Further studies on the role of afferent input from relatively large diameter fibers in transmission of nociceptive messages in humans

The effects of a sural nerve conditioning stimulation (S2) were studied on nociceptive flexion reflexes (RIII) from the biceps femoris muscle elicited by a test stimulation (S1) of the same sural nerve. The experimental procedure was designed so as to allow an indirect measurement of the conduction velocity (c.v.) of the conditioning inputs. When the S2 parameters produced an activation of afferent fibers belonging only to the A alpha beta group, it produced a facilitation of the RIII reflex which was maximal when 40 m/s conducting fibers were recruited. In contrast, when the S2 parameters activated also the A delta and C groups, the facilitatory effects were maximal when 20 m/s and 1 m/s conducting fibers were activated.     

Differential inhibition produced by peripheral conditioning stimulation on noxious mechanical and thermal responses of different classes of spinal neurons in the cat

The effect of conditioning stimulation of a peripheral nerve on responses of spinal neurons (dorsal horn cells and motoneurons) was studied in 16 decerebrate-spinal cats. The activity of dorsal horn cells was recorded with a microelectrode at the lumbosacral spinal cord and the single-unit activity of motoneurons was recorded from a filament of ventral rootlet divided from either the L7 or S1 ventral root. The responses of spinal neurons were evoked by noxious and innocuous mechanical stimuli and by noxious thermal stimuli applied to the receptive fields. The peripheral conditioning stimulation was applied to the tibial nerve with repetitive electrical pulses (2 Hz) at an intensity either suprathreshold for A delta or C fibers for 5 min. Applying conditioning stimulation to a peripheral nerve produced a powerful inhibition of the responses elicited by noxious stimuli, suggesting this inhibition is an antinociceptive effect. The inhibition produced by peripheral conditioning stimulation was differentially greater on the responses to noxious than to innocuous stimuli. Based on the results obtained from conditioning stimulation with graded strengths, afferent inputs from both myelinated and unmyelinated fibers seem to contribute to the production of the antinociceptive effect. The magnitude of the antinociceptive effect is bigger for the responses to noxious thermal than to mechanical stimuli. Furthermore, the reflex activity recorded in motor axons seemed to be more sensitive than in dorsal horn cells to the antinociceptive effect.     

Noxious stimulation of the toes evokes long-lasting, naloxone-reversible suppression of the sural-gastrocnemius reflex in the rabbit

Repetitive stimulation of the small myelinated and non-myelinated afferents of the common peroneal (c.p.) nerve evokes a long-lasting (20-25 min), naloxone-reversible inhibition of the sural-gastrocnemius reflex in the decerebrated and spinalized rabbit. Altering the number and frequency of stimuli applied to the c.p. nerve showed that this inhibition was dependent on temporal summation of afferent input from that nerve, and that the optimum frequency for producing the effect was between 2 and 10 Hz. Application of natural conditioning stimuli in and around the receptive field of the c.p. nerve showed that noxious, but not innocuous, mechanical and thermal stimuli could evoke long-lasting inhibition of the sural-gastrocnemius reflex. Thermal stimuli produced a biphasic change in the excitability of the reflex with facilitation followed by inhibition. The opioid antagonist naloxone (250 micrograms.kg-1) blocked all suppression resulting from these natural noxious stimuli. Chemical stimulation of the skin with mustard oil did not evoke naloxone-reversible inhibition of the reflex. These results indicate that intensely noxious stimuli can promote the release of opioid peptides in the spinal cord, and that one of the functions of these peptides may be to regulate the level of excitability in withdrawal reflex pathways.     

Responses of cardiac vagus and sympathetic nerves to excitation of somatic and visceral nerves

Somato-vagal and somato-sympathetic reflex responses were studied by recording simultaneously the activity of cardiac vagal and sympathetic efferents following excitation of various somatic (and 1 visceral) nerves in chloralose-anesthetized dogs.Stimulation of pure cutaneous (infraorbital, superficial radial, sural nerves), muscle (gastrocnemius, hamstring nerves) and mixed nerves (sciatic, brachial, intercostal, spinal) with short trains of pulses inhibited the activity of cardiac vagus nerve and excited that of cardiac sympathetic nerve after a latency of approximately 40–60 ms, depending on the nerve stimulated. These responses were followed by the opposite response, i.e. excitation of vagus and long-lasting inhibition (`silent period') of sympathetic nerve activity. These biphasic reflex responses recorded from both autonomic nerves had similar latencies so that a clear reciprocal relationship was observed. In addition to the above reflex responses which were observed in most instances, two peaks of excitation of short duration were recorded from the vagus nerve, in some instances, and an ‘early (spinal) reflex’ in sympathetic nerve was also observed. Both excitatory and inhibitory responses described above in either nerve were readily evoked by excitation of Group II (Aβ), but not Group I (Aα), afferent fibers and increased in magnitude when Group III (Aδ) afferents were also excited. Group IV (C) afferent contributed insignificantly to the somato-vagal reflex. The vagus nerve discharge evoked by sinus nerve stimulation was inhibited during reflex inhibition produced by somatic nerve stimulation. The latency of such inhibition was less than 20 ms and lasted for 100 ms after sural nerve stimulation. We conclude that, as in case of the baroreceptor reflex and autonomic component of the ‘defense reaction’, the somato-vagal and somato-sympathetic reflex responses are reciprocal in nature.     

Relative effectiveness of C primary afferent fibers of different origins in evoking a prolonged facilitation of the flexor reflex in the rat

Changes in the excitability of the hamstring flexor withdrawal reflex produced by conditioning stimuli applied to C-afferent fibers of different origins have been examined in the decerebrate spinal rat. In the absence of conditioning stimuli, the flexor reflex elicited by a standard suprathreshold mechanical stimulus to the toes is stable when tested repeatedly for hours. Three categories of conditioning stimuli have been used in an attempt to modify the excitability of the flexor reflex; electrical stimulation of a cutaneous (sural) nerve or a muscle (gastrocnemius-soleus) nerve at C-fiber strength; the application of mustard oil, a chemical irritant that activates chemosensitive C-afferents, to the skin or injected intramuscularly and intraarticularly; and the indirect activation of high-threshold muscle afferents by fused tetanic contractions of the tibial muscles. Conditioning stimuli of an intensity sufficient to activate C-afferent fibers result in a heterosynaptic facilitation of the flexor motoneuronal response to the standard test input, which lasts from 3 min to more than 3 hr, depending on the stimulus and the C-afferents activated. Pretreatment of the sciatic nerve with the C-fiber neurotoxin capsaicin abolishes all the postconditioning facilitations, which is an indication that it is likely that it is C-afferents that are primarily responsible for the facilitatory effects of the conditioning stimuli, although some A delta afferents may contribute. Capsaicin pretreatment does not modify the reflex response to the test stimulus. The most prolonged increase in the excitability of the flexor reflex resulted from intraarticular injections of 5 microliter mustard oil. Using the subsequent injection of lignocaine intraarticulary, it was found that the prolonged facilitation of the reflex is triggered by the afferent input generated by the conditioning stimulus and does not require an ongoing input for its maintenance. These results indicate that there is a spectrum of central changes in the stimulus response relations of the spinal cord resulting from the activation of C-fibers of different origins. The prolonged duration of some of these changes means that the peripheral activation of C-afferents will modify the functional response of the spinal cord to other inputs applied long after the conditioning input, and this may be responsible for some of the sensory and motor alterations found after peripheral tissue injury.     

Activation by high intensity peripheral nerve stimulation of adrenergic and opioidergic inhibition of a spinal reflex in the decerebrated rabbit

The short-latency sural to gastrocnemius reflex in the decerebrated rabbit was depressed for 20-30 min following high intensity conditioning stimulation of the common peroneal nerve. This effect was observed in animals with or without spinal section, but was greater in non-spinalized preparations. Graded conditioning stimuli showed that it was necessary to activate fine myelinated common peroneal axons to inhibit the reflex. In spinalized rabbits, maximal inhibition was achieved with conditioning stimulation of fine myelinated axons and was completely reversed by the opioid antagonist naloxone. In non-spinalized rabbits, maximal inhibition was only obtained with conditioning stimuli which activated non-myelinated axons. In these preparations the effects of common peroneal nerve stimuli were only blocked by co-administration of naloxone with the alpha 2-adrenoceptor antagonist idazoxan. Thus high intensity peripheral nerve stimuli activated a segmental opioidergic and a supraspinal adrenergic suppression of the sural-gastrocnemius withdrawal reflex. Such long-lasting suppression of reflex excitability may contribute to recovery from intensely noxious stimuli.     

A gentle mechanical skin stimulation technique for inhibition of micturition contractions of the urinary bladder

Effects of gentle skin stimulation of various segmental areas on the micturition contractions of the urinary bladder were examined in anesthetized male rats. The bladder was expanded by infusing saline via urethral cannula until the bladder produced rhythmic micturition contractions as a consequence of rhythmic burst discharges of vesical pelvic efferent nerves. Gentle stimulation was applied for 1 min by slowly rolling on top of skin with an elastomer "roller". Rolling on the perineal area inhibited both micturition contractions and pelvic efferent discharges during and after stimulation. Stimulation of the hindlimb, abdomen and forelimb inhibited micturition contractions after stimulation ended, in this order of effectiveness. During stimulation of the perineal skin, the reflex increase in pelvic efferent discharges in response to bladder distension to a constant pressure was also inhibited up to 45% of its control response. The inhibition of the micturition contractions induced by perineal stimulation was abolished, to a large extent by the opioid receptor antagonist naloxone and completely by severing cutaneous nerves innervating the perineal skin. We recorded unitary afferent activity from cutaneous branches of the pudendal nerve and found that the fibers excited by stimulation were low-threshold mechanoreceptive Aβ, Aδ and C fibers. Discharge rates of afferent C fibers (7.9 Hz) were significantly higher than those of Aβ (2.2 Hz) and Aδ (2.9 Hz) afferents. The results suggest that low frequency excitation of low threshold cutaneous mechanoreceptive myelinated and unmyelinated fibers inhibits a vesico-pelvic parasympathetic reflex, mainly via release of opioids, leading to inhibition of micturition contraction.     

Mechanism of the reflex inhibition of heart rate elicited by acupuncture-like stimulation in anesthetized rats

Acupuncture or acupuncture-like stimulation applied to different body areas can modify autonomic nerve activity to various organs, including gut, bladder, adrenal medulla, and the heart. We studied the reflex bradycardia in response to insertion into the skin and underlying muscles and twisting of an acupuncture needle in pentobarbital-anesthetized rats. We found that acupuncture-like stimulation of forelimb, hindlimb, chest, and abdomen all produced significant heart rate decreases. Rate minima were reached at the end of the 60-second stimulation episode and significant bradycardia persisted for about 40 s after stimulation ended. Heart rate decreases were paralleled by decreases in cardiac sympathetic nerve activity, and could be produced by electrical stimulation of group IV muscle afferent fibers (tibial nerve). Electrical stimulation of the tibial nerve at rates as low as 0.1-2 Hz was effective for eliciting heart rate decreases. Nerve fiber groups were defined by stimulation of and recording from tibial nerve. Activation of groups I, II, or III fiber was ineffective for eliciting the reflex bradycardia. Sympathectomy, high spinal transection, or infusion of the GABA(A) receptor antagonist, bicuculline, into the cisterna magna were all effective for disrupting the reflex bradycardia. Vagotomy and opioid receptor blockade were ineffective for disrupting the reflex pathway. We conclude that the reflex pathway to decrease heart rate by acupuncture-like stimulation consists of mainly group IV muscle afferent fibers whose activity (even very low rate of activity) leads to the activation of GABA-ergic neurons in the brainstem and an inhibition of sympathetic outflow to the heart.     

The digastric reflex evoked by tooth-pulp stimulation in the cat and its modulation by stimuli applied to the limbs

The digastric reflex evoked by electrical stimulation of tooth pulp in anaesthetized cats was studied together with the effects on this reflex of stimulating other parts of the body.The threshold for the digastric reflex generally lay in the range of stimulus intensities which would excite a large proportion of the pulpal afferent fibres which suggested that a large amount of central summation was required to evoke the reflex. During the course of 2527 experiments, the threshold for the reflex increased. It was also found that repeated application of suprathreshold stimuli produced first an increase and then a decrease in the reflex response.The application of noxious but not of non-noxious mechanical conditioning stimuli to the limbs produced strong, long-lasting depressions of the digastric reflex. Electrical conditioning stimuli applied to the limbs also depressed the reflex; this depression had a latency of onset of 20–50 ms and lasted for up to 500 ms. When conditioning stimuli were applied to the saphenous nerve, the depression of the reflex occurred only when the stimuli were of an intensity sufficient to excite fibres conducting at less than 40 m·s⁻¹; it may be assumed that some of these fibres would have been high threshold mechanoreceptors or nociceptors. These results show that noxious stimulation of anatomically remote structures can depress the activity of a population of trigeminal brainstem neurones.The opiate antagonist, naloxone, had no detectable effect on either the digastric reflex or the depression of the reflex produced by stimulating other parts of the body. The serotonin antagonists, methysergide and cinanserin, strongly depressed the digastric reflex but it was not clear whether these drugs also affected the depression of the reflex by the conditioning stimuli.     

Coexistent Hoffmann reflexes in human leg muscles are commonly due to volume conduction

The existence of "concomitant" (coexistent) electromyographic reflex responses in soleus and tibialis anterior muscles, produced by posterior tibial nerve stimulation, has been cited as evidence for "reciprocal excitation" between these antagonistic muscles normally reflexly linked by reciprocal inhibition. Using the Hoffmann reflex procedure and posterior tibial nerve stimulation, the existence of true reciprocal excitation was tested in six subjects with no neuromuscular impairment. Coexistent EMG responses were observed in all subjects. In no instance, however, could the tibialis anterior EMG volley to posterior tibial nerve stimulation of the soleus muscle be antidromically blocked by common peroneal nerve stimulation applied at 10 to 20 ms offset latencies. A second stimulation pulse applied to the common peroneal nerve at similar offset latencies did antidromically block a tibialis anterior reflex response to common peroneal nerve stimulation. Therefore, volume conduction of reflex activity from the posterior tibial compartment to the anterior tibial compartment was a common observance. We suggest that coexistent EMG reflex responses, presumed to reflect reciprocal excitation, should be tested by the procedure described to reject the possibility of EMG cross-talk as a confounding variable or as the actual variable under investigation.     

Inhibition from the plantar nerve to soleus muscle during the stance phase of walking

The purpose of this study is to investigate the effect and the circuit from the branch of tibial (plantar) nerve to soleus muscle and its modulation during walking in humans. Stimulation of the plantar nerve produced short latency inhibition of soleus EMG activity and the H-reflex in humans. The threshold of afferent fibers was lower than that of motor fibers. This inhibition did not converge to disynaptic reciprocal Ia inhibition nor did inhibition from the cutaneous nerve of the big toe, but to Ib inhibition from the medial gastrocnemius nerve. The inhibitory pathway from the plantar nerve therefore is considered to include Ib inhibitory interneurones. Modulation of the inhibition was investigated during walking. Less EMG depression after plantar nerve stimulation occurred in the stance phase of walking than for tonic or dynamic plantar flexion at similar background EMG activity level. The inhibition of the soleus H-reflex after plantar nerve stimulation was also decreased during the stance phase. For investigating the influence of load on the inhibition from the plantar nerve, more EMG depression occurred in the stance phase with body unloading. Similar findings were observed in Ib inhibition from the medial gastrocnemius nerve, but not in disynaptic reciprocal Ia inhibition to soleus muscle. It is concluded that transmission of inhibition from the plantar nerve to soleus muscle is modulated during walking. It would minimize this inhibition during the stance phase of walking and might enhance soleus muscle activity via this reflex pathway for the support of weight.     

Vagal afferent modulation of a nociceptive reflex in rats: involvement of spinal opioid and monoamine receptors

Modulation of the spinal nociceptive tail flick (TF) reflex by electrical stimulation of subdiaphragmatic or cervical vagal afferent fibers was characterized in rats lightly anesthetized with pentobarbital. Cervical vagal afferent stimulation (VAS) inhibited the TF reflex in a pulse width-, frequency-, and intensity-dependent fashion. The optimum parameters for inhibition of the TF reflex were determined to be 2.0 ms pulse width, 20 Hz frequency with a threshold (T) current of 60 microA. Cervical VAS at 0.2-0.6 T facilitated the TF reflex. Cervical VAS at T typically produced a depressor arterial blood pressure response, but inhibition of the TF reflex by VAS was not due to changes in blood pressure. Subdiaphragmatic VAS also inhibited the TF reflex and generally produced a pressor effect, but did not facilitate the TF reflex at intensities of stimulation less than T as did cervical VAS. The parameters of cervical VAS required for inhibition of TF reflex suggest that excitation of high-threshold, unmyelinated fibers are important in VAS-induced descending inhibition. The intrathecal administration of pharmacologic receptor antagonists into the subarachnoid space of the lumbar enlargement indicated that the opioid receptor antagonist naloxone produced a dose-dependent antagonism of cervical VAS-produced inhibition of TF reflex, but single doses of either phentolamine or methysergide (30 micrograms each) failed to affect the inhibition by VAS. Combined intrathecal injection of both phentolamine and methysergide (30 micrograms each), however, significantly attenuated inhibition of the TF reflex by cervical VAS. These results suggest that cervical VAS engages a spinal opioid system and co-activates descending serotonergic and noradrenergic systems to modulate spinal nociceptive processing.     

Neural organization of the viscero-cardiac reflexes

The reflex responses evoked in the postganglionic nerves to the heart were tested in chloralose-anaesthetized cats. Electrical stimulation of the A delta afferent fibres from the left inferior cardiac nerve evoked spinal and supraspinal reflex responses with the onset latencies of 36 ms and 77 ms respectively. The most effective stimulus was a train of 3-4 electrical pulses with the intratrain frequency of 200-300 Hz. Electrical stimulation of the high threshold afferent fibres (C-fibres) from the left inferior cardiac nerve evoked the reflex response with the onset latency of 200 ms. The C-reflex was present in intact animals and disappeared after spinalization. The most effective stimulus to evoke this reflex was a train of electrical pulses delivered at a frequency of 1-2 Hz with an intratrain frequency of 20-30 Hz. The most prominent property of the C-reflex was its marked increase after prolonged repeated electrical stimulation. We conclude that: (1) viscero-cardiac sympathetic reflexes may be organized at the spinal and supraspinal level; (2) viscero-cardiac sympathetic reflexes evoked by stimulation of the A delta and C afferent fibres from the left inferior cardiac nerve have different central organization.     

Somatosplanchnic reflex discharges in rats

Splanchnic efferent reflex discharges caused by electrical stimulation of limb afferent nerves or intercostal afferent nerves were studied in chloralose-urethane anesthetized rats. Stimulation of the limb afferent nerve produced late supraspinal reflex discharges via group II and III afferent excitation. Stimulation of the intercostal afferent nerve produced early spinal reflex discharges via group II and III afferent excitation and also late spinal reflex discharges via group IV afferent excitation. Intercostal afferent nerve stimulation seemed to strongly depress the splanchnic late supraspinal reflex discharges.     

Long-latency,nonreciprocal reflex responses of antagonistic hind limb muscles after cutaneous nerve stimulation in the cat

The time course of changes in monosynaptic reflex amplitude, after conditioning from both ipsi- and contralateral sural nerves at different stimulus strengths, was studied on two antagonistic motoneuronal pools acting on ankle muscles in spinal cats. Attention was focused on late effects, namely those appearing after a dely of more than 30 ms from the cutaneous stimulus. With low-threshold afferent activation, at conditioning-test intervals to 30 ms, the ipsilateral extensor monosynaptic reflex, recorded from the proximal stump of L7 to S1 ventral roots, showed marked inhibition; at longer intervals, a late facilitation period (LFP) lasting to 100 ms was observed. Increasing stimulus strength did not modify the time course of reflex excitability, but might enhance the amount of the facilitatory effecct. On the flexor monosynaptic reflex, sural conditioning induced, after the expected early facilitation, a second facilitatory period, starting at about 30 ms and recovering at about 130 ms. The excitability of antagonistic contralateral motoneuronal pools was also influenced, showing again a LFP with the same time course. The LFP was present after stimulation of the sural and saphenous nerves and was absent after stimulation of a muscle nerve. These late, long-lasting and nonreciprocal facilitatory effects on flexor and extensor ipsilateral motoneurons were quite distinct from the early reciprocal responses, and were evoked by large cutaneous fibers. An interpretation is put forward in light of the primary afferent hyperpolarization of Ia afferent terminals. A correlation is tentatively proposed with the mechanism subserving the stumbling corrective reaction.     

Peripheral nerve stimulation for producing the suppressive effect on the tooth pulp-evoked jaw opening reflex in rat: Relation between stimulus intensity and degree of suppression

To reveal the peripheral physiologic mechanisms involved in local electrical stimulation, effects of conditioning radial nerve stimulation (0.1-ms duration, 45-Hz frequency, for 15 min) on the tooth pulp-evoked jaw opening reflex, which was monitored by the digastric electromyogram (dEMG), were investigated in Wistar albino rats. The activity of the dEMG was gradually decreased after the onset of the conditioning stimulation and after its cessation, the activity slowly recovered to the control value. A good suppressive effect was observed with a conditioning stimulus intensity of only four or six times the Aα threshold of the radial nerve compound action potential. When the jaw opening reflex was evoked with the supramaximum intensity of tooth-pulp stimulation, radial-induced suppression was rarely observed. The present study showed that direct peripheral nerve stimulation produced the suppressive effect similar to acupuncture or transcutaneous electrical nerve stimulation and that the effect was presumably elicited by the impulses of large-diameter A fibers, not by those of small-diameter ones.     

Tonic descending inhibition of the spinal cardio-sympathetic reflex in the cat

Electrical stimulation of the left inferior cardiac nerve elicited a two-component reflex potential (spinal and supraspinal reflexes) in the ipsilateral white ramus T3 from which recordings were made in chloralose-anaesthetised cats. Reversible interruption of all spinal pathways achieved by cooling the spinal cord at C2/C3 produced an enhancement of the spinal reflex and abolished the supraspinal reflex, the latter usually being the more prominent reflex potential prior to spinal cord block. The spinal cord block-induced increase in the amplitude of the spinal reflex was, however, less than the increase observed during stimulation of the somatic intercostal nerve T4. Recordings of the afferent volley following cardiac nerve stimulation and analysis of the stimulus-reflex response relationship in neuraxis-blocked cats indicated that the spinal reflex as determined here was activated by A delta afferent fibres. However, if stimulus strength was raised above C-fibre threshold, spinal cord block revealed in addition a late spinal reflex response. In some cases, the appearance of this late potential was accompanied by a secondary decline of the earlier spinal reflex potential, possibly indicating C-fibre-mediated afferent inhibition. Neither baroreceptor activation nor denervation had any effect on spinal reflex amplitudes. Pharmacologically, clonidine given i.v. to cats with a blocked neuraxis reduced the spinal reflex amplitudes to pre-block values, an action which could be antagonised by the subsequent administration of the alpha 2-adrenoceptor antagonist rauwolscine. When given to non-pretreated cats with intact neuraxis, however, neither rauwolscine nor its analog yohimbine were capable of inducing a persistent release from tonic inhibition. The results suggest that both purely visceral and somato-visceral reflexes are subject to tonic descending inhibition, but they do not support the hypothesis that a catecholamine is the responsible transmitter mediating this inhibition.     

Discharge characteristics of sympathetic efferents to the knee joint of the cat

Sympathetic postganglionic neurons to the knee joint of the cat were studied to characterize the nerve supply and response to somatic stimulation. In halothane anesthetized cats, the sympathetic postganglionic units from a branch of medial articular nerve (MAN) were dissected. The other branch of MAN was left intact. Most of the central filaments of MAN showed spontaneous discharge. The frequency of the spontaneous discharge of single units ranged from 0.2 to 2.9 impulses per second. Cardiovascular rhythmic modulation was observed in most of the filaments tested. Phenylephrine-induced baroreceptor stimulation caused inhibition of the discharges. Repetitive stimulation of the lumbar sympathetic trunk or the peripheral cut end of MAN led to a decrease in the local temperature inside the joint. The frequency threshold for decreasing the temperature of the joint was approximately 1 Hz. Maximum effects were obtained with 5 Hz stimulation. Histograms of MAN sympathetic efferent fibre activity following electrical stimulation of afferent nerve fibres in the MAN exhibited two response periods. The first, of about 280 ms latency, was elicited by myelinated fibre excitation (the A-reflex) and the second, with a latency of approximately 700 ms, was evoked by unmyelinated fibre excitation (the C-reflex). Electrical stimulation of radial afferent nerve produced similar A- and C-reflex discharges in sympathetic fibres of MAN. Passive movement of the knee joint within its normal working range (flexion, extension or outward rotation) had very little effect on sympathetic efferent nerve activity in MAN, whereas noxious outward rotation of the joint produced a reflex increase in activity to about 140% of the prestimulus control level.(ABSTRACT TRUNCATED AT 250 WORDS)     

Slow changes in the flexion reflex of the rat following arthritis or tenotomy

(1) The flexor reflex was measured in control decerebrate spinal rats by recording the motor axon activity in the nerve to biceps femoris evoked by a pressure stimulus to the hindpaw. A brief (1 Hz, 20 s) conditioning stimulus to peripheral nerves with sufficient strength to activate C fibres results in a prolonged increase in this flexor reflex. If the conditioning stimulus is applied to a cutaneous nerve, the sural, the facilitation lasts 5 (5.0 +/- 1.6, n = 28) min. However, if the same conditioning stimulus is applied to the muscle nerve to gastrocnemius, the facilitation lasts up to an hour (54.0 +/- 8.3 min, n = 16). (2) Ankle joint urate arthritis was induced by the injection of 1.25 mg sodium urate crystals into one ankle joint. Two hours after the injection, conditioning of the flexor reflex by brief stimulation of the sural or gastrocnemius nerves produced the same effect as in control animals. However, 24 h after the injection, sural nerve conditioning produced the same effect on the flexor reflex as in controls but gastrocnemius nerve conditioning produced only 8 min (8.3 +/- 1.6, n = 8) of facilitation instead of the expected 54 min. (3) The decreased ability of the muscle nerve to produce prolonged facilitation was not dependent on a saturation of the flexor reflex since the reflex could still be enhanced briefly by the conditioning stimuli. It was also not dependent on a continuous input from the arthritic ankle since the decrease is still apparent when the ankle has been denervated 24 h after the beginning of the arthritis.(ABSTRACT TRUNCATED AT 250 WORDS)     

Can Ib axons be selectively activated by electrical stimuli in human subjects?

A tendon-vibration technique, used to raise the electrical threshold of muscle spindle Ia afferent fibers above that of Golgi tendon organ Ib afferent fibers in animals, was tested on human subjects. After prolonged tendon vibration, electrical stimulation of the posterior tibial nerve was ineffective or markedly less effective in eliciting Hoffmann (H-) reflexes in the soleus muscle at previbration threshold intensities. With stimulus intensity held constant at values between 1.0 to 1.4 X threshold, postvibration H-reflex myoelectric amplitudes returned to previbration values usually within 60 min. However, at higher electrical stimulus intensities (1.8 X threshold), postvibration H-reflex amplitudes were produced at or near previbration values irrespective of postvibration recovery time; in contrast, initial postvibration tendon tap reflexes were potentiated. Findings suggest that it is indeed possible to selectively activate Ib afferent fibers in humans by electrical stimuli.