Functional Transcranial Doppler (fTCD) investigation of brain lateralization following visual stimuli.

This preliminary investigation of the local cerebral perfusion evaluated by Transcranial Doppler (TCD) monitoring of the Posterior Cerebral Arteries shows that selective stimulation of visual hemifields evokes significantly different vascular responses. TCD can therefore allow for functional evaluation of lateralized enhancements in cerebral metabolism.Clinical Relevance-The cerebral lateralization evaluated with the functional TCD can be a useful and low-cost approach to evaluate the effectiveness of the rehabilitation therapy in post stroke subjects experiencing hemianopia or to assess cerebral reorganization after cerebrovascular accidents.

Repetitive muscle compression reduces vascular mechano-sensitivity and the hyperemic response to muscle contraction.

Animal studies have shown that the rapid hyperemic response to external muscle compression undergoes inactivation upon repetitive stimulation, but this phenomenon has never been observed in humans. The aim of the present study was to determine whether 1) the vascular mechano-sensitivity underlying muscle compression-induced hyperemia is inactivated in an inter-stimulus interval (ISI)-dependent fashion upon repetitive stimulation, as suggested by animal studies, and 2) whether such inactivation also attenuates contraction-induced hyperemia. Brachial artery blood flow was measured by echo Doppler sonography in 13 healthy adults in response to 1) single and repetitive cuff muscle compression (CMC) of the forearm (20 CMCs, 1 s ISI); 2) a sequence of CMC delivered at decreasing ISI from 120 to 2 s; and 3) electrically-stimulated contraction of the forearm muscles before and after repetitive CMC. The peak amplitude of hyperemia in response to CMC normalized to baseline decreased from 2.2 ± 0.6 to 1.4 ± 0.4 after repetitive CMC and, in general, was decreased at ISI < 240 s. The peak amplitude of contraction-induced hyperemia was attenuated after as compared to before repeated CMC (1.7 ± 0.4 and 2.6 ± 0.6, respectively). Mechano-sensitivity of the vascular network can be conditioned by previous mechanical stimulation, and such preconditioning may substantially decrease contraction-induced hyperemia.

Effects of gadolinium chloride on basal flow and compression-induced rapid hyperemia in the rabbit masseter muscle.

Aim of the present study is to investigate the role of mechano-sensitive channels on basal muscle blood flow and on the compression-induced rapid hyperaemia. To this aim, the mechano-sensitive channel blocker Gadolinium (Gd(3+)) is employed, which already proved to reduce the myogenic response in isolated vessels. Muscle blood flow (MaBF) was recorded from the masseteric artery in 8 urethane-anesthetized rabbits. Rapid hyperemic responses were evoked by 1-s lasting compressions of the masseter muscle (MC) delivered before and after close arterial infusion of Gd(3+) in the masseteric artery. Three infusions were performed at 1-h interval, producing estimated plasma concentration (EPC) of 0.045, 0.45 and 4.5 mM, in the masseteric artery. The amplitude of the hyperaemic response to MC, equal to 195±77% of basal flow in control condition, was reduced by 9.5±19.4% (p=0.18) and 45±28% (p<0.01) while basal MaBf increased by 10±3% (p=0.90) and by 68±30% (p<0.01) at EPC of 0.045 and 0.45 mM, respectively. At EPC of 4.5 mM a strong reduction in both MaBF (by 54±13%, p<0.01) and MC response (75±12%, p<0.01) was instead observed. These effects did not depend on time from infusion. At all doses employed Gd(3+) never affected arterial blood pressure, heart rate and contralateral MaBF. While the effects observed at the highest EPC likely result from blood vessel occlusion due to Gd(3+) precipitation, the effects observed at lower concentrations demonstrate that Gd(3+) affects musculo-vascular function by decreasing both resting vascular tone and responsiveness to mechanical stimuli. The results are compatible with a Gd(3+)-induced blockade of vascular mechano-sensitive channels.

Inactivation of mechano-sensitive dilatation upon repetitive mechanical stimulation of the musculo-vascular network in the rabbit.

Mechano-sensitivity of the vascular network is known to be implicated in the rapid dilatation at the onset of exercise, however, it is not known how this mechanism responds to repetitive mechanical stimulation. This study tests the hypothesis that the mechanically-induced hyperaemia undergoes some attenuation upon repetitive stimulation. Muscle blood flow was recorded from 9 masseteric arteries (5 right, 4 left) in 6 anesthetized rabbits. Two mechanical stimuli, masseter muscle compression (MC) and occlusion of the masseteric artery (AO), were provided in different combinations: A) repeated stimulation (0.5 Hz, for 40 s); B) single stimuli delivered at decreasing inter-stimulus interval (ISI) from 4 min to 2 s, C) single AO delivered before and immediately after a series of 20 MCs at 0.5 Hz, and vice-versa. Repetitive AO stimulation at 0.5 Hz produced a transient hyperaemia (378 ±189%) peaking at 4.5 ±1.4 s and then decaying before the end of stimulation. The hyperaemic response to individual AOs progressively decreased by 74 ±39% with decreasing ISI from 4 min to 2 s (p<0.01). Non significant differences were observed between AO and MC stimulation. Decreased response to AO was also provoked by previous repetitive MC stimulation, and vice-versa. The results provide evidence that the mechano-sensitivity of the vascular network is attenuated by previous mechanical stimulation. It is suggested that the mechano-sensitive dilatory mechanisms undergoes some inactivation whose recovery time is in the order of a few minutes.

Inconsistent detection of changes in cerebral blood volume by near infrared spectroscopy in standard clinical tests.

The attractive possibility of near infrared spectroscopy (NIRS) to noninvasively assess cerebral blood volume and oxygenation is challenged by the possible interference from extracranial tissues. However, to what extent this may affect cerebral NIRS monitoring during standard clinical tests is ignored. To address this issue, 29 healthy subjects underwent a randomized sequence of three maneuvers that differently affect intra- and extracranial circulation: Valsalva maneuver (VM), hyperventilation (HV), and head-up tilt (HUT). Putative intracranial ("i") and extracranial ("e") NIRS signals were collected from the forehead and from the cheek, respectively, and acquired together with cutaneous plethysmography at the forehead (PPG), cerebral blood velocity from the middle cerebral artery, and arterial blood pressure. Extracranial contribution to cerebral NIRS monitoring was investigated by comparing Beer-Lambert (BL) and spatially resolved spectroscopy (SRS) blood volume indicators [the total hemoglobin concentration (tHb) and the total hemoglobin index, (THI)] and by correlating their changes with changes in extracranial circulation. While THIe and tHbe generally provided concordant indications, tHbi and THIi exhibited opposite-sign changes in a high percentage of cases (VM: 46%; HV: 31%; HUT: 40%). Moreover, tHbi was correlated with THIi only during HV (P < 0.05), not during VM and HUT, while it correlated with PPG in all three maneuvers (P < 0.01). These results evidence that extracranial circulation may markedly affect BL parameters in a high percentage of cases, even during standard clinical tests. Surface plethysmography at the forehead is suggested as complementary monitoring helpful in the interpretation of cerebral NIRS parameters.

Detecting activation of the sympatho-adrenal axis from haemodynamic recordings, in conscious rabbits exposed to acute stress.

AIMS: When assessing sympathetic activation in acute stress, the attention is often limited to the sympatho-neural axis, whereas sympatho-adrenal activation, that can only be detected with poor time resolution from the concentration of plasma catecholamines, is often neglected. This study is aimed at re-investigating the role and the relevance of the sympatho-adrenal system in acute stress based on the analysis of haemodynamic responses in conscious rabbits. METHODS: Experiments were carried out on 19 rabbits implanted with chronic probes for arterial blood pressure and for blood flow in the facial artery. Cardiovascular responses to a randomized sequence of acute stressors (pinprick, air jet, oscillation of the cage, inhalation of formaldehyde vapours and im injection of hypertonic saline) were recorded before and after α-adrenergic blockade (phentolamine) and unilateral section of the cervical sympathetic trunk (decentralization). Plasma catecholamine concentrations were analysed in four animals. RESULTS: All stressors induced an increase in arterial blood pressure and a reduction of vascular conductance in the facial artery ranging on average from 24% (pinprick) to 55% (box oscillation). Such vasoconstrictor response was abolished by phentolamine. In decentralized arteries, the vasoconstriction was delayed by 10-15 s and decreased in magnitude in a stressor-dependent way, indicating an adrenaline-mediated effect in the late phase of the stress response that was confirmed by changes in plasma adrenaline concentration. CONCLUSIONS: In conscious rabbits, rapid release of adrenaline makes a prominent contribution to vasoconstriction in response to different stressors including box oscillation, muscle pain and air jet but not the nasopharyngeal stimulation.

A model for investigating the control of muscle blood flow: the masseteric artery in conscious rabbits.

The complex interplay of neural, metabolic, myogenic and mechanical mechanisms that regulate blood flow in skeletal muscle (MBF) is still incompletely understood. For the first time, a method is presented for high time-resolution recording of MBF from a purely muscular artery in physiological conditions. Ultrasound perivascular flow probes were implanted (n = 15) mono- or bilaterally around the masseteric branch of the facial artery in nine rabbits and tested up to 16 days after implant. Reliable and stable recordings were achieved in 50% of implants. Blood flow was observed to increase from a resting level of 0.2-0.3 ml min(-1) up to 4.0-6.0 ml min(-1) during spontaneous masticatory activity. In addition, within single masticatory cycles marked back flow transients could be observed (peak flow = -10 ml min(-1)) during powerful masticatory strokes but not during mild mastication. The possibility of (1) surgically removing the sympathetic supply to the relevant vascular bed and of (2) bilaterally monitoring the perfusion of masseter muscles thus allowing to use one side as control side for different types of interventions makes this model a useful tool for disentangling the different mechanisms involved in the control of MBF.

Responses of muscle spindles in feline dorsal neck muscles to electrical stimulation of the cervical sympathetic nerve.

Previous studies performed in jaw muscles of rabbits and rats have demonstrated that sympathetic outflow may affect the activity of muscle spindle afferents (MSAs). The resulting impairment of MSA information has been suggested to be involved in the genesis and spread of chronic muscle pain. The present study was designed to investigate sympathetic influences on muscle spindles in feline trapezius and splenius muscles (TrSp), as these muscles are commonly affected by chronic pain in humans. Experiments were carried out in cats anesthetized with alpha-chloralose. The effect of electrical stimulation (10 Hz for 90 s or 3 Hz for 5 min) of the peripheral stump of the cervical sympathetic nerve (CSN) was investigated on the discharge of TrSp MSAs (units classified as Ia-like and II-like) and on their responses to sinusoidal stretching of these muscles. In some of the experiments, the local microcirculation of the muscles was monitored by laser Doppler flowmetry. In total, 46 MSAs were recorded. Stimulation of the CSN at 10 Hz powerfully depressed the mean discharge rate of the majority of the tested MSAs (73%) and also affected the sensitivity of MSAs to sinusoidal changes of muscle length, which were evaluated in terms of amplitude and phase of the sinusoidal fitting of unitary activity. The amplitude was significantly reduced in Ia-like units and variably affected in II-like units, while in general the phase was affected little and not changed significantly in either group. The discharge of a smaller percentage of tested units was also modulated by 3-Hz CSN stimulation. Blockade of the neuromuscular junctions by pancuronium did not induce any changes in MSA responses to CSN stimulation, showing that these responses were not secondary to changes in extrafusal or fusimotor activity. Further data showed that the sympathetically induced modulation of MSA discharge was not secondary to the concomitant reduction of muscle blood flow induced by the stimulation. Hence, changes in sympathetic outflow can modulate the afferent signals from muscle spindles through an action exerted directly on the spindles, independent of changes in blood flow. It is suggested that such an action may be one of the mechanisms mediating the onset of chronic muscle pain in these muscles in humans.

Curvature affects Doppler investigation of vessels: implications for clinical practice.

In clinical practice, blood velocity estimations from Doppler examination of curved vascular segments are normally different from those of nearby straight segments. The observed "accelerations," sometimes considered as a sort of stochastic disturbances, can actually be related to very specific physical effects due to vessel curvature (i.e., the development of nonaxial velocity [NAV] components) and the spreading of the axial velocity direction in the Doppler sample volume with respect to the insonation axis. The relevant phenomena and their dependence on the radius of curvature of the vessels and on the insonation angle are investigated with a beam-vessel geometry as close as possible to clinical setting, with the simplifying assumptions of steady flow, mild vessel curvature, uniform ultrasonic beam and complete vessel insonation. The insonation angles that minimize the errors are provided on the basis of the study results.

Effects of sympathetic stimulation on the rhythmical jaw movements produced by electrical stimulation of the cortical masticatory areas of rabbits.

The somatomotor and sympathetic nervous systems are intimately linked. One example is the influence of peripheral sympathetic fibers on the discharge characteristics of muscle spindles. Since muscle spindles play important roles in various motor behaviors, including rhythmic movements, the working hypothesis of this research was that changes in sympathetic outflow to muscle spindles can change rhythmic movement patterns. We tested this hypothesis in the masticatory system of rabbits. Rhythmic jaw movements and EMG activity induced by long-lasting electrical cortical stimulation were powerfully modulated by electrical stimulation of the peripheral stump of the cervical sympathetic nerve (CSN). This modulation manifested itself as a consistent and marked reduction in the excursion of the mandibular movements (often preceded by a transient modest enhancement), which could be attributed mainly to corresponding changes in masseter muscle activity. These changes outlasted the duration of CSN stimulation. In some of the cortically evoked rhythmic jaw movements (CRJMs) changes in masticatory frequency were also observed. When the jaw-closing muscles were subjected to repetitive ramp-and-hold force pulses, the CRMJs changed characteristics. Masseter EMG activity was strongly enhanced and digastric EMG slightly decreased. This change was considerably depressed during CSN stimulation. These effects of CSN stimulation are similar in sign and time course to the depression exerted by sympathetic activity on the jaw-closing muscle spindle discharge. It is suggested that the change in proprioceptive information induced by an increase in sympathetic outflow (a) has important implications even under normal conditions for the control of motor function in states of high sympathetic activity, and (b) is one of the mechanisms responsible for motor impairment under certain pathological conditions such as chronic musculoskeletal head-neck disorders, associated with stress conditions.

Assessment of the effect of vessel curvature on Doppler measurements in steady flow.

Blood vessel curvature is responsible for the appearance of nonaxial velocity components and for minor changes in the pattern of the axial flow. All the velocity components are expected to contribute to the Doppler signal produced by the ultrasound (US) backscattered by the insonated blood cells, the axial velocity, contributing to the actual volumetric blood flow, and the transverse velocity, causing the recirculating vortices. A detailed, separate analysis of the velocity components is, therefore, mandatory to quantify how vessel curvature can affect results and clinical diagnosis. Both experimental in vitro measures and numerical simulations were performed on a curved tube and the Doppler power spectra so obtained were compared. The satisfactorily agreement of the above spectra shows that the nonaxial velocity components are easily detectable with clinical equipment and that their amplitude, as expected, is not negligible and can bias Doppler measurements and resulting clinical diagnosis.

Noradrenergic constriction of cerebral arteries as detected by transcranial Doppler (TCD) in the rabbit.

Interpretation of transcranial Doppler (TCD) recordings requires assumptions about flow or diameter of the insonated vessel. This study aimed at assessing if activation of the sympathetic system could affect blood velocity (bv) in basal cerebral arteries. In anaesthetized rabbits, stimulation of cervical sympathetic nerve (cervSN) was used selectively to activate the sympathetic pathway to the head while monitoring bv in all major cerebral arteries. cervSN stimulation at 10 Hz produced: 1. in internal carotid artery (ICA) and ICA-supplied arteries (ICA-s), a consistent bv increase ranging between 20 and 70%, 2. in the basilar artery, a transient decrease by 15-30%. These effects were mimicked, in both territories, by injection of phenylephrine into the ICA. Because cerebral blood flow is known to be reduced by cervSN stimulation, the increase in bv in ICA and ICA-s must be ascribed to constriction of the insonated vessels. These effects should be considered when monitoring bv during sympathetic activation tests or exercise.

Sympathetic modulation of muscle spindle afferent sensitivity to stretch in rabbit jaw closing muscles.

Previous reports showed that sympathetic stimulation affects the activity of muscle spindle afferents (MSAs). The aim of the present work is to study the characteristics of sympathetic modulation of MSA response to stretch: (i) on the dynamic and static components of the stretch response, and (ii) on group Ia and II MSAs to evaluate potentially different effects. In anaesthetised rabbits, the peripheral stump of the cervical sympathetic nerve (CSN) was stimulated at 10 impulses s(-1) for 45-90 s. The responses of single MSAs to trapezoidal displacement of the mandible were recorded from the mesencephalic trigeminal nucleus. The following characteristic parameters were determined from averaged trapezoidal responses: initial frequency (IF), peak frequency at the end of the ramp (PF), and static index (SI). From these, other parameters were derived: dynamic index (DI = PF - SI), dynamic difference (DD = PF - IF) and static difference (SD = SI - IF). The effects of CSN stimulation were also evaluated during changes in the state of intrafusal muscle fibre contraction induced by succinylcholine and curare. In a population of 124 MSAs, 106 units (85.4 %) were affected by sympathetic stimulation. In general, while changes in resting discharge varied among different units (Ia vs. II) and experimental conditions (curarised vs. non-curarised), ranging from enhancement to strong depression of firing, the amplitude of the response to muscle stretches consistently decreased. This was confirmed and detailed in a quantitative analysis performed on 49 muscle spindle afferents. In both the non-curarised (23 units) and curarised (26 units) condition, stimulation of the CSN reduced the response amplitude in terms of DD and SD, but hardly affected DI. The effects were equally present in both Ia and II units; they were shown to be independent from gamma drive and intrafusal muscle tone and not secondary to muscle hypoxia. Sympathetic action on the resting discharge (IF) was less consistent. In the non-curarised condition, IF decreased in most Ia units, while in II units decreases and increases occurred equally often. In the curarised condition, IF in group II units mostly increased. The results have important functional implications on the control of motor function in a state of 'high' sympathetic activity, like excessive stress, as well as in certain pathological conditions such as sympathetically maintained pain.

Doppler sonography to monitor flow in different cerebral arteries in the rabbit.

Most of the transcranial Doppler (TCD) experimental studies on cerebral haemodynamics have been performed in the rabbit because of the similarity between its Willis circle and that of the human, but these studies have mainly been limited to the basilar artery. The present study was aimed at extending the use of TCD sonography to all other large cerebral arteries. In anaesthetised rabbits, these arteries were insonated from three different recording sites, i.e. top-cranial, suboccipital and orbital, using a two-channel pulsed Doppler device equipped with 4 and 8 MHz probes. First, discrimination between intra- and extracranial arteries was achieved through a standard 'rebreathing' test (hypercapnic-hypoxic stimulation). The distinctive blood velocity response patterns, reflecting the different extents of metabolic reactivity in intra- and extracranial territories, are described and discussed. Intracranial arteries were then identified on the basis of their response to ipsi- and contralateral common carotid artery occlusion. This procedure allowed recording from the following arteries: anterior common trunk, anterior cerebral, internal carotid, middle cerebral and basilar; the latter could be simultaneously monitored with any of the others. This study provides an experimental model allowing investigation of regional differences in the haemodynamic response to neurogenic and pharmacological stimuli.

Modeling of surface myoelectric signals--Part II: Model-based signal interpretation.

Experimental electromyogram (EMG) data from the human biceps brachii were simulated using the model described in [10] of this work. A multichannel linear electrode array, spanning the length of the biceps, was used to detect monopolar and bipolar signals, from which double differential signals were computed, during either voluntary or electrically elicited isometric contractions. For relatively low-level voluntary contractions (10%-30% of maximum force) individual firings of three to four-different motor units were identified and their waveforms were closely approximated by the model. Motor unit parameters such as depth, size, fiber orientation and length, location of innervation and tendonous zones, propagation velocity, and source width were estimated using the model. Two applications of the model are described. The first analyzes the effects of electrode rotation with respect to the muscle fiber direction and shows the possibility of conduction velocity (CV) over- and under-estimation. The second focuses on the myoelectric manifestations of fatigue during a sustained electrically elicited contraction and the interrelationship between muscle fiber CV, spectral and amplitude variables, and the length of the depolarization zone. It is concluded that a) surface EMG detection using an electrode array, when combined with a model of signal propagation, provides a useful method for understanding the physiological and anatomical determinants of EMG waveform characteristics and b) the model provides a way for the interpretation of fatigue plots.

A technique for estimating activity in whole nerve trunks applied to the cervical sympathetic trunk, in the rabbit.

The changes in sympathetic outflow may be evaluated from the amplitude of the antidromic compound action potential (ACAP) according to the collision technique described by Douglas and Ritchie (Douglas, W.W. and Ritchie J.M., A technique for recording functional activity in specific groups of medullated and non-medullated fibers in whole nerve trunks. J. Physiol., 138(1957) 19-30). This technique was revised, taking into account the depressant action exerted by antidromic stimulation on sympathetic preganglionic neurones (SPNs). Cervical sympathetic nerve (CSN) of rabbits was used as experimental model. Stimulation frequencies of 0.2-0.5 Hz were found to be sufficiently low to avoid depressant actions on CSN spontaneous activity; they were employed to test the sensitivity of the technique during different experimental manoeuvres, such as changes in pulmonary-ventilation, baroreceptor unloading and arousal stimuli. In addition a procedure was devised to calibrate the ACAP amplitude: high frequency antidromic stimulation was used to induce a complete and transient inhibition of SPNs which allows to record the ACAP maximum amplitude. ACAPs recorded in various experimental conditions can then be expressed as percentage of this value.

Quantitative Doppler measures in coiled vessels: investigation on excised umbilical veins.

Quantitative assessment of umbilical venous blood velocity with Doppler ultrasound (US) must cope with the coiled structure of the vein inside the cord. Both an experimental and a theoretical approach showed remarkable variations in the insonation angle when the probe was moved along the vein, provided the inclination between the Doppler probe and the cord was kept constant. Inaccurate signal processing, stochastic variability and flow disturbances could, however, mask the influence of the geometry. The above hypotheses were assessed by investigating five cords in vitro a few hours after delivery from normal pregnancies at term. The Doppler signal was sampled at different sites along each cord and the mean Doppler shift estimated by FFT spectral analysis, both directly and through the noise rejection D'Alessio's algorithm, which proved effective in improving the Doppler shift estimate in condition of low signal-to-noise ratio (SNR).

Effect of generalised sympathetic activation by cold pressor test on cerebral haemodynamics in healthy humans.

There is no general agreement regarding several aspects of the role of the sympathetic system on cerebral haemodynamics such as extent of effectiveness, operational range and site of action. This study was planned to identify the effect of a generalised sympathetic activation on the cerebral haemodynamics in healthy humans before it is masked by secondary corrections, metabolic or myogenic in nature. A total of 35 healthy volunteers aged 20-35 underwent a 5 min lasting cold pressor test (CPT) performed on their left hand. The cerebral blood flow (CBF) velocity in the middle cerebral arteries and arterial blood pressure were recorded with transcranial Doppler sonography and with a non-invasive finger-cuff method, respectively. The ratio of arterial blood pressure to mean blood velocity (ABP/Vm) and Pulsatility Index (PI) were calculated throughout each trial. CPT induced an increase in mean ABP (range 2-54 mmHg depending on the subject) and only a slight, though significant, increase in blood velocity in the middle cerebral artery (+2.4 and +4.4% on ipsi- and contralateral side, respectively). During CPT, the ratio ABP/Vm increased and PI decreased in all subjects on both sides. These changes began simultaneously with the increase in blood pressure. The increase in ABP/Vm ratio is attributed to an increase in the cerebrovascular resistance, while the concomitant reduction in PI is interpreted as due to the reduction in the compliance of the middle cerebral artery. The results suggest that generalised increases in the sympathetic discharge, causing increases in ABP, can prevent concomitant increases in CBF by acting on both small resistance and large compliant vessels. This effect is also present when a slight increase in blood pressure occurs, which suggests a moderate increase in the sympathetic discharge, i.e. when ABP remains far below the upper limit of CBF autoregulation.

Sympathetic control of skeletal muscle function: possible co-operation between noradrenaline and neuropeptide Y in rabbit jaw muscles.

Stimulation of the cervical sympathetic nerve at 10/s increases by 12.9 +/- 0.7% peak tension of maximal twitches in the directly stimulated jaw muscles and markedly depresses (41.6 +/- 1.3%) the tonic vibration reflex (TVR) elicited in the same muscles by vibration of the mandible. Both effects are not significantly influenced by administration of beta-adrenoceptor antagonists. When both alpha- and beta-adrenergic receptors are blocked, sympathetic stimulation induces a very small increase in twitch tension (3.8 +/- 0.7%), while no detectable change in the TVR is observed. Close arterial injection of alpha 1-adrenoceptor agonist phenylephrine mimics the effects induced by sympathetic stimulation on twitch tension and TVR, dose-dependently. The noradrenaline co-transmitter neuropeptide Y also produces a long-lasting, dose-dependent increase in the twitch tension which is unaffected by blockade of adrenergic receptors as well as of the neuromuscular junctions. Contribution of neuropeptide Y to the sympathetically-induced reduction of the stretch reflex is not clearly demonstrated. These data suggest that co-operation between noradrenaline and neuropeptide Y may be effective in determining sympathetic modulation of skeletal muscle function.

Sympathetically-induced changes in microvascular cerebral blood flow and in the morphology of its low-frequency waves.

The effect of bilateral cervical sympathetic nerve stimulation on microvascular cerebral blood flow, recorded at various depths in the parietal lobe and in ponto-mesencephalic areas, was investigated by laser-Doppler flowmetry in normotensive rabbits. These areas were chosen as representative of the vascular beds supplied by the carotid and vertebro-basilar systems, which exhibit different degrees of sympathetic innervation, the former being richer than the latter. Sympathetic stimulation at 30 imp/s affects cerebral blood flow in 77% of the parietal lobe and in 43% of the ponto-mesencephalic tested areas. In both cases the predominant effect was a reduction in blood flow (14.7 +/- 5.1% and 4.1 +/- 2.4%, respectively). The extent of the reduction in both areas was less if the stimulation frequency was decreased. Sometimes mean cerebral blood flow showed a small and transient increase, mainly in response to low-frequency stimulation. The morphology was analysed of low-frequency spontaneous oscillations in cerebral blood flow, attributed to vasomotion. Present in 41% of the tested areas (frequency 4-12 cycles/min, peak-to-peak amplitude 10-40% of mean value), these waves decreased in amplitude and increased in frequency during sympathetic stimulation, irrespective of changes in mean flow. The possibility has been proposed that the sympathetic action on low-frequency spontaneous oscillations may contribute to the protective influence that this system is known to exert on the blood-brain barrier in hypertension.