Proximal and segmental motor nerve conduction in the sciatic nerve produced by percutaneous high voltage electrical stimulation

Comparison of Threshold and Tolerance Nociceptive Withdrawal Reflexes in Horses

Simple Summary

Nociception is the physiological basis of the complex experience of pain. An established model for its quantification in equine studies is based on the nociceptive withdrawal reflex evoked by electrical stimulation of a sensory nerve. The reflex is recorded via electromyography and it is common to determine the threshold at which a nociceptive-specific reflex activity can be observed. In the present study, the classical methodology was expanded for a deeper understanding of the physiology of nociceptive reflexes in horses. First, for each individual horse, a threshold was determined as the minimal stimulation intensity able to evoke a nociceptive withdrawal reflex. Second, the stimulation intensity was stepwise increased up to tolerance, which was defined as the stimulus that is able to elicit the maximal tolerable behavioral reaction. The characteristics of the reflex activity on the electromyographic records were compared for threshold and tolerance stimulation intensities. At tolerance, the reflex became faster and wider than at threshold, indicating that either a spinal summation mechanism or the recruitment of faster sensory fibers occurs in response to high-intensity noxious stimuli. A novel endpoint (i.e., tolerance) can now be considered when applying the nociceptive withdrawal reflex model in equine studies.

Abstract

The nociceptive withdrawal reflex (NWR) is used to investigate nociception in horses. The NWR threshold is a classical model endpoint. The aims of this study were to determine NWR tolerance and to compare threshold and tolerance reflexes in horses. In 12 horses, the NWR was evoked through electrical stimulation of the digital nerve and recorded via electromyography from the deltoid. Behavioral reactions were scored from 0 to 5 (tolerance). First, the individual NWR threshold was defined, then stimulation intensity was increased to tolerance. The median NWR threshold was 7.0 mA, whereas NWR tolerance was 10.7 mA. Upon visual inspection of the records, two main reflex components R1 (median latency 44 ms) and R2 (median latency 81 ms) were identified at threshold. Increasing stimulation intensity to tolerance led to a significant increase in the amplitude and duration of R1 and R2, whereas their latency decreased. At tolerance, a single burst of early, high-amplitude reflex activity, with a median latency of 39 ms, was detected in 15 out of 23 stimulations (65%). The results of this study suggest that (1) it is feasible to determine NWR tolerance in horses and (2) high-intensity stimuli initiate ultrafast bursts of reflex activity, which is well known in practice and has now been quantified using the NWR model.

Conduction velocity of nerve and muscle fiber action potentials after a space mission or a bed rest

OBJECTIVE

The purpose of the present investigation was to study the readaptation of a human muscle after a prolonged stay in microgravity and after a bed rest of several months.

METHODS

The surface electromyogram of the soleus muscle was recorded in 6 cosmonauts and 6 bed rest subjects at 5 different torque levels and, in addition, the direct muscle responses (M responses) to supramaximal stimulation of the posterior tibial nerve were also recorded in the bed rest subjects. In a supplementary experiment in normal subjects, M response was recorded with an array of electrodes.

RESULTS

The median frequency (MF) of the power spectrum of the surface electromyogram was reduced, at all torque levels, immediately after the test period. In the bed rest subjects, the latency of the M response peaks and the inter-peak interval increased during the test period. Recovery to normal occurred within about 10 days. In the normal subjects, the peaks of the M response were conducted along the muscle with a velocity between 21 and 30 m/s.

CONCLUSIONS

All these results point to a reduction of the conduction velocity in the branching axon terminals and in the muscle fibers during space missions and bed rest.

Interaction between cutaneous and muscle afferent activity in polysynaptic reflex pathways: a human experimental study

Interactions between the input from cutaneous and nociceptive muscle afferents in polysynaptic reflex pathways were investigated in man. Interaction was tested by evoking reflexes before, during, and after a period of muscle pain induced by intramuscular injection of hypertonic saline. Muscle pain was induced either in the ankle flexor (tibialis anterior, TA) or in the extensor (soleus, SOL) muscles by injection of 1 ml hypertonic saline. Electrical skin stimulation (1.1 x initial reflex threshold) at the dorsum of the foot over the tarsal joint was used to elicit cutaneo-muscular polysynaptic reflexes in the knee flexor semitendinosus (ST). The injected hypertonic saline evoked a robust muscle pain (the subjects made a continuous score of the muscle pain on a 0-10 cm VAS scale, and the mean VAS area was 1229+/-251 cm x s and lasting 390+/-30 s). In five of 12 subjects, the infusion of hypertonic saline into TA evoked referred pain to the dorsal aspect of the ankle. A significant inhibition (17+/-8.2%, P<0.05) of the ST-reflex by pain in SOL was observed. Pain in TA facilitated (92+/-36%, P<0.05) the short-latency part (50-70 ms post stimulation) of the reflex. The muscle pain did not modulate the perceived sensory intensity of the electrical stimuli. The findings indicate an interaction of input from thin muscle afferents and cutaneous group A-fibre afferents in polysynaptic segmental reflex pathways, which seems to depend on the location of the muscle pain.