Muscle Afferents Research Articles

Capsaicin-based analgesic balm attenuates the skeletal muscle metaboreflex in healthy humans.

The exercise pressor reflex (EPR) is comprised of group III and IV skeletal muscle afferents and is one of the principal mediators of the cardiovascular response to exercise. In animals, capsaicin-based analgesic balm (CAP) attenuates the pressor response to muscle contraction, indicating the transient receptor potential vanilloid 1 (TRPv1) receptor (localized on the group IV afferent neuron) as an important mediator of the EPR. However, whether these findings can be extrapolated to humans remains unknown. Here, we tested the hypothesis that CAP would attenuate blood pressure (BP) and muscle sympathetic nerve activity (MSNA) responses to isolated muscle metaboreflex activation in healthy men. MSNA (microneurography) and beat-to-beat heart hate (HR, by electrocardiography), and BP (finger photoplethysmography) were continuously measured in eight healthy males (23 ± 5 yr) at rest, during isometric handgrip exercise, and during postexercise ischemia (PEI). Trials were performed before and 30 and 60 min after the topical application of CAP (0.1%, CAPZASIN-HP) over the volar forearm of the subject's exercising arm. Isometric exercise evoked increases in mean BP (∆32 ± 4 mmHg) and MSNA (∆26 ± 5 bursts/min; ∆19 ± 5 bursts/100 heart beats). The increases in BP during handgrip were not affected by CAP, but the increase in MSNA was lower after 60 min of CAP application. During PEI, the increases in BP and MSNA were all significantly less than those before CAP (all P < 0.05). In conclusion, CAP attenuated BP and sympathetic responses evoked by PEI in humans. These data provide evidence that transient receptor potential vanilloid 1 receptors potentially contribute to the EPR in humans, via its metabolic component. NEW & NOTEWORTHY We found that topical application of capsaicin-based analgesic balm attenuates arterial blood pressure and muscle sympathetic nerve activity during isolated muscle metaboreflex activation following isometric handgrip exercise in healthy humans. These findings suggest that the transient receptor potential vanilloid 1 may contribute to the exercise pressor reflex in humans via its metabolic component.

Histamine‐Receptor Antagonists Affect Endurance Exercise Performance in Highly Competitive Cyclists

Histamine‐receptor antagonists (“antihistamines”) are widely used among endurance athletes and are not currently banned by the World Anti‐Doping Agency. Our lab has recently shown that a single dose of antihistamines prior to muscle‐damaging exercise attenuates strength loss and muscle pain in the 72 hours following exercise. Histamine, produced within muscle, may be a factor in exercise‐induced muscle pain/discomfort and, therefore, perception of effort, as group III/IV muscle afferents (nociceptive neurons) are sensitized via activation of H1 and H2 receptors. Histamine may have more influence on muscle pain and effort sensations during long‐duration exercise than short‐duration, because the activity of histidine decarboxylase (the enzyme that creates histamine) increases with exercise duration. Therefore, the purpose of this experiment was to determine if combination H1/H2 antihistamines increase endurance exercise performance. It was hypothesized that H1/H2 antihistamines would decrease the time to completion of a fixed‐distance time‐trial compared to placebo, and the effect would be greater following an endurance‐exercise bout. Eleven (3F) highly competitive cyclists (Cat 1–3) performed six 10 km time‐trials on separate days. The first two trials served as a familiarization, and repeatability was assessed by calculating a coefficient of variation (CV) between times to completion. The next 4 trials were performed in a randomized‐block order. Two were preceded by 120 min of seated rest and two by 120 min of steady‐state cycling at 50% VO2peak. Within those blocks, volunteers consumed either antihistamines (540 mg fexofenadine; a H1 receptor blocker, and 300 mg of ranitidine; a H2 blocker) or placebo 60 min prior to the start of rest/exercise. The main outcome variable was 10 km time to completion. Additionally, rating of perceived exertion (RPE), isometric quadriceps muscle strength, blood glucose, and blood lactate were measured prior to and following the time‐trials. Conventional statistics (2‐Way Repeated Measures ANOVA), effect size (ES = Cohen's dz), 95% CI, and CV from familiarization trials were used to determine the presence, strength, and meaningfulness of differences, respectively, between the trials. There was a significant reduction in performance with antihistamines compared to placebo (+10.5 ± 3.8 s, mean±SEM, drug effect p=0.002), the reduced performance tended to be exacerbated by prior exercise (p=0.057) but there was no drug by prior exercise interaction (p=0.716) (Figure 1). The day‐to‐day 10 km performance variability (CV) was 0.98%. The percent change between placebo and antihistamine was likely trivial for time‐trials following rest (mean −0.87%, 95%CI −2.02 to 0.29%) (ES=0.505) and potentially harmful following exercise (mean −1.2%, 95%CI −2.45 to 0.05%) (ES=0.646). There were no drug effects on changes (pre to post time‐trial) in isometric muscle strength (p=0.607), RPE (p=0.828), blood glucose (p=0.964) or lactate (p=0.402) between antihistamine and placebo conditions. Thus, contrary to our hypothesis, blocking histamine's receptors slowed performance of 10 km time‐trials in highly competitive cyclists and did not influence perception of effort.Support or Funding InformationEugene &amp; Clarissa Evonuk Memorial Graduate FellowshipThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

The Ventilatory Response to Muscle Metaboreflex Activation During Concurrent Hypercapnia is Attenuated by Local Muscle Training

Increased ventilation during post‐exercise circulatory occlusion (PECO) with concurrent hypercapnia suggests a synergistic interaction between muscle metaboreflex activation and chemoreflex stimulation. The exercise pressor reflex (EPR) response to contraction of a human calf muscle and subsequent PECO can be attenuated by training, indicating a decreased activation of muscle metaboreceptive afferents driving the EPR. We hypothesised that 6 weeks of one legged calf raise training would attenuate metaboreceptor activity, the EPR and if these afferents provide a drive to breathe, the ventilatory response seen during PECO when combined with mild hypercapnia. In the contralateral limb there would be no effect on muscle afferent input to the cardiovascular or respiratory control systems so no change in EPR or breathing would be expected during PECO.Eleven male subjects aged 19.5 ± 0.8 years (mean±SD) gave informed consent to participate in the study, approved by the local ethics committee. A standardized test of the cardiovascular and respiratory responses to isometric contraction of the calf muscle at 50% of maximum voluntary force (MVC) for 2 min was followed by 3 min of PECO. In random order left and right legs were tested separately under two conditions, 1) breathing room air 2) hypercapnia, 5% CO2 above end tidal level. A dynamic end tidal forcing system controlled the inspired gas composition and monitored ventilation. Heart rate (HR) and blood pressure (MAP) were measured by ECG and Portapres respectively. Right leg training consisted of 120 calf raises to maximum height in 4 sets of 30, one heel raise and lower per second, one minute rest between sets. Whenever 4 sets could be achieved loading of 10%, then increments of 5% body mass was added. After 6 weeks of training the initial tests were repeated.MVC did not change after training in either leg. During exercise of the trained leg during hypercapnia, average MAP rise from baseline was reduced from 18.1± 1.4, to 15.6±1.4 mmHg (mean±SEM), p&lt;0.05 after, training. Ventilation change from the hypercapnic baseline was also reduced from 17.5± 2.8 to 12.1±2.6 l.min−1, p&lt;0.05 after training.Table 1 shows the data from each leg in the PECO phase of the trials, performed before and after the training. In the room air and hypercapnia trials, the EPR was significantly reduced from control values in the trained leg. The increase in ventilation from baseline was reduced, by ~45%, in the trained limb during the hypercapnia trial. There were no alterations in the cardiovascular or ventilatory responses to PECO, during hypercapnia or room air breathing trials in the untrained limb, ruling out any central or peripheral adaptation in this leg, induced by the training period. In the trained leg the MAP data shows clear evidence of a decrease in the EPR after training, indicating reduced muscle metaboreflex activation during PECO. This is despite generating the same force‐time integral during the exercise period as before training. We conclude that in the trained limb the marked decrease in ventilation during PECO in the hypercapnia condition must be due to a decreased drive to breathe arising from metabosensitive muscle afferents.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Neural Responses to Mechanical Stimulation in Thin Muscle Afferents and Dorsal Root Ganglia are Sensitized by Insulin

In patients with type 2 diabetes mellitus (T2D), the circulatory responses to physical activity are abnormally exaggerated. Recent studies in patients with T2D have suggested that this exaggerated cardiovascular response is mediated, in part, by the exercise pressor reflex (EPR) which is a primary contributor to autonomic cardiovascular control during exercise. However, the mechanisms underlying this abnormal EPR function in T2D remain unknown. The pathophysiology of T2D is characterized by insulin resistance induced hyperinsulinemia. Further, evidence suggests that insulin increases the sensitivity of transient receptor potential vanilloid receptors localized to muscle afferent neurons that contribute to EPR activation. Importantly, sensitization of muscle receptors associated with the mechanically sensitive component of the EPR (i.e. the muscle mechanoreflex) has been shown to mediate reflex overactivity in other cardiovascular disease states such as heart failure and hypertension. Given this information, it is logical to suggest that excess levels of insulin may potentiate mechanoreflex function in T2D.PURPOSETo examine the effect of insulin on neuronal responses to mechanical stimulation in thin muscle afferents and dorsal root ganglia (DRG) of normal healthy rodents. We hypothesized that insulin sensitizes the activity of mechanically sensitive afferent neurons.METHODSMechanically activated neurons were assessed before (control) and after exposure to insulin 1) by single‐fiber recordings from rat muscle‐nerve preparations in vitro and 2) by whole cell patch‐clamp recordings from cultured mice DRG neurons. The magnitude of the response to mechanical stimulation as well as the mechanical threshold were assessed.RESULTSCompared to control, the mechanical threshold of thin muscle afferents was significantly decreased after insulin administration (58±16 vs. 15±17 mN, n=8, P&lt;0.05) while the response magnitude was not changed (66±22 vs. 68±16 spikes, n=8). In DRG cell culture, mechanically activated current was significantly augmented by insulin (−93 ±12 vs. −190±43 pA, n=16, P&lt;0.05) whereas the mechanical threshold was significantly decreased (4.6±0.4 vs. 2.6±0.3 steps, n=17, P&lt;0.05). Further, after pretreatment of DRG insulin receptors with the inhibitor GSK1838705, insulin failed to augment the mechanically activated current (−91±19 vs. −92±19 pA, n=14) or to reduce the mechanical threshold (3.5 ± 0.4 vs. 3.4 ± 0.5 steps, n=11).CONCLUSIONSThese data demonstrate that thin muscle afferent and DRG neuronal responses to mechanical stimulation are sensitized by the presence of insulin in normal healthy animals. Importantly, the findings provide support for the concept that chronic hyperinsulinemia enhances skeletal muscle mechanoreflex function in T2D contributing to the abnormal cardiovascular response during exercise in this disease.Support or Funding InformationSupported by the Lawson &amp; Rogers Lacy Research Fund in Cardiovascular Disease and JSPS KAKENHI JP17K01769This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

The Effects of Oral Contraceptive Use on Ergoreflex control of Blood Pressure and Heart Rate during Steady‐State Cycling in Healthy, Young Women

BackgroundErgoreflex, activation of group III/IV skeletal muscle afferents, is an important regulator of mean arterial pressure (MAP) and heart rate (HR) during exercise. Oral Contraceptive (OC) use in women has been shown to increase resting BP, but the effect of OC use on the ergoreflex is unclear.ObjectiveDetermine if OC use causes an exaggerated MAP and HR response with ergoreflex activation in young, healthy women.MethodsTwelve healthy women (mean ± SD; 24±2 yrs, 7 taking OCs) attended 2 experimental sessions; one during early follicular (low estrogen, days 1–4) and one during ovulation (high estrogen, days 10–14). Each session included 1) 3 minutes of light‐intensity steady‐state cycling (60 W at 65–70 rpm) and 2) 3 randomized, subsystolic circulatory occlusions applied bilaterally to the proximal thigh at 20, 60, and 100 mmHg of pressure to increase activation of group III/IV skeletal muscle afferents (ergoreflex). Occlusions lasted 2 min and were separated by 5 min of deflation with 2 min of recovery at the end (rest). MAP and HR were continuously monitored via non‐invasive blood pressure (Nexfin, Edwards Lifesciences, Irvine, CA) and a 3‐lead ECG (AD Instruments, Colorado Springs, CO), respectively.ResultsData did not differ significantly between women in the follicular or ovulatory phases; thus, the ovulatory phase data was used for comparisons. HR increased from rest to exercise more with women in the ovulatory phase (68±14 to 130±12 bpm) as compared with women taking OCs (69±14 to 112±10 bpm, exercise *OC interaction: p=0.02). However, HR increased from baseline exercise with vascular occlusion (condition: p&lt;0.01) similarly in women in the ovulatory phase and women taking OCs (20 mmHg: 3±9% vs. 1±2%, 60 mmHg: 4±3% vs 8±4%, and 100 mmHg: 9±17% vs. 20±8% respectively, condition*OC interaction: p=0.14). MAP increased from rest to exercise similarly in women in the ovulatory phase (83±8 to 92±14 mmHg) as compared with women taking OCs (88±7 to 92±8 mmHg, exercise*OC interaction: p=0.08). MAP increased from baseline exercise with vascular occlusion (condition: p=0.01) similarly in women in the ovulatory phase as compared with women taking OCs (20 mmHg: 7±19% vs. 2±8%, 60 mmHg: 15±16% vs. 2±9%, and 100 mmHg: 8±18% vs. 28±19%, condition*OC interaction: p=0.20).ConclusionPrevious studies have implicated that blood pressure tends to be greater with OC use, this study suggests that OCs do not cause an exaggerated increase in MAP during exercise or activation of group III/IV afferents. OCs attenuate the HR response to exercise, but does not appear to affect the HR response to group III/IV afferent activation.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Muscle Metaboreflex Modulation of Spontaneous Cardiac Baroreflex Sensitivity: Does Sex Matter?

It is well established that during exercise, the arterial baroreflex is reset to operate around the prevailing blood pressure in an exercise intensity‐dependent manner. This exercise‐induced modulation of baroreflex function is mediated by central signals and by afferent signals arising from working skeletal muscle that are composed of mechanically and metabolically sensitive afferents. Previous studies in humans examining the influence of metabolically sensitive skeletal muscle afferents (i.e., muscle metaboreflex) on cardiac baroreflex sensitivity (cBRS) have found equivocal results reporting increases, decreases, or no change in sensitivity. The reason behind these discrepancies could be attributable to variation in the methods used to assess baroreflex function, exercise modality and intensity, size of exercising muscle mass, and, less discussed, but also relevant, the inclusion of both men and women. This becomes important given a plethora of studies indicating that sex plays a pivotal role when it comes to blood pressure regulation. Given this, the aim of the present study was to investigate the effect of isolated muscle metaboreflex activation on cBRS and the potential sex‐related differences in this interaction. Forty healthy and physically active volunteers (20 men), aged 18–30 years were recruited. All women were non‐oral contraceptives users and performed experiments in the early follicular phase of the menstrual cycle. After 10 minutes rest period, the subjects performed 90s of isometric handgrip (IHG) at 40% of maximal voluntary contraction followed by 3‐min of post exercise ischemia (PEI) to isolate the muscle metaboreflex. Beat‐to‐beat heart rate (HR ‐ electrocardiography) and arterial blood pressure (finger photopletysmography) were continuously measured. Spontaneous cBRS was assessed for all combined sequences (Gainall), and also separately upward (Gainup) and downward (Gaindown) using the sequence technique. HR variability was measured in time (RMSSD ‐ standard deviation of the RR intervals) and frequency domains (LF – low and HF – high frequency power). Resting MBP was lower in women than men (P &lt; 0.01). MBP increased from rest during IHG and remained elevated during PEI in both men and women. However, the magnitude of these responses was greater in men. Resting cBRS was similar between men and women. During PEI, men exhibited an increase in cBRS (Gainall: Δ3.0 ± 1.1 ms mmHg‐1, P = 0.03; Gainup: Δ4.4 ± 2.1 ms mmHg‐1, P = 0.04; and Gaindown: Δ3.3 ± 1.4 ms mmHg‐1, P = 0.03). In contrast, cBRS was unchanged during PEI in women (Gainall: Δ0.6 ± 0.7 ms mmHg‐1, P = 0.10; Gainup: Δ0.3 ± 0.7 ms mmHg‐1, P = 0.27; and Gaindown: Δ0.2 ± 1.1 ms mmHg‐1, P = 0.43). At rest, HF and LF power were lower in women compared with men, whereas RMSSD and LF/HF ratio were similar. During PEI, men exhibited a highest increase in RMSSD and HF compared to women, while LF power was unchanged in both groups. In addition, LF/HF ratio decreased in men and was unchanged in women. These findings indicate that the muscle metaboreflex activation modulates spontaneous cBRS in a sex dependent manner.Support or Funding InformationBrazilian National Council of Scientific and Technological Development (CNPq); Foundation for Research Support of Federal District (FAPDF); Brazilian Federal Agency for Support and Evaluation of Graduate Education (CAPES).This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Exercise pressor reflex and carotid chemoreflex interaction: consequences for the cardiovascular response to exercise in healthy humans

BackgroundAlthough both the exercise pressor reflex (EPR) and the carotid chemoreflex (CC) have been documented as powerful autonomic feedback mechanisms involved in the cardiovascular control during exercise, the interaction of both reflexes and the subsequent consequence for the circulatory response during physical activity remains elusive.PurposeWe examined the actions and interactions of the EPR and CC on the cardiovascular response during exercise by independently and simultaneously manipulating these reflexes.MethodSeven healthy volunteers completed two experimental sessions in random order. In one session, subjects performed dynamic single leg knee‐extension exercise at 60% of peak power output for 4 min, both in normoxic control conditions (Norm‐C: SaO2 = 97%, PaO2 = 90 mmHg) and isocapnic hypoxia (Hypo‐C: SaO2 = 79%, PaO2 = 42 mmHg). In the other session, following lumbar intrathecal fentanyl administration attenuating feedback from group III/IV leg muscle afferents, subjects repeated the same exercise characterized by similar blood gases and Hb saturations (Norm‐F and Hypo‐F). Mean arterial blood pressure (MAP, radial arterial catheter), cardiac output (CO, Finometer) and femoral blood flow (QL, Doppler ultrasound) were quantified continuously during exercise. Comparisons were made between Norm‐F and Norm‐C (i.e., ΔNorm‐F/Norm‐C) and Norm‐F and Hypo‐F (i.e., ΔNorm‐F/Hypo‐F) to estimate the EPR and CC contribution to the cardiovascular responses during exercise, respectively. To determine the interaction of the reflexes, the cardiovascular responses during exercise with both the EPR and CC activated simultaneously (estimated from the differences between Norm‐F and Hypo‐C, i.e. ΔNorm‐F/Hypo‐C) were compared with the sum of the responses induced by the EPR and by the CC activation alone (i.e., calculated as ΔNorm‐F/Norm‐C + ΔNorm‐F/Hypo‐F).ResultsFentanyl blockade had no effect on resting cardiovascular responses. During exercise, activation of the EPR alone (ΔNorm‐F/Norm‐C) significantly increased MAP (+11 ± 3 mmHg) and QL (+0.5 ± 0.1 L·min−1), but did not alter CO (p = 0.41) and leg vascular conductance (LVC; p = 0.10). Activation of the CC alone (ΔNorm‐F/Hypo‐F) significantly increased CO (+1.2 ± 0.5 L·min−1), QL (+0.4 ± 0.1 L·min−1), and LVC (+3 ± 1 ml·min−1·mmHg−1), but did not alter MAP (p = 0.13). During co‐activation of the EPR and CC (ΔNorm‐F/Hypo‐C), the QL and LVC responses were significantly lower than the summated responses evoked by each reflex alone (QL: 0.5 ± 0.2 vs 0.8 ± 0.2 L·min−1; LVC: 1 ± 1 vs 5 ± 1 ml·min−1·mmHg−1), whereas the CO and MAP responses were similar (1.5 ± 0.5 vs 1.3 ± 0.7 L·min−1 and 15 ± 2 vs 14 ± 3 mmHg, respectively, p &gt; 0.12).ConclusionWhile QL and LVC responses to exercise undergo occlusive interaction upon co‐activation of the EPR and CC, CO and MAP responses exhibit additive interaction. Hence, when the EPR and CC are activated simultaneously, exercise‐induced increases in limb blood flow and vascular conductance are diminished, suggesting that the interaction between the two reflexes may result in augmented sympathetic vasoconstrictor tone.Support or Funding InformationThe authors were supported by National Heart, Lung, and Blood Institute grants (HL‐116579).This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Group III/IV muscle afferents play an independent role in arterial baroreflex resetting during exercise in humans: is it the end of the road?

Group III/IV muscle afferents play an independent role in arterial baroreflex resetting during exercise in humans: is it the end of the road?

Identifying the role of group III/IV muscle afferents in the carotid baroreflex control of mean arterial pressure and heart rate during exercise.

We investigated the contribution of group III/IV muscle afferents to carotid baroreflex resetting during electrically evoked (no central command) and voluntary (requiring central command) isometric knee extension exercise. Lumbar intrathecal fentanyl was used to attenuate the central projection of μ-opioid receptor-sensitive group III/IV leg muscle afferent feedback. Spontaneous carotid baroreflex control was assessed by loading and unloading the carotid baroreceptors with a variable pressure neck chamber. Group III/IV muscle afferents did not influence spontaneous carotid baroreflex responsiveness at rest or during exercise. Afferent feedback accounted for at least 50% of the exercise-induced increase in the carotid baroreflex blood pressure and heart rate operating points, adjustments that are critical for an appropriate cardiovascular response to exercise. These findings suggest that group III/IV muscle afferent feedback is, independent of central command, critical for the resetting of the carotid baroreflex blood pressure and heart rate operating points, but not for spontaneous baroreflex responsiveness. This study sought to comprehensively investigate the role of metabolically and mechanically sensitive group III/IV muscle afferents in carotid baroreflex responsiveness and resetting during both electrically evoked (EVO, no central command) and voluntary (VOL, requiring central command) isometric single-leg knee-extension (15% of maximal voluntary contraction; MVC) exercise. Participants (n=8) were studied under control conditions (CTRL) and following lumbar intrathecal fentanyl injection (FENT) to inhibit μ-opioid receptor-sensitive lower limb muscle afferents. Spontaneous carotid baroreflex control of mean arterial pressure (MAP) and heart rate (HR) were assessed following rapid 5s pulses of neck pressure (NP, +40mmHg) or suction (NS, -60mmHg). Resting MAP (87±10mmHg) and HR (70±8bpm) were similar between CTRL and FENT conditions (P>0.4). In terms of spontaneous carotid baroreflex responsiveness, FENT did not alter the change in MAP or HR responses to NP (+13±5mmHg, P=0.85; +9±3bpm; P=0.99) or NS (-13±5mmHg, P=0.99; -24±11bpm; P=0.49) at rest or during either exercise protocol, which were of a remarkably similar magnitude to rest. In contrast, FENT administrationreduced the exercise-induced resetting of the operating point for MAP and HR during both EVO (116±10mmHg to 100±15mmHg and 93±14bpm to 82±10bpm) and VOL (107±13mmHg to 100±17mmHg and 89±10bpm to 72±10bpm) exercise bouts. Together, these findings document that group III/IV muscle afferent feedback is critical for the resetting of the carotid baroreflex MAP and HR operating points, independent of exercise-induced changes in central command, but not for spontaneous carotid baroreflex responsiveness.

Mechano- and thermosensitivity of injured muscle afferents 20 to 80 days after nerve injury.

Chronic injury of limb nerves leading to neuropathic pain affects deep somatic nerves. Here the functional properties of injured afferent fibers in the lateral gastrocnemius-soleus nerve were investigated 20 and 80 days after suturing the central stump of this muscle nerve to the distal stump of the sural nerve in anesthetized rats. Neurophysiological recordings were made from afferent axons identified in either the sciatic nerve (87 A-, 63 C-fibers) or the dorsal root L4/L5 (52 A-, 26 C-fibers) by electrical stimulation of the injured nerve. About 70% of the functionally identified A-fibers had regenerated into skin by 80 days after nerve suture; the remaining A-fibers could be activated only from the injured nerve. In contrast, 93% of the functionally identified C-fibers could only be activated from the injured sural nerve after 80 days. Nearly half of the injured A- (45%) and C-fibers (44%) exhibited ongoing and/or mechanically or thermally evoked activity. Because ~50% of the A- and C-fibers are somatomotor or sympathetic postganglionic axons, respectively, probably all injured muscle afferent A- and C-fibers developed ectopic activity. Ongoing activity was present in 17% of the A- and 46% of the C-fibers. Mechanosensitivity was present in most injured A- (99%) and C-fibers (85%), whereas thermosensitivity was more common in C-fibers (cold 46%, heat 47%) than in A-fibers (cold 18%, heat 12%). Practically all thermosensitive A-fibers and C-fibers were also mechanosensitive. Thus, unlike cutaneous axons, almost all A- and C-fibers afferents in injured muscle nerves demonstrate ectopic activity, even chronically after nerve injury. NEW & NOTEWORTHY After chronic injury of a muscle nerve, allowing the nerve fibers to regenerate to the target tissue, 1) most afferent A-fibers are mechanosensitive and regenerate to the target tissue; 2) ectopic ongoing activity, cold sensitivity, and heat sensitivity significantly decrease with time after injury in A-afferents; 3) most afferent C-fibers do not regenerate to the target tissue; and 4) injured C-afferents maintain the patterns of ectopic discharge properties they already show soon after nerve injury.

(144) - Ischemia and reperfusion injury induces acute and chronic muscle pain-related behaviors through increased expression of the GFRα1 receptor

Muscle pain is a common complication of conditions that produce ischemia/reperfusion injuries (I/R), like peripheral artery disease (PAD) or sickle cell anemia. Previously, we have found that I/R injury in mouse induces increased pain-related behaviors that correlate with group III/IV muscle afferent sensitization and increased gene expression in the affected muscles and the dorsal root ganglia (DRG). In particular, glial cell line-derived neurotrophic factor (GDNF) expression was increased in the injured muscle tissue, while its receptor, GDNF family receptor α1 (GFRα1) was upregulated in DRGs. Both GDNF and GFRα1 have been linked to behavioral changes as well as afferent sensitization in other models of muscle pain. We hypothesized that enhanced expression of GFRα1 in sensory neurons regulates the development of pain-related behaviors after I/R. To test this, we selectively knocked down GFRα1 in mice by median and ulnar nerve injection of Penetratin1-linked GFRα1-targeting siRNAs in vivo prior to unilateral I/R injury of the forelimb. We then performed pain-related behavioral tests and single unit recordings of group III/IV muscle afferents along with real-time PCR on the I/R affected DRGs. Selective knockdown of GFRα1 expression in the DRGs effectively decreased the I/R-induced pain related behaviors and the observed reduction in mechanical thresholds in group III/IV afferents. GFRα1 inhibition also specifically prevented I/R-induced upregulation of the purinergic receptor, P2X5 and acid sensing ion channel 3 (ASIC3). Finally, repetitive I/R injury induced prolonged spontaneous pain-like behaviors (paw guarding) compared to animals with a single injury. This also correlated with increased GFRα1 expression in DRGs. These results suggest that GFRα1 upregulation may play an important role in the development of pain-related behaviors via group III/IV muscle afferent sensitization. GFRα1 and the receptors downstream of its signaling, may provide potential targets for therapies aimed at patients experiencing acute or chronic pain from ischemia/reperfusion injuries.

Cardiac modulation of alpha motoneuron discharges.

Recordings of alpha motoneuron discharges from branches of the intercostal and abdominal nerves in anesthetized cats were analyzed for modulation during the cardiac cycle. Cardiac modulation was assessed by the construction of cross-correlation histograms between the R-wave of the ECG and the largest amplitude efferent spikes. In all but two recordings (which were believed to have either no or few alpha spikes), the histograms showed relatively short duration peaks and/or troughs (widths at half amplitude 4-50 ms) at lags of 10-150 ms. These observations were deduced to result from activity in oligosynaptic pathways, probably from muscle spindle afferents, whose discharges are known to be synchronized to the cardiac pulse. The results suggest that onward transmission of the cardiac signal from thoracic muscle afferents (and possibly from other dynamically sensitive afferents) to other parts of the central nervous system is highly likely and that therefore these afferents could contribute to cardiac interoception. NEW & NOTEWORTHY It has been recognized since 1933 that muscle spindles respond to the cardiac pulse, but it is unknown whether this cardiac signal is transmitted to other levels in the nervous system. Here we show that a cardiac signal, likely arising from muscle spindles, is present in the efferent activities of thoracic and abdominal muscle nerves, suggesting probable onward transmission of this signal to higher levels and therefore that muscle spindles could contribute to cardiac interoception.

Normal tendon reflexes despite absent sensory nerve action potentials in CANVAS: a neurophysiological study

CANVAS is a recently defined progressive ataxic syndrome with impairment of vestibular, somatosensory and cerebellar function due to atrophic degeneration of dorsal root ganglia and dorsal columns, of cranial nerve somatosensory ganglia, of vestibular ganglia and vestibular nerves and of cerebellar Purkinje cells. While all patients eventually develop sensory impairment in a non-length dependent pattern and lose sensory nerve action potentials, some retain their tendon reflexes. Here we study 5 CANVAS patients with absent sensory nerve action potentials but intact, even brisk Achilles tendon reflexes and, in 4, preserved H reflexes in the upper and lower limbs. These findings imply that dorsal root ganglion neurons subserving cutaneous afferents more vulnerable than those subserving muscle afferents. Our findings have a clinical message: preservation of the Achilles tendon jerk does not exclude a large fibre peripheral neuronopathy.

Sex differences in primary muscle afferent sensitization following ischemia and reperfusion injury

BackgroundChronic pain conditions are more prevalent in women, but most preclinical studies into mechanisms of pain generation are performed using male animals. Furthermore, whereas group III and IV nociceptive muscle afferents provoke central sensitization more effectively than their cutaneous counterparts, less is known about this critical population of muscle nociceptors. Here, we compare the physiology of individual muscle afferents in uninjured males and females. We then characterize the molecular, physiological, and behavioral effects of transient ischemia and reperfusion injury (I/R), a model we have extensively studied in males and in females.MethodsResponse properties and phenotypes to mechanical, thermal, and chemical stimulation were compared using an ex vivo muscle/nerve/dorsal root ganglia (DRG)/spinal cord recording preparation. Analyses of injury-related changes were also performed by assaying evoked and spontaneous pain-related behaviors, as well as mRNA expression of the affected muscle and DRGs. The appropriate analyses of variance and post hoc tests (with false discovery rate corrections when needed) were performed for each measure.ResultsFemales have more mechanically sensitive muscle afferents and show greater mechanical and thermal responsiveness than what is found in males. With I/R, both sexes show fewer cells responsive to an innocuous metabolite solution (ATP, lactic acid, and protons), and lower mechanical thresholds in individual afferents; however, females also possess altered thermal responsiveness, which may be related to sex-dependent changes in gene expression within the affected DRGs. Regardless, both sexes show similar increases in I/R-induced pain-like behaviors.ConclusionsHere, we illustrate a unique phenomenon wherein discrete, sex-dependent mechanisms of primary muscle afferent sensitization after ischemic injury to the periphery may underlie similar behavioral changes between the sexes. Furthermore, although the group III and IV muscle afferents are fully developed functionally, the differential mechanisms of sensitization manifest prior to sexual maturity. Hence, this study illustrates the pressing need for further exploration of sex differences in afferent function throughout the lifespan for use in developing appropriately targeted pain therapies.

Variable impact of tizanidine on the medium latency reflex of upper and lower limbs.

Sudden limb displacement evokes a complex sequence of compensatory muscle activity. Following the short-latency reflex and preceding voluntary reactions is an epoch termed the medium-latency reflex (MLR) that could reflect spinal processing of group II muscle afferents. One way to test this possibility is oral ingestion of tizanidine, an alpha-2 adrenergic agonist that inhibits the interneurons transmitting group II signals onto spinal motor neurons. We examined whether group II afferents contribute to MLR activity throughout the major muscles that span the elbow and shoulder. MLRs of ankle muscles were also tested during walking on the same day, in the same participants as well as during sitting in a different group of subjects. In contrast to previous reports, the ingestion of tizanidine had minimal impact on MLRs of arm or leg muscles during motor actions. A significant decrease in magnitude was observed for 2/16 contrasts in arm muscles and 0/4 contrasts in leg muscles. This discrepancy with previous studies could indicate that tizanidine's efficacy is altered by subtle changes in protocol or that group II afferents do not substantially contribute to MLRs.

Peripheral Mechanisms of Ischemic Myalgia

Musculoskeletal pain due to ischemia is present in a variety of clinical conditions including peripheral vascular disease (PVD), sickle cell disease (SCD), complex regional pain syndrome (CRPS), and even fibromyalgia (FM). The clinical features associated with deep tissue ischemia are unique because although the subjective description of pain is common to other forms of myalgia, patients with ischemic muscle pain often respond poorly to conventional analgesic therapies. Moreover, these patients also display increased cardiovascular responses to muscle contraction, which often leads to exercise intolerance or exacerbation of underlying cardiovascular conditions. This suggests that the mechanisms of myalgia development and the role of altered cardiovascular function under conditions of ischemia may be distinct compared to other injuries/diseases of the muscles. It is widely accepted that group III and IV muscle afferents play an important role in the development of pain due to ischemia. These same muscle afferents also form the sensory component of the exercise pressor reflex (EPR), which is the increase in heart rate and blood pressure (BP) experienced after muscle contraction. Studies suggest that afferent sensitization after ischemia depends on interactions between purinergic (P2X and P2Y) receptors, transient receptor potential (TRP) channels, and acid sensing ion channels (ASICs) in individual populations of peripheral sensory neurons. Specific alterations in primary afferent function through these receptor mechanisms correlate with increased pain related behaviors and altered EPRs. Recent evidence suggests that factors within the muscles during ischemic conditions including upregulation of growth factors and cytokines, and microvascular changes may be linked to the overexpression of these different receptor molecules in the dorsal root ganglia (DRG) that in turn modulate pain and sympathetic reflexes. In this review article, we will discuss the peripheral mechanisms involved in the development of ischemic myalgia and the role that primary sensory neurons play in EPR modulation.

Sensorimotor Integration During Motor Learning: Transcranial Magnetic Stimulation Studies.

The effect of sensory signals coming from skin and muscle afferents on the sensorimotor cortical networks is entitled as sensory-motor integration (SMI). SMI can be studied electrophysiologically by the motor cortex excitability changes in response to peripheral sensory stimulation. These changes include the periods of short afferent inhibition (SAI), afferent facilitation (AF), and late afferent inhibition (LAI). During the early period of motor skill acquisition, motor cortex excitability increases and changes occur in the area covered by the relevant zone of the motor cortex. In the late period, these give place to the morphological changes, such as synaptogenesis. SAI decreases during learning the motor skills, while LAI increases during motor activity. In this review, the role of SMI in the process of motor learning and transcranial magnetic stimulation techniques performed for studying SMI is summarized.

Bioenergetics and ATP Synthesis during Exercise: Role of Group III/IV Muscle Afferents.

The purpose of this study was to investigate the role of the group III/IV muscle afferents in the bioenergetics of exercising skeletal muscle beyond constraining the magnitude of metabolic perturbation. Eight healthy men performed intermittent isometric knee-extensor exercise to task failure at ~58% maximal voluntary contraction under control conditions (CTRL) and with lumbar intrathecal fentanyl to attenuate group III/IV leg muscle afferents (FENT). Intramuscular concentrations of phosphocreatine (PCr), inorganic phosphate (Pi), diprotonated phosphate (H2PO4), adenosine triphosphate (ATP), and pH were determined using phosphorous magnetic resonance spectroscopy (P-MRS). The magnitude of metabolic perturbation was significantly greater in FENT compared with CTRL for [Pi] (37.8 ± 16.8 vs 28.6 ± 8.6 mM), [H2PO4] (24.3 ± 12.2 vs 17.9 ± 7.1 mM), and [ATP] (75.8% ± 17.5% vs 81.9% ± 15.8% of baseline), whereas there was no significant difference in [PCr] (4.5 ± 2.4 vs 4.4 ± 2.3 mM) or pH (6.51 ± 0.10 vs 6.54 ± 0.14). The rate of perturbation in [PCr], [Pi], [H2PO4], and pH was significantly faster in FENT compared with CTRL. Oxidative ATP synthesis was not significantly different between conditions. However, anaerobic ATP synthesis, through augmented creatine kinase and glycolysis reactions, was significantly greater in FENT than in CTRL, resulting in a significantly greater ATP cost of contraction (0.049 ± 0.016 vs 0.038 ± 0.010 mM·min·N). Group III/IV muscle afferents not only constrain the magnitude of perturbation in intramuscular Pi, H2PO4, and ATP during small muscle mass exercise but also seem to play a role in maintaining efficient skeletal muscle contractile function in men.

The physiological basis of neuromuscular fatigue during high intensity exercise

Introduction Neuromuscular fatigue refers to a reduction in maximal force generation capacity, and is categorized as central and peripheral. Central fatigue is defined as a reduction in the ability of the central nervous system to voluntarily activate muscles, and peripheral fatigue indicates a decrease in the contractile strength of muscle fibers. During high intensity exercise, motor neurons are involved in the recruitment of type IIB muscle fibers as they are fast-twitch, high glycolytic, and have low aerobic capacity. Furthermore, group III and IV muscle afferents detect the physiological circumstances in the body and convey signals to the brain that influence the onset of central and peripheral fatigue. Methods A PRISMA flow diagram was created to record relevant studies found from scholarly databases. Inclusion criteria required studies from 2005 to 2017, and subject grouping headings required key terms indicating that the presence of central and peripheral fatigue was analyzed on healthy adult subjects performing exercise. To ensure that high quality studies were analyzed, each article was independently rated using the National Institute of Health Quality Assessment Tool criteria. Discussion During low intensity exercise, asynchronous motor unit recruitment is involved in delaying the onset of muscle fatigue. However, this is not apparent in high intensity exercises, as maximal motor unit firing is required in order to sustain a maximal level of force output. Persistent firing of action potentials to maintain muscle contraction results in acetylcholine depletion at the motor end plate, initiating the process of central fatigue. Furthermore, due to prolonged metabolite accumulation in skeletal muscle fibers, group III and IV afferents convey signals to the motor cortex and cause a reduction in the action potential conduction velocities along the contracting muscle. This leads to the onset of peripheral fatigue. As high intensity exercise proceeds, electromyogram (EMG) measurements display this as an increase in amplitude to reflect heightened motor unit recruitment and a compressed power density spectrum alongside a decreased centre frequency. This is determined by the innervated muscle fiber’s conduction velocity and subsequent variations in the action potential waveform shape. Conclusion A record of current studies systematically display the overview of muscle fatigue and its underlying mechanisms during exercise. However, further research is yet to be conducted for a more comprehensive understanding regarding the onset and recovery of neuromuscular fatigue in varied population demographics and physiological circumstances. Likewise, the distinctive roles of group III and IV muscle afferents in supraspinal stimulation require further investigation in order to gain a holistic understanding of their involvement in central fatigue and resistance training. Additional research in this subject matter is currently being explored through technology involving imaging studies, as they have potential to elucidate motor cortex activity alongside other regions of the brain and portray neuromuscular muscle fatigue eminently.

Ankle muscle tenotomy does not alter ankle flexor muscle recruitment bias during locomotor-related repetitive limb movement in late-stage chick embryos.

In ovo, late-stage chick embryos repetitively step spontaneously, a locomotor-related behavior also identified as repetitive limb movement (RLM). During RLMs, there is a flexor bias in recruitment and drive of leg muscle activity. The flexor biased activity occurs as embryos assume an extremely flexed posture in a spatially restrictive environment 2-3 days before hatching. We hypothesized that muscle afferent feedback under normal mechanical constraint is a significant input to the flexor bias observed during RLMs on embryonic day (E) 20. To test this hypothesis, muscle afference was altered either by performing a tenotomy of ankle muscles or removing the shell wall restricting leg movement at E20. Results indicated that neither ankle muscle tenotomy nor unilateral release of limb constraint by shell removal altered parameters indicative of flexor bias. We conclude that ankle muscle afference is not essential to ankle flexor bias characteristic of RLMs under normal postural conditions at E20.