were cut in the transverse plane and had a mean somal diameter of 30.4 m?m, while five others sectioned in the horizontal plane had a mean of 38.2 m?m. These neurons produced an average of 6.2 primary dendrites (range: 4–10), many of which projected rostrally or caudally up to 1.0 mm from the cell bodies. These dendrites were oriented along the longitudinal axis; they ran parallel and ventral to the tractus solitarius. In general, all dendrites possessed numerous spines and appendages. Many axons could be traced for considerable distances within the medulla (in one instance, up to 8 mm). These axons were last discerned in the contralateral ventral medulla rostral to the level of their cell bodies. The axons of three neurons bifurcated in the ipsilateral medulla; one branch remained ipsilateral and projected caudally, while the other crossed the midline. A small number of counterstained cells of size similar to or larger than the HRP-stained neurons formed a column that constituted the vl-NTS. Based upon our observations of stained and counterstained cells, we conclude that the inspiratory neurons of the vl-NTS are few in number and represent a morphologically homogeneous population. The primary orientation of the dendritic arbors of vl-NTS inspiratory neurons appears to optimize the surface area available to receive synaptic contacts from sensory afferents emerging from the tractus solitarius.     

Projections from brainstem nuclei to the nucleus paragigantocellularis lateralis in the cat

Afferent projections from the pons and medulla to the nucleus paragigantocellularis lateralis (PGL) have been mapped in the cat using retrograde transport of horseradish peroxidase (HRP). In the caudal medulla, the major sources of afferents were the medial and lateral divisions of the solitary nuclei complex and the contralateral trigeminal nucleus caudalis. Labelled cells were also present in the dorsal column nuclei, nucleus intercalatus and praepositus hypoglossi but this may have been due to uptake of HRP into fibres of passage. In the dorsolateral medulla and pons, neurones in the vestibular complex and in the parabrachial nucleus were labelled bilaterally. Nucleus raphe magnus and raphe obscurus were both found to send projections to the PGL and labelled cells were also present throughout the pontine and medullary reticular nuclei as well as in PGL on the side opposite to the injection of HRP. These findings are discussed in relation to the role of the PGL in cardiovascular regulation and in the control of pain.     

Bulbospinal respiratory neurons are a source of double synapses onto phrenic motoneurons following cervical spinal cord hemisection in adult rats

The purpose of this study was to determine if the medullary neurons that provide the primary excitatiry drive to phrenic motoneurons (i.e., rostral ventral respiratory group, rVRG) are a source of double synapse formation in the phrenic nucleus after spinal cord hemisection. The axons of rVRG neurons either ipsilateral or contralateral to the hemisection were labeled by injection of a mixture of HRP and WGA-HRP into the rostral ventral respiratory group. Phrenic motoneurons ipsilateral and caudal to the hemisection were labeled by the retrograde transport of HRP. The ultrastructural results indicated that after hemisection, rVRG neurons from both sides of the medulla formed labelled double synapses in the phrenic nucleus.     

Retinal ganglion cell size groups projecting to the superior colliculus and the dorsal lateral geniculate nucleus in the North American opossum

The retinal ganglion cells projecting to the superior colliculus (SC) and dorsal lateral geniculate nucleus (LGNd) of the North American opossum (Didelphis virginiana) were studied by using the retrograde transport of horseradish peroxidase (HRP). The four ganglion cell size groups recognized previously were found to project in systematically different ways. After injections of HRP into the superior colliculus, labeled cells were seen in nasal retina contralateral to the injection and in temporal retina both ipsilateral and contralateral to the injection. In contralateral nasal retina cells of all size classes were labeled, while in contralateral temporal retina small (8-14 micrometers diameter), small-medium (15-19 micrometers diameter), and large (greater than 24 micrometers diameter) cells were labeled but few, if any, large-medium (20-24 micrometers diameter) cells were labeled. In ipsilateral temporal retina, soma size groups labeled included small-medium, large-medium, and large cells, but very few small cells. A nasal-temporal difference in the soma size of ganglion cells projecting to the SC was found: Labeled cells in temporal retina were 1.7-4.2 micrometers larger than their counterparts in nasal retina. Following injection of HRP into the LGNd, label was seen in contralateral nasal and ipsilateral temporal retina with no label seen in contralateral temporal retina. The labeled cells were small-medium, large-medium, and large. No small ganglion cells were labeled from the LGNd. A small nasal-temporal soma size difference in retinal ganglion cells projecting to the LGNd was seen: labeled cells in temporal retina were 1.0-2.1 micrometers larger than in nasal. It is concluded that all four ganglion cell size groups in the opossum project to the SC, but that only the three largest project to the LGNd.     

Amygdaloid and pontine projections to the ventromedial nucleus of the hypothalamus

Amygdaloid and pontine projections to the feline ventromedial nucleus of the hypothalamus (HVM) were studied with retrograde transport of horseradish peroxidase (HRP) and anterograde transport of tritiated amino acids. Following injections of HRP into HVM, amygdaloid neurons were labeled in the ipsilateral cortical and medial nuclei and the ventral portion of the parvocellular part of the basal nucleus. In experiments in which HRP was injected into the tuberal hypothalamus following stria terminalis lesions, it was determined that amygdaloid neurons projecting to HVM by way of the stria terminalis were located in the cortical and medial nuclei while those projecting through another route, presumably the ventral amygdalofugal pathway, were found in the rostral part of the medial nucleus and the parvocellular basal nucleus. Following HRP injection into lateral hypothalamus at the level of HVM, labeled neurons were seen in the magnocellular basal nucleus. After preoptic injections, neurons containing the HRP reaction product were in cortical and medial nuclei and magnocellular and parvocellular parts of the basal nucleus. In addition to cells in the amygdala, rostral pontine neurons were labeled after HRP injections into HVM. The cells were located ipsilateral to the injection, mostly in the dorsal nucleus of the lateral lemniscus, lateral and dorsolateral to the brachium conjunctivum. The pontine cells labeled following HVM injections of HRP were different from those labeled following lateral hypothalamic and preoptic region injections. The pontine projection to HVM was confirmed using axoplasmic transport autoradiography. A mixture of tritiated leucine and tritiated proline was injected into the lateral pontine region labeled after HRP injections into HVM. Labeled axons ascending in the medial forebrain bundle terminated throughout the rostro-caudal extent of HVM.     

Afferent projections to the dorsal and ventral respiratory nuclei in the medulla oblongata of the cat studied by the horseradish peroxidase technique

The central afferent projections to cells in the region of the dorsal and ventral groups of respiratory neurones of the medulla were studied in the cat using the retrograde axonal transport of horseradish peroxidase (HRP).HRP was injected electrophoretically either in the ventrolateral nucleus of the solitary tract (the dorsal group), or into the nucleus ambiguus and retroambigualis (the ventral group). Microelectrode recordings of the activity of the respiratory neurones in these locations were obtained prior to the iontophoretic injections of the enzyme.Projections from the parabrachial region of the pons (nucleus locus coeruleus and subcoeruleus, lateral and medial parabrachial nuclei, Kölliker-Fuse nucleus), nucleus reticularis pontis oralis, retrofacial nucleus, nucleus paragigantocellularis lateralis and ventrolateral nucleus of the solitary tract to the nucleus ambiguus and retroambigualis were identified.The ventrolateral nucleus of the solitary tract was found to receive an input from the retrofacial and lateral paragigantocellular nuclei, and to have strong reciprocal connections with the nucleus paragigantocellularis dorsalis of the medulla upon which pontine Kölliker-Fuse nucleus projects, and some of the primary respiratory and cardiovascular afferences are thought to converge.     

Medullary respiratory neurons in the guinea pig: localization and firing patterns

The location and firing patterns of medullary respiratory neurons have been described in a small number of species. The cat has been the most widely studied species, but some potentially important differences have recently been noted in others. A more complete survey of species is required to determine the significance of these differences. We describe the location and firing patterns of respiratory neurons in the medulla of anesthetized, paralyzed and mechanically ventilated adult guinea pigs. Extracellular single-unit recordings were made from the medulla, their phase relationship with phrenic nerve activity used to define them as respiratory and their location marked with fast green. Respiratory units were concentrated ventrolateral to the nucleus tractus solitarius (NTS) and within and surrounding the nucleus ambiguus (NA), corresponding to the dorsal respiratory group (DRG) and ventral respiratory group (VRG) of the cat, respectively. Most DRG respiratory units were inspiratory, while the VRG contained equal numbers of inspiratory and expiratory units. The DRG and VRG both contained early, late and constant-frequency inspiratory and expiratory units. In general, these findings are similar to those in other mammalian species examined, consistent with these basic aspects of the respiratory network being highly conserved.     

The distribution of afferent neurones in the mesencephalic nucleus of the fifth nerve in the cat

The objective of this study was to examine anatomically the distribution of afferent neurones in the mesencephalic nucleus of the fifth nerve (Mes V). HRP was applied, in separate experiments, to the inferior alveolar, infraorbital, and masseter nerves, and injected into the masseter muscle and periodontal ligament. Following application of HRP to the masseter muscle and masseter nerve, labelled cells were found in the ipsilateral motor nucleus of the fifth nerve and in the ipsilateral Mes V. Labelled cells in Mes V, identified as belonging to proprioceptor afferents from jaw-closing muscles, were distributed throughout the full extent of the nucleus. Following application of HRP to the inferior alveolar nerve, infraorbital nerve, and periodontal ligament, labelled cells were found in the ipsilateral trigeminal ganglion and Mes V, and the latter identified as belonging to periodontal receptor afferents. In contrast to the distribution of spindle afferent somata, they were restricted to the caudal region of Mes V. The differential distribution of afferent neurones within Mes V demonstrated in this study confirms previous electrophysiological findings, and its significance is considered.     

Afferents to the cerebellar lateral nucleus. Evidence from retrograde transport of horseradish peroxidase after pressure injections through micropipettes.

HRP was injected by pressure from glass capillary micropipettes unilaterally into the lateral nucleus of rat so as to encompass the entire nucleus, but without spread into the interpositus nuclei. The cells of origin of the afferents to the lateral nucleus were studied after retrograde transport of the HRP. The reticulotegmental nucleus of the pons was labelled bilaterally and is the major source of crossed and uncrossed reticular imputs. The pontine nuclei also provide extensive crossed and uncrossed afferents. The inferior olive gives a large crossed olivo-lateral nucleus projection and a minor uncrossed input. The trigeminal nuclear complex--the nucleus of the spinal tract and the mesencephalic, principal sensory, and motor nuclei--all provide uncrossed afferents. The rostral portion of the lateral reticular nucleus gives a small crossed and uncrossed projection while the perihypoglossal nuclei and the dorsal parabrachial body give crossed afferents to the lateral nucleus. The norepinephrine afferent system from the locus coeruleus is represented by one or two heavily labelled cells and the serotonin raphe systems come from at least five raphe subgroups, the dorsal, superior centralis, pontis, obscurus and magnus nuclei. No evidence was found for commissural fibers between ipsilateral or contralateral cerebellar nuclei, or afferent axons from the spinocerebellar nuclei and the paramedian retricular nucleus. The significance of these sources of afferent imputs to the lateral cerebellar nucleus is discussed. The question is raised of the direct relationship between size of terminal axonal arborization and the quantity of HRP granules present in a cell retrograde transport. The limitations of the HRP method for detecting subtle local differences in the distribution of afferents within the heterogeneous groups of neurons in the lateral nucleus are discussed.     

Nucleus prepositus neurons projecting to the oculomotor and trochlear nuclei in rabbits

In this study we have used retrogradely transported horseradish peroxidase (HRP) to investigate whether the nucleus prepositus hypoglossi of a lateral-eyed mammal projects to the oculomotor and trochlear nuclei. After the injection of HRP in the oculomotor nucleus of rabbits, labelled neurons were found bilaterally in the nucleus prepositus hypoglossi, though they were more numerous on the ipsilateral side. The majority of these neurons were labelled in the rostral part of the propositus nucleus and were dispersed predominantly in the lateral and ventral zone of the nucleus. Neurons were also labelled in the prepositus nucleus after injection of HRP in the trochlear nucleus. These data were compared with those for frontal-eyed mammals and birds and suggest that the said projections are less well developed in species that possess panoramic vision and a lesser degree of binocular yoking.     

Behavior of VRG neurons during the atonia of REM sleep induced by pontine carbachol in decerebrate cats.

The microinjection of carbachol into the pons of acute decerebrate cats elicits a REM sleep-like atonia and a profound suppression of respiratory motoneuronal activity (J. Appl. Physiol., 69 (1990) 2280-2289). To assess whether this suppression is mediated by medullary neurons that provide respiratory drive to motoneurons of the respiratory pump muscles (diaphragm and intercostals), we studied the effect of pontine carbachol on the activity of neurons of the ventral respiratory group (VRG) in decerebrate, vagotomized, paralyzed and artificially ventilated cats. VRG neurons were recorded extracellularly along with the activity of phrenic and intercostal (external and internal) nerves. Both inspiratory (I) and expiratory (E) VRG neurons had incrementing, ramp-like bursts of activity during their firing periods and were not vagal motoneurons. Carbachol produced a depression of the peak firing rate in most (42/57) neurons studied. However, five cells showed no change and ten had an increase in activity in spite of consistent depression at the motoneuronal level. For the total population of cells (34 I and 23 E), the peak firing was reduced to 88.5% +/- 16.3 (S.D.) of control. The simultaneously recorded phrenic activity was reduced to 77.9% +/- 11.5, while inspiratory intercostal activity fell to 63.4% +/- 21.6 and expiratory to 23.2% +/- 21.2 of control. The carbachol-induced changes in peak firing of both I and E cells were quantitatively similar, and positively correlated to changes in peak phrenic activity. Analysis of this correlation suggested that phrenic and intercostal activities will be depressed to some degree by carbachol even when the average VRG cell activity remains unchanged. In addition, our data show that VRG cells may receive a combination of inhibitory and excitatory inputs during the carbachol-induced depression of respiratory motoneurons. Thus, although some disfacilitation from VRG cells may occur, there must be additional inhibitory or disfacilitatory pathways that mediate the decrease in activity of both phrenic and intercostal motoneurons that accompanies the REM sleep-like atonia.     

Central projections of the rat radial nerve investigated with transganglionic degeneration and transganglionic transport of horseradish peroxidase

Transganglionic degeneration and transganglionic transport of HRP were used for investigation of the spinal cord and brainstem projections from the superficial, cutaneous (SR) and deep, muscular (DR) branches of the radial nerve. The HRP study included a numerical and size analysis of labelled dorsal root ganglion (DRG) cells. In degeneration experiments the SR nerve was found to project somatotopically to laminae III-IV, but degeneration was also found in lamina I and inconsistently in lamina II. Transection of the DR nerve was found to give rise to a small amount of degeneration, which in "sham" operations was established to result from the skin injury during dissection of the DR nerve. With the HRP method, the SR nerve was found to project somatotopically to laminae I-IV, whereas the DR nerve projected more diffusely to the medial part of laminae V-VII. HRP application to the SR and DR nerves resulted in labelling of a mean of 1,024 and 310 DRG cells, respectively. These labelled neurons had a median cell area of 381 and 562 micron 2 for the SR and DR nerves, respectively, and both small and large cells were labelled in both types of experiments. In the lower brainstem, projections from the SR nerve were found only in the ipsilateral dorsal part of the main cuneate nucleus (MCN) with both methods. Brainstem projections from the DR nerve that were found only with the HRP method were found in the ipsilateral ventral part of the MCN together with a projection to the ipsilateral external cuneate nucleus. No projections were found to the central cervical nucleus. The present results indicate that cutaneous compared to muscular primary sensory neurons are much more prone to react with transganglionic degeneration after peripheral nerve transection. Furthermore, in the rat the SR nerve projects somatotopically, whereas the DR nerve does not. Both nerve branches are connected to small and large spinal ganglion cells, although the median cell area is larger in muscular neurons.