Influence of forelimb afferents on the activity of lumbar spinal-cord motoneurons

Experiments were conducted on anesthetized cats with microelectrode recording to study the synaptic responses that develop in the lumbar motoneurons on stimulation of the afferent fibers of groups II and III in the nerves of the ipsilateral and contralateral forelegs. Stimulation of these afferents evoked predominantly inhibitory postsynaptic potentials (IPSP) in the extensor motoneurons and excitatory postsynaptic potentials (EPSP) in the flexor motoneurons. A basically inhibitory change in the rhythmic background activity developed under the influence of descending impulsation. The duration of the total inhibition of "spontaneous" motoneuron activity corresponded to the duration of the inhibitory influences exerted by the forelimb flexor-reflex afferents (FRA) on the interneurons. The interaction of the descending and segmental PSP resulted in inhibition and facilitation of the segmental responses in the motoneurons. The ultimate result of this interaction was determined by the shifts in the membrane potential of the motoneuron and by the effects created in the interneurons.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 3, No. 1, pp. 58–67, January–February, 1971.     

The effect of stimulation of Golgi tendon organs and spindle receptors from hindlimb extensor muscles on supraspinal descending inhibitory mechanisms

Experiments were performed in precollicular decerebrate cats to investigate whether proprioceptive volleys originating from Golgi tendon organs and muscle spindles may activate supraspinal descending inhibitory mechanisms. Conditioning stimulation of the distal stump of ventral root filaments of L7 or S1 leading to isometric contraction of the gastrocnemius-soleus (GS) muscle inhibited the monosynaptic reflex elicited by stimulation of the ipsilateral plantaris-flexor digitorum and hallucis longus (Pl-FDHL) nerve. The amount and the time course of this Golgi inhibition were greatly increased by direct cross-excitation of the intramuscular branches of the group Ia afferents due to ephaptic stimulation of the sensory fibers, which occurred when a large number of a fibers had been synchronously activated. The postsynaptic and the presynaptic nature of these inhibitory effects, as well as their segmental origin, have been discussed. In no instance, however, did the stimulation of Golgi tendon organs elicit any late inhibition of the test monosynaptic reflex, which could be attributed to a spino-bulbo-spinal (SBS) reflex. Conditioning stimulation of both primary and secondary endings of muscle spindles, induced by dynamic stretch of the lateral gastrocnemius-soleus (LGS) muscle, was unable to elicit any late inhibition of the medial gastrocnemius (MG) monosynaptic reflex. The only changes observed in this experimental condition were a facilitation of the test reflex during the dynamic stretch of the LGS, followed at the end of the stimulus by a prolonged depression. These effects however were due to segmental interactions, since they persisted after postbrachial section of the spinal cord. Intravenous injection of an anticholinesterase, at a dose which greatly potentiated the SBS reflex inhibition produced by conditioning stimulation of the dorsal root L6, did not alter the changes in time course of the test reflex induced either by muscle contraction or by dynamic muscle stretch. Conditioning stimulation of a muscle nerve activated the supraspinal descending mechanism responsible for the inhibitory phase of the SBS reflex only when the high threshold group III muscle afferents (innervating pressure-pain receptors) had been recruited by the electric stimulus. This finding contrasts with the great availability of the system to the low threshold cutaneous afferents. The proprioceptive afferent volleys originating from Golgi tendon organs as well as from both primary and secondary endings of muscle spindles, contrary to the cutaneous and the high threshold muscle afferent volleys, were apparently unable to elicit not only a SBS reflex inhibition, but also any delayed facilitation of monosynaptic extensor reflexes attributable to inhibition of the cerebellar Purkinje cells.     

Human group I excitatory projections from ankle dorsiflexors to quadriceps muscle

A short latency projection of group I afferent fibers from ankle dorsiflexors to knee extensor muscles has been categorized as species specific to humans. However, the effects of the pathway have only been inferred from conditioning homonymous reflexes in relaxed muscle. This study focused directly on the responses evoked in the electromyogram of the heteronymous muscles when active, in two experiments. In the first, preferential activation of group I afferents of ankle dorsiflexors, by electrical stimulation of the common peroneal nerve, excited both vastus medialis (mean latency, 26.3 ms) and rectus femoris (mean latency, 33.5 ms). No excitation or inhibition in either muscle was associated with stimulation of the tibial nerve. The second experiment compared vastus medialis responses with common peroneal nerve stimulation during three different movement conditions in which the muscle was equally contracted: rhythmic isotonic (pedalling); episodic isotonic; isometric contraction. Responses were identified in all three active states, with no significant differences in amplitude or latency. No responses were seen in the relaxed muscle.     

Synaptic processes in thoracic α-motoneurons evoked by segmental afferent stimulation

Synaptic processes in various functional groups of thoracic motoneurons (Th₉-Th₁₁) evoked by stimulation of segmental nerves were investigated in anesthetized and decerebrate cats. No reciprocal relations were found between these groups of motoneurons. Only excitatory mono- and polysynaptic responses were recorded in the motoneurons of the principal intercostal nerve following stimulation of the homonymous nerve. Activation of the afferents of the external intercostal muscle and dorsal branches does not cause noticeable synaptic processes in these motoneurons; much more rarely it is accompanied by the development of low-amplitude polysynaptic EPSP's. In motoneurons of the dorsal branches, stimulation of homonymous nerves leads to the appearance of simple, short-latent EPSP's. Late responses of the IPSP or EPSP - IPSP type with a predominance of the inhibitory component were observed in most motoneurons of this type following activation of the afferent fibers of the principal intercostal nerve. In other motoneurons of the dorsal muscles, stimulation of the main intercostal nerve (and nerve of the external intercostal muscle) did not evoke apparent synpatic processes. EPSP's (mono- and polysynaptic) appeared in the motoneurons of the external intercostal muscle following stimulation of the homonymous and main intercostal nerves. Activation of the afferents of the dorsal branches was ineffective. The character of the synaptic responses of the respiratory motoneurons to segmental afferent stimulation, investigated under conditions of spontaneous respiration, was different. The characteristics of synaptic activation of thoracic motoneurons by segmental afferents under conditions of hyperventilation apnea and during spontaneous breathing of the animals are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 2, No. 3, pp. 279–288, May–June, 1970.     

Reflex mechanisms of fore-hindlimb relations demonstrable in lumbar flexor centers of the cat spinal cord

The dynamics of interaction between segmental, propriospinal, and spino-bulbo-spinal components of reflex responses of lumbar flexor centers was investigated during activation of forelimb and hindlimb afferents in cats. The monosynaptic flexor reflex to activation of hindlimb afferents is facilitated up to 300% when it coincides in time with the reflex discharge evoked by activation of forelimb afferents, and it remains increased up to 120–140% for 40–50 msec during the aftereffect of such activation. Polysynaptic flexor reflexes of segmental, propriospinal, and spino-bulbo-spinal origin have both facilitatory and inhibitory effects on each other. Facilitation is observed only while the interacting responses coincide in time, inhibition when they do not coincide. Three type of inhibitory effects with durations of 7–15, 40–150, and 300–500 msec are observed. The possible neuronal mechanisms of interaction between these reflexes and their role in functional relations between the fore- and hindlimbs are discussed.     

Effects of peripheral inputs from hindlimb on the monosynaptic reflex of motoneurons innervating tail muscles.

The effects of group II muscle (PBSt, GS) and cutaneous afferent (Sur, SPc, Tib) inputs from the hindlimb on the monosynaptic reflexes of motoneurons innervating tail muscles were studied in lower spinalized cats. Stimulation of the cutaneous nerves at the conditioning-test stimulus interval of about 10-20 ms facilitated and inhibited the monosynaptic reflexes of ipsilateral and contralateral tail muscles, respectively. The effects of the muscle nerve stimulation were not so prominent as those elicited by cutaneous nerve stimulation. The monosynaptic reflex was also inhibited by muscle nerve stimulation at 10-50 ms intervals. The effects of conditioning stimulation of the hindlimb peripheral nerves at short intervals were depressed or blocked by section of the ipsilateral lateral funiculus at S1 spinal segment. These findings show that the neuronal pathway from hindlimb afferents to tail muscle motoneurons passed the lateral funiculus of the spinal cord and modulates the motoneuronal activity of tail muscles.     

Time-dependent effects of neuromuscular electrical stimulation on changes in spinal excitability are dependent on stimulation frequency: A preliminary study in healthy adults

Neuromuscular electrical stimulation (NMES) can be used as treatment for spasticity. The present study examined differences in time-dependent effects of NMES depending on stimulation frequency. Forty healthy subjects were separated into four groups (no-stim, NMES of 50, 100, and 200?Hz). The un-conditioned H-reflex amplitude and the H-reflex conditioning-test paradigm were used to measure the effectiveness on monosynaptic Ia excitation of motoneurons in the soleus (SOL) muscle, disynaptic reciprocal Ia inhibition from tibialis anterior (TA) to SOL, and presynaptic inhibition of SOL Ia afferents. Each trial consisted of a 30-min period of NMES applied to the deep peroneal nerve followed by a 30-min period with no stimulation to measure prolonged effects. Measurements were performed periodically. Stimulation applied at all frequencies produced a significant reduction in monosynaptic Ia excitation of motoneurons in the SOL muscle, however, only stimulation with 50?Hz showed prolonged reduction after NMES. NMES frequency did not affect the amount of disynaptic reciprocal Ia inhibition and presynaptic inhibition of Ia afferents. The results show a frequency-dependent effect of NMES on the monosynaptic Ia excitation of motoneurons. This result has implications for selecting the optimal NMES frequency for treatment in patients with spasticity.     

Convergence of forelimb afferent actions on C7-Th1 propriospinal neurones bilaterally projecting to sacral segments of the cat spinal cord

Propriospinal neurones located in the cervical enlargement and projecting bilaterally to sacral segments of the spinal cord were investigated electrophysiologically in eleven deeply anaesthetized cats. Excitatory or inhibitory postsynaptic potentials from forelimb afferents were recorded following stimulation of deep radial (DR), superficial radial (SR), median (Med) and ulnar (Uln) nerves. 26 cells were recorded from C7, 22 from C8 and 3 from Th1 segments. The majority of the cells were located in the Rexed's laminae VIII and the medial part of the lamina VII. In 10 cases no afferent input from the forelimb afferents was found. In the remaining neurones effects were evoked mostly from DR (88%) and Med (63%), less often from SR (46%) and Uln (46%). Inhibitory actions were more frequent than excitatory. The highest number of IPSPs was evoked from high threshold flexor reflex afferents (FRA)--all connections were polysynaptic. However, inhibitory actions were often evoked from group I or II muscle afferents (polysynaptic or disynaptic) and, less frequently, from cutaneous afferents (mostly polysynaptic). Di- or polysynaptic IPSPs often accompanied monosynaptic EPSPs from group I or II muscle afferents. Disynaptic or polysynaptic EPSPs from muscle and cutaneous afferents were also recorded in many neurones, while polysynaptic EPSPs from FRA were observed only exceptionally. Various patterns of convergence in individual neuronal subpopulations indicate that they integrate different types of the afferent input from various muscle and cutaneous receptors of the distal forelimb. They transmit this information to motor centers controlling hind limb muscles, forming a part of the system contributing to the process of coordination of movements of fore--and hind--limbs.     

Spinal reflexes in the long-tailed stingray, Himantura fai

We have exploited the segregation of motor and sensory axons into peripheral nerve sub-compartments to examine spinal reflex interactions in anaesthetized stingrays. Single, supra-maximal electrical stimuli delivered to segmental sensory nerves elicited compound action potentials in the motor nerves of the stimulated segment and in rostral and caudal segmental motor nerves. Compound action potentials elicited in segmental motor nerves by single stimuli delivered to sensory nerves were increased severalfold by prior stimulation of adjacent sensory nerves. This facilitation of the segmental reflex produced by intense conditioning stimuli decreased as it was applied to more remote segments, to approximately the same degree in up to seven segments in the rostral and caudal direction. In contrast, an asymmetric response was revealed when test and conditioning stimuli were delivered to different nerves, neither of which was of the same segment as the recorded motor nerve: in this configuration, conditioning volleys generally inhibited the responses of motoneurons to stimuli delivered to more caudally located sensory nerves. This suggests that circuitry subserving trans-segmental interactions between spinal afferents is present in stingrays and that interneuronal connections attenuate the influence that subsequent activity in caudal primary afferents can have on the motor elements.     

Interlimb Reflexes Induced by Electrical Stimulation of Cutaneous Nerves after Spinal Cord Injury

Whether interlimb reflexes emerge only after a severe insult to the human spinal cord is controversial. Here the aim was to examine interlimb reflexes at rest in participants with chronic (>1 year) spinal cord injury (SCI, n = 17) and able-bodied control participants (n = 5). Cutaneous reflexes were evoked by delivering up to 30 trains of stimuli to either the superficial peroneal nerve on the dorsum of the foot or the radial nerve at the wrist (5 pulses, 300 Hz, approximately every 30 s). Participants were instructed to relax the test muscles prior to the delivery of the stimuli. Electromyographic activity was recorded bilaterally in proximal and distal arm and leg muscles. Superficial peroneal nerve stimulation evoked interlimb reflexes in ipsilateral and contralateral arm and contralateral leg muscles of SCI and control participants. Radial nerve stimulation evoked interlimb reflexes in the ipsilateral leg and contralateral arm muscles of control and SCI participants but only contralateral leg muscles of control participants. Interlimb reflexes evoked by superficial peroneal nerve stimulation were longer in latency and duration, and larger in magnitude in SCI participants. Interlimb reflex properties were similar for both SCI and control groups for radial nerve stimulation. Ascending interlimb reflexes tended to occur with a higher incidence in participants with SCI, while descending interlimb reflexes occurred with a higher incidence in able-bodied participants. However, the overall incidence of interlimb reflexes in SCI and neurologically intact participants was similar which suggests that the neural circuitry underlying these reflexes does not necessarily develop after central nervous system injury.     

Inhibitory effects of reticulospinal fibers of the lateral funiculus on neurons of thoracic spinal reflex pathways

Experiments on anesthetized cats with partial transection of the spinal cord showed that reticulo-spinal fibers in the ventral part of the lateral funiculus participate in the inhibition of polysynaptic reflexes evoked by stimulation of the ipsi- and contralateral reticular formation. The reticulo-fugal wave in the ventrolateral funiculus evoked comparatively short (up to 70 msec) IPSPs in some motoneurons of the internal intercostal nerve investigated and at the same time evoked prolonged (up to 500 msec) inhibition of IPSPs caused by activation of high-threshold segmental afferents. This wave also led to the appearance of IPSPs in 14 of 91 (15.5 %) thoracic spinal interneurons studied. The duration of these IPSPs did not exceed 100 msec; meanwhile, segment excitatory responses of 21 of 43 interneurons remained partly suppressed for 120–500 msec. It is concluded that the inhibitory action of the lateral reticulo-spinal system on segmental reflexes is due to several synaptic mechanisms, some of them unconnected with hyperpolarization of spinal neurons. The possible types of mechanisms of this inhibition are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 10, No. 2, pp. 162–172, March–April, 1978.     

Multiple gate control of the descending statocyst-motor pathway in the crayfish Procambarus clarkii Girard

1. Synaptic responses of uropod motoneurons and interneurons to magnetic field stimulation of the statocyst were studied in a whole animal preparation using intracellular recording and staining techniques to characterize the descending statocyst pathways controlling uropod steering behavior. 2. When the animal was engaged in abdominal postural movement, all uropod motoneurons received sustained excitatory input. Motoneurons which were to be activated during steering behavior showed excitatory responses to the stimulus superimposed on the sustained excitation. In the resting state, they showed weaker responses or no visible responses to the same stimulation. 3. Motoneurons to be suppressed during steering showed inhibitory responses to the stimulus only during abdominal movement. These included both active inhibition as well as disfacilitatory suppression of excitatory input to the motoneurons. 4. Premotor nonspiking interneurons, like motoneurons, showed greater responses to the stimulus during abdominal movement than at rest. Unlike motoneurons, however, they did not always receive sustained input during abdominal movement. 5. Descending axons which responded to statocyst stimulation independent of abdominal movement were found in the 4th and 5th abdominal ganglia. Other axons showed greater responses during abdominal movement than at rest. 6. A number of intersegmental descending interneurons with cell bodies and dendrites in the 4th or 5th ganglion were found to receive excitatory inputs from both the statocyst and the motor system controlling abdominal posture. These responses were found to summate with each other to generate spikes. 7. Statocyst signals are thus transmitted to uropod motoneurons by two types of descending pathways: one whose operation is affected by the abdominal system and the other which operates independently. The former pathway functions by recruiting intersegmental abdominal interneurons and makes stronger connections with motoneurons than the latter.     

Interactions between inhibitions of sympathetic nerve discharge to successive stimulations of baroreceptor and cutaneous afferents

Interactions between inhibitions of sympathetic nerve discharge (SND) to consecutive stimulations of A- and C-fibres in the aortic nerve and superficial peroneal (SP) nerve have been studied in rabbits anaesthetized with urethane and chloralose. SND was recorded from the renal nerve. At interval of 10 s following conditioning stimulation of SP nerve the test response of SND to activation of C-fibres (interaction SP-C) amounts to 83.7 +/- 7.5% (means +/- SE) of control. After applying both stimulations to the same SP nerve (interaction SP-SP) the test response of SND is significantly lower amounting to 56.3 +/- 3.9%. Comparable decrease of the effect of conditioning stimulation of one type of afferents on the test response to activation of other type of fibres was observed at interval of 10 s in two other interactions (SP-A and C-A). In seven interactions the conditioning stimulation of one type of fibres had no effect on the test response to stimulation of other type of fibres. This was found in one interaction at interval of 10 s (C-SP), in four interactions at interval of 20 s (SP-A, C-SP, SP-C and C-A) and in two interactions at interval of 40 s (SP-A and SP-C). Conditioning stimulations used in above interactions depressed the test responses to activation of the same type of fibres. It is suggested that the decrease or suppression of the effect of conditioning stimulation of one type of afferents on the test response of SND to activation of other type of fibres is related to disinhibition of this response.     

Neuronal pathways from low-threshold muscle and cutaneous afferents innervating tail to trunk muscle motoneurons in the cat

We studied neuronal pathways from low-threshold muscle (group I, II) and cutaneous afferents (group A(alpha)beta) innervating the tail to motoneurons innervating trunk muscles (m. iliocostalis lumborum and m. obliquus externus abdominus) in 18 spinalized cats. Stimulation of group I muscle afferents produced excitatory postsynaptic potentials or excitatory postsynaptic potentials followed by inhibitory postsynaptic potentials in all motoneurons innervating the m. iliocostalis lumborum which showed effects (32%), and predominantly inhibitory postsynaptic potentials in motoneurons innervating the m. obliquus externus abdominus (47%). Stimulation of group I+II afferents produced significant increases of the incidence of motoneurons showing postsynaptic potentials (the notoneurons innervating the m. iliocostalis lumborum, 87%; the motoneurons innervating the m. obliquus externus abdominus, 82%). The effects of low threshold cutaneous afferents were bilateral, predominantly producing inhibitory postsynaptic potentials in motoneurons innervating both muscles. These results suggest that neuronal pathways from muscle afferents to back muscle motoneurons mainly increase the stiffness of the trunk to maintain its stability, while those to abdominal muscles help to extend the dorsal column by decreasing their activities. The results also indicate that neuronal pathways from cutaneous afferents to trunk motoneurons functionallY disconnect the tail from the trunk.     

Effects of repetitive stimulation of the locus coeruleus on spinal inhibitory responses to suprasegmental stimulation in cats

Effects of repetitive stimulation of the locus coeruleus on spinal responses to activation of cortico-, reticulo-, and vestibulospinal tracts were studied in decerebellate cats anesthetized with chloralose. Descending influences of these structures were assessed from changes in amplitude of extensor and flexor monosynaptic discharges or from the magnitude of postsynaptic potentials recorded from the corresponding motoneurons. Stimulation of the motor cortex or modullary reticular formation as a rule evoked two-component inhibitory responses in extensor motoneurons and excitatory-inhibitory responses in flexor motoneurons. Stimulation of locus coeruleus effectively depressed the amplitude of the late component and, to a lesser degree, that of the early component of inhibition arising after stimulation of the cerebral cortex or reticular formation. During stimulation of the locus coeruleus no marked changes were found in inhibitory responses evoked by vestibulospinal influences in flexor motoneurons, and also in excitatory responses arising after stimulation of the above-mentioned descending pathways in both groups of motoneurons.     

A possible coordinating role for presynaptic inhibition

Conditioning stimuli were applied to the common peroneal or superficial peroneal nerve in acute experiments on anesthetized cats. Changes in the N₁-component of the dorsal cord potential evoked by stimulation of one of these nerves or of other nerves (tibial, deep peroneal) and changes in the amplitude of antidromic action potentials in the afferent fibers of these nerves were investigated. The degree of reinforcement of antidromic action potentials, reflecting the degree of depolarization of the afferent terminals, was found to be greater for the passive nerve than for the active to which the conditioning stimulus was applied. Inhibition of the N₁-component of the dorsal cord potential was deeper when a pair of stimuli was applied to two different nerves (under these conditions only the mechanism of presynaptic inhibition was activated) than when they were applied to the same nerve. It is concluded that presynaptic inhibition, by selectively controlling afferent volleys, can evidently play a coordinating role.     

Late long-lasting discharges in hind limb motor nerves and their connection with the locomotor rhythm in thalamic immobilized cats

Effects of stimulation of flexor reflex afferents were studied in decerebrate immobilized cats. Stimulation of ipsilateral afferents evoked late long-lasting discharges in the nerves to the flexors, whereas stimulation of contralateral afferents led to similar discharges in nerves to both extensors and flexors. Compared with spinal animals, early segmental reflexes in thalamic cats were tonically depressed. Similar tonic inhibition of segmental reflexes took place in spinal animals after injection of dopa. Segmental reflexes were clearly modulated during late or rhythmic discharges. The possible central mechanisms of these changes in the segmental reflexes are discussed on the basis of data in the literature.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 11, No. 2, pp. 137–145, March–April, 1979.     

Modulation of spinal inhibitory reflexes depends on the frequency of transcutaneous electrical nerve stimulation in spastic stroke survivors

Neurophysiological studies in healthy subjects suggest that increased spinal inhibitory reflexes from the tibialis anterior (TA) muscle to the soleus (SOL) muscle might contribute to decreased spasticity. While 50?Hz is an effective frequency for transcutaneous electrical nerve stimulation (TENS) in healthy subjects, in stroke survivors, the effects of TENS on spinal reflex circuits and its appropriate frequency are not well known. We examined the effects of different frequencies of TENS on spinal inhibitory reflexes from the TA to SOL muscle in stroke survivors. Twenty chronic stroke survivors with ankle plantar flexor spasticity received 50-, 100-, or 200-Hz TENS over the deep peroneal nerve (DPN) of the affected lower limb for 30?min. Before and immediately after TENS, reciprocal Ia inhibition (RI) and presynaptic inhibition of the SOL alpha motor neuron (D1 inhibition) were assessed by adjusting the unconditioned H-reflex amplitude. Furthermore, during TENS, the time courses of spinal excitability and spinal inhibitory reflexes were assessed via the H-reflex, RI, and D1 inhibition. None of the TENS protocols affected mean RI, whereas D1 inhibition improved significantly following 200-Hz TENS. In a time-series comparison during TENS, repeated stimulation did not produce significant changes in the H-reflex, RI, or D1 inhibition regardless of frequency. These results suggest that the frequency-dependent effect of TENS on spinal reflexes only becomes apparent when RI and D1 inhibition are measured by adjusting the amplitude of the unconditioned H-reflex. However, 200-Hz TENS led to plasticity of synaptic transmission from the antagonist to spastic muscles in stroke survivors.     

Inhibition of jaw opening reflexes by stimulation of the central gray matter and raphe nuclei in cats

The effect of stimulation of the mesencephalic central gray matter and raphe nuclei on jaw opening reflexes evoked by excitation of high-threshold (dental pulp) and low-threshold (A-alpha) fibers of the infraorbital nerve afferents was studied in cats anesthetized with chloralose and pentobarbital. The jaw opening reflex evoked by stimulation of the dental pulp was shown to be effectively suppressed by conditioning stimulation of the central gray matter and raphe nuclei. The reflex evoked by stimulation of low-threshold infraorbital nerve afferents also was depressed (but less deeply and for a shorter period than the reflex evoked by stimulation of the dental pulp) during stimulation of the raphe nuclei and caudal zone of the central gray matter, but was unchanged after stimulation of the points located in the rostral zone of the central gray matter. Application of single stimuli or bursts of five stimuli with a frequency of 100 Hz had no effect on the reflexes studied. Short-term stimulation with a burst of 10–20 stimuli with a following frequency of 200–400 Hz led to inhibition of the reflexes, which lasted 450–1000 msec. Long-term stimulation of the central gray matter and raphe nuclei for 30 sec with a frequency of 50 Hz caused inhibition of jaw opening reflexes evoked by stimulation of both high- and low-threshold afferents for 60 min. Impulses from the central gray matter and raphe nuclei thus have a mainly inhibitory action on the jaw opening reflex evoked by stimulation of high-threshold afferents, but they act less effectively on the reflex evoked by stimulation of low-thres-hold afferents. The duration of inhibition depends essentially on the parameters of stimulation.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 16, No. 3, pp. 374–387, May–June, 1984.     

Post-tetanic potentiation of reciprocal Ia inhibition in human lower limb.

The purpose of this study was to investigate how reciprocal Ia inhibition is changed during muscle fatigue of lower limb muscle, induced with a voluntary contraction or height frequency electrical stimulation. Reciprocal Ia inhibition from ankle flexors to extensors has been investigated in 12 healthy subjects. Hoffmann reflex (H-reflex) in the soleus muscle was used to monitor changes in the amount of reciprocal Ia inhibition from common peroneal nerve as demonstrated during voluntary dorsi or planterflexion and 50 Hz electrical stimulation induced dorsi or planterflexion. The test soleus H-reflex was kept at 20-25% of maximum directly evoked motor response (M response) and the strength of the conditioning common peroneal nerve stimulation was kept at 1.0 x motor threshold. At rest, weak la inhibition was demonstrated in 12 subjects, maximal inhibition from the common peroneal nerve was 28.8%. During voluntary dorsiflexion and 50 Hz electrical stimulation induced dorsiflexion, there absolute amounts of inhibition increased as compared to at rest, and decreased or disappeared during voluntary planterflexion and 50 Hz electrical stimulation induced planterflexion as compared to at rest. During voluntary or electrical stimulation induced agonist muscle fatigue, the inhibition of the soleus H-reflex from the common peroneal nerve was greater during voluntary dorsiflexion (maximal, 11.1%) and 50 Hz (maximal, 6.7%) electrical stimulation induced dorsiflexion than at rest. The inhibition was decreased or disappeared during voluntary planterflexion 50 Hz electrical stimulation induced planterflexion. It was concluded that the results were considered to support the hypothesis that alpha-motoneurones and la inhibitory intemeurones link to antagonist motoneurones in reciprocal inhibition. The diminished reciprocal Ia inhibition of voluntary contraction during muscle fatigue as compared to electrical stimulation, is discussed in relation to its possible contribution to ankle stability.