Multi- and single-fibre mesenteric and renal sympathetic responses to chemical stimulation of intestinal receptors in cats

The superior cervical ganglia modulate ventilatory responses to hypoxia independently of preganglionic drive from the cervical sympathetic chain

Superior cervical ganglia (SCG) postganglionic neurons receive preganglionic drive via the cervical sympathetic chains (CSC). The SCG projects to structures like the carotid bodies (e.g., vasculature, chemosensitive glomus cells), upper airway (e.g., tongue, nasopharynx), and to the parenchyma and cerebral arteries throughout the brain. We previously reported that a hypoxic gas challenge elicited an array of ventilatory responses in sham-operated (SHAM) freely moving adult male C57BL6 mice and that responses were altered in mice with bilateral transection of the cervical sympathetic chain (CSCX). Since the CSC provides preganglionic innervation to the SCG, we presumed that mice with superior cervical ganglionectomy (SCGX) would respond similarly to hypoxic gas challenge as CSCX mice. However, while SCGX mice had altered responses during hypoxic gas challenge that occurred in CSCX mice (e.g., more rapid occurrence of changes in frequency of breathing and minute ventilation), SCGX mice displayed numerous responses to hypoxic gas challenge that CSCX mice did not, including reduced total increases in frequency of breathing, minute ventilation, inspiratory and expiratory drives, peak inspiratory and expiratory flows, and appearance of noneupneic breaths. In conclusion, hypoxic gas challenge may directly activate subpopulations of SCG cells, including subpopulations of postganglionic neurons and small intensely fluorescent (SIF) cells, independently of CSC drive, and that SCG drive to these structures dampens the initial occurrence of the hypoxic ventilatory response, while promoting the overall magnitude of the response. The multiple effects of SCGX may be due to loss of innervation to peripheral and central structures with differential roles in breathing control.NEW & NOTEWORTHY We present data showing that the ventilatory responses elicited by a hypoxic gas challenge in male C57BL6 mice with bilateral superior cervical ganglionectomy are not equivalent to those reported for mice with bilateral transection of the cervical sympathetic chain. These data suggest that hypoxic gas challenge may directly activate subpopulations of superior cervical ganglia (SCG) cells, including small intensely fluorescent (SIF) cells and/or principal SCG neurons, independently of preganglionic cervical sympathetic chain drive.

Loss of Cervical Sympathetic Chain Input to the Superior Cervical Ganglia Affects the Ventilatory Responses to Hypoxic Challenge in Freely-Moving C57BL6 Mice

The cervical sympathetic chain (CSC) innervates post-ganglionic sympathetic neurons within the ipsilateral superior cervical ganglion (SCG) of all mammalian species studied to date. The post-ganglionic neurons within the SCG project to a wide variety of structures, including the brain (parenchyma and cerebral arteries), upper airway (e.g., nasopharynx and tongue) and submandibular glands. The SCG also sends post-ganglionic fibers to the carotid body (e.g., chemosensitive glomus cells and microcirculation), however, the function of these connections are not established in the mouse. In addition, nothing is known about the functional importance of the CSC-SCG complex (including input to the carotid body) in the mouse. The objective of this study was to determine the effects of bilateral transection of the CSC on the ventilatory responses [e.g., increases in frequency of breathing (Freq), tidal volume (TV) and minute ventilation (MV)] that occur during and following exposure to a hypoxic gas challenge (10% O₂ and 90% N₂) in freely-moving sham-operated (SHAM) adult male C57BL6 mice, and in mice in which both CSC were transected (CSCX). Resting ventilatory parameters (19 directly recorded or calculated parameters) were similar in the SHAM and CSCX mice. There were numerous important differences in the responses of CSCX and SHAM mice to the hypoxic challenge. For example, the increases in Freq (and associated decreases in inspiratory and expiratory times, end expiratory pause, and relaxation time), and the increases in MV, expiratory drive, and expiratory flow at 50% exhaled TV (EF₅₀) occurred more quickly in the CSCX mice than in the SHAM mice, although the overall responses were similar in both groups. Moreover, the initial and total increases in peak inspiratory flow were higher in the CSCX mice. Additionally, the overall increases in TV during the latter half of the hypoxic challenge were greater in the CSCX mice. The ventilatory responses that occurred upon return to room-air were essentially similar in the SHAM and CSCX mice. Overall, this novel data suggest that the CSC may normally provide inhibitory input to peripheral (e.g., carotid bodies) and central (e.g., brainstem) structures that are involved in the ventilatory responses to hypoxic gas challenge in C57BL6 mice.

Autonomic regulation of cellular immune function

The nervous system and the immune system (IS) are two integrative systems that work together to detect threats and provide host defense, and to maintain/restore homeostasis. Cross-talk between the nervous system and the IS is vital for health and well-being. One of the major neural pathways responsible for regulating host defense against injury and foreign antigens and pathogens is the sympathetic nervous system (SNS). Stimulation of adrenergic receptors (ARs) on immune cells regulates immune cell development, survival, proliferative capacity, circulation, trafficking for immune surveillance and recruitment, and directs the cell surface expression of molecules and cytokine production important for cell-to-cell interactions necessary for a coordinated immune response. Finally, AR stimulation of effector immune cells regulates the activational state of immune cells and modulates their functional capacity. This review focuses on our current understanding of the role of the SNS in regulating host defense and immune homeostasis. SNS regulation of IS functioning is a critical link to the development and exacerbation of chronic immune-mediated diseases. However, there are many mechanisms that need to be further unraveled in order to develop sound treatment strategies that act on neural-immune interaction to resolve or prevent chronic inflammatory diseases, and to improve health and quality of life.

Sympathetic nervous system and inflammation: a conceptual view

The peripheral sympathetic nervous system is organized into function-specific pathways that transmit the activity from the central nervous system to its target tissues. The transmission of the impulse activity in the sympathetic ganglia and to the effector tissues is target cell specific and guarantees that the centrally generated command is faithfully transmitted. This is the neurobiological basis of autonomic regulations in which the sympathetic nervous system is involved. Each sympathetic pathway is connected to distinct central circuits in the spinal cord, lower and upper brain stem and hypothalamus. In addition to its conventional functions, the sympathetic nervous system is involved in protection of body tissues against challenges arising from the environment as well as from within the body. This function includes the modulation of inflammation, nociceptors and above all the immune system. Primary and secondary lymphoid organs are innervated by sympathetic postganglionic neurons and processes in the immune tissue are modulated by activity in these sympathetic neurons via adrenoceptors in the membranes of the immune cells (see Bellinger and Lorton, 2014). Are the primary and secondary lymphoid organs innervated by a functionally specific sympathetic pathway that is responsible for the modulation of the functioning of the immune tissue by the brain? Or is this modulation of immune functions a general function of the sympathetic nervous system independent of its specific functions? Which central circuits are involved in the neural regulation of the immune system in the context of neural regulation of body protection? What is the function of the sympatho-adrenal system, involving epinephrine, in the modulation of immune functions?

Glutamatergic activities in neonatal rat spinal cord heterogeneously regulate single-fiber splanchnic nerve discharge

Kynurenic acid (KYN) is a metabolite of tryptophan and is involved in various neurological disorders. Using whole-bundle nerve recording techniques, we previously observed that applications of KYN to block endogenous ionotropic glutamate receptor activities in neonatal rat spinal cords in vitro cause a reversible fluctuation of splanchnic sympathetic nerve discharge (SND). We hypothesized that the SND fluctuation was due to a heterogeneous single-fiber response. To detail individual fiber activities, we used the so-called 'oligofiber recordings'. Spontaneous single-fiber activities were recorded from the collagenase-dissociated splanchnic nerve fascicles. Applications of KYN increased, decreased or did not change firing rates. The heterogeneous responses in spontaneous spiking activities were confirmed by applications of APV or CNQX, suggesting an effect mediated by endogenous NMDA- or non-NMDA receptor activities. In addition to changes in firing rates, apparent drug-induced changes in firing patterns were also observed in some fiber activities. Using the oligofiber recording techniques, we confirmed a differential role of endogenous ionotropic glutamate receptor activities in regulating sympathetic outflows from the spinal cord of neonatal rats. Fine-tuning of ionotropic glutamate receptor activities in the spinal cord may serve as a simple way for heterogeneous regulation of various sympathetic-targeting tissues.

Predominant postglomerular vascular resistance response to reflex renal sympathetic nerve activation during ANG II clamp in rabbits

We have shown previously that a moderate reflex increase in renal sympathetic nerve activity (RSNA) elevated glomerular capillary pressure, whereas a more severe increase in RSNA decreased glomerular capillary pressure. This suggested that the nerves innervating the glomerular afferent and efferent arterioles could be selectively activated, allowing differential control of glomerular capillary pressure. A caveat to this conclusion was that intrarenal actions of neurally stimulated ANG II might have contributed to the increase in postglomerular resistance. This has now been investigated. Anesthetized rabbits were prepared for renal micropuncture and RSNA recording. One group (ANG II clamp) received an infusion of an angiotensin-converting enzyme inhibitor (enalaprilat, 2 mg/kg bolus plus 2 mg·kg−1·h−1) plus ANG II (∼20 ng·kg−1·min−1), the other vehicle. Measurements were made before (room air) and during 14% O2. Renal blood flow decreased less during ANG II clamp compared with vehicle [9 ± 1% vs. 20 ± 4%, interaction term (PGT) < 0.05], despite a similar increase in RSNA in response to 14% O2in the two groups. Arterial pressure and glomerular filtration rate were unaffected by 14% O2in both groups. Glomerular capillary pressure increased from 33 ± 1 to 37 ± 1 mmHg during ANG II clamp and from 33 ± 2 to 35 ± 1 mmHg in the vehicle group before and during 14% O2, respectively (PGT< 0.05). During ANG II clamp, postglomerular vascular resistance was still increased in response to RSNA during 14% O2, demonstrating that the action of the renal nerves on the postglomerular vasculature was independent of the renin-angiotensin system. This further supports our hypothesis that increases in RSNA can selectively control pre- and postglomerular vascular resistance and therefore glomerular ultrafiltration.

DIFFERENTIAL NEURAL CONTROL OF GLOMERULAR ULTRAFILTRATION

The renal nerves constrict the renal vasculature, causing decreases in renal blood flow (RBF) and glomerular filtration rate (GFR). Whether renal haemodynamics are influenced by changes in renal nerve activity within the physiological range is a matter of debate. We have identified two morphologically distinct populations of nerves within the kidney, which are differentially distributed to the renal afferent and efferent arterioles. Type I nerves almost exclusively innervate the afferent arteriole whereas type II nerves are distributed equally on the afferent and efferent arterioles. We have also demonstrated that type II nerves are immunoreactive for neuropeptide Y, whereas type I nerves are not. This led us to hypothesize that, in the kidney, distinct populations of nerves innervate specific effector tissues and that these nerves may be selectively activated, setting the basis for the differential neural control of GFR. In physiological studies, we demonstrated that differential changes in glomerular capillary pressure occurred in response to graded reflex activation of the renal nerves, compatible with our hypothesis. Thus, sympathetic outflow may be capable of selectively increasing or decreasing glomerular capillary pressure and, hence, GFR by differentially activating separate populations of renal nerves. This has important implications for our understanding of the neural control of body fluid balance in health and disease.

Resting discharge of human muscle spindles is not modulated by increases in sympathetic drive

There is evidence in experimental animals that, in addition to receiving fusimotor drive, muscle spindles are subject to modulation by the sympathetic nervous system. We examined the validity of this idea in human subjects by recording from muscle spindles in the relaxed ankle and toe extensor muscles during a strong and sustained physiological activation of muscle sympathetic outflow. Unitary recordings were made from 20 primary and 17 secondary muscle spindle afferents via a tungsten microelectrode inserted percutaneously into the peroneal nerve in 10 awake, healthy subjects seated with the legs supported in the extended position. ECG, blood pressure, respiration and calf circumference were also recorded. The majority of the muscle spindles were spontaneously active at rest; a background discharge was induced in four silent spindles by vibrating the tendon. A sustained increase in muscle vasoconstrictor activity, an increase in calf volume and a fall in pulse pressure were produced by subjects performing a 30-40 s maximal inspiratory breath-hold. Despite this strong increase in muscle sympathetic outflow no significant changes occurred in the discharge of either primary or secondary muscle spindle afferents, measured as a change in mean frequency and variability over sequential 5 s epochs and compared with the preceding period of rest. Strong chemoreceptor-driven sympathetic bursts during sustained expiratory breath-holds also failed to modulate the firing of 14 spindle endings. We conclude that a sustained, physiological increase in muscle sympathetic activity causes no detectable change in muscle spindle firing, lending no support to the concept that the sympathetic nervous system can influence the sensitivity of human muscle spindles directly.

Differential responses of regional sympathetic activity and blood flow to visceral afferent stimulation

The sympathetic nervous system is essential for the cardiovascular responses to stimulation of visceral afferents. It remains unclear how the reflex-evoked sympathetic output is distributed to different vascular beds to initiate the hemodynamic changes. In the present study, we examined changes in regional sympathetic nerve activity and blood flows in anesthetized cats. Cardiovascular reflexes were induced by either electrical stimulation of the right splanchnic nerve or application of 10 μg/ml of bradykinin to the gallbladder. Blood flows were measured using colored microspheres or the Transonic flow meter system. Sympathetic efferent activity was recorded from the left splanchnic, inferior cardiac, and tibial nerves. Stimulation of visceral afferents decreased significantly blood flows in the celiac (from 49 ± 4 to 25 ± 3 ml/min) and superior mesenteric (from 35 ± 4 to 23 ± 2 ml/min) arteries, and the vascular resistance in the splanchnic bed was profoundly increased. Consistently, stimulation of visceral afferents decreased tissue blood flows in the splanchnic organs. By contrast, activation of visceral afferents increased significantly blood flows in the coronary artery and portal vein but did not alter the vascular resistance of the femoral artery. Furthermore, stimulation of visceral afferents increased significantly sympathetic efferent activity in the splanchnic (182 ± 44%) but not in the inferior cardiac and tibial nerves. Therefore, this study provides substantial new evidence that stimulation of abdominal visceral afferents differentially induces sympathetic outflow to the splanchnic vascular bed.

Neural control of the kidney: functionally specific renal sympathetic nerve fibers

The sympathetic nervous system provides differentiated regulation of the functions of various organs. This differentiated regulation occurs via mechanisms that operate at multiple sites within the classic reflex arc: peripherally at the level of afferent input stimuli to various reflex pathways, centrally at the level of interconnections between various central neuron pools, and peripherally at the level of efferent fibers targeted to various effectors within the organ. In the kidney, increased renal sympathetic nerve activity regulates the functions of the intrarenal effectors: the tubules, the blood vessels, and the juxtaglomerular granular cells. This enables a physiologically appropriate coordination between the circulatory, filtration, reabsorptive, excretory, and renin secretory contributions to overall renal function. Anatomically, each of these effectors has a dual pattern of innervation consisting of a specific and selective innervation by unmyelinated slowly conducting C-type renal sympathetic nerve fibers in addition to an innervation that is shared among all the effectors. This arrangement permits the maximum flexibility in the coordination of physiologically appropriate responses of the tubules, the blood vessels, and the juxtaglomerular granular cells to a variety of homeostatic requirements.

Specificity in the organization of the autonomic nervous system: a basis for precise neural regulation of homeostatic and protective body functions

Experimental investigations of the lumbar sympathetic outflow to skin, skeletal muscle and viscera and the thoracic sympathetic outflow to the head and neck have shown that each target organ and tissue is supplied by one or two separate pathways which consists of sets of pre- and postganglionic neurons with distinct patterns of reflex activity. This probably applies to all sympathetic and parasympathetic systems. The specificity of the messages that these peripheral pathways transmit from the central nervous system arises from integration within precisely organized pathways in the neuraxis. The messages in these discrete functional pathways are transmitted to the target tissues often via organized neuroeffector junctions. Modulation in the periphery can occur within each pathway, both in ganglia and at the level of the effector organs. This organization is the basis not only for precise neural regulations of all homeostatic body functions in which the autonomic nervous system is involved but also the basis of one main component in the regulation of protective body functions: (a) Elementary defense behaviors which are organized in the mesencephalon (confrontational defense, flight, quiescence), (b) regulation of the immune system by the sympathetic nervous system, and (c) adaptive autonomic motor responses during basic emotions require precisely working autonomic, in particular sympathetic, systems. In this sense, the concept of the functioning of the sympathetic nervous system in an "all-or-none" fashion, without distinction between different effector organs, and of simple functional antagonistic organization between sympathetic and parasympathetic nervous system is misleading, inadequate and untenable.

Firing properties of single vasoconstrictor neurones in human subjects with high levels of muscle sympathetic activity

1. Single-unit recordings were made from 19 postganglionic muscle vasoconstrictor axons via tungsten microelectrodes in the peroneal nerve in seven healthy subjects with many multi-unit sympathetic discharges at rest ('high group', 75 +/- 5 multi-unit bursts per 100 heart beats, mean +/- s.e.m.). The results were compared with previous data from 14 units in subjects with 21 +/- 2 multi-unit bursts per 100 heart beats ('low group'). 2. In the 'high group' the units fired spontaneously in 35 +/- 4 % of all cardiac intervals. One unit only ever fired once per cardiac interval, 14 units (74 %) generated maximally two to three spikes, and four units (21 %) up to four to five spikes. Of those cardiac intervals in which a unit fired, a single spike occurred in 78 %, two spikes in 18 %, three spikes in 4 % and four spikes in less than 1 % of cardiac intervals. Measured as the inverse of all interspike intervals, the mean rate was 0.33 +/- 0.04 Hz and the mean intraburst frequency 22.2 +/- 1.6 Hz. Most results were similar to those in the 'low group', but in the 'low group' heart rate was higher (64.5 vs. 50.4 beats min-1) and mean firing frequency was higher (0.49 +/- 0.06 Hz). 3. During increases of multi-unit burst activity evoked by sustained inspiratory-capacity apnoea the firing probability of nine units in the 'high group' increased from 33 +/- 6 to 56 +/- 3 % of the cardiac intervals. Simultaneously, the incidence of single spikes decreased and the incidence of multiple spikes per cardiac interval increased, resulting in an increase of mean firing frequency from 0. 23 +/- 0.04 Hz at rest to 1.04 +/- 0.14 Hz during the apnoea. 4. We conclude that single muscle vasoconstrictor neurones usually fire only a solitary spike during sympathetic bursts both in subjects with a high and in subjects with a low number of bursts at rest. Presumably, differences in the numbers of bursts are due mainly to differences in firing probability and recruitment of sympathetic fibres. During acute increases of multi-unit activity, both increases in discharge frequency and recruitment of additional neurones contribute to the increased intensity of an individual sympathetic burst.

Respiratory and cardiac modulation of single sympathetic vasoconstrictor and sudomotor neurones to human skin

1. The firing of single sympathetic neurones was recorded via tungsten microelectrodes in cutaneous fascicles of the peroneal nerve in awake humans. Studies were made of 17 vasoconstrictor neurones during cold-induced cutaneous vasoconstriction and eight sudomotor neurones during heat-induced sweating. Oligounitary recordings were obtained from 8 cutaneous vasconstrictor and 10 sudomotor sites. Skin blood flow was measured by laser Doppler flowmetry, and sweating by changes in skin electrical resistance within the innervation territory on the dorsum of the foot. 2. Perispike time histograms revealed respiratory modulation in 11 (65 %) vasoconstrictor and 4 (50 %) sudomotor neurones. After correcting for estimated conduction delays, the firing probability was higher in inspiration for both classes of neurone. Measured from the oligounitary recordings, the respiratory modulation indices were 67. 7 +/- 3.9 % for vasoconstrictor and 73.5 +/- 5.7 % for sudomotor neurones (means +/- s.e.m.). As previously found for sudomotor neurones, cardiac rhythmicity was expressed by 7 (41 %) vasoconstrictor neurones, 5 of which showed no significant coupling to respiration. Measured from the oligounitary records, the cardiac modulation of cutaneous vasoconstrictor activity was 58.6 +/- 4.9 %, compared with 74.4 +/- 6.4 % for sudomotor activity. 3. Both vasoconstrictor and sudomotor neurones displayed low average firing rates (0.53 and 0.62 Hz, respectively). The percentage of cardiac intervals in which units fired was 38 % and 35 %, respectively. Moreover, when considering only those cardiac intervals when a unit fired, vasoconstrictor and sudomotor neurones generated a single spike 66 % and 67 % of the time. Rarely were more than four spikes generated by a single neurone. 4. We conclude that human cutaneous vasoconstrictor and sudomotor neurones share several properties: both classes contain subpopulations that are modulated by respiration and/or the cardiac cycle. The data suggest that the intensity of a multi-unit burst of vasoconstrictor or sudomotor impulses is probably governed primarily by firing incidence and the recruitment of additional neurones, rather than by an increase in the number of spikes each unit contributes to a burst.

Reflex effects on components of synchronized renal sympathetic nerve activity

The effects of peripheral thermal receptor stimulation (tail in hot water, n = 8, anesthetized) and cardiac baroreceptor stimulation (volume loading, n = 8, conscious) on components of synchronized renal sympathetic nerve activity (RSNA) were examined in rats. The peak height and peak frequency of synchronized RSNA were determined. The renal sympathoexcitatory response to peripheral thermal receptor stimulation was associated with an increase in the peak height. The renal sympathoinhibitory response to cardiac baroreceptor stimulation was associated with a decrease in the peak height. Although heart rate was significantly increased with peripheral thermal receptor stimulation and significantly decreased with cardiac baroreceptor stimulation, peak frequency was unchanged. As peak height reflects the number of active fibers, reflex increases and decreases in synchronized RSNA are mediated by parallel increases and decreases in the number of active renal nerve fibers rather than changes in the centrally based rhythm or peak frequency. The increase in the number of active renal nerve fibers produced by peripheral thermal receptor stimulation reflects the engagement of a unique group of silent renal sympathetic nerve fibers with a characteristic response pattern to stimulation of arterial baroreceptors, peripheral and central chemoreceptors, and peripheral thermal receptors.

Pattern of ongoing discharge of single renal sympathetic neurons in the rabbit

Ongoing discharge in single renal sympathetic neurons was first studied in vagotomized rabbits without baroreceptor information (60 min after section of the aortic nerves). Under urethane + chloralose anaesthesia interspike-interval histograms were compiled and discharge rates were measured in 79 neurons. The following parameters were analysed: (a) the shortest, (b) the preferred and (c) the longest interspike-intervals, (d) discharge rate, (e) spread of a histogram, (f) coefficient of symmetry, and (g) coefficient of variability. The type of distribution of histograms and 9 correlations between some parameters were also assessed. These parameters were considered to make up the pattern of the ongoing discharge. The shortest, preferred and longest interspike-intervals of the ongoing discharge were: 14.1 +/- 0.9, 30.4 +/- 3.5 and 1672 +/- 82 ms. The mean rate of discharge amounted to 1.78 +/- 0.08 spikes/s. Three out of 9 correlation coefficients between the above parameters were statistically significant. In a second part, the effect of section of the aortic nerves and of 4-aminopyridine (a drug known to enhance synaptic transmission) on the pattern of ongoing discharge were also studied. Ten minutes after section of the aortic nerves the rate of discharge significantly increased, the shortest interspike-interval diminished and coefficient of variability was not changed. The number of significant correlations rose from 3 to 9.4-Aminopyridine significantly increased the discharge rate, did not alter the shortest interspike-interval and increased the coefficient of variability. These data show that assessing several parameters of ongoing discharge making up its pattern may differentiate between the excitatory effects of section of the aortic nerves and administration of 4-aminopyridine and in this way help to elucidate the mechanisms of action of various factors affecting renal sympathetic discharge.

Cardiovascular and hormonal responses to food ingestion in humans with spinal cord transection

In sympathetic denervation due to primary autonomic failure, ingestion of food causes a fall in blood pressure (BP) and exacerbates postural hypotension. It is not known whether these responses occur in tetraplegics with physiologically complete cervical spinal cord transection, who also have sympathetic dysfunction because of disruption of descending spinal sympathetic pathways. We, therefore, studied the effect of a liquid meal on BP, heart rate (HR) and neurohormonal levels in tetraplegics. Paraplegics with low lesions and without sympathetic dysfunction served as controls. After food ingestion, there was no fall in BP in tetraplegics or in controls. HR did not change in either group. After fund, plasma noradrenaline was unchanged in tetraplegics, but rose in controls, while plasma renin activity (PRA) rose in tetraplegics but not in controls. The fall in BP and rise in HR on head-up tilt after the meal in tetraplegics was similar to that before the meal. There was no change in PRA following pre-prandial tilt in either group; post-prandial tilt raised levels in the tetraplegics, unlike in controls. Thus there is considerable variance in the responses to food between tetraplegics and paraplegic controls, and even greater differences when compared with published data in other autonomic disorders with sympathetic dysfunction; this may relate to the site and the nature of the sympathetic lesion and the ability to activate compensatory mechanisms.

Identification of sympathetic preganglionic neurons controlling the small intestine in hamsters using a recombinant herpes simplex virus type-1

Sympathetic preganglionic neurons (SPNs) may be organized topographically within the spinal cord for selective control of visceral organs. We used a recombinant herpes simplex virus type-1 (rHSV-1) to identify SPNs innervating the small intestine in hamsters. These SPNs were distributed bilaterally in the cord from the fifth thoracic spinal segment to the second lumbar segment, but predominantly in thoracic segments 5-10. They had morphology similar to that of renal and adrenal SPNs infected with HSV-1. The majority of intestinal SPNs were found in the intermediolateral cell column, with a few located in the lateral funiculus. The SPNs labelled following duodenal injection of rHSV-1 were in the same spinal segments as the SPNs labelled following jejunal or ileal injections, suggesting lack of a relation between target topography and the topographic organization of these neurons. In addition, intestinal SPNs were located in the same spinal segments, and autonomic nuclei as renal and adrenal SPNs suggesting that SPNs controlling the abdominal viscera are not organized viscerotopically for discrete control of different organs.

The discharge behaviour of single sympathetic neurones supplying human sweat glands

Firing properties of single sudomotor axons were studied via tungsten microelectrodes inserted percutaneously into cutaneous fascicles of the peroneal nerve in awake subjects. Sweating was induced by radiant heat and measured by changes in skin electrical resistance within the innervation territory on the dorsum of the foot. Eight units were classified as sudomotor neurones because spike-triggered averaging revealed a time-locked relationship between the unitary discharge and the subsequent decrease in skin resistance (1.12 +/- 0.05 s), but no relationship to skin blood flow (measured by a laser-doppler probe). Sudomotor units usually fired only one (maximum six) spike(s) in a sympathetic burst. The mean firing rate was 0.62 Hz, but instantaneous frequencies above 50 Hz could be generated. R-wave triggered histograms and coherence analysis revealed significant coupling between the firing of three sudomotor neurones and the ECG. Moreover, the firing of four sudomotor neurones showed a weak but significant correlation with the spontaneous fluctuations in cardiac interval, diastolic pressure, or the rate of fall in arterial pressure. We conclude that the discharge of human sudomotor neurones is modulated by baroreceptor input.

The discharge behaviour of single vasoconstrictor motoneurones in human muscle nerves

1. The discharge behaviour of fourteen single sympathetic vasoconstrictor efferents was studied using a tungsten microelectrode inserted percutaneously into a motor fascicle of the radial or peroneal nerve in eight awake supine subjects. Units were classified as vasoconstrictor because their firing properties correlated appropriately to changes in cardiac interval and arterial pressure. 2. On average, individual vasoconstrictor units discharged in only 21% of heart beats, with an overall mean frequency of 0.47 Hz. Usually only one spike was generated per cardiac cycle. Calculated from cardiac cycles in which a unit fired from two to seven spikes, the mean within-burst firing rate was 18.8 +/- 2.5 Hz (mean +/- S.E.M.); but instantaneous frequencies above 50 Hz were occasionally observed. 3. Measured from a defined R-wave of the ECG, the spike onset latency varied over 358 +/- 33 ms, suggesting considerable variation of synaptic delays in the baroreflex arc. This latency had a relatively uniform temporal relationship with the burst onset or peak latency, compatible with a fixed recruitment order of individual sympathetic neurones. 4. In view of the low average firing rate of individual units we suggest that the variable instantaneous firing rates may optimize the contractile responses of vascular smooth muscle.

Human spinal lateralization assessed from motoneurone synchronization: dependence on handedness and motor unit type

1. Motoneurone synchronization as a means of investigating synaptic connectivity was studied in the extensor carpi radialis muscles of the preferred and non-preferred arms of healthy right- and left-handed human subjects. The activities of pairs of motor units recorded during voluntary isometric contractions were analysed by cross-correlation to detect any synchronous motor unit firing in the form of central peaks in the cross-correlation histograms. 2. The synchronization peaks were compared first in the case of 273 motor unit pairs tested in the preferred and non-preferred arms of two left-handed subjects and two right-handed subjects. The percentage of synchronized motor unit pairs was found to be significantly higher in the preferred arm with synchronization peaks significantly larger and broader than in the non-preferred arm. The narrow peaks (< 7.5 ms) likely to reflect the activity of common inputs to motoneurones were also found to be significantly larger in the preferred arm of all four subjects. 3. The handedness-related differences in synchronization were definitely confirmed in a total of 275 pairs of motor units tested in the left extensor carpi radialis muscles of fourteen right-handed subjects using their non-preferred arm and six left-handed subjects using their preferred arm. In order to determine whether the differences in synchronization were dependent on the motor unit type, each motor unit was characterized on the basis of its recruitment threshold and on the basis of the contraction time of its twitch extracted from the overall muscle force using the spike-triggered averaging method. Two populations of motor units were distinguished, namely the 'slow' motor units (recruitment thresholds < 0.4 N, contraction times > 40 ms) and the 'fast' motor units (recruitment thresholds > 0.6 N, contraction times < or = 40 ms). 4. In the non-preferred arm, the synchronization peaks were always fairly narrow, whatever the motor unit's biomechanical properties; whereas in the preferred arm, broad peaks were found to be particularly common among the pairs including one or two fast motor units, which also showed the largest rate of synchronization occurrence. 5. The narrow peaks (< 7.5 ms) were found to be consistently larger in the preferred than the non-preferred arm whatever the categories of motor unit pairs. In both arms, however, the amplitude of the narrow peaks tended to increase as the recruitment threshold of the motor unit decreased and as their contraction time increased.(ABSTRACT TRUNCATED AT 400 WORDS)

The frequency content of common synaptic inputs to motoneurones studied during voluntary isometric contraction in man

1. The discharges of pairs of individual motor units were recorded from intrinsic hand muscles in man. Single motor unit recordings were obtained either when both members of the motor unit pair were within first dorsal interosseous muscle (1DI:1DI recordings) or where one motor unit was within 1DI and the other in second dorsal interosseous muscle (1DI:2DI recordings). The pairs of motor unit spike trains were cross-correlated in the time domain and the results compared to those of coherence analysis performed on the same spike train data. Central peaks were present in the cross-intensity functions, indicating the presence of common synaptic input to the motoneurone pair. Coherence analysis of these data indicated significant association between motor unit firing in the frequency ranges 1-12 and 16-32 Hz. 2. Analysis of sequential non-overlapping segments of data recorded from individual motor unit pairs, demonstrated that both the central cross-intensity peak and coherence in the frequency bands 1-12 and 16-32 Hz were consistent features throughout a long recording. In these sequential recordings, the size of the central cross-intensity peak and the maximal value of coherence in the frequency band 16-32 Hz covaried from segment to segment. Analysis of the entire population of motor unit pairs confirmed a positive relationship between the magnitude of peak coherence and the size of the central cross-intensity peak. 3. Voluntary sinusoidal co-modulation of the firing rates of pairs of individual motor units recorded from within 1DI was found to produce significant values of coherence corresponding to the frequency of the common modulation. However, firing rate co-modulation was not found to affect either the size of the central cross-intensity peak or the maximum value of coherence in the frequency band 16-32 Hz. 4. Pairs of single motor units were recorded from within 1DI and biceps brachii muscles of healthy subjects. The number and size of the central cross-intensity peaks and coherence peaks detected were compared for the two muscles. The incidence and size of central cross-intensity peaks and the incidence and magnitude of 16-32 Hz coherence peaks were both found to be greater for 1DI recordings when compared to biceps brachii recordings. 5. Single motor unit recordings were made from the intrinsic hand muscles of a patient with severe peripheral deafferentation. Time- and frequency-domain analysis of these recordings revealed cross-intensity peaks and frequency bands of coherence similar to those seen in healthy subjects.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of renal sympathetic preganglionic neurons in hamsters using transsynaptic transport of herpes simplex type 1 virus

Herpes viruses have been used as retrograde transsynaptic tracers to identify pathways from the CNS to specific target tissues. We used herpes simplex virus to identify central nervous system neurons responsible for control of the kidney. Herpes simplex type 1 or herpes simplex type 2 was injected into rat kidneys and herpes simplex type 1 was microinjected into hamster and guinea pig kidneys. After three to seven days, ganglia, spinal cords and brains were examined using immunohistochemistry to visualize the virus-infected neurons. Our first experiments demonstrated that rats were not susceptible to infection with neurotropic strains of herpes simplex type 1. Injections of a wildtype strain of herpes simplex type 2 into rat kidneys led to nonspecific infection of many central nervous system neurons and glia. In contrast, herpes simplex type 1 injections in hamsters and guinea pigs caused specific infection of limited numbers of neurons in approximately one-third of the animals and the study was continued using hamsters. Sympathetic preganglionic neuron labelling was found in the ipsilateral intermediolateral cell column of the spinal cord as well as the lateral funiculus. Most infected preganglionic neurons were located in the seventh to the ninth thoracic spinal segments. Infected neurons were not found in the dorsal or ventral horn of the spinal gray matter and only one or two cells were found in the brainstem. Sympathetic preganglionic neuron morphology was usually normal, showing detailed dendritic arborizations, and lysis was infrequent. Small infected cells were sometimes observed close to sympathetic preganglionic neurons. Because herpes simplex type 1 virus was not detected immunocytochemically in ganglionic neurons in these same hamsters, the polymerase chain reaction was used in some additional hamsters to detect viral DNA in the T12 and T13 chain ganglia and splanchnic ganglia ipsilateral to the kidney injected with herpes simplex type 1. Finally, the overall distribution of renal postganglionic and splanchnic preganglionic neurons in hamsters was examined for comparison to the number and locations of virus-labelled neurons. Retrograde transport of the fluorescent dye FluoroGold demonstrated that (i) renal postganglionic neurons are distributed in the T10-L1 chain ganglia and in the prevertebral splanchnic ganglion and (ii) splanchnic preganglionic neurons are located in the T3-T12 spinal segments, predominantly in the intermediolateral and funicular spinal autonomic nuclei. In conclusion, herpes simplex type 1 virus infected an exclusive population of "renal" neurons in hamsters without lysis and with little cellular reaction to the infection after a survival period of three days, permitting these neurons to be studied in detail.

The perceptions of force and of movement in a man without large myelinated sensory afferents below the neck

1. Motor memory and the sense of effort have been investigated in a man with a complete large fibre sensory neuropathy for over 16 years. The perceptions of pain, heat, cold and muscular fatigue remained but he was without perceptions of light touch and proprioception below the neck. 2. The subject was able to discriminate weights held in the hand with an accuracy only slightly worse than control subjects (20 g in 200 g) when forearm movement and visual inspection were allowed. With eyes shut however he could only distinguish a weight of 200 g from 400 g. It is concluded that a crude sense of effort remains which may have a peripheral origin. 3. A limited motor memory was also present, which allowed him to maintain a posture or continue a simple repetitive movement. No novel movement was possible without visual feedback. 4. Differences in movement ability between this subject and others with similar if less pure sensory neuropathies are ascribed to rehabilitation.

Evidence for descending tonic inhibition specifically affecting sympathetic pathways to the kidney in rats

1. The present study investigated the possibility that pre- and postganglionic neurones innervating the kidney and spleen in rats are affected by descending inhibitory as well as descending excitatory influences. This hypothesis was tested by comparing the effects of cervical spinal cord transection to the effects of blockade of tonic activity of excitatory neurones in the rostral ventrolateral medulla (RVLM). 2. Electrical discharge of multifibre postganglionic renal and splenic and preganglionic greater splanchnic nerves and 13th thoracic (T13) white rami was recorded in artificially respired, urethane-anaesthetized rats. In one group of rats, descending supraspinal pathways were interrupted by cervical spinal cord transection. In another group, tonic activity of rostral ventrolateral medulla (RVLM) neurones was blocked by bilateral microinjections of the inhibitory amino acid glycine. The effects of spinal cord transection were compared to effects of this bilateral RVLM blockade and to effects of unilateral RVLM blockade described in a previous study. 3. Spinal cord transection caused decreases in preganglionic greater splanchnic and postganglionic splenic nerves which were of the same magnitude as those caused by bilateral blockade of the RVLM. 4. In contrast, discharge of renal nerves was decreased more by bilateral RVLM blockade than by cervical spinal cord transection. Similarly, even unilateral RVLM blockade caused greater decreases in discharge of T13 white rami than were caused by spinal cord transection. 5. These findings suggest that renal nerves and their preganglionic inputs (T13 white rami) are controlled in part by tonic sympathoinhibitory influences which can be unmasked by blockade of the RVLM. These sympathoinhibitory influences do not appear to affect the activity of splanchnic and splenic nerves.

Correlation between the discharges of motor units recorded from the same and from different finger muscles in man

1. Cross-correlation analysis of the discharges of individual motor units recorded from various different finger muscles has been performed during weak, isometric, voluntary contractions in man. 2. The dominant feature in 88% of the cross-correlograms studied was a narrow, central peak, the area of which significantly exceeded that expected for independent processes (P less than 0.001). The highest bin counts in these central peaks were mostly within 5 ms of time zero in the histograms, and the base of these peaks extended between 5 and 31 ms (modal value = 13 ms with 90% of the values lying between 8 and 18 ms). The width and displacement of the central cross-correlogram peaks were similar irrespective of whether the contributory spike trains were recorded from motor units active in the same finger muscle or recorded from motor units in different, co-activated finger muscles. 3. The time course of the central peaks in this study was found to be consistent with the hypothesis that it is generated by the joint occurrence of EPSPs evoked in motoneurones by branches of common stem presynaptic fibres using the theoretical model developed by Kirkwood (Kirkwood & Sears, 1978). The model parameters providing the best fit with our experimental data imply that synaptic contacts on motoneurones made by these common inputs lie on average peripherally in the dendritic tree and generate small (less than 300 microV) EPSPs superimposed on a high level of background synaptic noise. 4. Minima (troughs) were found either side of the central peak in 27% of the cross-correlograms studied, and their appearance was invariably associated with a large central peak. These secondary features could not be modelled with the same operator parameters that describe the central peaks. Their presence was particularly noticed in association with very regular discharges from the output motoneurones. 5. Smaller and broader secondary peaks symmetrically displaced 30-55 ms either side of the large, narrow central peak were observed in 7% of the cross-correlograms studied. We suggest that these secondary features which were found at lags shorter than the interspike interval of the contributory motor unit spike trains reflect the autocorrelation functions of the spike trains of common input fibres. On this basis the observed displacement of these secondary peaks from the primary feature in the cross-correlogram indicate firing rates for common input fibres in the range 18-33 impulses s-1. 6. In a small number of cases (1.4%) the cross-correlogram was flat and indistinguishable from the results of cross-correlating independent spike train data.

Central nervous pathways underlying synchronization of human motor unit firing studied during voluntary contractions

1. Motor unit firing has been studied during weak voluntary isometric contractions with pairs of needle electrodes in normal human subjects. 2. Pre- and post-stimulus time histograms of the firing time of firing of one event unit before and after the time of firing of another reference (stimulus) unit showed a clear central peak, indicative of synchronization. 3. Synchronization was seen in all the muscles studied. The mean strength of synchronization, expressed as the number of concomitant discharges of the two units as a proportion of the number of stimulus unit discharges, was 0.095 extra event unit spikes/reference unit spike (range 0.042-0.28) for first dorsal interosseous muscle, 0.016 extra event unit spikes per reference unit spike (range 0-0.043) for medial gastrocnemius and 0.056 extra event unit spikes per reference unit spike range 0.016-0.079) for tibialis anterior. 4. The mean duration of synchronization was 11.3 ms (range 5.0-21.0 ms) for first dorsal interosseous, 10.3 ms (range 3.5-21.7 ms) for medial gastrocnemious and 13.5 ms (range 3.0-25.0) for tibialis anterior. 5. Seven patients with radiographically and clinically identified central strokes were studied while they made weak voluntary isometric contractions. The duration of synchronization was significantly prolonged compared to that found in normal subjects. In these stroke patients the mean duration of synchronization on the affected side was longer than that seen in the normal subjects, and in first dorsal interosseous muscle was 35.4 ms (range 12.0-65.0 ms), in medial gastrocnemius was 21.3 ms (range 4.0-43.0 ms) and in tibialis anterior was 28.8 ms (range 14.0-49.0 ms). 6. The mean strength of synchronization of motor unit discharge was found to be greater in the stroke patients than that seen in the normal subjects for first dorsal interosseous muscle (0.161 extra event unit spikes per reference unit spike, range 0.017-0.391) and for medial gastrocnemius (0.030 extra event unit spikes per reference unit spike) but only significantly so when pooled data was compared. There was no difference in the strength of motor unit synchronization in tibialis anterior between stroke patients and normal subjects. 7. Broad duration synchronization among first dorsal interosseous motor units was also found in a patient with a rostral cervical spine lesion (total duration range 43-46 ms; n = 2), but not in a patient with a caudal (thoracic) spinal lesion.(ABSTRACT TRUNCATED AT 400 WORDS)

Mirror movements studied in a patient with Klippel-Feil syndrome

1. Electromyographic (EMG) recordings have been made from upper limb muscles in a patient with well-defined congenital mirror movements occurring in association with Klippel-Feil syndrome and the results compared to those obtained in normal control subjects. 2. In the patient, liminal percutaneous electrical or magnetic brain stimulation applied over either hemisphere elicited bilateral and symmetrical short-latency muscle responses in relaxed intrinsic hand muscles. In the normal subjects unilateral brain stimulation only elicited contralateral muscle responses. 3. F response and H reflex studies for the patient's ulnar-supplied intrinsic hand muscles were normal. No crossed responses were recorded in the homologous muscles of the contralateral hand. 4. Scalp-recorded somatosensory-evoked responses following ulnar or median nerve stimulation were of normal latency and distribution in the patient. 5. In the patient, cross-correlation analysis of on-going single and multiunit needle EMGs recorded between muscles of left and right hands revealed a central peak in the cross-correlogram. No cross-correlogram peaks were found between left- and right-hand muscles in normal subjects. The magnitude and time course of the central peaks in the cross-correlograms constructed between the firing of motor units on opposite sides of the body in the patient were similar to those found in cross-correlograms constructed between the firing of motor units from muscles on the same side of the body in the patient and in normal subjects. 6. The magnitude of cross-correlogram peaks detected within a muscle and those detected between left and right homologous muscles showed a gradient in which the largest peaks were found in the intrinsic hand and forearm extensor muscles. The smallest peaks were observed in the forearm flexor muscles. No peaks were detected between left and right biceps brachii muscles. In intrinsic hand muscles, the size of the cross-correlogram peak detected between the EMGs of homologous muscle pairs was greater than that found for non-homologous muscle pairs. 7. Cutaneous reflex responses were recorded from first dorsal interosseous muscle following unilateral electrical stimulation of the digital nerves of the index finger. In the patient, this produced an early excitatory (E1) response on the stimulated side. Later excitatory (E2 and E3) responses, of approximately equal size and latency, were distributed bilaterally. In the normal subjects, reflex responses were confined to the stimulated side.(ABSTRACT TRUNCATED AT 400 WORDS)

Motor unit discharge characteristics and short term synchrony in paraplegic humans

Frequency of firing and regularity of discharge of human motor units, and short term synchrony between pairs of motor units, have been assessed in extensor digitorum communis (EDC) and tibialis anterior (TA) muscles in control subjects and in clinically complete paraplegic subjects. The discharge pattern of TA motor units in paraplegia ranged from extremely regular to very irregular for different motor units whereas in the control population, and in EDC of both groups, there was a narrow, but intermediate, range of regularity. There was little difference in the incidence and degree of short term synchrony (STS) in EDC between paraplegic and normal subjects. In contrast, virtually no STS of motor units was observed in the TA muscles of the paraplegic group whereas control subjects exhibited approximately the same amount of STS in their TA and EDC muscles. It is concluded that the extra burden placed on arm muscles in paraplegia does not change the amount of synchronisation between motor units. Furthermore, section of the spinal cord does not increase STS as predicted from lesions of the reticulospinal tract in cats. This may reflect the coincidental removal of supraspinal synchronising inputs of motoneurons or the reorganisation of synaptic inputs in chronic paraplegia.

Selective control of sympathetic pathways to the kidney, spleen and intestine by the ventrolateral medulla in rats

1. Electrical activity of multifibre renal, splenic, mesenteric and greater splanchnic nerves and 13th thoracic white rami was recorded in artificially respired, urethane-anaesthetized rats. Discharge of neurones in the rostral ventrolateral medulla was blocked by unilateral microinjections of the inhibitory amino acid glycine and effects on the electrical activity of these sympathetic nerves were compared. 2. Blockade of the rostral ventrolateral medulla caused greater decreases in discharge of renal than splenic nerves and had no consistent effect on mesenteric nerves. This blockade also decreased the discharge of the preganglionic white rami more than that of the preganglionic splanchnic nerves. 3. Postganglionic responses to rostral ventrolateral medulla blockade were always greater than preganglionic responses. 4. The arterial pressure and renal nerve responses to rostral ventrolateral medulla blockade in urethane-anaesthetized rats were not different from those in rats anaesthetized with alpha-chloralose. 5. These findings demonstrate that pre- and postganglionic sympathetic pathways to the kidney are more dependent upon excitatory drive from the rostral ventrolateral medulla than pathways directed to the spleen and intestine.

Visceral pain: a review of experimental studies

This paper reviews clinical and basic science research reports and is directed toward an understanding of visceral pain, with emphasis on studies related to spinal processing. Four main types of visceral stimuli have been employed in experimental studies of visceral nociception: (1) electrical, (2) mechanical, (3) ischemic, and (4) chemical. Studies of visceral pain are discussed in relation to the use and 'adequacy' of these stimuli and the responses produced (e.g., behavioral, pseudoaffective, neuronal, etc.). We propose a definition of an adequate noxious visceral stimulus and speculate on spinal mechanisms of visceral pain.

Peripheral presynaptic and central effects of clonidine, yohimbine and rauwolscine on the sympathetic nervous system in rabbits

The function of presynaptic alpha 2-autoreceptors at postganglionic sympathetic neurones under conditions of normal, ongoing sympathetic impulse traffic was studied in anaesthetized rabbits (alfadolone + alfaxalone). Clonidine was used as an alpha 2-adrenoceptor agonist, and yohimbine and rauwolscine were used as antagonists. Mean arterial pressure, postganglionic renal sympathetic firing rate, arterial plasma noradrenaline concentration and heart rate were measured before (basal values) and at the end of 3-min infusions of sodium nitroprusside and phenylephrine, which were given to modulate efferent activity in the sympathetic nervous system through the baroreflex. The nitroprusside- and phenylephrine-induced changes of mean arterial pressure produced the expected changes in sympathetic nerve activity, plasma noradrenaline and heart rate. Clonidine (5 micrograms kg-1 + 0.5 micrograms kg-1 min-1) reduced the basal mean arterial pressure, sympathetic nerve activity and heart rate. It also reduced the nitroprusside-induced increase in the plasma noradrenaline level without changing the nitroprusside-induced increase in sympathetic firing. These results, as well as the mean arterial pressure-sympathetic nerve activity and the sympathetic nerve activity-plasma noradrenaline function curves indicate that clonidine inhibited both sympathetic tone centrally and the average release of noradrenaline per action potential peripherally. Yohimbine (1 mg kg-1 + 0.1 mg kg-1 h-1) and rauwolscine (0.5 mg kg-1 + 0.1 mg kg-1 h-1) increased the basal plasma noradrenaline level without any increase of renal sympathetic nerve activity. They also enhanced the nitroprusside-induced increase in plasma noradrenaline without any enhancement of the nitroprusside-induced increase in sympathetic firing. The hypotensive response to nitroprusside was attenuated, whereas the heart rate response was augmented. These results, as well as the mean arterial pressure-sympathetic nerve activity and the sympathetic nerve activity-plasma noradrenaline function curves indicate that the main effect of yohimbine and rauwolscine was to increase the average release of noradrenaline per action potential. The simultaneous measurement of postganglionic sympathetic nerve activity and the arterial plasma noradrenaline concentration proved suitable to differentiate central (or ganglionic; this distinction was not possible) effects of alpha 2-adrenoceptor ligands from peripheral presynaptic effects. The results show that endogenous presynaptic, alpha 2-adrenergic autoinhibition of noradrenaline release from postganglionic sympathetic neurones operates physiologically in anaesthetized rabbits with ongoing, uninterrupted sympathetic nerve activity. The results also indicate that blockade of alpha 2-autoreceptors enhances the sympathetic reflex compensatory response to a hypotensive stimulus.

Synchronization of motor unit firing during different respiratory and postural tasks in human sternocleidomastoid muscle

1. Motor unit firing has been studied in human sternocleidomastoid muscle. 2. Two needle electrodes were inserted into the muscle and the activity of pairs of motor units recorded during (a) reflex hypercapnic obstructed breathing, (b) eucapnic voluntary copying of (a) against the same inspiratory resistance and (c) voluntary copying of (a) without any resistance, accompanied by isometric neck rotation. 3. Cross-correlation histograms of the firing of unit pairs showed a clear central peak, indicative of synchronization. The mean duration of the peak during voluntary breathing was 25 ms (range 9-40 ms). There was no difference in duration of synchronization during the different tasks. 4. For the duration of the synchronization peak, the mean strength of synchronization expressed as the number of concomitant discharges of the two units as a proportion of the total number of discharges was 0.026 (range 0.011-0.058) for reflex hypercapnic obstructed breathing. For the same unit pairs the strength of synchronization for isometric neck rotation was the same as that during reflex hypercapnic breathing but for voluntary obstructed breathing it was, on average, threefold greater. 5. In three out of twenty-two motor units studied, 'discharge' occurred with an interval of less than 10 ms ('doublet' firing) at the onset of each inspiration during both types of obstructed breathing; this was rarely observed during neck rotation. 6. The results are interpreted in terms of different synaptic drives to the motor units during the three different tasks.

Local application of neurotensin to abdominal organs triggers cardiovascular reflexes in guinea pigs: possible mechanisms

Intraabdominal (IAB) injections or topical application of neurotensin (NT) to the serosal surface of the ileum or stomach evoked dose-dependent increases of blood pressure and of heart rate in anesthetized guinea pigs. These effects were markedly reduced by prior animal treatment with a ganglion blocker, alpha and beta adrenoceptor blockers, as well as by exposure of the abdominal organs to lidocaine, a local anesthetic. The blood pressure and heart rate responses to IAB injections or topical application of NT to the ileum or stomach were both inhibited by animal pretreatment with capsaicin. Cervical vagotomy or atropine pretreatment did not prevent or alter the cardiovascular responses to IAB injections of NT. These results suggest the presence in some organs and/or tissues of the abdominal cavity of sympathetic, capsaicin-sensitive sensory nerve fibers which, upon stimulation by NT, produce reflex increases of blood pressure and of heart rate.

Ventrolateral medullary neurones: effects on magnitude and rhythm of discharge of mesenteric and renal nerves in cats

1. Discharge of whole mesenteric and renal nerves was recorded in eighteen chloralose-anaesthetized, artificially respired cats. 2. Inhibition of tonic activity of neurones within the rostral ventrolateral medulla (RVLM blockade) by bilateral application of glycine caused significant reductions in discharge of renal and mesenteric nerves, arterial blood pressure and heart rate. The decrease in discharge of renal nerves was significantly greater than that of mesenteric nerves. 3. During the response to glycine application, the spinal cord was transected at the first cervical segment. The magnitude of renal nerve discharge after transection was not different from that during blockade of the RVLM. On the other hand, mesenteric nerve activity increased following spinal cord transection, returning to control levels. 4. Power spectral analysis revealed that mesenteric and renal nerves discharged with periodicities ranging from 1 to 6 Hz. Application of glycine to the RVLM reduced the slow rhythm in firing of mesenteric and renal nerves similarly. Transection of the spinal cord resulted in further reduction in the rhythmicity in discharge of both nerves. 5. The results indicate that excitatory drive from the RVLM is crucial for the maintenance of on-going discharge of renal, but not of mesenteric nerves. However, such inputs are apparently essential to maintain the slow rhythm in firing of both nerves.

Persistent firing of splenic and renal nerves after acute decentralization but failure to produce ganglionic reflexes

Experiments were done to evaluate the contribution of peripheral neural circuits to generation of ongoing splenic and renal sympathetic discharge as well as to the reflex alteration of this discharge by chemical stimulation of receptors of intestinal afferent nerves. After decentralization of the celiac and superior mesenteric ganglia, low amplitude spikes with low discharge rates still were observed in both nerves. Stimulation of intestinal receptors with bradykinin or capsaicin did not alter this residual firing. Cholinergic blockade eliminated most of this discharge. The source of the residual firing and its contribution to basal discharge of splenic and renal nerves remains to be determined.

Characteristics of ongoing and reflex discharge of single splenic and renal sympathetic postganglionic fibres in cats

1. Electrical discharge of thirty-nine single splenic and renal postganglionic nerve fibres was recorded in artificially respired, chloralose-anaesthetized cats. 2. Ongoing discharge rates, averaged over 10 s periods, did not differ between renal and splenic fibres. All neurones of both groups had irregular discharge frequencies. 3. Half of the splenic population and all renal fibres had cardiac-related discharge patterns. Of those tested for respiratory-related firing, 30% of the splenic fibres and 69% of the renal fibres exhibited this pattern. 4. Firing of splenic fibres was less inhibited than that of renal fibres by stimulation of pressoreceptors with phenylephrine-induced increases in blood pressure. Firing of splenic fibres also was less excited than that of renal fibres by unloading pressoreceptors with depressor doses of sodium nitroprusside. 5. Chemical stimulation of splenic afferent nerves with bradykinin consistently elicited greater increases in splenic than renal nerve discharge by causing large increases in firing of all splenic fibres and smaller excitatory responses in 75% of the renal fibres. 6. Application of bradykinin to the intestinal serosa produced greater increases in renal than splenic nerve discharge by consistently causing increased firing of renal fibres and by causing excitation, inhibition, or no change in splenic fibre discharge. 7. Responses of splenic and renal fibres to stimulation of splenic and intestinal afferent nerves after spinal cord transection were similar to those responses elicited when the neuraxis was intact. 8. In conclusion, the differential reflex responses of splenic and renal neuronal populations can be due to the heterogeneity or to the intensity of responses within a neuronal population.