Is the long-latency stretch reflex in human masseter transcortical?

A long-latency stretch reflex (LLSR) has been described in the human masseter muscle, but its pathway remains uncertain. To investigate this, the excitability of corticomotoneuronal (CM) cells projecting to masseter motoneurons during the LLSR was assessed with transcranial magnetic stimulation (TMS). A facilitated response to TMS would be evidence of a LLSR pathway that traverses the motor cortex. Surface electromyogram electrodes were placed over the left or right masseter, and subjects (n=10) bit on bars with their incisor teeth at 10% of maximal electromyographic activity (EMG). Servo-controlled displacements were imposed on the lower jaw to evoke a short- and long-latency stretch reflex in masseter. TMS intensity was just suprathreshold for a response in contralateral masseter. Trials consisted of: (1) stretch alone, (2) TMS alone, and (3) TMS with a preceding conditioning stretch at varied conditioning-testing (C-T) intervals chosen to combine TMS with the short-latency stretch reflex (3 ms, 5 ms) and the LLSR (23–41 ms). Masseter EMG was rectified and averaged. With TMS alone, mean (± SE) MEP area above baseline was 56±9%. The area of masseter MEPs above baseline in the C-T trials was calculated from each EMG average following subtraction of the response to stretch alone. Conditioning muscle stretch had no significant effect on masseter MEPs evoked by TMS with any C-T interval (ANOVA; P=0.90). In addition, subjects were unable to modify the SLSR or LLSR by voluntary command. It is concluded that the long-latency stretch reflex in the masseter does not involve the motor cortex and is not influenced by "motor set". Electronic Publication     

Sympathoinhibition to Bezold–Jarisch reflex is attenuated in protein malnourished rats

Malnutrition affects cardiovascular reflexes, including chemoreflex and baroreflex. In this study we assessed the hypothesis that malnourishment changes the responses in mean arterial pressure (MAP), heart rate (HR) and renal sympathetic nerve activity (RSNA) evoked from Bezold–Jarisch reflex (BJR). Fischer rats were fed diets containing either (6% malnourished or 14% control) protein for 35 days after weaning. There were no differences in baseline MAP (102 ± 4 vs. 95 ± 3 mmHg) whereas higher baseline HR (478 ± 18 vs. 360 ± 11 bpm; P < 0.05,) and reduced sympathoinhibition (ΔRSNA = −54 ± 9 vs. −84 ± 7%; P = 0.0208) to BJR activation were found in malnourished rats. We conclude that malnutrition affects the sympathetic control of BJR.     

Have we stopped looking for a red reflex in newborn screening?

Best medical evidence indicates that surgical treatment of significant congenital cataracts is required within the first 3 months of life for optimal visual outcome. The aim of the present study was to review when the diagnosis of congenital cataracts was made in our region, by whom it was made, and the visual outcome at 2 years of age or more. This was a retrospective study in a region with a population of 546,000 and approximately 8500 births per annum, served by a single Regional Ophthalmology centre. All children under 15 years, diagnosed with Congenital Cataract over a 10-year period (1991-2002), were identified using the Hospital In-Patient Enquiry [HIPE] database. Children with cataract(s) from infancy from a congenital cause and those first presenting outside infancy but with salient clinical features indicating early cataract were included in the study. 27 cases of congenital and infantile cataract 15 (56%) males, 12 (44%) females were retrieved. 17 infants (63%) were diagnosed with bilateral disease, while the remainder were unilateral 10 (37%). Most of the cases 17 (63%) were diagnosed following presentation with parental/carer concerns about visual function (usually a squint). However only 2 of these 17 cases presented before 3 months of age. The remaining cases of congenital cataracts were diagnosed by general practitioners 8 (24%), paediatricians 4 (12%), ophthalmologists 3 (9%) or School Medical Officer (1, 3%). No case of congenital cataract was diagnosed by newborn screening examination. Six of 8 infants diagnosed with congenital cataracts before three months of age had a good visual outcome, (visual acuity < 6/24 at 2 years or more). In contrast only 3 of 19 cases who were diagnosed after 3 months of age had good visual outcomes. Despite their relative rarity, it is imperative that congenital cataracts are diagnosed and treated within 3 months of birth. The onus of diagnosis rests with newborn screening examiners at birth and with general practitioners at the 6-8 week checks. Parental concerns about a possible squint should be addressed by performing a red light reflex examination and urgent specialist ophthalmological referral if a flaw is detected.     

Is there a parotid-salivary reflex response to fat stimulation in humans?

The perception of fats in foods may involve gustatory, olfactory or textural cues. There is contradictory evidence as to whether the orosensory perception of fat is as a basic quality of taste or related to the physical characteristics of fat. A dose-response reflex parotid-salivary secretion has, however, been shown for the accepted basic taste qualities. The aim of this study was to establish whether varying fat concentration in two food types causes an associated dose-response reflex parotid secretion in humans. Parotid salivary flow was recorded using Lashley cups and cannulae connected to an instantaneous flow meter. Gustatory stimuli were achieved using 3 ml of skimmed (0.1% fat), semi-skimmed (1.7% fat) or full (3.6% fat) milk (Sainsbury) or 5 g of extra-light (5% fat), light (16% fat) or original (24% fat) cream cheese (Kraft). No significant differences in salivary flow rate were shown within the milk group (n=10, P=.93) or within the cream-cheese group (n=11, P=.82). Furthermore, no correlation was observed between increasing fat concentration and flow within either the milk (P=.98) or the cream-cheese group (P=.69; Pearson Product Moment Correlation). These results do not support the hypothesis that there is a fat-specific dose-response parotid reflex.     

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).     

The reflex to “time” in the artificial electrodefensive reactions of dogs

Summary Experimental investigation of reflex to the time of presentation of defensive signal stimuli (sound and light) was carried out by V.P. Petropavlovskii's method. Experiments were performed on 5 dogs over a period of 3 years. It was established that the reflex to time was revealed at first in the voval and respiratory components and only then in the motor component of the conditioned reaction (the raising of the right forepaw).In the absence of a definite sequence of signal stimulations, but with fixed intervals between them, the conditioned reflex to the time of their presentation appeared irrespective of the position of the stimuli in the experiment. With fixed sequence of the positive and inhibitory stimuli the reflex to time appeared only before the positive stimuli.It was established that not only the time of presentation of the stimulus had a signal action, but also time of the reinforcement (current) acted as a signal.Presented by Active Member Acad. Med. Sci. USSR V.N. Chernigovskii     

Neuroanatomical basis of Sandifer’s syndrome: A new vagal reflex?

Sandifer's syndrome is a gastrointestinal disorder with neurological features. It is characterized by reflex torticollis following deglutition in patients with gastroesophageal reflux and/or hiatal hernia. The authors believe that neurological manifestations of the syndrome are the consequence of vagal reflex with the reflex center in nucleus tractus solitarii (NTS). Three models for the neuroanatomical basis of the hypothetic reflex arc are presented. In the first one the hypothetic reflex arc is based on the classic hypothesis of two components nervus accessorius (n.XI) - radix cranialis (RC) and radix spinalis (RS) The nervous impulses are transmitted by nervus vagus (n.X) general visceral afferent (GVA) fibers to NTS situated in medulla oblongata, then by interneuronal connections on nucleus ambiguus (NA) and nucleus dorsalis nervi vagi (NDX). Special visceral efferent fibers (SVE) impulses from NA are in part transferred to n.XI ramus externus (RE) (carrying the majority of general somatic efferent (GSE) fibers) via hypothetic anastomoses in the region of foramen jugulare. This leads to contraction of trapezius and sternocleidomastoideus muscles, and the occurrence of intermittent torticollis. In the second suggested neuroanatomical model the hypothetic reflex arc is organized in the absence of n.XI RC, the efferent part of the reflex arc continues as NA, which is motor nucleus of nervus glossopharyngeus (n.IX) and n.X in this case while distal roots of n.XI that appear at the level of the olivary nucleus lower edge represent n.X roots. In the third presented model the hypothetic reflex arc includes no jugular transfer and could be realized via interneuronal connections directly from NTS to the spinal motoneurons within nucleus radicis spinalis nervi accessorii (NRS n.XI) or from NA to NRS n.XI. The afferent segment of the postulated reflex arc in all three models is mediated via n.X. We conclude that Sandifer's syndrome is a clinical manifestation of another vagal reflex that could be termed a "vagocervical" or "esophagocervical" reflex, based on the neuroanatomical hypotheses elaborated in this paper.     

Anal cytology: Is there a role for reflex HPV DNA testing?

There is an increased incidence of anal squamous carcinoma and its precursor lesions (anal intraepithelial neoplasia [AIN]) among persons who engage in anal-receptive sex. Analogous to cervical cancer screening, anal Papanicplaou (Pap) smears currently are used to screen these high-risk populations. Human papilloma virus (HPV) has been implicated in anal carcinoma pathogenesis and this study was performed to assess the potential role of HPV DNA testing as an adjunct to anal cytology. We correlated cytological diagnoses and HPV DNA (Digene Hybrid Capture [HC II] assay) in anal specimens collected in SurePath liquid medium from 118 patients; 54.8% of cases diagnosed as atypical squamous cells of undetermined significance (ASC-US) and 87.8% diagnosed as low-grade squamous intraepithelial lesion (LSIL) or above tested positive for high- risk HPV DNA (B+). High-grade SIL (HSIL) was present in 31 of the 51 patients with follow-up. Although a cytological diagnosis of ASC-US or above was a reliable indicator for AIN, cytology frequently did not accurately predict the grade of SIL in subsequent biopsy. Our findings suggest that reflex HPV DNA testing would be helpful in triaging patients diagnosed with ASC-US. However, patients diagnosed with LSIL or above should go directly to ansocopic biopsy.     

Participation of β-endorphin in the regulation of conditioned reflex activity of cats

The role of -endorphin in the regulation of conditioned instrumental food-procuring reactions and more complex forms of nervous activity such as reflexes of choice of the side of reinforcement was studied in cats. It was established that the subcutaneous injection of small doses of -endorphin (10 g/kg, 15 ·10^–6 g/kg) exerts a facilitatory nonspecialized effect on positive and negative food-procuring conditioned reflexes that has an overall adaptive character overall. The influence of the same doses of -endorphin on conditioned reflexes of choice is more complex in character, depending upon the initial level of conditioned reflex activity and on the typological features of the experimental animals. A possible mechanism of the influence of -endorphin on higher nervous activity is discUSSed.Deceased.Translated from Fiziologicheskii Zhurnal SSSR imeni I. M. Sechenova, Vol. 76, No. 8, pp. 992–1000, August, 1990.     

What do reflex and voluntary mean? Modern views on an ancient debate

Are the words reflex and voluntary useful scientific concepts, or are they prescientific terms that should be discarded? Physiologists use these words routinely in their publications, in laboratory experiments and, indeed, like most lay people, in their daily lives. The tacit assumption is that we all know, more or less, what they mean. However, the issue has a rich history of philosophical and scientific debate; and, as this article demonstrates, present-day researchers still cannot reach a consensus on the meaning of the words and on whether it is possible to draw a scientific distinction between them. The five authors present five quite different analyses. In broad terms, they split into two camps: those who equate voluntary behaviours with consciousness and suppressibility and those who view all behaviours as sensorimotor interactions, the complexity of which determines whether they are reflexive or voluntary. According to the first view, most movements of daily life are neither purely reflex nor purely voluntary. They fall into the middle ground of automatic motor programs. According to the second view, as neuroscience advances the class of reflex behaviours will grow and the class of voluntary behaviours will shrink.     

Plastic alteration of vestibulo-cardiovascular reflex induced by 2 weeks of 3-G load in conscious rats

Previous studies conducted in our laboratory have demonstrated that the vestibular system plays a significant role in controlling arterial pressure (AP) in conscious rats under conditions of transient microgravity. The vestibular system is known to be highly plastic, and on exposure to different gravitational environments, the sensitivity of the vestibular system-mediated AP response might be altered. In order to test this hypothesis, rats were maintained in a 3-G or a normal 1-G environment for 2 weeks, and the AP responses to free drop-induced microgravity were determined. In 1-G rats, the microgravity increased the AP by 37 ± 3 mmHg; this pressor response was significantly attenuated by vestibular lesion (VL) (24 ± 3 mmHg) or body stabilization (29 ± 2 mmHg). Thus, the microgravity-induced pressor response was mediated by both the vestibular and nonvestibular systems; the input of the latter system was blocked by body stabilization. In the 3-G rats, the pressor responses were significantly suppressed compared to those in the corresponding 1-G rats; i.e., the AP increased by 24 ± 2 mmHg in freely moving 3-G rats, by 10 ± 4 mmHg in 3-G rats with VL, and by 13 ± 4 mmHg in stabilized 3-G rats. Furthermore, there was no difference between the 1- and 3-G rats in terms of the pressor response induced by stressors such as a loud noise or an air jet. These results indicate that pre-exposure to 3-G for 2 weeks induces plasticity in both the vestibular- and nonvestibular-mediated AP responses to microgravity.     

The reflex nature of the phenomenon of “lagging” of respiratory movements in unilateral lung lesions

Summary In the presence of a unilateral focal pulmonary lesion the electric activity of the diaphragm is decreased on the homolateral side and increased on the contralateral side. This reflex lies at the basis of the physical sign known as lagging of respiratory movements on the side of the pulmonary lesion. The absence of compensatory increase of respiratory movements on the side of the lesion is evidently connected with active suppression of diaphragmatic activity taking place in the phrenic nerve center.Presented by Active Member AMN SSSR V. N. Chernigovskii     

Excitability of the soleus reflex arc during intensive stretch–shortening cycle exercise in two power-trained athlete groups

In several explosive types of sport events the leg extensor muscles are subjected to very high impact loads. Thus, extreme requirements exist for the neuromuscular system to develop sufficient muscle stiffness in the lower extremities in order to tolerate these high impact loads. Therefore, it would be challenging to measure reflex modulation during high impact activities, and with different athlete populations. In the present experiment, H-reflex and short latency reflex (M1) sensitivity was measured during drop jump exercises among high jumpers and sprinters. The changes in both reflex peak-to-peak amplitudes showed a significant (P < 0.05) reduction towards the end of the exercise for the sprinters. In addition, the same subject group showed a remarkable increase in serum creatine kinase (CK) activity 2 h after the jumps. Similar changes could not be observed for the high jumpers. These results clearly indicate different neural adaptation strategies for the two athlete groups. Reduction in H-reflex sensitivity and an increase in CK-activity in sprinters were taken as evidence for presynaptic inhibition, probably induced by substances related to muscle damage. Since high jump training includes more high impact loading, it was assumed that it could lead to some structural adaptation and, thus, prevents exercise induced reflex modification to a certain extent.     

Estimating reflex responses in large populations of motor units by decomposition of the high‐density surface electromyogram

Key points

• Reflex responses of single motor units have been used for the study of spinal circuitries but the methods employed are invasive and limited to the assessment of a relatively small number of motor units.
• We propose a new approach to investigate reflexes on individual motor units based on high‐density surface electromyography (HDsEMG) decomposition.
• The decomposition of HDsEMG has been previously validated in voluntary isometric contractions but never during reflex activities.
• The use of HDsEMG decomposition for reflex studies at the individual motor unit level, during constant force contractions, with excitatory and inhibitory stimuli, was validated here by the comparison of results with concurrently recorded intramuscular EMG signals.
• The validation results showed that HDsEMG decomposition allows an accurate quantification of reflex responses for a large number of individual motor units non‐invasively, for both excitatory and inhibitory stimuli.

Abstract

We propose and validate a non‐invasive method that enables accurate detection of the discharge times of a relatively large number of motor units during excitatory and inhibitory reflex stimulations. High‐density surface electromyography (HDsEMG) and intramuscular EMG (iEMG) were recorded from the tibialis anterior muscle during ankle dorsiflexions performed at 5%, 10% and 20% of the maximum voluntary contraction (MVC) force, in nine healthy subjects. The tibial nerve (inhibitory reflex) and the peroneal nerve (excitatory reflex) were stimulated with constant current stimuli. In total, 416 motor units were identified from the automatic decomposition of the HDsEMG. The iEMG was decomposed using a state‐of‐the‐art decomposition tool and provided 84 motor units (average of two recording sites). The reflex responses of the detected motor units were analysed using the peri‐stimulus time histogram (PSTH) and the peri‐stimulus frequencygram (PSF). The reflex responses of the common motor units identified concurrently from the HDsEMG and the iEMG signals showed an average disagreement (the difference between number of observed spikes in each bin relative to the mean) of 8.2 ± 2.2% (5% MVC), 6.8 ± 1.0% (10% MVC) and 7.5 ± 2.2% (20% MVC), for reflex inhibition, and 6.5 ± 4.1%, 12.0 ± 1.8% and 13.9 ± 2.4%, for reflex excitation. There was no significant difference between the characteristics of the reflex responses, such as latency, amplitude and duration, for the motor units identified by both techniques. Finally, reflex responses could be identified at higher force (4 of the 9 subjects performed contraction up to 50% MVC) using HDsEMG but not iEMG, because of the difficulty in decomposing the iEMG at high forces. In conclusion, single motor unit reflex responses can be estimated accurately and non‐invasively in relatively large populations of motor units using HDsEMG. This non‐invasive approach may enable a more thorough investigation of the synaptic input distribution on active motor units at various force levels.

Abbreviations

CoV_ISI

coefficient of variation of the inter‐spike interval

CPN

common peroneal nerve

CUSUM

cumulative sum

HDsEMG

high‐density surface electromyography

iEMG

intramuscular EMG

MU

motor unit

MVC

maximum voluntary contraction

PNR

pulse‐to‐noise ratio

PSF

peri‐stimulus frequencygram

PSTH

peri‐stimulus time histogram

RA

rate of agreement

SIR

signal‐to‐interference ratio

TA muscle

tibialis anterior muscle

TN

tibial nerve

     

A diverse pattern of the spike threshold changes in feline gastrocnemius–soleus motoneurons during stretch reflex activation

The aim of the study was to define the spike threshold changes in motoneurons during naturally evoked activation. Spike activity was evoked in the gastrocnemius–soleus motoneurons of decerebrate cats by controlled stretches of the homonymous muscles. The spike thresholds were defined by using the first derivative of the membrane potential and statistical boundaries of its change. The evoked firing was analyzed in terms of the spike thresholds and the membrane potential trajectories between spikes, which were dependent on the intensity of the firing. The main inference of the study is the absence of a single-valued dependence between the spike rates and thresholds; the last ones can both increase and decrease with a rise in the firing rate; moreover, opposite directions in the threshold changes are often observed at various phases of the stretch-evoked firing. The dependency of the spike thresholds on firing rates was checked in 42 of the total set of 57 motoneurons. In a group of cells (n = 27) showing statistically significant correlation (P < 0.05), this parameter was negative in 18 and positive in 9 motoneurons. The obtained results indicate that the threshold-crossing models with a single-valued dependency of the spike thresholds on the firing rates are not completely suitable to analyze the spike generation processes in the motor unit records.     

Are cardiac syndrome X,irritable bowel syndrome and reflex sympathetic dystrophy examples of lateral medullary ischaemic syndromes?

Altered pain appreciation and autonomic function are hallmarks of Cardiac syndrome X, Irritable bowel syndrome and Reflex sympathetic dystrophy. Both pain appreciation and autonomic function are controlled by the lateral medulla. This hypothesis proposes that lateral medullary ischaemia at a microvascular level is responsible for these syndromes and could also be linked to other conditions where autonomic dysfunction is a major feature such as late-onset asthma, type 2 diabetes and essential hypertension. Autonomic function is controlled by the nucleus tractus solitarius, which acts as the main viscero-afferent nucleus in the brain stem regulating vagal tone. It is particularly susceptible to ischaemia since it is highly metabolically active and lies in a medullary arterial watershed zone. The anatomical route of the vertebral artery through cervical vertebra makes it vulnerable to injury from whiplash with or without any genetic predisposition to atheroma formation. This could make microvascular occlusion commonplace and a plausible explanation for the above syndromes. Ischaemia rather than infarction occurs because of the excellent collateral blood supply in the brainstem. In support of this hypothesis, a new Transcranial doppler ultrasonography arterial signal has been described called small vessel knock, the ultrasound signal of small vessel occlusion. Recent evidence has shown that ultrasound targeting of this signal in the vertebral artery improves clinical symptoms in these syndromes which supports this hypothesis. Two such cases are discussed.     

Major contribution of Aδ-fibres to increased reflex transmission in the feline spinal cord during acute muscle inflammation

The contribution of activated nociceptive muscle afferents to pathologically increased muscle tone remains obscure. The aim of the present study was to investigate whether an acute myositis of the gastrocnemius-soleus (GS) influences spinal reflex activity and to test whether Aδ-fibre or C-fibre were mainly responsible for any effects. In high spinal cats monosynaptic reflexes (MRs) of flexors and extensors and transmission in reflex pathways from group III and IV muscle afferents (activated by intra-arterial KCl injection) were investigated. After infiltration of GS with carrageenan there was a distinct increase in the MRs of flexors and extensors, coupled with facilitation of the flexors, induced by chemically activated group III and IV afferents. The inhibition evoked in extensors by these afferents was also mainly enhanced but less consistently. The reflex effects of carrageenan started within 1 h and reached their maximum after about 1.5 h. After blocking the input of all myelinated A-fibres, including Aδ-fibres, from the inflamed muscle by TTX, only a small facilitatory effect on MRs remained and the facilitation of excitatory transmission in the excitatory pathway to the flexor PBSt was abolished. The results show that the action of carrageenan-induced inflammation on spinal reflex function derives mainly from Aδ-fibres.     

Modulation of reflex responses in hand muscles during rhythmical finger tasks in a subject with writer’s cramp

The objective of this study was to examine phase- and task-dependent modulation of stretch reflexes during repetitive finger movements in writer’s cramp, and compare them with normal controls from our previous study. A subject with writer’s cramp conducted two rhythmic tasks, index finger abduction (RFA) and a pen-squeezing (RPS) task akin to handwriting. Stretch reflexes were evoked by mechanical perturbations at random phases of each task. Surface electromyograms (EMG) were recorded from two hand muscles, first dorsal interosseous (FDI) and flexor digitorum superficialis (FDS). The reflex response and background EMG activity of each muscle were modulated in a phase-dependent manner in both tasks. However, they varied largely in phase during the RFA task, but in approximately inverse phase-relationship during RPS. Reflex sensitivity, as represented by the slope of the linear regression between response and background, was much lower for both muscles in the ‘writing’ task (RPS) than in the RFA task with its positively correlated responses. These phase- and task-related modulation patterns differed dramatically from those observed in our control subjects, where reflex responses were modulated largely in phase with background activity and reflex sensitivity was much higher, particularly in FDI during RFA and FDS during RPS. The altered reflex modulation patterns in writer’s cramp may reflect deficiencies of integration of proprioceptive afferent inputs and reduced inhibition at cortical and spinal levels during writing performance. Results from this case study support clinically identified task-specific feature of focal hand dystonia.     

Functional changes in autonomic nervous system and baroreceptor reflex induced by 14 days of 6° head-down bed rest

We studied the effects of 14 days of 6° head-down bed rest (HDBR) in 16 healthy male subjects to examine the functional changes in the autonomic nervous system and cardiac baroreceptor reflex response with an emphasis on dynamic changes during HDBR. Beat-by-beat RR intervals (RRIs) and systolic arterial pressures (SAPs) were measured non-invasively from simultaneous, continuous recordings of ECG and arterial pressure waves in supine resting postures. A power spectrum analysis by the fast Fourier transform was applied to a data set composed of interpolated 512 RRIs and 512 SAPs (256 s in duration). Three indices of cardiac baroreceptor reflex sensitivity (BRS) were obtained by applying a sequence technique and a cross-spectrum analysis technique to the spontaneous RRIs and SAPs. The high-frequency band power of RRI variability (HF_RRI) decreased significantly in the latter part of HDBR and persisted until the initial stage of the post-HDBR period (POST). The low-frequency band power of SAP variability decreased significantly only during the mid-part of HDBR. The BRS_sequence obtained by the sequence technique showed a significant increase temporarily on the initial day of HDBR. The BRS_sequence and the estimate of BRS obtained by the cross-spectrum analysis handling the high-frequency band were both significantly decreased on the initial day of POST. Each of the BRS estimates correlated negatively with heart rate and positively with HF_RRI during HDBR and POST. These results suggest the following: (1) cardiac spontaneous baroreceptor reflex sensitivity might be transiently increased at the initial stage of HDBR, (2) the reduction in vagal modulation on the sinus node occurs from the latter part of HDBR to the initial stage of POST, (3) sympathetic vasomotor control is probably slightly inhibited during the mid-part of HDBR, and (4) the enhancement in cardiac sympathetic modulation and the impairment in cardiac spontaneous baroreceptor reflex sensitivity may occur in the initial stage of POST.