Cardiac rhythmicity of skin sympathetic activity recorded from peripheral nerves in man

Vaso-Occlusion in Sickle Cell Disease: Is Autonomic Dysregulation of the Microvasculature the Trigger?

Sickle cell disease (SCD) is an inherited hemoglobinopathy characterized by polymerization of hemoglobin S upon deoxygenation that results in the formation of rigid sickled-shaped red blood cells that can occlude the microvasculature, which leads to sudden onsets of pain. The severity of vaso-occlusive crises (VOC) is quite variable among patients, which is not fully explained by their genetic and biological profiles. The mechanism that initiates the transition from steady state to VOC remains unknown, as is the role of clinically reported triggers such as stress, cold and pain. The rate of hemoglobin S polymerization after deoxygenation is an important determinant of vaso-occlusion. Similarly, the microvascular blood flow rate plays a critical role as fast-moving red blood cells are better able to escape the microvasculature before polymerization of deoxy-hemoglobin S causes the red cells to become rigid and lodge in small vessels. The role of the autonomic nervous system (ANS) activity in VOC initiation and propagation has been underestimated considering that the ANS is the major regulator of microvascular blood flow and that most triggers of VOC can alter the autonomic balance. Here, we will briefly review the evidence supporting the presence of ANS dysfunction in SCD, its implications in the onset of VOC, and how differences in autonomic vasoreactivity might potentially contribute to variability in VOC severity.

Neck movement but not neck position modulates skin sympathetic nerve activity supplying the lower limbs of humans

We previously showed that dynamic, but not static, neck displacement modulates muscle sympathetic nerve activity (MSNA) to lower limbs of humans. However, it is not known whether dynamic neck displacement modulates skin sympathetic nerve activity (SSNA). Tungsten microelectrodes inserted into the common peroneal nerve were used to record SSNA in 5 female and 4 male subjects lying supine on a table that fixed their head in space but allowed trapezoidal ramp (8.1 ± 1.2°/s) and hold (17.5° for 53 s) or sinusoidal (35° peak to peak at 0.33-0.46 Hz) horizontal displacement of the body about the head. SSNA recordings were made before, during, and after trapezoidal and sinusoidal displacements of the body. Spike frequency analysis of trapezoidal displacements and cross-correlation analysis during sinusoidal displacements revealed that SSNA was not changed by trapezoid body-only displacement but was cyclically modulated during sinusoidal angular displacements (median, 95% CI: 27.9%, 19.6-48.0%). The magnitude of this modulation was not statistically ( P > 0.05) different from that of cardiac and respiratory modulation at rest (47.1%, 18.7-56.3% and 48.6%, 28.4-59.3%, respectively) or during sinusoidal displacement (10.3%, 6.2-32.1% and 26.9%, 13.6-43.3%, respectively). Respiratory frequency was entrained above its resting rate (0.26 Hz, 0.2-0.29 Hz) during sinusoidal neck displacement; there was no significant difference ( P > 0.05) between respiratory frequency (0.38 Hz, 0.25-0.49 Hz) and sinusoidal displacement frequency (0.39 Hz, 0.35-0.42 Hz). This study provides evidence that SSNA is modulated during neck movement, raising the possibility that neck mechanoreceptors may contribute to the cutaneous vasoconstriction and sweat release associated with motion sickness. NEW & NOTEWORTHY This study demonstrates that dynamic, but not static, stretching of the neck modulates skin sympathetic nerve activity in the lower limbs.

Vestibular Modulation of Sympathetic Nerve Activity to Muscle and Skin in Humans

We review the existence of vestibulosympathetic reflexes in humans. While several methods to activate the human vestibular apparatus have been used, galvanic vestibular stimulation (GVS) is a means of selectively modulating vestibular afferent activity via electrodes over the mastoid processes, causing robust vestibular illusions of side-to-side movement. Sinusoidal GVS (sGVS) causes partial entrainment of sympathetic outflow to muscle and skin. Modulation of muscle sympathetic nerve activity (MSNA) from vestibular inputs competes with baroreceptor inputs, with stronger temporal coupling to the vestibular stimulus being observed at frequencies remote from the cardiac frequency; “super entrainment” was observed in some individuals. Low-frequency (<0.2 Hz) sGVS revealed two peaks of modulation per cycle, with bilateral recordings of MSNA or skin sympathetic nerve activity, providing evidence of lateralization of sympathetic outflow during vestibular stimulation. However, it should be noted that GVS influences the firing of afferents from the entire vestibular apparatus, including the semicircular canals. To identify the specific source of vestibular input responsible for the generation of vestibulosympathetic reflexes, we used low-frequency (<0.2 Hz) sinusoidal linear acceleration of seated or supine subjects to, respectively, target the utricular or saccular components of the otoliths. While others had discounted the semicircular canals, we showed that the contributions of the utricle and saccule to the vestibular modulation of MSNA are very similar. Moreover, that modulation of MSNA occurs at accelerations well below levels at which subjects are able to perceive any motion indicates that, like vestibulospinal control of posture, the vestibular system contributes to the control of blood pressure through potent reflexes in humans.

Effects of postural change from supine to head-up tilt on the skin sympathetic nerve activity component synchronised with the cardiac cycle in warmed men

KEY POINTS

Humans are unique in controlling body temperature in a hot environment by a large amount of skin blood flow; however, the decrease in total peripheral resistance due to systemic cutaneous vasodilatation and the reduction of venous return to the heart due to blood pooling in the cutaneous vein threatens blood pressure maintenance in the upright position, and occasionally causes heat syncope. Against this condition, cutaneous vasodilatation is reportedly suppressed to maintain arterial pressure; however, the nerve activity responsible for this phenomenon has not been identified. In the present study, we found that the skin sympathetic nerve activity component that was synchronised with the cardiac cycle increased in hyperthermia, but the increase was suppressed when the posture was changed from supine to head-up tilt. The profile of the component agreed with that of cutaneous vasodilatation. Thus, the component might contribute to the prevention of heat syncope in humans.

ABSTRACT

In humans, the cutaneous vasodilatation response to hyperthermia has been suggested to be suppressed by baroreflexes to maintain arterial pressure when the posture is changed from supine to upright, and if the reflexes do not function sufficiently, it can cause heat syncope. However, the efferent signals of the reflexes have not been identified. To identify the signals, we continuously measured skin sympathetic nerve activity (SSNA; microneurography), right atrial volume (RAV; echocardiography, the baroreceptors for the reflexes are reportedly located in the right atrium), cutaneous vascular conductance on the chest (CVC_chest ; laser Doppler flowmetry), and oesophageal temperature (T_oes ; thermocouple) in young men before and after passive warming with a perfusion suit, during which periods the posture was changed from supine to 30 deg head-up tilt positions. During these periods, we also simultaneously measured muscle sympathetic nerve activity (MSNA) to distinguish the SSNA from MSNA. We found that an increase in T_oes by ∼0.7°C (P < 0.0001) increased the total SSNA (P < 0.005); however, the head-up tilt in hyperthermia did not change the total SSNA (P > 0.26) although an increase in CVC_chest (P < 0.019) was suppressed and RAV was reduced (P < 0.008). In contrast, the SSNA component synchronised with the cardiac cycle increased in hyperthermia (P < 0.015), but decreased with the postural change (P < 0.017). The SSNA component during the postural change before and after warming was highly correlated with the CVC_chest (r = 0.817, P < 0.0001), but the MSNA component was not (r = 0.359, P = 0.085). Thus, the SSNA component synchronised with the cardiac cycle appeared to be involved in suppressing cutaneous vasodilatation during postural changes.

Skin Sympathetic Nerve Activity is Modulated during Slow Sinusoidal Linear Displacements in Supine Humans

Low-frequency sinusoidal linear acceleration (0.08 Hz, ±4 mG) modulates skin sympathetic nerve activity (SSNA) in seated subjects (head vertical), suggesting that activation of the utricle in the peripheral vestibular labyrinth modulates SSNA. The aim of the current study was to determine whether SSNA is also modulated by input from the saccule. Tungsten microelectrodes were inserted into the common peroneal nerve to record oligounitary SSNA in 8 subjects laying supine on a motorized platform with the head aligned with the longitudinal axis of the body. Slow sinusoidal (0.08 Hz, 100 cycles) linear acceleration-decelerations (peak ±4 mG) were applied rostrocaudally to predominately activate the saccules, or mediolaterally to predominately activate the utricles. Cross-correlation histograms were constructed between the negative-going sympathetic spikes and the positive peaks of the sinusoidal stimuli. Sinusoidal linear acceleration along the rostrocaudal axis or mediolateral axis both resulted in sinusoidal modulation of SSNA (Median, IQR 27.0, 22-33% and 24.8, 17-39%, respectively). This suggests that both otolith organs act on sympathetic outflow to skin and muscle in a similar manner during supine displacements.

Sympathetic regulation during thermal stress in human aging and disease

Humans control their core temperature within a narrow range via precise adjustments of the autonomic nervous system. In response to changing core and/or skin temperature, several critical thermoregulatory reflex effector responses are initiated and include shivering, sweating, and changes in cutaneous blood flow. Cutaneous vasomotor adjustments, mediated by modulations in sympathetic nerve activity (SNA), aid in the maintenance of thermal homeostasis during cold and heat stress since (1) they serve as the first line of defense of body temperature and are initiated before other thermoregulatory effectors, and (2) they are on the efferent arm of non-thermoregulatory reflex systems, aiding in the maintenance of blood pressure and organ perfusion. This review article highlights the sympathetic responses of humans to thermal stress, with a specific focus on primary aging as well as impairments that occur in both heart disease and type 2 diabetes mellitus. Age- and pathology-related changes in efferent muscle and skin SNA during cold and heat stress, measured directly in humans using microneurography, are discussed.

Modulation of muscle sympathetic nerve activity by low-frequency physiological activation of the vestibular utricle in awake humans

We recently showed that selective stimulation of one set of otolithic organs-those located in the utricle, sensitive to displacement in the horizontal axis-causes a marked entrainment of skin sympathetic nerve activity (SSNA). Here, we assessed whether muscle sympathetic nerve activity (MSNA) is similarly modulated. MSNA was recorded via tungsten microelectrodes inserted into cutaneous fascicles of the common peroneal nerve in 12 awake subjects, seated (head vertical, eyes closed) on a motorised platform. Slow sinusoidal accelerations-decelerations (±4 mG) were applied in the X (antero-posterior) or Y (medio-lateral) direction at 0.08 Hz. Cross-correlation analysis revealed partial entrainment of MSNA: vestibular modulation was 32 ± 3 % for displacements in the X-axis and 29 ± 3 % in the Y-axis; these were significantly smaller than those evoked in SSNA (97 ± 3 and 91 ± 5 %, respectively). For each sinusoidal cycle, there were two peaks of modulation-one associated with acceleration as the platform moved forward or to the side and one associated with acceleration in the opposite direction. We believe the two peaks reflect inertial displacement of the stereocilia within the utricle during sinusoidal acceleration, which evokes vestibulosympathetic reflexes that are expressed as vestibular modulation of MSNA as well as of SSNA. The smaller vestibular modulation of MSNA can be explained by the dominant modulation of MSNA by the arterial baroreceptors.

Cardiorespiratory coupling of sympathetic outflow in humans: a comparison of respiratory and cardiac modulation of sympathetic nerve activity to skin and muscle

NEW FINDINGS

What is the central question of this study?Muscle sympathetic nerve activity (MSNA) is well known to be modulated by the arterial baroreceptors and respiration, but what are the magnitudes of cardiac and respiratory modulation of skin sympathetic nerve activity (SSNA), which primarily subserves thermoregulation?What is the main finding and what is its importance?Using direct microelectrode recordings of MSNA and SSNA in awake humans, we show that the magnitude of respiratory modulation of SSNA is identical to that of MSNA, the primary difference between the two sources of sympathetic outflow being the greater cardiac modulation of MSNA. This emphasises the role of the baroreceptors in entraining sympathetic outflow to muscle. It is well known that microelectrode recordings of skin sympathetic nerve activity (SSNA) in awake human subjects reveal spontaneous bursts of activity with no overt modulation by changes in blood pressure or respiration, in contrast to the clear cardiac and respiratory modulation of muscle sympathetic nerve activity (MSNA). However, cross-correlation analysis has revealed that, like individual muscle vasoconstrictor neurones, the firing of individual cutaneous vasoconstrictor neurones is temporally coupled to both the cardiac and respiratory rhythms during cold-induced cutaneous vasoconstriction, and the same is true of single sudomotor neurones during heat-induced sweating. Here, we used cross-correlation analysis to determine whether SSNA exhibits cardiac and respiratory modulation in thermoneutral conditions and to compare respiratory and cardiac modulation of SSNA with that of MSNA. Oligounitary recordings of spontaneous SSNA (n = 20) and MSNA (n = 18) were obtained during quiet, unrestrained breathing. Respiration was recorded by a strain-gauge transducer around the chest and ECG recorded by surface electrodes. Respiratory and cardiac modulation of SSNA and MSNA were quantified by fitting polynomial equations to the cross-correlation histograms constructed between the sympathetic spikes and respiration or ECG. The amplitude of the respiratory modulation (52.5 ± 3.4%) of SSNA was not significantly different from the amplitude of the cardiac modulation (46.6 ± 3.2%). Both were comparable to the respiratory modulation of MSNA (47.7 ± 4.2%), while cardiac modulation of MSNA was significantly higher (89.8 ± 1.5%). We conclude that SSNA and MSNA share similar levels of respiratory modulation, the primary difference between the two sources of sympathetic outflow being the marked cardiac modulation of MSNA provided by the baroreceptors.

Low-frequency physiological activation of the vestibular utricle causes biphasic modulation of skin sympathetic nerve activity in humans

We have previously shown that sinusoidal galvanic vestibular stimulation, a means of selectively modulating vestibular afferent activity, can cause partial entrainment of sympathetic outflow to muscle and skin in human subjects. However, it influences the firing of afferents from the entire vestibular apparatus, including the semicircular canals. Here, we tested the hypothesis that selective stimulation of one set of otolithic organs-those located in the utricle, which are sensitive to displacement in the horizontal axis-could entrain sympathetic nerve activity. Skin sympathetic nerve activity (SSNA) was recorded via tungsten microelectrodes inserted into cutaneous fascicles of the common peroneal nerve in 10 awake subjects, seated (head vertical, eyes closed) on a motorised platform. Slow sinusoidal accelerations-decelerations (~4 mG) were applied in the X (antero-posterior) or Y (medio-lateral) direction at 0.08 Hz; composite movements in both directions were also applied. Subjects either reported feeling a vague sense of movement (with no sense of direction) or no movement at all. Nevertheless, cross-correlation analysis revealed a marked entrainment of SSNA for all types of movements: vestibular modulation was 97 ± 3 % for movements in the X axis and 91 ± 5 % for displacements in the Y axis. For each sinusoidal cycle, there were two major peaks of modulation-one associated with acceleration as the platform moved forward or to the side, and one associated with acceleration in the opposite direction. We interpret these observations as reflecting inertial displacement of the stereocilia within the utricle during acceleration, which causes a robust vestibulosympathetic reflex.

Human sympathetic outflows to skin and muscle target organs fluctuate concordantly over a wide range of time-varying frequencies

Frequency-domain analyses of simultaneously recorded skin and muscle sympathetic nerve activities may yield unique information on otherwise obscure central processes governing human neural outflows. We used wavelet transform and wavelet phase coherence methods to analyse integrated skin and muscle sympathetic nerve activities and haemodynamic fluctuations, recorded from nine healthy supine young men. We tested two null hypotheses: (1) that human skin and muscle sympathetic nerve activities oscillate congruently; and (2) that whole-body heating affects these neural outflows and their haemodynamic consequences in similar ways. Measurements included peroneal nerve skin and tibial nerve muscle sympathetic activities; the electrocardiogram; finger photoplethysmographic arterial pressure; respiration (controlled at 0.25 Hz, and registered with a nasal thermistor); and skin temperature, sweating, and laser-Doppler skin blood flow. We made recordings at ∼27°C, for ∼20 min, and then during room temperature increases to ∼38°C, over 35 min. We analysed data with a wavelet transform, using the Morlet mother wavelet and wavelet phase coherence, to determine the frequencies and coherences of oscillations over time. At 27°C, skin and muscle nerve activities oscillated coherently, at ever-changing frequencies between 0.01 and the cardiac frequency (∼1 Hz). Heating significantly augmented oscillations of skin sympathetic nerve activity and skin blood flow, arterial pressure, and R-R intervals, over a wide range of low frequencies, and modestly reduced coordination between skin and muscle sympathetic oscillations. These results suggest that human skin and muscle sympathetic motoneurones are similarly entrained by external influences, including those of arterial baroreceptors, respiration, and other less well-defined brainstem oscillators. Our study provides strong support for the existence of multiple, time-varying central sympathetic neural oscillators in human subjects.

Skin sympathetic nerve activity component synchronizing with cardiac cycle is involved in hypovolaemic suppression of cutaneous vasodilatation in hyperthermia

Although cutaneous vasodilatation in hyperthermia was suppressed during hypovolaemia, the efferent neural pathway mediating this suppression has not been identified. To determine the electrical nerve signals which account for the suppression of cutaneous vasodilatation during hypovolaemia, skin sympathetic nerve activity (SSNA; microneurography) from the peroneal nerve, laser-Doppler blood flow (LDF) on the ipsilateral dorsal foot, mean arterial pressure (MAP; sonometry) and oesophageal temperature (T(oes)) were measured before and during 45 min of passive warming in 20 healthy subjects during normovolaemia (n = 10) or hypovolaemia (n = 10) conditions. Hypovolaemia was achieved by diuretic administration. Cutaneous vascular conductance (CVC = LDF/MAP), SSNA burst frequency and total SSNA obtained from rectified and filtered SSNA signal increased as T(oes) increased by ~0.5°C by the end of warming in both groups. The increase in CVC was significantly lower in hypovolaemia than normovolaemia (P < 0.0001), but with no significant difference in the increase in burst frequency and total SSNA between groups (P > 0.32). However, using an alternative analysis that constructed spike incidence histograms from the original signal using 0.05 s bins during the 5 s following a given R-wave, we found a SSNA component synchronized with the cardiac cycle with a 1.1-1.3 s latency. This component increased with an increase in T(oes) and the increase was significantly suppressed by hypovolaemia (P < 0.0001). In conclusion, hypovolaemic suppression of cutaneous vasodilatation during hyperthermia might be caused by a reduction in the SSNA component synchronized with cardiac cycle.

Vestibular and pulse-related modulation of skin sympathetic nerve activity during sinusoidal galvanic vestibular stimulation in human subjects

We have previously shown that sinusoidal galvanic vestibular stimulation (sGVS), a means of a selectively modulating vestibular afferent input without affecting other inputs, can cause partial entrainment of muscle sympathetic nerve activity (MSNA). Given that motion sickness causes sweating and pallor, we tested the hypothesis that sGVS also entrains skin sympathetic nerve activity (SSNA), but that the optimal frequencies are closer to those associated with slow postural changes (0.2 Hz). SSNA was recorded via tungsten microelectrodes inserted into the common peroneal nerve in 11 awake-seated subjects. Bipolar binaural sinusoidal GVS (+/-2 mA, 200 cycles) was applied to the mastoid processes at frequencies of 0.2, 0.5, 0.8, 1.1, 1.4, 1.7 and 2.0 Hz. All subjects reported strong postural illusions of 'rocking in a boat' or 'swaying in a hammock'. Sinusoidal GVS caused a marked entrainment of SSNA at all frequencies. Measured as the modulation index, vestibular modulation ranged from 81.5 +/- 4.0% at 0.2 Hz to 76.6 +/- 3.6% at 1.7 Hz; it was significantly weaker at 2.0 Hz (63.2 +/- 5.4%). Interestingly, pulse-related modulation of SSNA, which is normally weak, increased significantly during sGVS but was stronger at 0.8 Hz (86.2 +/- 2.0%) than at 0.2 Hz (69.3 +/- 8.3%), the opposite of the pattern seen with vestibular modulation of MSNA. We conclude that vestibular inputs can entrain the firing of cutaneous sympathetic neurones and increase their normally weak pulse-related rhythmicity.

Mean body temperature does not modulate eccrine sweat rate during upright tilt

Conflicting reports exist about the role of baroreflexes in efferent control of eccrine sweat rate. These conflicting reports may be due to differing mean body temperatures between studies. The purpose of this project was to test the hypothesis that mean body temperature modulates the effect of head-up tilt on sweat rate and skin sympathetic nerve activity (SSNA). To address this question, mean body temperature (0.9.internal temperature + 0.1.mean skin temperature), SSNA (microneurography of peroneal nerve, n = 8), and sweat rate (from an area innervated by the peroneal nerve and from two forearm sites, one perfused with neostigmine to augment sweating at lower mean body temperatures and the second with the vehicle, n = 12) were measured in 13 subjects during multiple 30 degrees head-up tilts during whole body heating. At the end of the heat stress, mean body temperature (36.8 +/- 0.1 to 38.0 +/- 0.1 degrees C) and sweat rate at all sites were significantly elevated. No significant correlations were observed between mean body temperature and the change in SSNA during head-up tilt (r = 0.07; P = 0.62), sweating within the innervated area (r = 0.06; P = 0.56), sweating at the neostigmine treated site (r = 0.04; P = 0.69), or sweating at the control site (r = 0.01; P = 0.94). Also, for each tilt throughout the heat stress, there were no significant differences in sweat rate (final tilt sweat rates were 0.69 +/- 0.11 and 0.68 +/- 0.11 mg.cm(-2).min(-1) within the innervated area; 1.04 +/- 0.16 and 1.06 +/- 0.16 mg.cm(-2).min(-1) at the neostigmine-treated site; and 0.85 +/- 0.15 and 0.85 +/- 0.15 mg.cm(-2).min(-1) at the control site, for supine and tilt, respectively). Hence, these data indicate that mean body temperature does not modulate eccrine sweat rate during baroreceptor unloading induced via 30 degrees head-up tilt.

Comparison of the firing patterns of human postganglionic sympathetic neurones and spinal alpha motoneurones during brief bursts

Focal recordings from individual postganglionic sympathetic neurones in awake human subjects have revealed common firing properties. One of the most striking features is that they tend to fire only once per sympathetic burst. Why this should be so is not known, but we propose that the short duration of the burst may limit the number of times a sympathetic neurone can fire. Indeed, while the normal variation in cardiac interval and burst duration is too narrow to reveal a correlation between burst duration and the number of spikes generated, we know that spike generation is doubled when burst duration is doubled following ectopic heart beats. To test the hypothesis that the burst duration constrains the firing of individual sympathetic neurones to one per burst, we used the human skeletomotor system as a model for the sympathetic nervous system, which allowed us to vary burst duration and amplitude experimentally. Intramuscular recordings were made from 27 single motor units (alpha motoneurones) in the tibialis anterior or soleus muscles of seven subjects; multiunit EMG activity was recorded via surface electrodes and blood pressure was recorded continuously. Subjects were instructed to generate EMG bursts of varying amplitude in the intervals between heart beats. By constraining the firing of alpha motoneurones to brief ( approximately 400 ms) bursts we could emulate real sympathetic bursts. Individual motoneurones generated 0-7 spikes during the emulated sympathetic bursts, with firing patterns similar to those exhibited by real sympathetic neurones. Eleven motor units showed significant positive linear correlations between the number of spikes they generated within a burst and its amplitude, whereas for 17 motor units there were significant positive correlations between the number of spikes and burst duration. This indicates that burst duration is a major determinant of the number of times an alpha motoneurone will fire during a brief burst, and we suggest that the same principle may explain the firing pattern typical of human sympathetic neurones.

Neurophysiological analysis of target-related sympathetic pathways--from animal to human: similarities and differences

The sympathetic nervous system regulates many different target tissues in the somatic and visceral domains of the body in a differentiated manner, indicating that there exist separate sympathetic pathways that are functionally defined by their target cells. Signals generated by central integration and channelled through the preganglionic neurons into the final sympathetic pathways are precisely transmitted through the para- and prevertebral ganglia and at the neuroeffector junctions to the effector cells. Neurophysiological recordings of activity in postganglionic neurons in skin and muscle nerves using microneurography in human subjects and in skin, muscle and visceral nerves, using conventional recording techniques in anaesthetized animals, clearly show that each type of sympathetic neuron exhibits a discharge pattern that is characteristic for its target cells and, therefore, its function. These findings justify labelling the neurons as muscle vasoconstrictor, cutaneous vasoconstrictor, sudomotor, lipomotor, cardiomotor, secretomotor neurons, etc. The discharge patterns monitor aspects of the central organization of the respective sympathetic system in the neuraxis and forebrain. They can be dissected into several distinct reflexes (initiated by peripheral and central afferent inputs) and reactions connected to central signals (related to respiration, circadian and other rhythms, command signals generated in the forebrain, etc). They are functional markers for the sympathetic final pathways. These neurophysiological recordings of the discharge patterns from functionally identified neurons of sympathetic pathways in the human and in animals are the ultimate reference for all experimental investigations that aim to unravel the central organization of the sympathetic systems. The similarities of the results obtained in the in vivo studies in the human and in animals justify concluding that the principles of the central organization of sympathetic systems are similar, if not identical, at least in the neuraxis, in both species. Future progress in the analysis of the central neuronal circuits that are associated with the different final sympathetic pathways will very much depend on whether we are able to align the human models and the animal models. Human models using microneurography have the advantage to work under awake conditions. The activity in the postganglionic neurons can be correlated with various other (afferent, centrally generated) signals, effector responses, perceptions, central changes monitored by imaging methods, etc. However, human models have considerable limitations. Animal models can be divided into in vivo models and various types of reduced in vitro models. Animal models allow using various methodological approaches (e.g., neurophysiological, pharmacological, modern anatomical tracing methods; behavioural animal models; transgenic animals), which cannot be used in the human. Interaction of the research performed in the human and animals will allow to design animal models that are relevant for diseases in which the sympathetic nervous systems is involved and to trace down the underlying pathophysiological mechanisms. The scientific questions to be asked are formulated on the basis of clinical observations resulting in testable hypotheses that are investigated in the in vivo human and animal models. Results obtained in the in vivo models lead to the formulation of hypotheses that are testable in reduced in vivo and particularly in vitro animal models. Microneurographic recordings from sympathetic postganglionic fibres in the human will keep its place in the analysis of the sympathetic nervous system in health and disease although only relatively few laboratories in the world will be able to keep the standards and expertise to use this approach. Experimental investigation of the organization of the sympathetic nervous system in animal models has changed dramatically in the last 15 years. The number of in vitro models and the methodological diversity have increased. In vivo experimentation on larger animals has almost disappeared and has been replaced by experimentation on rats, which became the species for practically all types of studies on the central organization of the sympathetic nervous system.

Why do human postganglionic neurones primarily only fire once during a sympathetic burst?

AIM

Single-unit recordings from muscle vasoconstrictor, cutaneous vasoconstrictor and sudomotor neurones in awake human subjects have shown that they tend to fire only once per sympathetic burst. We review the firing properties of human sympathetic neurones and examine the idea that the short duration of a sympathetic burst may limit the number of times a neurone can fire, using the human skeletomotor system as a model for the sympathetic nervous system.

RESULTS

It is known that human alpha motor neurones usually fire in long trains during voluntary contractions, but what of their pattern when constrained by a brief burst? We recorded from single motor units in the tibialis anterior muscle while subjects generated brief electromyogram bursts in the intervals between heart beats, with a duration similar to that of muscle sympathetic bursts. Eight motor units fired mostly one spike per burst, with a pattern identical to that of sympathetic neurones.

CONCLUSION

These results suggests that were it not for the constraint of the bursting pattern, individual sympathetic neurones would--like alpha motor neurones--tend to fire in long trains.

Non-thermoregulatory modulation of sweating in humans

Sweating in humans is critical for appropriate thermoregulation during exercise and/or exposure to warm environmental temperatures. In addition to thermal controllers of sweating, a number of non-thermal factors modulate the sweating response. This review summarizes the primary non-thermal neural modifiers of sweating in humans.

Sympathetic nerve activity to amputated lower leg in humans. Evidence of altered skin vasoconstrictor discharge

Transection of a peripheral nerve in the cat is known to cause a regional change in sympathetic impulse pattern. Our aim was to determine whether microneurography can be used to study sympathetic activity in the transected nerve of human amputees, and whether any such activity shows an abnormal pattern similar to that observed in the cat. Seven successful sympathetic recording sessions were performed in the peroneal nerve of four subjects with posttraumatic transtibial leg amputation; one of them was studied on four occasions. Muscle nerve sympathetic activity (MSA) was detected in all four subjects. Skin nerve sympathetic activity (SSA) was found in one patient only, but on three occasions. It was more difficult to obtain high quality sympathetic recordings than for intact nerves, particularly in patients amputated many years before our studies. MSA showed a qualitatively normal pattern at rest and during various manoeuvres. In three recordings from skin nerve fascicles without innervation zone, SSA displayed normal characteristics at room temperature and qualitatively normal responses to arousal stimuli and various manoeuvres. During body cooling there was an abnormal shift in SSA pattern with a reduction in burst duration instead of the increase occurring normally. Cardiac rhythmicity of SSA was more pronounced during body cooling than during body heating. This is also a reversal of the normal pattern. The abnormal SSA pattern during body cooling suggests increased baroreflex regulation of cutaneous vasoconstrictor neurones, similar to the change after nerve transection in the cat. This is the first time that human nerve recordings support the hypothesis of a regional alteration in sympathetic impulse pattern following a nerve lesion. The implications of this phenomenon for pain conditions remains to be explored; our patients did not suffer from phantom or stump pain.

Firing properties of single postganglionic sympathetic neurones recorded in awake human subjects

For over three decades, the technique of microneurography has allowed us to record sympathetic neural outflow directly from postganglionic axons in awake human subjects. But because sympathetic axons are clustered within a nerve fascicle, such recordings have been limited to the analysis of multi-unit neural activity. To improve the information content of intraneural recordings, we developed the single-unit approach, in which focal recordings can be made from a single C-fibre via a high-impedance tungsten microelectrode. In this review, we describe our methodology for analyzing unitary sympathetic activity and discuss the similarities in the firing properties of individual muscle vasoconstrictor, cutaneous vasoconstrictor and sudomotor neurones.

Absence of arterial baroreflex modulation of skin sympathetic activity and sweat rate during whole-body heating in humans

1. Prior findings suggest that baroreflexes are capable of modulating skin blood flow, but the effects of baroreceptor loading/unloading on sweating are less clear. Therefore, this project tested the hypothesis that pharmacologically induced alterations in arterial blood pressure in heated humans would lead to baroreflex-mediated changes in both skin sympathetic nerve activity (SSNA) and sweat rate. 2. In seven subjects mean arterial blood pressure was lowered (approximately 8 mmHg) and then raised (approximately 13 mmHg) by bolus injections of sodium nitroprusside and phenylephrine, respectively. Moreover, in a separate protocol, arterial blood pressure was reduced via steady-state administration of sodium nitroprusside. In both normothermia and heat-stress conditions the following responses were monitored: sublingual and mean skin temperatures, heart rate, beat-by-beat blood pressure, skin blood flow (laser-Doppler flowmetry), local sweat rate and SSNA (microneurography from peroneal nerve). 3. Whole-body heating increased skin and sublingual temperatures, heart rate, cutaneous blood flow, sweat rate and SSNA, but did not change arterial blood pressure. Heart rate was significantly elevated (from 74 +/- 3 to 92 +/- 4 beats x min(-1); P < 0.001) during bolus sodium nitroprusside-induced reductions in blood pressure, and significantly reduced (from 92 +/- 4 to 68 +/- 4 beats x min(-1); P < 0.001) during bolus phenylephrine-induced elevations in blood pressure, thereby demonstrating normal baroreflex function in these subjects. 4. Neither SSNA nor sweat rate was altered by rapid (bolus infusion) or sustained (steady-state infusion) changes in blood pressure regardless of the thermal condition. 5. These data suggest that SSNA and sweat rate are not modulated by arterial baroreflexes in normothermic or moderately heated individuals.

Differential control of sympathetic outflow

With advances in experimental techniques, the early views of the sympathetic nervous system as a monolithic effector activated globally in situations requiring a rapid and aggressive response to life-threatening danger have been eclipsed by an organizational model featuring an extensive array of functionally specific output channels that can be simultaneously activated or inhibited in combinations that result in the patterns of autonomic activity supporting behavior and mediating homeostatic reflexes. With this perspective, the defense response is but one of the many activational states of the central autonomic network. This review summarizes evidence for the existence of tissue-specific sympathetic output pathways, which are likely to include distinct populations of premotor neurons whose target specificity could be assessed using the functional fingerprints developed from characterizations of postganglionic efferents to known targets. The differential responses in sympathetic outflows to stimulation of reflex inputs suggest that the circuits regulating the activity of sympathetic premotor neurons must have parallel access to groups of premotor neurons controlling different functions but that these connections vary in their ability to influence different sympathetic outputs. Understanding the structural and physiological substrates antecedent to premotor neurons that mediate the differential control of sympathetic outflows, including those to noncardiovascular targets, represents a challenge to our current technical and analytic approaches.

T (tail)-rhythm oscillators: principles of nervous control of the vasculature revealed?

This review focuses on the nervous control of the caudal ventral artery of the rat tail, and aims to convince the reader that sympathetic control of the vasculature can be mediated via neural oscillators intrinsic to the sympathetic nervous system. The definitive functional significance of these oscillators is unknown at present. However, it is expected that through dynamic relationships with modulating and driving inputs, such oscillators would permit graded vascular responses.

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.

Effects of aortic nerve stimulation on discharges of sympathetic neurons innervating rat tail artery and vein

Activity was recorded from postganglionic sympathetic neurons (PSNs) innervating either the caudal ventral artery (CVA) or a lateral vein (LV) of the tail circulation of anesthetized rats. The study sought to determine whether sympathetic activity directed at the CVA and LV was influenced by cardiovascular mechanoreceptor afferents and whether this effect was differential. Cardiac rhythmicity was not a robust component of either CVA PSN activity or LV PSN activity. Stimulation of an aortic nerve with short trains was followed by a decreased probability of discharge in both CVA and LV PSNs that was followed by a series of peaks that showed a constant periodicity that was not significantly different from that revealed by autocorrelogram analysis over the same data set. The latter dominant periodicity is referred to in this and related previous publications as the T rhythm. Furthermore, blood volume expansion and long-train aortic nerve stimulation produced a significant decrease in the frequency of the T rhythm. It is concluded that the CVA and LV sympathetic activity can be influenced by inputs from cardiovascular mechanoreceptors and that this effect is mediated in part by a modulation of the T rhythm.

Inhibition of muscle sympathetic outflow following transcranial cortical stimulation

The possible contribution of cerebral cortical activity to sympathetic outflow to the muscle vascular bed was assessed in normal human subjects. Muscle sympathetic activity was recorded from motor fascicles of the peroneal nerve in 8 subjects while transcranial magnetic stimulation was applied over the vertex, or unilaterally over the hand area of cortex. By triggering the cortical stimulus from the R-wave of the ECG and introducing delays of 0-600 ms between the trigger and the stimulus, we found that a single cortical stimulus delayed by 200-400 ms caused a pronounced inhibition of one pulse-synchronous sympathetic burst. Stimulation over the vertex was more effective than stimulation over the hand area of cortex. In addition to this inhibition of muscle sympathetic outflow, brain stimulation caused an increase in cutaneous sympathetic activity, both sudomotor (sweating) and vasoconstrictor (decrease in skin blood flow). We suggest that the cerebral cortex may normally suppress muscle sympathetic outflow and speculate that lesions that interrupt this source of inhibition (such as those caused by stroke) may result in an augmented muscle sympathetic outflow.

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.

Respiratory modulation of blood flow in normal and sympathectomized skin in humans

Sympathetic vasoconstrictor neurons innervating hairless skin of the cat show a respiratory rhythm of activity discharging in inspiration. The following questions arise: (1) Is it possible to detect respiratory variations in cutaneous blood flow in humans? (2) Are these variations actively mediated by rhythmic activity in vasoconstrictor neurons (active rhythms), or do they depend on blood flow changes induced passively due to respiratory blood pressure waves (passive rhythms)? Three patients who had been sympathectomized unilaterally and four healthy controls were studied. Cutaneous blood flow was measured bilaterally using a laser-Doppler flowmeter during physiological breathing (14/min, tidal volume 500-600 ml. minute volume 81/min) and during slower respiratory rate with a higher tidal and smaller minute volume (5/min, 11, 51/min). The temporal pattern of skin blood flow was analyzed with respect to respiration by constructing peri-event-time histograms after summation and averaging of 10-15 respiratory cycles. During physiological breathing no or minimal variation of cutaneous blood flow could be detected. During slower respiratory rate with higher tidal and smaller minute volume a potentiation of variations appeared. In controls the inspiratory phase was followed by a considerable decrease in cutaneous blood flow with a latency of 4.6 s. Identical rhythms were also present on the unoperated side of the patients. In contrast, on the sympathectomized side a respiratory rhythm appeared that was lower in amplitude and phase shifted by about half a cycle. We conclude: (1) Respiration related cutaneous blood flow variations can be detected, in particular if slower respiratory rates, higher tidal and smaller minute volumes are present. (2) Passive oscillations can be differentiated from active rhythms due to sympathetic vasoconstrictor activity by their temporal pattern. (3) The observations suggest that the neurons responsible for the active rhythm discharge during inspiration.

Central blood volume influences sympathetic sudomotor nerve traffic in warm humans

The objective of this study was to test whether changes in central blood volume can induce reflex effects on sweating. Multi-unit skin sympathetic nerve activity (SSA) was recorded from the posterior cutaneous nerve of the forearm or radial nerve branches in 11 healthy volunteers. Skin electrical resistance and skin blood flow were recorded in the area innervated by the impaled nerve fascicle. Sudomotor nerve traffic and sweating was induced by whole body heating. Lower body negative pressure (LBNP) and tilting (30 degrees head up) was used for blood volume displacement from the chest to the lower body. Low levels of LBNP (5 and 10 mmHg) had no effect on blood pressure, heart rate or skin blood flow but induced a prompt inhibition of SSA and a reduced number of transient skin resistance changes (n = 9), both rapidly returning to control levels after cessation of LBNP. Quantitatively, the effect was similar at both levels of LBNP. Head up tilting also reduced SSA (n = 3, 19 tilt manoeuvres). A capacity for mental stress-induced SSA increase remained during LBNP. Spontaneous fluctuations in blood pressure did not affect SSA, arguing against arterial (high-pressure) baroreceptors modulating SSA. Consequently, the present results indicate that unloading of cardiopulmonary (low-pressure) volume receptors reduces sympathetic sudomotor nerve traffic and sweating in warm subjects. It is suggested that the reflex contributes to counteracting hypovolaemia.

Independent control of skin and muscle sympathetic nerve activity in patients with heart failure

BACKGROUND

Sympathetic excitation characterizes heart failure, but the underlying mechanisms remain unknown. Abnormal baroreflex restraint of sympathetic neural outflow has been proposed, since baroreflexes are known to be abnormal in heart failure. The purpose of this study was to determine if sympathetic activation in humans with heart failure is limited to regions governed by the baroreflexes or is generalized to other regions free from baroreflex control.

METHODS AND RESULTS

We report the first direct recordings of skin sympathetic nerve activity (free from baroreflex control) in humans with heart failure and compare simultaneous skin and muscle (baroreflex-dependent) sympathetic peroneal nerve activity in six patients with severe heart failure (mean left ventricular ejection fraction, 0.19 +/- 0.06) and in six age-matched normal control subjects. Although muscle sympathetic nerve activity was markedly increased in heart failure patients (heart failure versus controls, 69 +/- 3 versus 21 +/- 2 bursts per minute; P < .001), skin sympathetic nerve activity was not increased (heart failure versus controls, 12 +/- 1 versus 15 +/- 1 bursts per minute; P = NS).

CONCLUSIONS

The finding that skin sympathetic nerve activity in contrast to muscle sympathetic nerve activity is not increased in heart failure supports the concept that an altered reflex system, such as the baroreflexes, with nonuniform effects on muscle and skin sympathetic nerve activity, underlies sympatho-excitation in heart failure.

Interactions between non-symmetric mechanical vector forces in the body and the autonomic nervous system: basic requirements for any mechanical technique to engender long-term improvements in autonomic function as well as in the functional efficiency of the respiratory, cardiovascular, and brain systems

There are known anatomical asymmetries in the respiratory, cardiovascular, and nervous system. The coupling mechanisms between each of these systems--lungs-heart, heart-brain, and lungs-brain--are also asymmetrical. There is a growing body of literature indicating that mechanical pressure asymmetrically applied to certain areas of the human body produces changes in the balance of autonomic parameters. These findings implicitly indicate that not only magnitude but also the direction and point of application of the force play a role in its influence upon the autonomic nervous system. Therefore, we suggest that asymmetrical vector forces resulting from the mechanical activity of the lungs, heart and blood moving throughout the circulatory system, will also produce a lateralization effect in autonomic balance. We postulate the existence of negative feedback loops between brain autonomic control and mechanical functions in the body as a fundamental part of the body's homeostatic mechanisms. It follows that any mechanical assist to the respiratory or cardiovascular system will be significantly reduced or even eliminated if these homeostatic mechanisms are not taken into account. Our hypothesis predicts that a long-term improvement in autonomic balance as well as in respiratory, cardiovascular, and brain function can be achieved if mechanical forces are applied to the body with the aim of reducing existing imbalances of mechanical force vectors. This technique implies continually controlling for precise timings resulting from physiological periodical forces as well as factors derived from anatomical and coupling asymmetries in the respiratory, cardiovascular, and nervous systems.

Baroreflex control of the cutaneous active vasodilator system in humans

Cutaneous arterioles are controlled by vasoconstrictor and active vasodilator sympathetic nerves. To find out whether the active vasodilator system is under baroreceptor control, laser-Doppler velocimetry and the local iontophoresis of bretylium were combined to allow selective study of the active vasodilator system. Each of six subjects had two forearm sites (0.64 cm2) treated with bretylium to abolish adrenergic vasoconstrictor control. Laser-Doppler velocimetry was monitored at those sites and at two adjacent untreated sites. Subjects underwent 3 minutes of lower-body negative pressure (LBNP) and 3 minutes of cold stress to verify blockade of vasoconstrictor nerves. They were then subjected to whole-body heat stress (water-perfused suits), and the 3 minutes of LBNP was repeated. Finally, subjects were returned to normothermia, and LBNP and cold stress were repeated to verify the persistence of blockade. During the application of LBNP in normothermia, cutaneous vascular conductance (CVC) fell at untreated sites by 22.7 +/- 4.7% (p less than 0.01) but was unaffected at bretylium-treated sites (p greater than 0.20). During cold stress, CVC at untreated sites fell by 30.2 +/- 1.7% (p less than 0.01) and at treated sites rose by 0.7 +/- 4.6% (p greater than 0.10). Both control and bretylium-treated sites reflexly vasodilated in response to hyperthermia. With LBNP during hyperthermia, CVC at untreated sites fell by 23.3 +/- 7.1% (p less than 0.05) and at treated sites 17.9 +/- 9.2% (p less than 0.05) with no significant difference between sites (p greater than 0.10). After return to normothermia, neither LBNP application nor cold stress caused CVC to fall at treated sites (p greater than 0.10). Thus, the vasoconstrictor system was blocked by bretylium treatment throughtout the study, whereas the active vasodilator response to heat stress was intact. Because LBNP in hyperthermia induced similar falls in CVC at both sites, we conclude that baroreceptor unloading elicits a withdrawal of active vasodilator tone and that the baroreflex has control of the active vasodilator system.

Interactions between non-symmetric mechanical vector forces in the body and the autonomic nervous system: basic requirements for any mechanical technique to engender long-term improvements in autonomic function as well as in the functional efficiency of the respiratory, cardiovascular, and brain systems

There are known anatomical asymmetries in the respiratory, cardiovascular, and nervous system. The coupling mechanisms between each of these systems--lungs-heart, heart-brain, and lungs-brain--are also asymmetrical. There is a growing body of literature indicating that mechanical pressure asymmetrically applied to certain areas of the human body produces changes in the balance of autonomic parameters. These findings implicitly indicate that not only magnitude but also the direction and point of application of the force play a role in its influence upon the autonomic nervous system. Therefore, we suggest that asymmetrical vector forces resulting from the mechanical activity of the lungs, heart and blood moving throughout the circulatory system, will also produce a lateralization effect in autonomic balance.

Hot flushes are induced by thermogenic stimuli

Hot flushes were caused by hot drinks, alcohol, radiant heaters and thermal blankets in men undergoing treatment for carcinoma of the prostate and in menopausal women. Avoiding or changing these commonplace stimuli appears to reduce the frequency of flushing.

Neuroendocrine responses to stimulation of the splanchnic nerves in bursts in conscious, adrenalectomized, weaned lambs

1. Effects of stimulation of the peripheral ends of the splanchnic nerves below behavioural threshold at either 4 or 7 Hz continuously for 10 min, or at 40 or 70 Hz for 1 s at 10 s intervals for 10 min. have been compared in conscious adrenalectomized lambs. 2. Both patterns of stimulation resulted in an abrupt rise in mean aortic blood pressure of closely similar extent which was associated with reflex bradycardia. 3. At the lower frequencies both patterns of stimulation elicited a closely similar rise in mean plasma glucose, glucagon and pancreatic polypeptide concentration, but the fall in mean plasma insulin concentration was significantly greater during continuous stimulation. 4. Unlike other species in which the release of NPY and bombesin-like immunoreactivity (BLI) is potentiated by intermittent high-frequency stimulation, no significant differences were produced by changing the pattern of stimulation. The release of BLI was found to be frequency related over the ranges tested (4-7 Hz continuously and 40-70 Hz in bursts) whereas the release of NPY was not. 5. Splanchnic nerve stimulation also produced detectable rises in the mean plasma concentrations of noradrenaline and adrenaline. The mean average concentration of noradrenaline during stimulation in bursts was significantly higher than that during continuous stimulation (P less than 0.02). There was also a steady rise in mean plasma 3,4-dihydroxyphenylacetic acid (DOPAC) during stimulation followed by a further rise to significantly higher values (P less than 0.02) following stimulation in bursts at 40 Hz. 6. It is concluded that the pattern of stimulation is a less important determinant of autonomic responses to splanchnic nerve stimulation in sheep than in certain other species.

Direct evidence of neurally mediated vasodilatation in hairy skin of the human foot

1. Intraneural stimulation (i.n.s.) was made in the superficial peroneal nerve at the ankle in seventeen healthy subjects. The effect on skin blood flow was monitored by laser-doppler flowmeters and photo-electrical pulse plethysmographs inside and outside the innervation zone of the stimulated nerve fascicle. I.n.s. was applied before and after proximal local anaesthesia of the stimulated nerve. 2. Painful i.n.s. (stimulation strength 0.3-4 V) induced skin vasodilatation with the following characteristics: (a) it occurred on the dorsal side of both feet, (b) the blood flow increase on the opposite foot was blocked by local anaesthesia of the nerve supplying the skin area under study, (c) the blood flow increase on the stimulated foot was abolished by proximal local anaesthesia of the stimulated nerve. The findings show that the vasodilatation was due to activation of a reflex pathway. 3. The reflex vasodilatation was bigger in the stimulated than in the opposite foot. At the same time there were signs of skin vasoconstriction in the fingers. The reflex vasodilatation in the foot was enhanced by body cooling. It was unaffected by atropine or propranolol. 4. After local anaesthesia of the nerve proximal to the stimulation site, i.n.s. with 2-6 times increased stimulation strength produced skin vasodilatation restricted to the innervation zone of the stimulated nerve fascicle. This response had greater amplitude and longer duration than the reflex vasodilatation. 5. Intravenously administered atropine and propranolol did not affect the local dilatation to i.n.s. but after chronic treatment of the skin with capsaicin (1% in ethanol), i.n.s. after a proximal nerve block induced skin vasoconstriction. In five of seven experiments subcutaneous injection of terbutaline (0.25 mg) in the innervation zone abolished the local dilatation. 6. It is suggested that (a) the reflex vasodilatation is of sympathetic nature and is induced by stimulation of thin (A delta?) afferent fibres, (b) the local vasodilatation is due to centrifugally conducted impulses in (afferent?) non-myelinated fibres.

Skin nerve sympathetic activity during insulin-induced hypoglycaemia

Microelectrode recordings of skin nerve sympathetic activity, consisting of sudomotor and vasoconstrictor signals, were performed in the peroneal nerve in seven healthy subjects during insulin-induced hypoglycaemia. The nerve activity was recorded at rest and for 90 min after intravenous injection of 0.15 IU insulin/kg body weight. The net outflow of skin nerve sympathetic activity was increased during hypoglycaemia, with the exception of one subject who exhibited a high initial level of activity. In all subjects a change of the temporal pattern of the outflow was found, suggesting a shift from mixed (sudomotor and vasoconstrictor) to pure sudomotor activity. This change coincided with a sensation of warmth, sweating and varying degrees of cutaneous vasodilatation, and was followed by a fall in body temperature. It is concluded that hypoglycaemia has a differential effect on sympathetic activity in skin nerves, with a strong increase of sudomotor impulses and simultaneous inhibition of vasoconstrictor signals. Thus, neurally mediated thermoregulatory adjustment contributes to heat loss during hypoglycaemia.

Discharge patterns of sympathetic neurons supplying skeletal muscle and skin in man and cat

Skeletal muscle and skin of humans and cats are supplied by various sympathetic systems: vasoconstrictor neurons, sudomotor neurons, vasodilator neurons and pilomotor neurons. Only vasoconstrictor and sudomotor neurons have resting activity in both species. Discharge patterns of spontaneous activity in postganglionic neurons with resting activity as well as their reflex responses to various stimuli have been compared for both species: (1) muscle vasoconstrictor neurons react similarly in both species; they are under dominant control of arterial baroreceptors; (2) in both species cutaneous vasoconstrictor neurons are under no or weak control of arterial baroreceptors and they are influenced in similar ways by thermal stimuli. In contrast, other (somatic and visceral) stimuli elicit largely inhibition in these neurons in cats but excitation in humans. This may be due to the different experimental situations (anesthesia, etc.); (3) sudomotor neurons in humans are involved in thermoregulation, in cats they are not. The differences in the reflexes may also be due to the different experimental situations and to species differences; (4) the implications of studies on sympathetic neurons in humans and animals for the progress of research in this field have been discussed.