Effect of the tendon reflex on motoneurons of the antagonist muscle in man

The knee jerk was elicited during regular firing of relatively low-threshold motor units of the biceps femoris muscle (during weak voluntary contraction). Besides the reflex response of the rectus femoris muscle, synchronous discharges of motor units of the biceps femoris muscle and activation of new motor units also were observed. Poststimulus histograms and statistical analysis of interspike intervals of motor units of the biceps femoris muscle revealed well-marked excitatory influences synchronous with the reflex response of the rectus femoris. This result can be explained by the presence of excitatory inputs of Ia afferents on motoneurons of the antagonist muscle. In the knee jerk, excitation of motoneurons of the antagonist was followed by later inhibitory influences which evidently correspond to the "silent period" of motoneurons of the agonist muscle during the elicitation of its tendon reflex.Institute for Problems of Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 8, No. 6, pp. 624–632, November–December, 1976.     

Corticospinal control of soleus motoneurons in man

Electrical or magnetic stimulation of the human motor cortex causes a strong, short latency facilitation of tibialis anterior (TA) motoneurons but only weak, longer latency changes in the excitability of soleus (SOL) motoneurons. The facilitation of TA motoneurons has been attributed to the monosynaptic action of the "fast" corticospinal pathway. The present study further investigates the cortical control of soleus motoneurons in man. In tests of reaction time to auditory stimuli, normal subjects took significantly longer to activate soleus motoneurons than tibialis anterior motoneurons. Thus we could not demonstrate the existence of a "fast" pathway from the brain to SOL motoneurons that, for some reason, is not activated by magnetic stimulation. The hypothesis that the cortex might control soleus motoneurons indirectly by modulation of the Ia input from muscle spindles was tested. Magnetic stimulation of the cortex was used to condition the facilitation of soleus motoneurons resulting from the stimulation of group I fibres in the tibial nerve. There were no consistent changes in Ia facilitation. We conclude (i) that there is no evidence so far that SOL motoneurons are excited by a direct pathway from the cortex (similar to that projecting to TA motoneurons) and (ii) that the observed changes in firing probability of soleus motoneurons produced by magnetic stimulation over the motor cortex do not result from modulation of presynaptic inhibition of Ia afferents.     

Modulation of the human nociceptive reflex by cyclic movements

During static conditions the nociceptive reflex is known to vary as a function of, for example, the stimulus position, stimulus intensity, and muscle contraction. The aim of the present human study was to investigate whether the reflex and the corresponding perception of pain are modulated by cyclic movements of the limb involved. Reflexes, evoked by nociceptive electric stimulation of the sural nerve, were recorded from the biceps femoris and the rectus femoris muscles in eight volunteers. Four different experiments were performed to compare the nociceptive reflex and pain score elicited during active isometric/dynamic flexion/extension of the knee joint. The amplitudes of the reflexes were largest for the dynamic conditions. The reflexes, evoked during dynamic extension and isometric contraction of the rectus femoris muscle, had the shortest latencies but the recordings from the biceps femoris muscle were larger than from the rectus femoris muscle. Knee joint angle recordings showed that the largest angle variations occurred for the dynamic conditions and were only marginally disturbed for the isometric conditions. A given stimulus intensity evoked the highest pain intensity during isometric contractions. This indicates that there would seem to be no causal relationship between the size of the nociceptive reflex and the pain intensity.     

Detection and analysis of recurrent inhibition of firing motoneurons in man

During regular firing of "small" motor units, activated during weak voluntary contraction of the human soleus muscle, thick efferent fibers of n. tibialis were stimulated (a small M response was evoked, in which the small units did not participate). Peristimulus histograms of potentials of single motor units were constructed and the effect of stimulation on interspike interval duration was analyzed. The firing rate of the motor units was 4.5–7.6 spikes/sec. Stimulation of the nerve led to a sharp decrease in probability of their discharge or even complete temporary cessation of firing, i.e., it had a well marked inhibitory effect (lasting 10–20 msec). The latent period of inhibition (35–40 msec) was only a little longer than the latent period of the monosynaptic reflex of the soleus muscle. The effect of an inhibitory volley on duration of the interspike interval of the motor units depended on the time when the volley arrived during the interval. Lengthening of the interval was observed only if the inhibitory volley arrived in the second half or at the end of the interval. It is concluded that inhibition of firing of small motor units is due to Renshaw cells, activated on stimulation of axons of large motoneurons. The efficiency of a short (compared with the duration of the interspike interval) inhibitory volley reaching a motoneuron firing at low frequency characteristic of its adequate activation, is discussed.Institute for Problems of Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 16, No. 1, pp. 88–96, January–February, 1984.     

Synaptic potentials of motoneurons of the masseter muscle

Postsynaptic potentials of 93 motoneurons of the masseter muscle evoked by stimulation of different branches of the trigeminal nerve were studied. Stimulation of the most excitable afferent fibers of the motor nerve of the masseter muscle evoked monosynaptic EPSPs with a latent period of 1.2–2.0 msec, changing into action potentials when the strength of stimulation was increased. A further increase in the strength of stimulation produced an antidromic action potential in the motoneurons with a latent period of 0.9 msec. In some motoneurons polysynaptic EPSPs and action potentials developed following stimulation of the motor nerve to the masseter muscle. The ascending phase of synaptic and antidromic action potentials was subdivided into IS and SD components, while the descending phase ended with definite depolarization and hyperpolarization after-potentials. Stimulation of cutaneous branches of the trigeminal nerve, and also of the motor nerve of the antagonist muscle (digastric) evoked IPSPs with a latent period of 2.7–3.5 msec in motoneurons of the masseter muscle. These results indicate the existence of functional connections between motoneurons of the masseter muscle and its proprioceptive afferent fibers, and also with proprioceptive afferent fibers of the antagonist muscle and cutaneous afferent fibers.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 1, No. 3, pp. 262–268, November–December, 1969.     

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.     

Activity of fusimotor neurons during reflex muscle contraction

The influence of fusimotor activity via the gamma-loop on reflex responses of motoneurons to stretch or vibration stimulation of mm. triceps surae was studied in decerebrate cats. Action potentials of single fusimotor neurons were derived from thin filaments isolated from nerves innervating this muscle group, leaving their main nerve supply intact. Most fusimotor neurons tested were found to be coactivated with motor units during reflex muscle contraction. In the initial period of development of reflex muscle contraction a weak autogenetic inhibitory effect on discharge of fusimotor neurons was found. The results suggest that reduction of the reflex motor signal, leading to a "silent period," is partly the result of a transient decrease in the fusimotor output effect on contracting muscles. A study of changes in fusimotor discharge generation during the ascending phase of reflex muscle contraction may provide data useful for identification of autogenetic reflex influences on these motoneurons and for elucidating the conditions necessary for servoassistance of muscle contractions.Medical Research Institute, Belgrade, Yugoslavia. Translated from Neirofiziologiya, Vol. 16, No. 5, pp. 630–637, September–October, 1984.     

Proprioceptive proximo-distal relationships between the quadriceps and soleus muscles in man

Electrical stimulation of femoral nerve modulates voluntary tonic activity o of ipsilateral soleus muscle. Stimulus time-locked inhibitory and facilitatory phases can be distinguished. EMG temporal analysis suggests that early perturbations are correlated with spinal effects of centripetal electrical activity. The inhibitory effects which momentarily abolish voluntary soleus activity are thought to result from quadriceps Ib fibres recruitment. While no heteronymous activity is induced at rest, femoral nerve Ia fibres activation can produce soleus muscle reflex when soleus motor nucleus excitability is increased by voluntary command. Recurrent discharge resulting from soleus reflex response enhances inhibition initially due to quadriceps Ib volley. Secondary effects of isometric quadriceps contraction (and soleus contraction when the femoral stimulus elicits a reflex in this muscle) have their own effects later. These findings suggest that proprioceptive relationships of the two muscular groups are efficient during tonic isometric voluntary command.     

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.     

Functional coordination of motor activity in colonic smooth muscles in rat experimental model

Spontaneous and electrically-elicited motor activity was recorded by triple organ bath in rat segment-model preparation as display of excitation of local nerve networks and ascending or descending reflex pathways underlying contractile potency and functional coordination of colonic longitudinal and circular muscles. Spontaneous high-amplitude contractions, but not relaxations, appeared synchronously in both muscles. Electrical field stimulation applied to proximal or distal part of segments elicited both tetrodotoxin (0.1 microM)-sensitive local motor responses of the stimulated part and ascending or descending motor responses of the contralateral, nonstimulated part of the preparations. Contractions characterized the local response of longitudinal muscle. The circular muscle responded with relaxation followed by contraction. Synchronous ascending contractions and descending contraction of the longitudinal muscle and relaxation followed by contraction of the circular muscle were observed when the middle part of segments was stimulated, thus indicating that locally-induced nerve excitation propagated via intrinsic ascending or descending nerve pathways that could be synchronously coactivated by one and the same stimulus. The ascending motor responses were more pronounced and the motor responses of longitudinal muscle were expressed more than those of circular muscle suggesting an essential role of ascending reflex pathways and longitudinal muscle in the coordinated motor activity of colon.     

Changes in the reflex excitability of the soleus spinal center in humans performing various motor tasks

The reflex excitability of the soleus spinal motoneurons was assessed in healthy subjects performing different types of motor tasks: voluntary contraction of the flexor (dorsal flexion) and extensor (plantar flexion) muscles of the foot. The effect of the contraction strength of these muscles was also evaluated. During dorsal flexion of the ipsi-and contralateral feet, changes in the reflex ecitability of the soleus motoneurons were unidirectional: the excitability decreased. The decrease in the reflex excitability was more profound during dorsal flexion with the maximum strength than with the half-maximum strength. During the plantar flexion of the ipsi-and contralateral feet, the excitability of the soleus motoneurons changed in opposite directions: in some subjects it increased, while in the others it decreased. The reflex excitability of the soleus motoneurons changed to a greater extent during dorsal or plantar flexion of the ipsilateral foot. In the case of plantar flexion, the soleus motor center is possibly affected by a broader spectrum of influences than in the case of dorsal flexion, which can explain the variations in the reflex excitability changes during plantar flexion.     

Capsaicin-sensitive muscle afferents modulate the monosynaptic reflex in response to muscle ischemia and fatigue in the rat

The role of muscle ischemia and fatigue in modulating the monosynaptic reflex was investigated in decerebrate and spinalized rats. Field potentials and fast motoneuron single units in the lateral gastrocnemious (LG) motor pool were evoked by dorsal root stimulation. Muscle ischemia was induced by occluding the LG vascular supply and muscle fatigue by prolonged tetanic electrical stimulation of the LG motor nerve. Under muscle ischemia the monosynaptic reflex was facilitated since the size of the early and late waves of the field potential and the excitability of the motoneuron units increased. This effect was abolished after L3-L6 dorsal rhizotomy, but it was unaffected after L3-L6 ventral rhizotomy. By contrast, the monosynaptic reflex was inhibited by muscle fatiguing stimulation, and this effect did not fully depend on the integrity of the dorsal root. However, when ischemia was combined with repetitive tetanic muscle stimulation the inhibitory effect of fatigue was significantly enhanced. Both the ischemia and fatigue effects were abolished by capsaicin injected into the LG muscle at a dose that blocked a large number of group III and IV muscle afferents. We concluded that muscle ischemia and fatigue activate different groups of muscle afferents that are both sensitive to capsaicin, but enter the spinal cord through different roots. They are responsible for opposite effects, when given separately: facilitation during ischemia and inhibition during fatigue; however, in combination, ischemia enhances the responsiveness of the afferent fibres to fatigue.     

The nature of facilitation of leg muscle motor evoked potentials by knee flexion

Knee flexion is a movement that initiates rising from a sitting position, which is a common therapeutic exercise for patients unable to ambulate. We investigated how voluntary isometric biceps femoris contraction affects motor evoked potential (MEP) amplitude following transcranial magnetic stimulation, background electromyographic (EMG) amplitude, and H-reflex amplitude in ipsilateral leg muscles. Subjects were seated on the edge of a bed with their hips and knees flexed at 90°, and the soles of their feet on the floor. MEP and background EMG were recorded from the tibialis anterior (TA) and soleus (SOL), and H reflexes from SOL of 30 volunteers. Background EMG and MEP also were recorded while voluntarily contracting tested muscles. Biceps femoris contraction increased MEP and background EMG for TA and SOL ( p < 0.01). Maximal background EMG and MEP increased with increasing voluntary contraction of tested muscles ( p < 0.005). Regression slope differed little between TA and SOL. Biceps femoris contraction facilitated MEP comparably for TA and SOL, while SOL background EMG exceeded that of TA ( p < 0.02). The relationship between MEP facilitation and background EMG changed to favor more efficient facilitation in TA ( p < 0.05), but not SOL ( p > 0.1). MEP recorded from TA and SOL with subthreshold stimuli using needle electrodes were more frequent with biceps femoris contraction ( p < 0.04). H-reflex amplitude of SOL decreased during biceps femoris contraction ( p < 0.001). We concluded that biceps femoris contraction affects leg muscle MEP, background EMG, and H reflexes differently.     

Long-lasting inhibition of the soleus muscle H reflex related to realization of voluntary arm movements in humans

In healthy humans, we recorded the H reflex induced by transcutaneous stimulation of the tibial nerve (recording from the soleus muscle). In subjects in the lying position, we studied changes in the H reflex values after preceding voluntary arm movements realized with a maximum velocity after presentation of an acoustic signal. On the 200th to 300th msec after forearm flexion, long-lasting inhibition of the H reflex developed following a period of initial facilitation and reached the maximum, on average, 700 msec from the moment of the movement. Flexion of the contralateral upper limb in the elbow joint induced deeper inhibition than analogous movement of the ipsilateral arm. Long-lasting clear inhibition of the H reflex developed after arm flexion in the elbow joint but was slightly expressed after finger clenching. After inhibition reached the maximum, its time course was satisfactorily approximated by a logarithmic function of the time interval between the beginning of the conditioning voluntary movement and presentation of the test stimulus. Durations of inhibition calculated using a regression equation were equal to 6.6 sec and 8.5 sec after ipsilateral and contralateral elbow-joint flexions, respectively. Inhibition was not eliminated under conditions of tonic excitation of motoneurons of the tested muscle upon voluntary foot flexion. Long-lasting inhibition of the H reflex was also observed after electrical stimulation-induced flexions of the upper limb. The obtained data indicate that movements of the upper limb cause reflex long-lasting presynaptic inhibition of the soleus-muscle H reflex that can play a noticeable role in redistribution of the muscle tone during motor activity. Neirofiziologiya/Neurophysiology, Vol. 40, No. 3, pp. 221–227, May–June, 2008.     

Isometric training of young rats — Effects upon hind limb muscles

The soleus, rectus femoris, and gastrocnemius muscles of young rats trained isometrically for 4 weeks were studied by light and electron microscopy.--The percentage of fast-twitch oxidative muscle fibers decreased at the cost of the fast-twitch glycolytic fibers in the rectus femoris muscle. The percentages of the slow-twitch oxidative fibers did not change significantly in any of the muscles studied. The changes in the areas of the muscle fibers were specific for the muscle and the fiber type and indicate geometrical rearrangements of the fibers in the trained muscles. The Z and M lines were broader in the soleus (containing about 85% slow-twitch oxidative fibers) than in the rectus femoris muscle (containing about 90% fast-twitch glycolytic fibers), while the sarcomere length and the pseudo-H zone were similar. The length of the myosin filaments appeared to be slightly shorter in the fast rectus femoris than in the slow soleus muscle.--The hypothesis on the temporal progress of muscle adaptation to training (Müller, 1974) was substantiated. Correlations between biochemical (Exner et al., 1973a) and histochemical parameters measuring the oxidative capacity were preserved during adaptation to training. The comparison of the histochemical results with the physiological data on similar animals (Exner et al., 1973a) suggests a complex relationship between the contraction time and the percentage of fast-twitch muscle fibers.     

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.     

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.     

Muscular reflexes elicited by electrical stimulation of the anterior cruciate ligament in humans.

Anterior cruciate ligament (ACL)-deficient knees have impaired proprioception, and, although mechanoreceptors have been found in the ACL, the existence of a reflex elicited from these receptors has not been directly demonstrated in humans. In eight patients that underwent knee arthroscopy and had no sign of ACL disease, thin wire electrodes were inserted into the proximal and mid parts of the ACL. Postoperatively, the sensory nerve fibers inside the ACL were stimulated electrically while motor activity in the knee muscles was recorded using electromyography. In seven of the eight patients, a muscular contraction of the semitendinosus muscle could be elicited with stimulus trains consisting of at least two stimuli. The latency was 95 +/- 35 ms. Stimulation during isometric contraction of either extensor or flexor muscles elicited a short, complete inhibition of the muscle activity in the contracting muscles. The latency of the inhibitory responses was 65 +/- 20 ms in the semitendinosus muscle and 70 +/- 15 ms in the rectus femoris muscle.     

Mechanical properties of fast and slow skeletal muscle with special reference to collagen and endurance training

The mechanical properties of the slow soleus and the fast rectus femoris muscle under passive stretching were studied in endurance trained, untrained and lathyritic rats, aged 3 months. The soleus muscle with more abundant and cross-linked collagen had higher ultimate tensile strength and tangent modulus compared to the fast rectus femoris muscle which, on the other hand, had higher maximum strain. The inhibition of collagen cross-linking by lathyrism resulted in decreased tensile strength and stiffness, especially in the soleus muscle, whereas endurance training showed the opposite effects. It is supposed that the properties of collagen partly explain the capacity of slow muscles to maintain posture and to perform prolonged dynamic work. The effects of training on the tensile properties further indicate the close relationship between intramuscular collagen and the endurance capacity of muscles.     

Electrophysiological and psychophysical quantification of temporal summation in the human nociceptive system

Animal experiments have shown that the nociceptive reflex can be used as an indicator of central temporal integration in the nociceptive system. The aim of the present study on humans was to investigate whether the nociceptive reflex, evoked by repetitive strong electrical sural nerve stimuli, increased when summation was reported by the volunteers. The reflexes were recorded from the biceps femoris and rectus femoris muscles in eight volunteers following a series of stimulations at 0.1, 1, 2, and 3 Hz. Each series consisted of five consecutive stimuli. Using 0.1- and 1-Hz stimulation, the reflex was not facilitated in the course of the five consecutive stimuli. Following 2- and 3-Hz stimulation, the reflex size (root mean square amplitude) increased significantly during the course of the fifth stimulus. This reflex facilitation was followed by a significant increase (summation) in the pain magnitude when compared with 1- and 0.1-Hz stimulation. Furthermore, the threshold for psychophysical summation could be determined. This threshold (stimulus intensity) decreased when the stimulus frequency (1–5 Hz) of the five consecutive stimuli was increased. The nociceptive reflex and the psychophysical summation threshold might be used to clarify and quantify aspects of temporal summation within the human nociceptive system.