Distribution of corticospinal motor fibres within the cervical spinal cord with special reference to the phrenic nucleus: a WGA-HRP anterograde transport study in the cat.

The anterograde transport of wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP) was used to demonstrate the corticospinal fibres which originate in the motor cortex and project to the cervical spinal cord, specifically to the phrenic nucleus, in the cat. Following injections of WGA-HRP into the pericruciate cortex large numbers of fibres were labelled in the contralateral lateral and ventral funiculi and fewer fibres were labelled in the ipsilateral lateral and ventral funiculi. Labelled corticospinal motor fibres entered the gray matter laterally in laminae V and VI and terminated within these two laminae as well as throughout the entire extent of lamina VII. A few labelled fibres were present in medial lamina VIII and also in lamina IX where they were in close association with the phrenic motoneuron pool. Labelling was present in the gray matter at both sides, with a stronger labelling contralaterally. Labelled axons were also seen crossing from each side of the gray matter to the other side. The results suggest that in the cat the corticospinal motor fibres have a wider distribution in the spinal gray matter than has been previously shown, and that corticospinal motor axons may be in direct contact with phrenic motoneurons.

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].

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)

Brainstem projections to the phrenic nucleus: a HRP study in the cat.

Brainstem neurones which project to the phrenic nucleus were identified using retrogradely transported horseradish peroxidase (HRP) as a marker. Following iontophoretic injection of HRP into the phrenic nucleus, labelled cells were encountered throughout large areas of the medulla and pons, but occurred with characteristic high densities in those regions known to contain phasic respiratory neurones: namely, the ventrolateral solitary tract nucleus (vl-NTS), known as the dorsal respiratory group (DRG), the ambiguus complex or ventral respiratory group (VRG) and the parabrachial pontine nuclei (BCM-KF). In 12 cats a total of 1540 cells was identified within these regions, the relative contralateral and ipsilateral contributions were respectively 72%:28% (vl-NTS), 65%:35% for the ambiguus complex, and 5%:95% (BCM-KF). In addition, labelled cells, predominantly ipsilateral, were observed in the pontine and medullary reticular formation and the vestibular nuclei. The labelled cells of the DRG had round, oval or triangular perikarya. Their mean soma diameter was 18.3 micrometers. The HRP-positive cells of the VRG had slightly larger somas (mean 21.2 micrometers) and they were fusiform and triangular. The neurones labelled in the BCM-KF nuclei were more heterogeneous with a mean soma size of 14.9 micrometers. The bilateral projections to the phrenic nucleus from the DRG and the VRG, and the predominantly ipsilateral projection from the BCM-KF are discussed in relation to current electrophysiological and autoradiographic findings.

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.

CO2-sensitivity of stretch receptors in the marsupial lung.

Discharge activity in single fibres of the vagus of the brush tailed possum was examined for evidence of pulmonary CO2 receptors by artificially ventilating the lungs with gas mixtures which preserved, abolished or reversed the normal tidal oscillation in FCO2. No specific CO2 receptors were observed. A quantitative study of the CO2 sensitivity of thirteen pulmonary stretch receptors was carried out after stabilizing FACO2 at high (6.4-7.8%), low (1.4-2.5%) as well as intermediate values. In addition receptor responses to a series of sustained augmenting inflations were examined at different intrapulmonary CO2 concentrations. All thirteen receptors showed CO2 sensitivity, their frequency of discharge being reduced by hypercapnia and increased by hypocapnia. Five were low threshold receptors which discharged throughout the ventilatory cycle while the remaining eight were only phasically active. High threshold receptors were more sensitive to FACO2 changes than were low threshold units. The results from the series of augmenting inflations suggest that it is the receptor's threshold, but not its sensitivity, to tracheal pressure that is modulated by the co2 signal.

Inhibition of cuneate neurones: its afferent source and influence on dynamically sensitive "tactile" neurones.

1. Responses were recorded in decereberate, unanaesthetized cats from individual cuneate neurones in order to determine firstly, the afferent sources of inhibition on cuneate neurones and secondly, the influence of afferent-induced inhibition on those response features of dynamically sensitive tactile neurones which determine their capacity to code information about parameters of tactile stimuli.2. For all cuneate neurones which displayed afferent-induced inhibition from areas surrounding or within their excitatory receptive field (71% of the sample) it was consistently found that 300 Hz vibration at low amplitudes (< 25-50 mum) which selectively engages Pacinian corpuscles was an effective source of inhibition. In contrast, steady indentation which activates slowly adapting tactile afferents was quite ineffective, as was low frequency vibration (30 Hz) at amplitudes of < 50-100 mum. The latter stimulus can be used to engage rapidly adapting receptors either within glabrous skin (presumed to be Meissners corpuscles) or in association with hair follicles. It is concluded that afferents from Pacinian corpuscles are the dominant or exclusive source of afferent-induced inhibition of cuneate neurones.3. For dynamically sensitive neurones responsive to low frequency cutaneous vibration (30 Hz) there was a reduction in the slope of stimulus-response relations with afferent-induced inhibition, but no expansion of the range of stimulus amplitudes over which the neurone responded.4. The influence of afferent-induced inhibition on the phase-locking of impulse activity to a cutaneous vibratory wave form was examined by constructing post-stimulus time histograms and cycle histograms. Measures of dispersion of impulse activity around the preferred point of firing in the vibratory waveform indicated that the capacity of individual cuneate neurones to code information about the frequency of the cutaneous vibration was not systematically changed in the presence of afferent-induced inhibition.

Central and peripheral neural respiratory activity in the mature sheep foetus and newborn lamb.

Activity from respiratory neurones in the medulla, the phrenic and intercostal nerves was recorded in 25 foetal sheep, exteriorized a few days before term from ewes given a spinal anaesthetic, and from nine newborn lambs, anaesthetized with an allobarbitone-urethane mixture. In 12 foetuses, there was little or no sustained respiratory activity, central activity consisting of tonically discharging expiratory and other neurones and silent inspiratory neurones. In the remaining 13 foetuses, respiratory activity was periodic or continuous and it was possible to confirm that the motor component of the respiratory reflex was mature, that apnoea was not due to medullary depression, that foetal respiration was unaffected by chemoreceptor stimulation, by noise, by light, by electrical stimulation of the sciatic nerve and only slightly by inflation or deflation of the lungs. All these stimuli were effective shortly after birth. Occlusion of the umbilical cord caused poorly sustained gasps in the "non-breathing" foetuses and in the "breathing" foetuses, abolition of inspiratory and expiratory activity in the medulla and the onset of gasps and flattening of the electro-corticogram. Rhythmic respiration resumed after release of the cord with a latency which varied with the duration and severity of the asphyxia. This type of respiratory depression was not reflex but due to a direct, central action of hypoxia. The sequence of respiratory events at birth is discussed.

Studies of the pulmonary vagal control of central respiratory rhythm in the absence of breathing movements.

1. Breuer's hypothesis that the vagus nerves exert a tonic control of respiratory rhythm, in addition to the phasic control, was examined.2. Closed-chest cardiopulmonary bypass was instituted in dogs weighing 20-30 kg anaesthetized with chloralose. Respiratory rhythm was recorded from a phrenic electroneurogram.3. Complete muscular paralysis induced with gallamine triethiodide produced an increase in the duration of inspiration and an increase in the amplitude of the integrated phrenic electroneurogram. There was no consistent effect on expiratory duration. Gallamine produced no effect when given after vagotomy.4. In the paralysed state, an increase in lung volume of 25-100 ml. for 30-60 sec produced a sustained increase in the duration of expiration and a decrease of respiratory rate: there was little effect on inspiratory duration, or the amplitude of the integrated phrenic electroneurogram.5. A decrease in lung volume of the same order of magnitude for the same period produced a sustained decrease in the duration of expiration and an increase of respiratory rate: there was little effect on inspiratory duration or the amplitude of the integrated phrenic electroneurogram.6. The phenomena described in (5) and (6) constitute a high gain respiratory frequency controller. They did not occur after bilateral cervical vagotomy.7. Bilateral cervical vagotomy during complete muscular paralysis produced a further increase in the duration of inspiration and in the amplitude of the integrated phrenic electroneurogram; there was no consistent effect on expiratory duration.8. The results confirmed Breuer's hypothesis and showed that inspiratory duration and expiratory duration are controlled independently.

Studies on the central effects of Hering-Breuer reflexes.

In our previous experiments performed with rabbits, it was found out that the strength of both inflation and deflation Hering-Breuer reflexes depends on the level of anaesthesia and ventilation as correlative factors in spontaneously and artificially ventilated animals, though this dependence was quantitatively and qualitatively different in these two groups. On the basis of the recent evidence pointing to the rapidly conducting fibres and thus indirectly the pulmonary stretch receptors as the pathway along which both Hering-Breuer reflexes are transmitted, an attempt has been made to show how both parameters of ventilation are centrally controlled in response to afferent information concerning such mechanical factors as resistance and compliance of the respiratory system. The results presented seem to allow the following conclusions: (i) An important role may be ascribed to the pulmonary stretch receptors in transmission of both Hering-Breuer reflexes. (ii) In quiet breathing the central mechanisms of the Hering-Breuer inflation reflex are set to regulate tidal volume on the basis of analysis of the compliance of the respiratory apparatus at the given moment. Their role to the control of the respiratory frequency is of minor importance. (iii) The contribution of the pulmonary stretch receptors to the control of respiratory frequency is based on information about the resistance of the airways. This parameter influences the ratio of duration of the activity phase to silent phase in discharges of pulmonary stretch receptors.