Recordings of alpha motoneuron discharges from branches of the intercostal and abdominal nerves in anesthetized cats were analyzed for modulation during the cardiac cycle. Cardiac modulation was assessed by the construction of cross-correlation histograms between the R-wave of the ECG and the largest amplitude efferent spikes. In all but two recordings (which were believed to have either no or few alpha spikes), the histograms showed relatively short duration peaks and/or troughs (widths at half amplitude 4-50 ms) at lags of 10-150 ms. These observations were deduced to result from activity in oligosynaptic pathways, probably from muscle spindle afferents, whose discharges are known to be synchronized to the cardiac pulse. The results suggest that onward transmission of the cardiac signal from thoracic muscle afferents (and possibly from other dynamically sensitive afferents) to other parts of the central nervous system is highly likely and that therefore these afferents could contribute to cardiac interoception. NEW & NOTEWORTHY It has been recognized since 1933 that muscle spindles respond to the cardiac pulse, but it is unknown whether this cardiac signal is transmitted to other levels in the nervous system. Here we show that a cardiac signal, likely arising from muscle spindles, is present in the efferent activities of thoracic and abdominal muscle nerves, suggesting probable onward transmission of this signal to higher levels and therefore that muscle spindles could contribute to cardiac interoception.

A previous neurophysiological investigation demonstrated an increase in functional projections of expiratory bulbospinal neurons (EBSNs) in the segment above a chronic lateral thoracic spinal cord lesion that severed their axons. We have now investigated how this plasticity might be manifested in thoracic motoneurons by measuring their respiratory drive and the connections to them from individual EBSNs. In anesthetized cats, simultaneous recordings were made intracellularly from motoneurons in the segment above a left-side chronic (16 wk) lesion of the spinal cord in the rostral part of T8, T9, or T10 and extracellularly from EBSNs in the right caudal medulla, antidromically excited from just above the lesion but not from below. Spike-triggered averaging was used to measure the connections between pairs of EBSNs and motoneurons. Connections were found to have a very similar distribution to normal and were, if anything (nonsignificantly), weaker than normal, being present for 42/158 pairs, vs. 55/154 pairs in controls. The expiratory drive in expiratory motoneurons appeared stronger than in controls but again not significantly so. Thus we conclude that new connections made by the EBSNs following these lesions were made to neurons other than α-motoneurons. However, a previously unidentified form of functional plasticity was seen in that there was a significant increase in the excitation of motoneurons during postinspiration, being manifest either in increased incidence of expiratory decrementing respiratory drive potentials or in an increased amplitude of the postinspiratory depolarizing phase in inspiratory motoneurons. We suggest that this component arose from spinal cord interneurons.

Internal intercostal and abdominal motoneurons are strongly coactivated during expiration. We investigated whether that synergy was paralleled by synergistic Group I reflex excitation. Intracellular recordings were made from motoneurons of the internal intercostal nerve of T8 in anesthetized cats, and the specificity of the monosynaptic connections from afferents in each of the two main branches of this nerve was investigated. Motoneurons were shown by antidromic excitation to innervate three muscle groups: external abdominal oblique [EO; innervated by the lateral branch (Lat)], the region of the internal intercostal muscle proximal to the branch point (IIm), and muscles innervated from the distal remainder (Dist). Strong specificity was observed, only 2 of 54 motoneurons showing excitatory postsynaptic potentials (EPSPs) from both Lat and Dist. No EO motoneurons showed an EPSP from Dist, and no IIm motoneurons showed one from Lat. Expiratory Dist motoneurons fell into two groups. Those with Dist EPSPs and none from Lat (group A) were assumed to innervate distal internal intercostal muscle. Those with Lat EPSPs (group B) were assumed to innervate abdominal muscle (transversus abdominis or rectus abdominis). Inspiratory Dist motoneurons (assumed to innervate interchondral muscle) showed Dist EPSPs. Stimulation of dorsal ramus nerves gave EPSPs in 12 instances, 9 being in group B Dist motoneurons. The complete absence of heteronymous monosynaptic Group I reflex excitation between muscles that are synergistically activated in expiration leads us to conclude that such connections from muscle spindle afferents of the thoracic nerves have little role in controlling expiratory movements but, where present, support other motor acts.

Cross-correlation of neural discharges was used to investigate the connections between expiratory bulbospinal neurons (EBSNs) in the caudal medulla and expiratory motoneurons innervating thoracic and abdominal muscles in anesthetized cats. Peaks were seen in the cross-correlation histograms for around half of the EBSN-nerve pairs for the following: at T8, the nerve branches innervating internal intercostal muscle and external abdominal oblique muscle and a more distal branch of the internal intercostal nerve; and at L1, a nerve branch innervating internal abdominal oblique muscle and a more distal branch of the ventral ramus. Fewer peaks were seen for the L1 nerve innervating external abdominal oblique, but a paucity of presumed α-motoneuron discharges could explain the rarity of the peaks in this instance. Taking into account individual EBSN conduction times to T8 and to L1, as well as peripheral conduction times, nearly all of the peaks were interpreted as representing monosynaptic connections. Individual EBSNs showed connections at both T8 and L1, but without any discernible pattern. The overall strength of the monosynaptic connection from EBSNs at L1 was found to be very similar to that at T8, which was previously argued to be substantial and responsible for the temporal patterns of expiratory motoneuron discharges. However, we argue that other inputs are required to create the stereotyped spatial patterns of discharges in the thoracic and abdominal musculature.

The cardioinhibitory effects of cardiac vagal motoneurons (CVMs) are mediated by activation of postganglionic neurons in the epicardial ganglia which have been shown to exert functionally selective effects on heart rate and atrioventricular conduction in the rat. Here we investigate whether CVMs producing these responses may occupy different rostrocaudal positions within the nucleus ambiguus. Excitation of CVMs was attempted by microinjections of glutamate into the nucleus ambiguus of an arterially perfused preparation in a grid extending over 2mm in the rostrocaudal plane using the obex as a reference point. Microinjections were paired, one made during pacing to measure changes in atrioventricular conduction (P-R interval) independent of changes in heart rate and the other looking for changes in heart period (P-P interval) un-paced. Although evidence of a differential distribution was found in 7 cases, in the majority (13/20), sites producing maximal effects on both variables coincided. Maximal changes in atrioventricular conduction resulted from more rostral sites in 6 cases and from a more caudal site in only one. Overall, the ratio of the change in atrioventricular conduction to the change in heart rate for a given site was significantly greater 1mm rostral to the obex than at either end of the test grid. We conclude that while CVMs controlling atrioventricular conduction are distributed with a peak somewhat rostral to those controlling heart rate in a number of animals, there is a significant overlap and much greater variability in this distribution in the rat than in cats and dogs.

Propriospinal interneurons in the thoracic spinal cord have vital roles not only in controlling respiratory and trunk muscles, but also in providing possible substrates for recovery from spinal cord injury. Intracellular recordings were made from such interneurons in anesthetized cats under neuromuscular blockade and with the respiratory drive stimulated by inhaled CO(2). The majority of the interneurons were shown by antidromic activation to have axons descending for at least two to four segments, mostly contralateral to the soma. In all, 81% of the neurons showed postsynaptic potentials (PSPs) to stimulation of intercostal or dorsal ramus nerves of the same segment for low-threshold (≤ 5T) afferents. A monosynaptic component was present for the majority of the peripherally evoked excitatory PSPs. A central respiratory drive potential was present in most of the recordings, usually of small amplitude. Neurons depolarized in either inspiration or expiration, sometimes variably. The morphology of 17 of the interneurons and/or of their axons was studied following intracellular injection of Neurobiotin; 14 axons were descending, 6 with an additional ascending branch, and 3 were ascending (perhaps actually representing ascending tract cells); 15 axons were crossed, 2 ipsilateral, none bilateral. Collaterals were identified for 13 axons, showing exclusively unilateral projections. The collaterals were widely spaced and their terminations showed a variety of restricted locations in the ventral horn or intermediate area. Despite heterogeneity in detail, both physiological and morphological, which suggests heterogeneity of function, the projections mostly fitted a consistent general pattern: crossed axons, with locally weak, but widely distributed terminations.

PROPRIOSPINAL INTERNEURONS IN THE THORACIC SPINAL CORD 2 3 4 S.A. Saywell, T.W. Ford, C.F. Meehan, A.J. Todd and P.A. Kirkwood 5 6 7 1 Sobell Department of Motor Neuroscience and Movement Disorders, UCL Institute of Neurology, 8 Queen Square, London WC1N 3BG, UK. 9 10 2 Faculty of Biomedical and Life Sciences, West Medical Building, University of Glasgow, 11 Glasgow G12 8QQ, UK. 12 13 14 15 16 Running head Thoracic interneuron morphology 17 18 19 Correspondence Dr PA Kirkwood 20 Sobell Department 21 UCL Institute of Neurology 22 Queen Square 23 London WC1N 3BG 24 UK. 25 Tel. +44 (0)20 7837 3611 Ext 4189 26 Fax. +44 (0)20 7813 3107 27 E-mail pkirkwoo@ion.ucl.ac.uk 28 29 30 31 † Deceased 18th August 2010 32 33 34 35 36 Articles in PresS. J Neurophysiol (November 24, 2010). doi:10.1152/jn.00738.2010

The descending control of respiratory-related motoneurones in the thoracic spinal cord remains the subject of some debate. In this study, direct connections from expiratory bulbospinal neurones to identified motoneurones were investigated using spike-triggered averaging and the strengths of connection revealed were related to the presence and size of central respiratory drive potentials in the same motoneurones. Intracellular recordings were made from motoneurones in segments T5-T9 of the spinal cord of anaesthetized cats. Spike-triggered averaging from expiratory bulbospinal neurones in the caudal medulla revealed monosynaptic EPSPs in all groups of motoneurones, with the strongest connections to expiratory motoneurones with axons in the internal intercostal nerve. In the latter, connection strength was similar irrespective of the target muscle (e.g. external abdominal oblique or internal intercostal) and the EPSP amplitude was positively correlated with the amplitude of the central respiratory drive potential of the motoneurone. For this group, EPSPs were found in 45/83 bulbospinal neurone/motoneurone pairs, with a mean amplitude of 40.5 mu V. The overall strength of the connection supports previous measurements made by cross-correlation, but is about 10 times stronger than that reported in the only previous similar survey to use spike-triggered averaging. Calculations are presented to suggest that this input alone is sufficient to account for all the expiratory depolarization seen in the recorded motoneurones. However, extra sources of input, or amplification of this one, are likely to be necessary to produce a useful motoneurone output.

Anatomical studies have shown a monosynaptic projection from nucleus retroambiguus (NRA) to semimembranosus (Sm) motor nucleus in female cats, which is stronger in oestrus. Expiratory bulbospinal neurones are the best documented functional cell type in the NRA. If these cells participate in this projection, an expiratory drive would be expected in Sm motoneurones and this drive would be expected to be stronger in oestrus. In anaesthetized, paralyzed, ovariohysterectormized female cats, artificially ventilated to produce a strong respiratory drive (as monitored by phrenic nerve discharges), intracellular recordings were made from Sm motoneurones and from motoneurones in the surrounding hindlimb motor nuclei that are outside the focus of the NRA projection. The animals comprised two groups: either treated for 7 days with oestradiol benzoate (oestrous) or untreated (non-oestrous).Central respiratory drive potentials (CRDPs) were observed in most motoneurones of both groups, with amplitudes larger for the oestrous than for the non-oestrous group (1.58 +/- 1.34 mV versus 0.89 +/- 0.79 mV, mean +/- S.D.). However, the CRDPs most often consisted of a maximum depolarization in early expiration, which declined in late expiration and into inspiration. This pattern is different from the incrementing firing pattern of most expiratory bulbospinal neurones. The CRDPs in Sm and semitendinosus motoneurones (located in the same motor column) were of similar size and frequency to CRDPs in motoneurones outside that column. The hypothesis that expiratory bulbospinal neurones are significantly involved in the projection was rejected. Alternative sources and possible functional roles for the CRDPs are discussed. (c) 2006 Elsevier Inc. All rights reserved.

Nucleus retroambiguus (NRA), in the caudal medulla, projects to all spinal levels. One physiological role is abdominal pressure control, evidenced by projections to intercostal and abdominal motoneurons from expiratory bulbospinal neurons (EBSNs) within NRA. The roles of NRA projections to the lumbosacral cord are less certain, although those to limb motoneurons may relate to mating behavior and those to Onuf's nucleus (ON) to maintaining continence. To clarify this we physiologically characterized NRA projections to the lumbosacral cord. Extracellular recordings were made in NRA under anesthesia and paralysis in estrus cats. Administered CO2gave a strong respiratory drive. Antidromic unit responses were recorded to stimulation of the contralateral ventrolateral funiculus of L6, L7, or sacral segments and to microstimulation in the region of semimembranosus motor nucleus or ON. All units were found at sites showing expiratory discharges. Units that showed collisions between antidromic and spontaneous spikes (all in late expiration) were identified as EBSNs. These were common from the ventrolateral funiculus (VLF) of L6(42.5%) or L7(32.9%), but rare from the sacral VLF or the motor nuclei. Antidromic latencies revealed a subthreshold respiratory drive in some non-EBSNs. This group had lower conduction velocities than the EBSNs. The remainder, with a negligible respiratory drive, had even lower conduction velocities. A new population of NRA neurons has thus been defined. They are not active even with a strong respiratory drive, but may provide most of the synaptic input from NRA to lower lumbar and sacral segments and could subserve functions related to mating behavior.

Retrograde transport of horseradish peroxidase, applied to cut peripheral nerves, was used to determine the rostrocaudal distribution of motoneurones supplying different branches of the ventral ramus for a single mid‐ or caudal thoracic segment in the cat. The motoneurones occupied a length of spinal cord equal to the segmental length but displaced rostrally from the segment as defined by the dorsal roots, with the number of motoneurones per unit length of cord higher in the rostral part of a segment (close to the entry of the most rostral dorsal root) than in the caudal part. The cross‐sectional area of the ventral horn showed a rostrocaudal variation that closely paralleled the motoneurone distribution. The ratio between the number of motoneurones per unit length in the caudal and rostral regions of a segment (0.70) was similar to the ratio previously reported for the strength of functional projections of expiratory bulbospinal neurones (0.63). This is consistent with the motoneurones being the main targets of the bulbospinal neurones. J. Comp. Neurol. 472:281–291, 2004. © 2004 Wiley‐Liss, Inc.

Vagal cardioinhibition is exerted through a reduction not only in the heart rate but also in the rate of propagation of the cardiac action potential and in myocardial contractility. In several species, such effects can be produced independently by selective activation of ganglia in identified 'fat pads'. In this study we investigate differential control of heart rate and atrioventricular conduction by two ganglionic clusters in the rat, a species increasingly important in studies of cardiovascular control. Epicardial sites producing low-threshold changes in P-P and P-R interval of the ECG in an arterially perfused preparation were explored with concentric bipolar stimulating electrodes. Stimulation sites centred on two principal ganglia, the sinoatrial (SA) ganglion at the junction of the right superior vena cava and right atrium, and the atrioventricular (AV) ganglion at the junction of the inferior pulmonary veins and left atrium. Stimulation of the SA ganglion decreased heart rate in all preparations, with little or no effect on AV conduction in one-third. Stimulation of the AV ganglion consistently slowed conduction without eliciting a comparable bradycardia. Responses survived blockade of ganglionic transmission by trimetaphan, with an enhanced chronotropic selectivity to SA ganglion stimulation, suggesting that co-excitation of preganglionic elements en passant may have contributed to the earlier mixed responses. Effective stimulation sites were precisely circumscribed and corresponded to principal ganglionic clusters confirmed histologically. We conclude that cardiac vagal ganglia in the rat show a topographical functional organisation and are amenable to investigation using the arterially perfused preparation.

Central respiratory drive potentials (CRDPs) were observed in intracellular recordings from hindlimb motoneurones of anaesthetized, paralysed, ovariohysterectomized female cats, artificially ventilated so as to have a strong respiratory drive under hyperoxic hypercapnea. The animals comprised two groups: three animals treated for 7 days with oestradiol benzoate (oestrous), and three untreated (non-oestrous). Several features of the CRDPs suggested the involvement of voltage-sensitive channels, including variability in the CRDP amplitude and potentiation of the CRDP by depolarizing currents. Clear plateau potentials, related to or triggered by depolarizing phases of the CRDPs, occurred in six motoneurones, three in response to depolarizing currents, three spontaneously. This is the first report of plateau potentials in mammalian motoneurones under anaesthesia. The mechanisms accessed here by the respiratory inputs are deduced to be particularly potent, perhaps because of a distal dendritic location. Plateau-like effects were more common in the oestrous animals, which suggests a specific role for the plateau potentials in the production of the stereotyped movements of female receptive behaviour.

1. Plasticity in functional connections of expiratory bulbospinal neurones was investigated by measurement of terminal potentials (TPs) and focal synaptic potentials (FSPs), recorded with spike-triggered averaging in the thoracic spinal cord of anaesthetized, paralysed cats. These measurements were made in normal cats and in those which had previously been subjected to spinal cord lesions that transected the axons of the bulbospinal neurones in the segment below that under investigation, either about 2 weeks or about 16 weeks previously.2. In both groups of operated animals bulbospinal neurones with firing properties and conduction velocities similar to normal were present. The extracellular recordings that were averaged to reveal TPs and FSPs were made on two standard grids, each consisting of eight sites spaced 0.25 mm apart on two electrode tracks. One grid was positioned at a rostral and one at a caudal location within one segment (T7-T9).3. In the normal animals TPs and FSPs were larger and/or more common at rostral sites than at caudal sites, by a factor of about 1.7. In both 2 week and 16 week animals, TPs and FSPs were observed, both showing normal time courses and latencies. At rostral sites in both the 2 week and 16 week animals the amplitudes and/or the frequency of occurrence of TPs and FSPs were similar to normal, as was the case for caudal sites in the 2 week animals. However, at caudal sites in the 16 week animals the FSPs were more common and/or significantly larger than normal, with the increase particularly marked on the lateral track, being equivalent to a factor of about 2. A corresponding increase in the amplitude and/or frequency of occurrence of TPs at caudal lateral sites was also seen, but was not significant.4. The results are interpreted as evidence for short-range sprouting of the bulbospinal axons and the formation by them of new connections in the caudal parts of the segments concerned.