Observations on the arterial baroreflex in neurally mediated vasodepressor syncope

The arterial baroreflex was studied in subjects who had recently had an episode of vasodepressor syncope. This was determined using 2–3 mcg/kg intravenous boluses of phenylephrine and assessing the bradycardic response. The values were measured in ms/mmHg and expressed as the angular coefficient of the regression line between the increase in R—R interval on the electrocardiograph and the systolic arterial pressure. In subjects examined immediately after the vasodepressor syncope episode the bradycardic response was much more marked than in controls (p < 0.01) and in the subjects themselves 6 months after the episode, provided that they were symptom-free (p < 0.01). It is concluded that in vasodepressor syncope there is a phase in which the baroreflex is highly sensitive and that this is due not to a lowering of the stimulation threshold but to a gain in the efferent arc, which explains a vagotonic response.     

Vascular and electromyographic responses evoked in forearm muscle by isometric contraction of the contralateral forearm

There is controversy over whether isometric contraction of the forearm evokes vasoconstriction or vasodilatation in the muscles of the contralateral forearm. In the present study we have investigated in normal man, the effects of isometric contraction of one arm at 75, 50 and 25% maximum voluntary contraction (MVC) on arterial pressure, heart rate, blood flow and vascular resistance of the contralateral forearm and on electromyographic (EMG) activity recorded from that same arm with sensitive, surface electrodes.When EMG activity was not being recorded from the resting arm, isometric contraction of the contralateral arm for 2 min evoked increases in arterial pressure and heart rate whose magnitudes were graded with % MVC and an increase in forearm blood flow and a decrease in forearm vascular resistance at 75, 50 and 25% MVC, indicating vasodilatation. Further experiments in which EMG activity was recorded from the resting arm demonstrated that the decrease in forearm vascular resistance evoked by 75% MVC was associated with a substantial increase in EMG activity of the extensor and flexor muscles of that arm. By contrast, when forearm contraction was performed at 75% MVC whilst subjects viewed the EMG activity in the resting arm on an oscilloscope and kept EMG activity minimal, vascular resistance increased in that arm, indicating vasoconstriction. Further, when subjects performed contraction at 25% MVC whilst showing minimal EMG activity in the contralateral arm, vascular resistance in that same arm increased (from 78 ± 16 to 124 ± 29 mmHg/ml/min/100 ml tissue). These results are discussed in relation to those of previous studies. We propose, that in normal man, isometric contraction of the forearm evokes primary vasoconstriction in the muscles of the contralateral forearm, but that this response may be overcome by muscle vasodilatation occurring secondary to unintended muscle contraction or as part of the alerting response to acute stress.     

Does digoxin sensitize left ventricular mechanoreceptors?

Summary Afferent nerve fibre activity from left ventricular mechanoreceptors was recorded in 10 anaesthetized cats before and after two intravenous injections of 15 g/kg digoxin at 1 hour interval. These receptors are activated by coronary artery occlusion and induce a depressor cardiovascular reflex resulting in bradycardia and hypotension.Neither the spontaneous activity of the receptor's afferent nerve fibres nor their maximum activity during temporary coronary artery occlusion was affected by digoxin. The results show that digoxin in therapeutic doses has no sensitizing effect on left ventricular mechanoreceptors with vagal afferent fibres. The sensitization of cardiopulmonary baroreflexes by digitalis glycosides shown in previous investigations is thus more likely to be mediated by a central nervous effect of the drug.     

The trigeminocardiac reflex – a comparison with the diving reflex in humans

The trigeminocardiac reflex (TCR) has previously been described in the literature as a reflexive response of bradycardia, hypotension, and gastric hypermotility seen upon mechanical stimulation in the distribution of the trigeminal nerve. The diving reflex (DR) in humans is characterized by breath-holding, slowing of the heart rate, reduction of limb blood flow and a gradual rise in the mean arterial blood pressure. Although the two reflexes share many similarities, their relationship and especially their functional purpose in humans have yet to be fully elucidated. In the present review, we have tried to integrate and elaborate these two phenomena into a unified physiological concept. Assuming that the TCR and the DR are closely linked functionally and phylogenetically, we have also highlighted the significance of these reflexes in humans.     

Withdrawal reflexes in the upper limb adapt to arm posture and stimulus location

Introduction: Withdrawal reflexes in the leg adapt in a context‐appropriate manner to remove the limb from noxious stimuli, but the extent to which withdrawal reflexes adapt in the arm remains unknown. Methods: We examined the adaptability of withdrawal reflexes in response to nociceptive stimuli applied in different arm postures and to different digits. Reflexes were elicited at rest, and kinetic and electromyographic responses were recorded under isometric conditions, thereby allowing motorneuron pool excitability to be controlled. Results: Endpoint force changed from a posterior–lateral direction in a flexed posture to predominantly a posterior direction in a more extended posture [change in force angle (mean ± standard deviation) 35.6 ± 5.0°], and the force direction changed similarly with digit I stimulation compared with digit V (change = 22.9 ± 2.9°). Conclusions: The withdrawal reflex in the human upper limb adapts in a functionally relevant manner when elicited at rest. Muscle Nerve 49 : 716–723, 2014     

硬膜外麻醉下异丙酚靶控抑制胃手术牵拉反应的研究

目的:对比观察胃溃疡穿孔修补术用硬膜外麻醉时,辅助麻醉用药对胃的牵拉反应抑制效果.方法:40例男性ASA Ⅰ～Ⅱ级胃溃疡穿孔病人行穿孔修补手术,随机分成二组:在硬膜外麻醉效果满意后,分组静脉给药.DPK组:哌替啶0.86 mg/kg,异丙嗪0.43 mg/kg,氯胺酮0.43 mg/kg单次静脉注射.TCI组:TCI泵以异丙酚1.5 μg/ml为初始靶浓度静脉注入,至患者的镇静深度稳定于OAA/S评分3 min后开始手术,手术至关腹后停用异丙酚.分别记录各时段病人SpO2、R、P、BP;记录手术时间、术毕苏醒例数、术中躁动数、追加辅助用药例数、术中恶心、呕吐例数.记录药物的副作用.结果:两组对胃牵拉反应的抑制效果均满意.TCI组对血流动力学的影响小于DPK组,TCI组术毕患者清醒程度优于DPK组.结论:当胃手术选用硬膜外麻醉时,为抑制牵拉反应,异丙酚靶控方法安全,效果确切,可作为主要的辅助麻醉方式.     

Short latency cutaneous reflex responses of γ-efferents in the decerebrate cat

Summary The effect of single shock stimulation, up to 20 × threshold (T), of the sural nerve on the discharges of triceps surae -efferents was investigated in decerebrate cats. Units were classified as static (12) or dynamic (7) on the basis of their resting discharge rates (Murphy et al. 1984). All neurones were excited at short latency by sural nerve stimulation and response size was graded with stimulus intensity. Short latency mixed or inhibitory responses were not evident. Although reflex effects first occurred at low stimulus strengths (<-1.5T) in both types of efferent, most responses appeared at higher intensities (> 1.5T). The estimated central delays of the responses of static (3.0 ±1.1 ms, mean± SD) and dynamic (3.4 ± 1.0 ms) -motoneurones were not significantly different and are consistent with spinal oligosynaptic pathways. The present results differ from those of the only previous study (Johansson and Sojka 1985) of the short latency responses of triceps surae static and dynamic -motoneurones to sural nerve stimulation, in which mixed and inhibitory effects were common in anaesthetised cats. Although differences in recording techniques and sampling may account for the apparent disparity between these studies, it is also feasible that a difference in the setting of interneuronal pathways in the two types of preparation is responsible. The results are discussed in relation to the control of -motoneurones with particular reference to the final common input hypothesis (Johansson 1981; Appelberg et al. 1983).     

Central connections of the lingual-tonsillar branch of the glossopharyngeal nerve and the superior laryngeal nerve in lamb

Afferent and efferent central connections of the lingual-tonsillar branch of the glossopharyngeal nerve (LT-IX) and the superior laryngeal nerve (SLN) in the lamb were traced with horseradish peroxidase (HRP) histochemistry. After entering the brainstem, most LT-IX and SLN afferent fibers turned caudally in the solitary tract (ST). Some afferent fibers of LT-IX terminated in the medial nucleus of the solitary tract slightly caudal to their level of entry. The remaining fibers projected to the dorsolateral, ventrolateral, and interstitial areas of the nucleus of the solitary tract (NST) at the level of the area postrema. Superior laryngeal nerve afferent fibers terminated extensively in the medial and ventral NST at levels near the rostral pole of the area postrema. Further caudal, near the level of obex, SLN afferent terminations were concentrated in the region ventrolateral to the ST and in the interstitial NST. The caudal extent of LT-IX and the rostral extent of SLN terminals projected to similar levels of the NST, but only a relatively small proportion of the total projections overlapped. Lingual-tonsillar and SLN fibers also coursed rostrally to terminate in the caudal pons within and medial to the dorsomedial principal sensory trigeminal nucleus. Other labeled afferent fibers traveled caudally in the dorsal spinal trigeminal tract to terminate in the dorsal two-thirds of the spinal trigeminal nucleus at the level of obex. Large numbers of labeled cells with fibers in the LT-IX or SLN were located in the ipsilateral rostral nucleus ambiguus and surrounding reticular formation. Fewer labeled cells were observed in the inferior salivatory nucleus following HRP application to either the LT-IX or SLN. The LT-IX and SLN projections to areas of the NST associated with upper airway functions, like swallowing and respiration, suggest an important role for these two nerves in the initiation and control of airway reflexes.     

Peripheral reinnervation patterns and dorsal root ganglion cell topography in skin-grafted rana pipiens frogs

Behavioral responses to mechanical stimulation of the skin were observed in unoperated, 180° skin-rotated and sham-grafted Rana pipiens frogs before and after crushing various cutaneous nerve bundles. In the two control groups, wiping responses directed towards the animal's dorsum were mediated solely via dorsomedial (DM) and dorsolateral (DL) nerve trunks. In skin-rotated frogs, DM and DL nerve trunks were responsible for almost all of the misdirected responses elicited from dorsal cutaneous areas. Six frogs (2 sham- and 4 skin-rotated) possessed some areas of plical skin from which responses could only have been mediated via more ventrally located nerve trunks. However, such dorsal expansions of ventral receptive fields never included the midline skin areas from which misdirected responses had been elicited in the intact animal. The topographic arrangement of cells within the DRG appears to be weakly polarized in cobalt-stained sections, but with large individual variations in size and observed number of neuronal profiles. For the skin-rotated group the ratio of cobalt-filled profiles in the dorsal as compared with the ventral half of the ganglion was 1.6, which did not differ significantly from the control value of 1.9. Taken as a whole, the results of the present study indicate that selective (re)innervation of displaced skin by the original nerve fibers cannot explain the origin of misdirected responses in Rana pipiens.