Magnitude Of Reflex Research Articles

Effect of knock out of ASIC3 on cardiovascular reflexes arising from hindlimb muscle in ligated rats

The exercise pressor reflex (EPR) arises from contracting the skeletal muscle and is manifested by increases in arterial pressure, heart rate and blood flow to working muscles. Group III and IV afferents comprise the sensory arm of the exercise pressor reflex. The exercise pressor reflex is exaggerated in patients with peripheral artery disease, which is an atherosclerotic process that occludes blood flow to working muscles. The exaggeration of the exercise pressor reflex is believed to be caused by a metabolite whose concentration is increased when the working muscles are inadequately perfused. Previous work has shown that pharmacological blockade of acid‐sensing ion channel 3, which is found on the endings of group III and IV afferents, prevents the exaggeration of the exercise pressor reflex in rats with simulated peripheral artery disease. In the present experiments, we compared the magnitude of the exercise pressor reflex in decerebrated rats with a functional knockout of the ASIC3 (KO) with the magnitude of the reflex in their decerebrated wild type counterparts (WT). We found that the exercise pressor reflex in ASIC3 KO rats was significantly smaller than the exercise pressor reflex in their WT counterparts (p<0.05). In addition, ASIC 3 KO rats demonstrated significantly smaller pressor responses to injection of diprotonated phosphate (86mM; pH 6.0), lactic acid (12mM; pH 2.85), and capsaicin (0.2μg; pH 7.2) into the arterial supply of the hindlimb (p< 0.05). In contrast, both ligated WT and ASIC3 KO rats displayed similar pressor responses to calcaneal tendon stretch (p>0.05). We conclude that ASIC3 plays an important role in evoking the exaggerated exercise pressor reflex seen in rats with peripheral artery disease.Support or Funding InformationNIH Grants R01‐AR‐059397 and P01‐HL‐134609

The magnitude of the exercise pressor reflex is influenced by the active skeletal muscle mass in the decerebrate rat.

The exercise pressor reflex is composed of two components, namely the muscle mechanoreflex and the muscle metaboreflex. The afferents evoking the two components are either thinly myelinated (group III) or unmyelinated (group IV); in combination they are termed "thin fiber afferents." The exercise pressor reflex is often studied in unanesthetized, decerebrate rats. However, the relationship between the magnitude of this reflex and the number of thin fiber afferents stimulated by muscle contraction is unknown. This lack of knowledge prompted us to test the hypothesis that the magnitude of the exercise pressor reflex was directly proportional to the amount of muscle mass activated. Muscle mechanoreceptors were stimulated by stretching the calcaneal tendon. Likewise, muscle metaboreceptors were stimulated by injecting lactic acid into the arterial supply of the hindlimb muscles. In addition, both muscle mechanoreceptors and metaboreceptors were stimulated by statically contracting the hindlimb muscles. We found that simultaneous bilateral (both hindlimbs) stimulation of thin fiber afferents with stretch, lactic acid, and static contraction evoked significantly greater pressor responses than did unilateral (one hindlimb) stimulation of these afferents. In addition, the magnitude of the pressor responses to bilateral simultaneous stimulation of thin fiber afferents evoked by stretch, lactic acid, and contraction was not significantly different from the magnitude of the sum of the pressor responses evoked by unilateral stimulation of these afferents by stretch, lactic acid, and contraction. We conclude that the magnitude of the exercise pressor reflex and its two components is dependent on the number of afferents stimulated.

Comparing targeted thrust manipulation with general thrust manipulation in patients with low back pain. A general approach is as effective as a specific one. A randomised controlled trial

BackgroundSpinal manipulation is commonly used to treat back pain. The application of spinal manipulation has traditionally involved an element of targeting the technique to a level of the spine where...

Associations between self-report and physiological measures of emotional reactions to food among women with disordered eating

Individuals with eating disorders have exhibited both positive and negative emotional responses to food when assessed via self-report and psychophysiology. These mixed findings may be explained by a lack of association between self-report and physiological measures, and the degree of association may differ based on core eating disorder symptoms like dietary restriction and binge eating. Women from the community (N = 82) were recruited based on the presence or absence of dietary restriction and binge eating. We examined the startle eyeblink reflex, a physiological measure of defensive motivation, in relation to self-reported valence, arousal, and craving ratings of emotional (positive, neutral, negative) and food (high- and low-calorie) images. Dietary restriction and binge eating were investigated as moderators of self-report/physiology relationships. Replicating extant literature, valence ratings of emotional images were correlated with startle blink reflex magnitude, with more unpleasant ratings related to higher startle eyeblink reflex magnitudes. Increased craving, but not valence, ratings of food images were related to lower startle blink reflex magnitudes. Dietary restriction and binge eating did not moderate the relationship between self-report ratings and startle blink magnitude to food. Our findings suggest that self-reported appetitive motivation towards food relates to a decrease in physiologically measured aversion towards food. Future research should examine the extent to which self-report ratings correlate with physiological indices of positive emotion (e.g., postauricular reflex, zygomaticus major) during the viewing of food images in both patients with eating disorders and healthy controls.

Effect of knockout of the ASIC3 on cardiovascular reflexes arising from hindlimb muscle in decerebrated rats.

The exercise pressor reflex is initiated by the contraction-induced activation of group III and IV muscle afferents. The reflex is manifested by increases in arterial blood pressure and cardiac output, which, in turn, are generated by increases in the sympathetic outflow to the heart and vasculature and decreases in the vagal outflow to the heart. In previous experiments, we used a pharmacological approach to assess the role played by the acid-sensing ion channel 3 (ASIC3) on group III and IV afferents in evoking the exercise pressor reflex. In the present experiments, we used an alternative approach, namely functional knockout (KO) of the ASIC3 gene, to confirm and extend our previous finding that pharmacological blockade of the ASIC3 had only a small impact on the expression of the exercise pressor reflex when the arterial supply to the contracting hindlimb muscles of rats was patent. Using this alternative approach, we compared the magnitude of the exercise pressor reflex evoked in ASIC3 KO rats with that evoked in their wild-type (WT) counterparts. We found both WT and ASIC3 KO rats displayed similar pressor responses to static contraction (WT, n = 10, +12 ± 2 mmHg; KO, n = 9, +11 ± 2 mmHg) and calcaneal tendon stretch (WT, n = 9, +13 ± 2 mmHg; KO, n = 7, +11 ± 2 mmHg). Likewise, both WT and ASIC3 KO displayed similar pressor responses to intra-arterial injection of 12 mM lactic acid (WT, n = 9, +14 ± 3 mmHg; KO, n = 8, +18 ± 5 mmHg), 24 mM lactic acid (WT, n = 9,+24 ± 2 mmHg; KO, n = 8, +20 ± 5 mmHg), capsaicin (WT, n = 9,+27 ± 5 mmHg; KO, n = 10, +29 ± 5 mmHg), and diprotonated phosphate ([Formula: see text]; WT, n = 6,+22 ± 3 mmHg; KO, n = 6, +32 ± 6 mmHg). We conclude that redundant receptors are responsible for evoking the pressor reflexes arising from group III and IV afferents.

Stumbling Reactions in Partial Gravity - Neuromechanics of Compensatory Postural Responses and Inter-Limb Coordination During Perturbation of Human Stance.

Spontaneous changes in gravity play a significant role in interplanetary space missions. To preserve the astronauts’ capability to execute mission-critical tasks and reduce the risk of injury in transit and on planetary surfaces, a comprehensive understanding of the neuromuscular control of postural responses after balance deterioration in hypo- or hyper-gravity conditions is essential. Therefore, this study aimed to evaluate the effect of acute gravitational variation on postural adjustments in response to perturbations. Gravitational changes were induced using parabolic flight. Postural set was manipulated by randomly providing unilateral left, bilateral or split perturbations which require balance corrections to restore postural stability. In six subjects, postural reactions were recorded after anterior and posterior surface perturbations for progressively increased gravitational conditions spanning from 0.25 to 1.75 g. Ankle and knee joint kinematics and electromyograms (EMG) of eight leg muscles were recorded prior (PRE) and after perturbation onset. Muscle activation onset latencies and amplitudes in the short-, medium-, and long-latency responses (SLR, MLR, LLR) were assessed. Results demonstrate an increased muscle activity (p < 0.05) and co-contraction in the lower extremities (p < 0.05) prior to perturbation in hypo- and hyper-gravity. After perturbation, reduced muscle onset latencies (p < 0.05) and increased muscle activations in the MLR and LLR (p < 0.05), concomitant with an increased co-contraction in the SLR, were manifested with a progressive rise in gravity. Ankle and knee joint deflections remained unaffected, whereas angular velocities increased (p < 0.05) with increasing gravitation. Effects were more pronounced in bi- compared to unilateral or split perturbations (p < 0.05). Neuro-mechanical adaptations to gravity were more distinct and muscle onset latencies were shorter in the displaced compared to the non-displaced leg. In conclusion, the timing and magnitude of postural reflexes involved in stabilization of bipedal stance are gravity-dependent. The approximately linear relationship between gravity and impulse-directed EMG amplitudes or muscle onset latencies after perturbation indicates that the central nervous system correctly predicts the level of gravity. Moreover, it accurately governs contractions in the antigravity musculature to counterbalance the gravitational pull and to regain upright posture after its disturbance. Importantly, unilateral perturbations evoked fast reflex responses in the synergistic muscles of the non-displaced contralateral leg suggesting a synchronized inter-limb coordination mediated by spinal circuitries.

Emotional Modulation of Pain and Spinal Nociception in Sexual Assault Survivors.

Sexual assault (SA) is associated with an increased risk for chronic pain and affective distress. Given that emotional processes modulate pain (e.g., negative emotions enhance pain, positive emotions inhibit pain), increased pain risk in SA survivors could stem from a disruption of emotional modulation processes. A well-validated affective picture-viewing paradigm was used to study emotional modulation of pain in 33 healthy, pain-free SA survivors and a control group of 33 healthy, pain-free individuals with no reported history of SA (matched on age, sex, race, and number of non-SA traumas). Unpleasant (mutilation), neutral, and pleasant (erotic) pictures were presented, while painful electrocutaneous stimulations were delivered at the ankle. Pain intensity ratings and nociceptive flexion reflex (NFR) magnitudes (a physiologic measure of spinal nociception) were recorded in response to electric stimuli. Multilevel models were used to analyze the data with group (SA versus non-SA) and content (mutilation, neutral, erotic) as independent variables. Both groups demonstrated similar emotional modulation of pain (FGroupbyContent(2,646.52) = 0.44, p = .65), but a main effect of group (FGroup(1,65.42) = 4.24, p = .043) indicated the SA group experienced more overall pain from electric stimuli (hyperalgesia). A significant group by content interaction for NFR (p = .035) indicated that emotional modulation of NFR was present for the non-SA group (FContentSimpleEffect(2,684.55) = 12.43, p < .001), but not the SA group (FContentSimpleEffect(2,683.38) = 1.71, p = .18). These findings suggest that SA survivors have difficulty emotionally engaging brain-to-spinal cord mechanisms to modulate spinal nociception. A disruption of descending inhibition plus hyperalgesia could contribute to comorbidity between sexual trauma and chronic pain.

The Impact of Alcohol and Social Context on the Startle Eyeblink Reflex.

Researchers have long sought to understand how individuals respond to alcohol in social settings with the aim of elucidating pathways of risk for alcohol use disorder (AUD). But studies that incorporate a social context are still outnumbered by those that examine alcohol's subjective effects among participants drinking alcohol in isolation. Further, perhaps due to the challenges of capturing automatic affective processes in these settings, prior studies of alcohol response in social context have relied mainly on self-report measures, and so relatively little is known about alcohol's psychophysiological effects in social settings. Using a novel paradigm that integrated alcohol-administration procedures, startle methodology, and social context, this study examined the impact of alcohol and social context on startle eyeblink reflex among 40 social drinkers. Results indicated that there was a significant effect of group presence, indicating that startle magnitude was larger when people were alone than with others. There was a significant group presence-by-alcoholic beverage interaction, with the effect of alcohol being significantly larger when people were alone versus with others. These effects were found both for the startle habituation data and during the picture-viewing task. Results of this study highlight the importance of considering the presence of other individuals for understanding alcohol response and mechanisms of AUD risk. Findings are discussed in light of both emotional and cognitive correlates of startle reflex magnitude. Future research should examine these effects within larger samples of participants and further explore mechanisms that might underlie these effects.

Enhanced pupillary light reflex in infancy is associated with autism diagnosis in toddlerhood

Autism spectrum disorder (ASD) is a neurodevelopmental condition affecting around 1% of the population. We previously discovered that infant siblings of children with ASD had stronger pupillary light reflexes compared to low-risk infants, a result which contrasts sharply with the weak pupillary light reflex typically seen in both children and adults with ASD. Here, we show that on average the relative constriction of the pupillary light reflex is larger in 9–10-month-old high risk infant siblings who receive an ASD diagnosis at 36 months, compared both to those who do not and to low-risk controls. We also found that the magnitude of the pupillary light reflex in infancy is associated with symptom severity at follow-up. This study indicates an important role of sensory atypicalities in the etiology of ASD, and suggests that pupillometry, if further developed and refined, could facilitate risk assessment in infants.

Cortical modulation of brainstem circuits is abnormal in cervical dystonia

ObjectiveIn healthy subjects, magnitude of hand-evoked blink reflex (BR) is increased if the stimulated hand is positioned in the peripersonal space. Here, we aimed to investigate the modulation of hand-evoked BR in peripersonal space to understand alterations of cortical modulations of subcortical structures in cervical dystonia. Patients and methodWe recruited 23 patients with idiopathic cervical dystonia and 21 healthy subjects, all of whom had hand-evoked BR. The reflex was recorded while stimulated hand was close to face (‘peripersonal space) and far away from face (extrapersonal space). Comparisons were done between two conditions in each group. ResultsIn extrapersonal space, magnitude of the reflex was bigger in patients compared to healthy subjects. In peripersonal space, magnitude of the reflex was increased in healthy subjects whereas it was reduced in patients. Comparisons of patients with and without sensory trick disclosed reflex magnitude was reduced in peripersonal space in patients with sensory trick whereas it did not change between two conditions in patients without sensory trick. DiscussionEnhancement of hand-evoked BR is a normal phenomenon that provides evidence for top-down modulation of its neural circuitry. In cervical dystonia, the normal modulation is lost which suggests abnormal modulatory effect of higher-order centers upon brainstem circuits. The pattern of alteration also differs if there is no sensory trick.

History-dependence of muscle slack length following contraction and stretch in the human vastus lateralis.

In reduced muscle preparations, the slack length and passive stiffness of muscle fibres have been shown to be influenced by previous muscle contraction or stretch. In human muscles, such behaviours have been inferred from measures of muscle force, joint stiffness and reflex magnitudes and latencies. Using ultrasound imaging, we directly observed that isometric contraction of the vastus lateralis muscle at short lengths reduces the slack lengths of the muscle-tendon unit and muscle fascicles. The effect is apparent 60s after the contraction. These observations imply that muscle contraction at short lengths causes the formation of bonds which reduce the effective length of structures that generate passive tension in muscles. In reduced muscle preparations, stretch and muscle contraction change the properties of relaxed muscle fibres. In humans, effects of stretch and contraction on properties of relaxed muscles have been inferred from measurements of time taken to develop force, joint stiffness and reflex latencies. The current study used ultrasound imaging to directly observe the effects of stretch and contraction on muscle-tendon slack length and fascicle slack length of the human vastus lateralis muscle in vivo. The muscle was conditioned by (a) strong isometric contractions at long muscle-tendon lengths, (b) strong isometric contractions at short muscle-tendon lengths, (c) weak isometric contractions at long muscle-tendon lengths and (d) slow stretches. One minute after conditioning, ultrasound images were acquired from the relaxed muscle as it was slowly lengthened through its physiological range. The ultrasound image sequences were used to identify muscle-tendon slack angles and fascicle slack lengths. Contraction at short muscle-tendon lengths caused a mean 13.5degree (95% CI 11.8-15.0degree) shift in the muscle-tendon slack angle towards shorter muscle-tendon lengths, and a mean 5mm (95% CI 2-8mm) reduction in fascicle slack length, compared to the other conditions. A supplementary experiment showed the effect could be demonstrated if the muscle was conditioned by contraction at short lengths but not if the relaxed muscle was held at short lengths, confirming the role of muscle contraction. These observations imply that muscle contraction at short lengths causes the formation of bonds which reduce the effective length of structures that generate passive tension in muscles.

Altered Neuromodulatory Drive May Contribute to Exaggerated Tonic Vibration Reflexes in Chronic Hemiparetic Stroke.

Exaggerated stretch-sensitive reflexes are a common finding in elbow flexors of the contralesional arm in chronic hemiparetic stroke, particularly when muscles are not voluntarily activated prior to stretch. Previous investigations have suggested that this exaggeration could arise either from an abnormal tonic ionotropic drive to motoneuron pools innervating the paretic limbs, which could bring additional motor units near firing threshold, or from an increased influence of descending monoaminergic neuromodulatory pathways, which could depolarize motoneurons and amplify their responses to synaptic inputs. However, previous investigations have been unable to differentiate between these explanations, leaving the source(s) of this excitability increase unclear. Here, we used tonic vibration reflexes (TVRs) during voluntary muscle contractions of increasing magnitude to infer the sources of spinal motor excitability in individuals with chronic hemiparetic stroke. We show that when the paretic and non-paretic elbow flexors are preactivated to the same percentage of maximum prior to vibration, TVRs remain significantly elevated in the paretic arm. We also show that the rate of vibration-induced torque development increases as a function of increasing preactivation in the paretic limb, even though the amplitude of vibration-induced torque remains conspicuously unchanged as preactivation increases. It is highly unlikely that these findings could be explained by a source that is either purely ionotropic or purely neuromodulatory, because matching preactivation should control for the effects of a potential ionotropic drive (and lead to comparable tonic vibration reflex responses between limbs), while a purely monoaminergic mechanism would increase reflex magnitude as a function of preactivation. Thus, our results suggest that increased excitability of motor pools innervating the paretic limb post-stroke is likely to arise from both ionotropic and neuromodulatory mechanisms.

(152) - Conditioned modulation of pain and NFR in trauma exposed adults

Trauma exposure is associated with hyperalgesia and chronic pain, but it is unknown if descending modulation of spinal nociception is involved. To address this, the current study examined conditioned pain modulation (CPM) in 198 healthy, pain-free participants that varied in their trauma exposure. Prior to testing, participants filled out the Life Events Checklist (LEC-5) to assess the number of different lifetime traumatic events they had experienced. Responses were used to form 4 levels of exposure (none [0], low [1], medium [2], high [ ≥ 3]). CPM was assessed from suprathreshold electric (test) stimuli delivered over the sural nerve at the ankle before, during, and after immersion of the hand in painful 10°C water (conditioning stimulus). Pain ratings and nociceptive flexion reflex magnitudes (NFR, a marker of spinal nociception) were assessed in response to each electric stimulation. Linear mixed ANOVAs found that all groups demonstrated significant conditioned inhibition of pain, but individuals with a high degree of exposure to trauma showed significantly higher pain ratings during the baseline and water immersion suggesting generalized hyperalgesia. Conversely, those with no, low, or medium trauma exposure showed NFR inhibition during or after the conditioning stimulus, but those with high exposure showed NFR facilitation. These results survived even after controlling for sex or catastrophizing. Together, these findings indicate that multiple trauma exposures may alter descending modulation of spinal modulation which in turn could contribute to the development of hyperalgesia and chronic pain. Future research is needed to determine the mediators of the relationship between trauma exposure and disrupted modulation (eg, allostatic load) so that interventions can be developed to prevent maladaptive sequelae.

Endocannabinoid-mediated potentiation of nonnociceptive synapses contributes to behavioral sensitization.

Endocannabinoids, such as 2-arachidonoyl glycerol (2-AG) and anandamide, can elicit long-term depression of both excitatory and inhibitory synapses. This latter effect will result in disinhibition and would therefore be expected to produce an increase in neural circuit output. However, there have been no examples directly linking endocannabinoid-mediated disinhibition to a change in a functional neurobehavioral circuit. The present study uses the well-characterized central nervous system of the medicinal leech, Hirudo verbana, to examine the functional/behavioral relevance of endocannabinoid modulation of an identified afferent synapse. Bath application of 2-AG potentiates synaptic transmission by pressure-sensitive sensory neurons (P cells) as well as the magnitude of the defensive shortening reflex elicited by P-cell stimulation. This potentiation requires activation of TRPV-like channels. Endocannabinoid/TRPV signaling was found to produce sensitization of the shortening reflex elicited by either direct stimulation of nearby nociceptive afferents (N cells) or noxious stimulation applied to skin several segments away. In both cases, heterosynaptic potentiation of P-cell synapses was observed in parallel with an increase in the magnitude of elicited shortening and both synaptic and behavioral effects were blocked by pharmacological inhibition of 2-AG synthesis or TRPV-like channel activation. Serotonin (5-HT) is known to play a critical role in sensitization in Hirudo and other animals, and the 5-HT2 receptor antagonist ritanserin also blocked behavioral sensitization and the accompanying synaptic potentiation. These findings suggest a novel, endocannabinoid-mediated contribution to behavioral sensitization that may interact with known 5-HT-dependent modulatory processes. NEW & NOTEWORTHY There is considerable interest in the analgesic potential of cannabinoids. However, there is evidence that the cannabinoid system can have both pro- and antinociceptive effects. This study examines how an endogenous cannabinoid transmitter can potentiate nonnociceptive synapses and enhance their capacity to elicit a nocifensive behavioral response.

Startle reflex modulation during threat of shock and "threat" of reward.

During threat of shock, the startle reflex is potentiated, suggesting modulation by defensive mobilization. To determine whether startle potentiation is specific to aversive anticipation, startle reflexes were measured in the context of either aversive or appetitive anticipation in a between-subject study. Participants wore a device on the wrist that could deliver electrical shock (n = 49), or vibrotactile stimulation indicating monetary reward (n = 48). Cues signaling "threat" or "safe" periods were presented alone, or accompanied by presentation of affective and neutral pictures on half of the trials. Results indicated that the startle reflex was significantly potentiated when anticipating either shock or reward, compared to safe periods, both when no picture was presented, as well as during picture viewing. The difference between threat and safety in both reflex magnitude and skin conductance changes was larger for those anticipating shock, suggesting that the aversive context was more motivationally engaging. The pattern of reflex modulation as a function of picture valence varied under threat and safety, but was identical in the shock and reward groups, consistent with a hypothesis that anticipation of either aversive or appetitive events prompts heightened perceptual vigilance, potentiating the acoustic startle reflex.

Monitoring voluntary blink magnitude through a wearable eye-tracking system: A preliminary study.

This study proposes a novel approach to measure the contractile force of eye blink that is generally obtained from the orbicularis oculi activity through Ocular ElectroMyo-Graphy (O-EMG). Here, O-EMG is compared with the eye information acquired through a wearable head-mounted eye-tracking system in order to investigate the possibility of using the eye-tracking in place of the O-EMG. Eight subjects were simultaneously monitored through an O-EMG and the eye-tracker while they were performing a structured protocol implying a variation in the blink contractile strength. Results showed that eye-tracking features were able to statistically discriminate three kinds of contractile forces similarly to EMG features. The consequent correlation analysis revealed that all the EMG-related features were significantly correlated with the eye-tracking ones with a p-value <;10-6. Moreover, considering the extracted eye-tracking features, i.e. Integrated Gaze Path (IGP) and Eye-closed Duration (ECD), IGP reported a higher Spearman's correlation values with eye-blink reflex magnitude (EBM) than ECD. These encouraging results suggest that the ocular information extracted from the eye-tracking could be profitably used in non-invasive ecological environments where wearability and comfortability play a crucial role in detecting spontaneous response.

The Effects of Muscle Damage on Muscle Spindle Function

The effects of eccentric muscle damage on extrafusal muscle fibers and motor function have been well studied. However, very few studies have examined the effects of muscle damage on sensory function, specifically, whether eccentric contractions damage the intrafusal fibers muscle spindles are wrapped around. PURPOSE: To investigate the effects of fatigue and muscle damage on the premotor latency (PML) and reflex magnitude (normalized reflex Torque; nRT) of patellar tendon reflexes. METHODS: Fifteen males (mean ± SD: age = 24.1 ± 2.9 yrs.) and fifteen females (age = 21.67 ± 2.1 yrs.) participated in a familiarization trial followed by two experimental sessions, separated by seven (±1) days. The 2 conditions consisted of either fatiguing (CON) or muscle damaging (ECC/CON) contractions at 60°· s-1 until 70% of peak torque (PT) was achieved. Patellar tendon taps were performed at a knee angle of 90° and a reflex hammer fastened to a swing arm was used to provide uniform impacts. PML was calculated as the time between the strike of the hammer to the onset of myoelectric activity in the rectus femoris, and nRT was normalized [% of Pre maximal voluntary contraction (MVC)] at all time points. Reflex assessments and MVCs were completed before (Pre) and following (Post) both the CON and ECC/CON protocols until recovery of PT to Pre values (Recov). A three-way repeated measures ANOVA [gender (males vs females) × condition (CON vs ECC/CON) × time (Pre vs Post vs Recov)] was used to analyze all reflex data. An alpha value of P ≤ 0.05 was considered statistically significant for all comparisons. RESULTS: A main effect for time (P = 0.003) was observed in which PML was greater at Recov compared to Pre (P = 0.01). Additionally, a two-way gender × time interaction was observed in which nRT was lower at Post and Recov compared to Pre (P = 0.001-0.002) for the females. No differences were observed across time for the males (P = 1.000). CONCLUSION: These findings revealed that changes in PML may not be sensitive to muscle damaging contractions but may have delayed responses as a result of exercise-induced fatigue. Furthermore, reflex magnitude deficits may be evident in females following fatiguing exercise but not likely as a result of muscle damage.

Day-to-day features of soleus stretch reflexes in sub-acute stroke patients

The aim of the study was to assess the reliability and variability of stretch reflex magnitude (SRmag) in sub-acute stroke patients. For testing, rapid dorsiflexion stretches were induced 24 h apart in 22 patients and 34 controls. SRmag between sessions in patients and controls was not different and the SRmag on the more-affected side was significantly larger than the less-affected, dominant, and non-dominant sides. The SRmag was consistent between sessions. Therefore, patients were not as variable between sessions as we had hypothesized.

Foreknowledge of an impending startling stimulus does not affect the proportion of startle reflexes or latency of StartReact responses.

During a simple reaction time (RT) task, movements can be initiated early and involuntarily through presentation of a loud startling acoustic stimulus (SAS), a phenomenon termed the StartReact effect. In order to infer that activity in startle-related structures led to the early response triggering, it is important to observe a concurrent startle reflex in sternocleidomastoid. It is generally accepted that to consistently elicit a startle reflex, the SAS must be both intense and unpredictable. However, it remains unclear what effect explicit foreknowledge of an impending SAS has on the effectiveness of a SAS to elicit a startle reflex when preparing a motor response. To test this, participants completed two separate blocks of a simple RT task (counterbalanced order), where the control auditory go-signal was replaced with a SAS on 20% of trials. In an unwarned block, knowledge of the trial type (SAS vs. control) was not provided in advance, while in a warned block, the trial type was forewarned. Results revealed that while foreknowledge of an impending SAS reduced the magnitude of the startle reflex, it did not affect the proportion of startle reflexes elicited or the magnitude of the StartReact effect. An increase in control trial RT was observed during the unwarned block, but only when it was performed first. These results indicate that preparation of a motor response leads to sufficiently increased activation in startle-related neural structures such that even with explicit knowledge of an upcoming SAS, participants are unable to proactively gate the upcoming sensory input.

Effect of stress and attention on startle response and prepulse inhibition

The startle reflex magnitude can be modulated when a weak stimulus is presented before the onset of the startle stimulus, a phenomenon termed prepulse inhibition (PPI). Previous research has demonstrated that emotional processes can modulate PPI and startle intensity, but the available evidence is inconclusive. In order to obtain additional evidence in this domain, we conducted two experiments intended to analyze the effect of induced stress and attentional load on PPI and startle magnitude. Specifically, in Experiment 1 we used a between subject strategy to evaluate the effect on startle response and PPI magnitude of performing a difficult task intended to induce stress in the participants, as compared to a group exposed to a control task. In Experiment 2 we evaluated the effect of diverting attention from the acoustic stimulus on startle and PPI intensity. The results seem to indicate that induced stress can reduce PPI, and that startle reflex intensity is reduced when attention is directed away from the auditory stimulus that induces the reflex.