On the role of NK-2 tachykinin receptors in the mediation of spinal reflex excitability in the rat.

The effects of intrathecal administration of neurokinin A, substance P and [Tyr5, D-Trp6,8,9 Arg10]neurokinin A-(4-10) (Men 10207), a specific NK-2 receptor antagonist, on the spinal nociceptive flexor reflex were studied in decerebrate, spinalized, unanesthetized rats. Intrathecal neurokinin A and substance P facilitate the flexor reflex in a similar manner. The reflex facilitation to intrathecal neurokinin A, but not substance P, is dose-dependently blocked by pretreatment with Men 10207. The NK-2 receptor antagonist by itself facilitates the flexor reflex with a potency about 10 times less than that of neurokinin A, indicating a partial agonistic property. Reversible depression of the flexor reflex, which is not due to nonspecific spinal blockade, is observed after 700 pmol Men 10207. Further increasing the dose of Men 10207 to 7 nmol for 20 s at an intensity that activates unmyelinated (C) fibers stimulation of peripheral nerves at 1 Hz for 20 s at an intensity that activates unmyelinated (C) fibers facilitates the ipsilateral flexor reflex. The duration of the facilitation after conditioning stimulation of the cutaneous sural nerve is several minutes and about 1 h after conditioning stimulation of the gastrocnemius muscle nerves. Pretreatment with Men 10207 (70-700 pmol) has no effect on facilitation by the sural nerve conditioning stimulation, but effectively blocks the long-term reflex facilitation to the gastrocnemius nerve stimulation. The present results indicate a distinct role for NK-2 tachykinin receptors in mediation of spinal reflex excitability in the rat. Neurokinin A may be involved in the long-term increase of spinal reflex excitability after activation of unmyelinated fibers innervating muscle.     

The effect of intrathecal galanin on the flexor reflex in rat: increased depression after sciatic nerve section

The effect of intrathecal galanin (GAL) on the hamstring flexor reflex to sural nerve stimulation was compared in rats with intact and unilaterally sectioned sciatic nerves. GAL had a biphasic effect on the flexor reflex in rats with intact nerves, including facilitation, facilitation followed by depression and pure depression, in a dose-dependent manner. In axotomized rats, the depressive effect of GAL was significantly increased, occurring at lower drug concentrations. Furthermore, the degree of depression was significantly stronger with a more rapid onset after nerve section. It is concluded that along with an increase in GAL-like immunoreactivity in primary afferents, the inhibitory function of this neuropeptide is enhanced following axotomy. This functional change may be due to both pre- and postsynaptic mechanisms.     

Spinal substance P and N-methyl-D-aspartate receptors are coactivated in the induction of central sensitization of the nociceptive flexor reflex.

We have studied the effects and interactions of the neurokinin-1 receptor antagonist CP-96,345 and the N-methyl-D-aspartate receptor/channel blocker MK-801, both applied intravenously, on the flexor reflex and on the facilitation of the flexor reflex by conditioning stimulation of cutaneous C-afferents in decerebrate, spinalized, unanesthetized rats. The flexor reflex was evoked by subcutaneous electrical stimuli applied to the sural nerve innervation area 1/min at an intensity that activated C-fibers and was recorded as electromyogram from the ipsilateral hamstring muscles. The magnitude of the baseline flexor reflex was usually highly stable in the course of the experiments without experimental manipulations. The same stimulus was used as a conditioning train (0.9 Hz, 20 shocks) and caused a brief facilitation of the flexor reflex, which was maximal 0.5 and 1 min after stimulation (255.1 +/- 23.6% over baseline). During the course of the conditioning stimulus train, the reflex magnitude was gradually increased (wind-up). MK-801 (0.1 and 0.5 mg/kg) consistently depressed the polysynaptic flexor reflex. At a dose of 0.5 mg/kg, but not 0.1 mg/kg, MK-801 reduced the wind-up and blocked the facilitation of the flexor reflex induced by the conditioning stimulus by 90%. The facilitatory effect of 7 pmol intrathecal substance P was also partially reduced by MK-801. CP 96,345 (1 and 3 mg/kg) did not depress the flexor reflex, but dose-dependently antagonized reflex facilitation by the conditioning stimulus train, similarly to its antagonism of intrathecally applied 7 pmol substance P-induced facilitation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Spinal motor actions of the μ-opioid receptor agonist DAMGO in the cat

For further evaluation of opioidergic spinal motor functions the action of the μ-opioid receptor agonist DAMGO was tested on transmission in different non-nociceptive and nociceptive spinal reflex pathways from flexor reflex afferents (FRA), and in non-FRA reflex pathways in spinal cats. The action of DAMGO was complex, not following a simple pattern with selective depression of nociceptive pathways compared to non-nociceptive ones. Monosynaptic reflexes of the flexor posterior biceps semitendinosus (PBSt) and transmission in nociceptive as well as non-nociceptive excitatory FRA pathways to PBSt were depressed, while the specific excitatory nociceptive non-FRA pathway from the central foot pad to foot extensors was mainly not depressed but rather facilitated by DAMGO. DAMGO caused a facilitation of monosynaptic reflexes to the extensor gastrocnemius soleus (GS) and partly a reversal of inhibitory to excitatory conditioning effects from cutaneous afferents to GS. FRA interneurones could show either an increase or a cessation of their spontaneous activity, but responsiveness to nociceptive and non-nociceptive afferent activation was blocked by DAMGO. The main DAMGO action is generated via interneuronal systems rather than on motoneurones themselves. The results indicate that opioidergic spinal functions are extensively involved in spinal motor control exceeding a mere suppression of nociceptive motor withdrawal reactions.     

Intramuscular localization of monosynaptic Ia reflex effects in the cat biceps femoris muscle

Intracellular recordings from biceps femoris (BF) motoneurons were made in anesthetized low spinal cats during periods of electrical stimulation of the nerve branches supplying the anterior, middle and posterior portions of the BF muscle and the nerves to semimembranosus and semitendinosus. Measurements were made of each cell's composite intrahomonymous and heteronymous monosynaptic Ia-EPSP responses to stimulation of the test nerves (branches). We have found evidence for an intramuscular localization of these monosynaptic Ia reflex effects not only when comparing responses between the two functional components of the BF muscle as is well established [6] but, in addition, when comparing responses between different parts of each functional (hip extensor and knee flexor) component as well. It is argued that both somatotopic and neuronal recognition factors may contribute to the localization of these monosynaptic reflex effects.     

Interaction between peripheral afferent activity and presynaptic inhibition of ia afferents in the cat

It has been demonstrated in man that the H-reflex is more depressed by presynaptic inhibition than the stretch reflex. Here we investigated this finding further in the alpha-chloralose-anesthetized cat. Soleus monosynaptic reflexes were evoked by electrical stimulation of the tibial nerve or by stretch of the triceps surae muscle. Conditioning stimulation of the posterior biceps and semitendinosus nerve (PBSt) produced a significantly stronger depression of the electrically than the mechanically evoked reflexes. The depression of the reflexes has been shown to be caused by presynaptic inhibition of triceps surae Ia afferents. We investigated the hypothesis that repetitive activation of peripheral afferents may reduce their sensitivity to presynaptic inhibition. In triceps surae motoneurones, we measured the effect of presynaptic inhibition on excitatory postsynaptic potentials (EPSPs) produced by repetitive activation of the peripheral afferents or by fast and slow muscle stretch. EPSPs evoked by single electrical stimulation of the tibial nerve or by fast muscle stretch were significantly depressed by PBSt stimulation. However, the last EPSP in a series of EPSPs evoked by a train of electrical stimuli (5-6 shocks, 150-200 Hz) was significantly less depressed by the conditioning stimulation than the first EPSP. In addition, the last part of the long-lasting EPSPs evoked by a slow muscle stretch was also less depressed than the first part. A single EPSP evoked by stimulation of the medial gastrocnemius nerve was less depressed when preceded by a train of stimuli applied to the same nerve than when the same train of stimuli was applied to a synergistic nerve. The decreased sensitivity of the test EPSP to presynaptic inhibition was maximal when it was evoked within 20 ms after the train of EPSPs. It was not observed at intervals longer than 30 ms. These findings suggest that afferent activity may decrease the efficiency of presynaptic inhibition. We propose that the described interaction between afferent nerve activity and presynaptic inhibition may partly explain why electrically and mechanically evoked reflexes are differently sensitive to presynaptic inhibition.     

Tachykinins mediate changes in spinal reflexes after activation of unmyelinated muscle afferents in the rat.

The effect of intrathecally applied tachykinin antagonist D-NicLys1, 3-Pal3, D-Cl2Phe5, Asn6, D-Trp7.9, Nle11-substance P, spantide II, on the long-term increase of spinal cord excitability after activation of unmyelinated muscle afferents was studied in decerebrate, spinalized, unanaesthetized rats. A conditioning stimulus train (1 Hz, 20 s) that activated unmyelinated fibres in the gastrocnemius muscle nerve facilitated the flexor reflex for about 1 h, which was strongly blocked by pretreatment with spantide II (3 micrograms). The present results indicate that the facilitation of the flexor reflex by conditioning stimulation of a muscle nerve is mediated by tachykinins and possibly other neuropeptides which may be released from the central terminals of these unmyelinated afferents.     

Only minor spinal motor reflex effects from feline group IV muscle nociceptors

The contribution of group III and IV muscle nociceptors activated by injection of KCl or bradykinin into the muscle artery (i.a.) of the gastrocnemius-soleus muscle to spinal motor reflex pathways was investigated in high spinal cats. Group I-III fibres were completely blocked by TTX, leaving group IV-fibre conduction intact. Thus, effects from i.a. KCl or bradykinin injection persisting after TTX were attributed to TTX resistant group IV fibres while the contribution of group III fibres was approximately defined by the difference between those effects and the control effects before TTX. Confirming former findings the chemical activation of group III and IV muscle afferents induced distinct reflex facilitation of the flexor posterior biceps semitendinosus and inhibition of the extensor quadriceps. After the block of all myelinated fibres by TTX the same stimuli induced only minor reflex effects mediated by the persistently conducting TTX resistant group IV afferents. It is concluded that the main functional meaning of group IV muscle afferents, which respond preferentially with a higher threshold to mechanical stimuli, is probably less related to reflex motor control than that of group III afferents.     

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.     

Sensitivity of monosynaptic test reflexes to facilitation and inhibition as a function of the test reflex size: a study in man and the cat

Summary In parallel experiments on humans and in the cat it was investigated how the sensitivity of monosynaptic test reflexes to facilitation and inhibition varies as a function of the size of the control test reflex itself. In man the monosynaptic reflex (the Hoffmann reflex) was evoked in either the soleus muscle (by stimulation of the tibial nerve) or the quadriceps muscle (by stimulation of the femoral nerve). In the decerebrate cat monosynaptic reflexes were recorded from the nerves to soleus and medial gastrocnemius muscles; they were evoked by stimulation of the proximal ends of the sectioned L7 and S1 dorsal roots. Various excitatory and inhibitory spinal reflex pathways were used for conditioning the test reflexes (e.g. monosynaptic Ia excitation, disynaptic reciprocal inhibition, cutaneous inhibition, recurrent inhibition, presynaptic inhibition of the Ia fibres mediating the test reflex). It was shown that the additional number of motoneurones recruited in a monosynaptic test reflex by a constant excitatory conditioning stimulus was very much dependent on the size of the test reflex itself. This dependency had the same characteristic pattern whatever the conditioning stimulus. With increasing size of the test reflex the number of additionally recruited motoneurones first increased, then reached a peak (or plateau) and finally decreased. A similar relation was also seen with inhibitory conditioning stimuli. The basic physiological factors responsible for these findings are discussed. Finally, the implications for the interpretation of experiments in man with the H-reflex technique are considered.     

Organization of recurrent inhibition and facilitation in motoneuron pools innervating dorsiflexors of the cat hindlimb

 The incidence of recurrent inhibition and facilitation in motor nuclei innervating the dorsiflexors of the ankle and digits was examined in spinalized, decerebrate cats. Motoneurons innervating the anterior and posterior portions of the tibialis anterior (TAa and TAp, respectively) received strong recurrent inhibition following stimulation of either of the homonymous muscle nerves. Both motoneuron species received substantial recurrent inhibition from the semitendinosus (St), but stimulation of the nerve to the extensor digitorum longus (EDL), an ankle flexor synergist, evoked smaller recurrent IPSPs. TA motoneurons received mainly facilitation from hindlimb extensors of the hip and ankle. Motoneurons of the EDL and extensor digitorum brevis (EDB), synergists which share mechanical action at the metatarsophalangeal joint and the digits, received little recurrent inhibition in response to stimulation of the nerve to either muscle. Overall, stimulation of heteronymous flexor nerves (including TAa, TAp, and St) failed to evoke responses in most of the EDB and EDL neurons tested (50–83%), and the amplitude of recurrent inhibitory responses was small. Recurrent facilitation from the extensors was more common in these motor nuclei. Most responses recorded in EDB motoneurons following either flexor or extensor nerve stimulation were recurrent facilitations. The sensitivity of this facilitation in EDB motoneurons to injection of polarizing current and its central latency indicate that it is mediated by a disinhibitory, trisynaptic pathway. Stimulation of the nerve to EDB produced recurrent IPSPs in some flexor motoneurons, but these potentials were infrequent and their amplitude was usually small. Based on a comparison of the distribution of recurrent inhibition to published reports of the activities of TAa, TAp, EDL, and EDB during different forms of locomotion, we conclude that recurrent inhibition is large for motor nuclei that exhibit stereotypical activity, while motor nuclei that are activated independently receive and produce little recurrent inhibition. Despite the absence of recurrent inhibition in some motor nuclei, recurrent circuits may still participate in their control through disinhibitory, facilitatory mechanisms. Received: 18 November 1997 / Accepted: 23 November 1998     

Differential synaptic effects on physiological flexor hindlimb motoneurons from cutaneous nerve inputs in spinal cat.

1. We previously demonstrated in the spinal cat that superficial peroneal cutaneous nerve stimulation produced strong reflex contraction in tibialis anterior (TA) and semitendinosus (St) muscles but unexpectedly produced mixed effects in another physiological flexor muscle, extensor digitorum longus (EDL). The goal of the present study was to further characterize the organization of ipsilateral cutaneous reflexes by examining the postsynaptic potentials (PSPs) produced in St, TA, and EDL motoneurons by superficial peroneal and saphenous nerve stimulation in decerebrate, spinal cats. 2. In TA and St motoneurons, low-intensity cutaneous nerve stimulation that activated only large (A alpha) fibers [i.e., approximately 2-3 times threshold (T)], typically produced biphasic PSPs consisting of an initial excitatory phase and subsequent inhibitory phase (EPSP, IPSP). Increasing the stimulus intensity to activate both large (A alpha) and small (A delta) myelinated cutaneous fibers supramaximally (15-45 T) tended to enhance later excitatory components in TA and St motoneurons. 3. In EDL motoneurons, 2-3 T stimulation of the superficial peroneal nerve evoked initial inhibition (of variable magnitude) in 7/10 EDL motoneurons tested, with either excitation (n = 2) or mixed effects (n = 1) observed in the remaining EDL motoneurons. Saphenous nerve stimuli produced excitation either alone, or preceded by an inhibitory phase in EDL. Increasing the stimulus intensity enhanced later inhibitory influences from superficial peroneal and excitatory influences both from superficial peroneal and saphenous nerve inputs in EDL motoneurons. 4. Short-latency (less than 1.8 ms) EPSPs were observed in a few motoneurons in all reflex pathways examined, except for EPSPs in EDL motoneurons evoked by saphenous stimulation. IPSPs with central latencies less than 1.8 ms were also produced by both saphenous (TA, n = 1; EDL, n = 2) and superficial peroneal (EDL, n = 4) nerve stimulation. 5. The results, in comparison with other reports employing spinal and nonspinal preparations, suggest that removal of influences from higher centers reveals inhibitory circuits from the superficial peroneal and saphenous nerves to EDL motoneurons in the spinal preparation. The inhibitory inputs observed are thought to reflect the activation of "specialized" reflex pathways. Additionally, the demonstration of short-latency EPSPs and IPSPs suggest that the minimal linkage in both the excitatory and inhibitory cutaneous reflex pathways examined is disynaptic. The results are discussed in relation to previous studies on classically conditioned flexion reflex facilitation in spinal cat.     

Contribution of TTX-resistant C-fibres and Adelta-fibres to nociceptive flexor-reflex and non-flexor-reflex pathways in cats

The contribution of Adelta-fibres and C-fibres activated by noxious heat stimulation of the central pad of the foot to nociceptive spinal flexor reflex pathways (FRA-type) and to nociceptive excitatory reflex pathways to foot extensors (non-FRA type) was investigated in high spinal cats. A-fibres were completely blocked by tetrodotoxin (TTX), leaving C-fibre conduction intact. Thus, effects persisting after TTX were attributed to nociceptive C-fibres while the contribution of nociceptive Adelta-fibres was defined by the difference between those effects and the control effects before TTX. The initial action of noxious stimulation on both types of reflex action was mediated predominantly by Adelta-fibres, while the later action was mainly mediated by C-fibres. In two (out of seven) experiments Adelta-fibres exerted a significant inhibitory influence on the C-fibre action in FRA pathways, but such an inhibitory interaction between the two fibre groups was absent in the non-FRA reflex pathways. The technique of TTX application at the peripheral nerve proved to be a reliable method for a long-lasting selective investigation of C-fibre effects. The results revealed that both Adelta- and C-fibres contributed to nociceptive FRA and non-FRA reflex pathways.     

Study of cutaneous reflex compensation during locomotion after nerve section in the cat

In the cat, section of all cutaneous nerves of the hindfeet except the tibial (Tib) nerve supplying the plantar surface results in a long-lasting decrease in the intensity of Tib stimulation needed for a threshold response in flexor muscles and an increase in the amplitude of the phase-dependent responses recorded in various muscles during locomotion. Stimulating through chronically implanted nerve cuffs ensured a stable stimulation over time. The increase in reflex amplitude was well above the small increase in the amplitude of the locomotor bursts themselves that results from the denervation. Short latency responses (P1) were seen in flexor muscles, especially at the knee (semitendinosus) and ankle (tibialis anterior and extensor digitorum longus), with stimuli applied in the swing phase and also to a lesser degree in the later part of the cycle. Longer latency responses (P2) were increased in hip, knee, and ankle flexors, as well as in a contralateral extensor (vastus lateralis) when applied in late stance. Responses evoked from stimulating the proximal end of sectioned nerves were not larger than before neurectomy. This suggests that the increased responsiveness to Tib stimulation is not simply caused by an increase in motoneuron excitability, because this would have resulted in a nonspecific increase of responses to stimulation of any nerve. It is concluded that the adult locomotor system is capable of central reorganization to enhance specific remaining cutaneous reflex pathways after a partial cutaneous denervation of the paw.     

Tachykinns mediate changes in spinal reflexes after activation of unmyelinated muscle afferents in the rat

The effect of intrathecally applied tachykinin antagonist D-NicLys¹, 3-Pal³, D-Cl² Phe⁵, Ama⁶, Trp^7,8, Nle¹¹-substance P, spantide II, on the long-term increase of spinal cord excitability after activation of unmyelinated muscle afferents was studied in decerebrate, spinalized, unanaesthetized rats. A conditioning stimulus train (1 Hz, 20 s) that activated unmyelinated fibres in the gastrocnemius muscle nerve facilitated the flexor reflex for about 1 h, which was strongly blocked by pretreatment with spantide II (3 µg). The present results indicate that the facilitation of the flexor reflex by conditioning stimulation of a muscle nerve is mediated by tachykinins and possibly other neuropeptides which may be released from the central terminals of these unmyelinated afferents.     

Matching of facilitation at the neuromuscular junction of the lobster: a possible case for influence of muscle on nerve

1. The facilitation of neuromuscular transmission, which occurs during repetitive activation, was examined in the proximal accessory flexor muscle in walking legs of the lobster using electrophysiological techniques.2. Post-synaptic potentials (p.s.p.s) in different muscle fibres facilitated to markedly different degrees. P.s.p.s in some fibres did not facilitate at all, while in others they increased in size by 20-30 times during stimulation at 20 Hz even though all the excitatory neuromuscular synapses are made by a single axon.3. Stimulation of widely separated groups of synapses on any single muscle fibre evoked p.s.p.s with closely matched facilitation properties. Extracellular p.s.p.s recorded from single synaptic spots showed the same characteristics of facilitation as those of intracellular p.s.p.s in the same muscle fibre, suggesting that individual synaptic contacts on any single fibre are similar to each other.4. Facilitation can be accounted for by an increase in the number of quanta released from the nerve terminals. There is no evidence for an increase in post-synaptic membrane sensitivity.5. Low Ca solutions reduce transmitter release with comparatively little change in facilitation, while Cs solutions increase the size of p.s.p.s without increasing the amplitude of spontaneous miniature potentials. Thus, at poorly facilitating synapses it is unlikely that the absence of facilitation is caused by the saturation of some post-synaptic process.6. It is concluded that the excitatory presynaptic nerve terminals on a single muscle fibre have matching facilitation characteristics. Some interaction between individual muscle fibres and their associated nerve endings may be required to establish or maintain this matching.     

Motor cortical modulation of cutaneous reflex responses in the hindlimb of the intact cat

We have used the technique of spatial facilitation to examine the interactions between the signals conveyed by the corticospinal tract and those of cutaneous afferents in the hindlimb of the intact, walking cat. Microstimulation was applied to 20 cortical sites in the hindlimb representation of the motor cortex and to three different cutaneous nerves innervating the hindpaw in four cats. Conditioning stimuli to the motor cortex induced both facilitation and depression of cutaneous reflexes evoked by stimulation of nerves in the hindlimb contralateral to the cortical stimulation site. Facilitation was most frequently evoked by conditioning stimuli in the range of 10-30 ms before the cutaneous stimulation; depression was normally evoked by shorter and longer conditioning delays. Similar changes were observed after conditioning stimuli to the pyramidal tract, suggesting that the changes were independent of any changes in cortical excitability. Modulation of reflex activity varied according to the muscle under study, the cutaneous nerve used to evoke the reflex and the cortical site used to condition the reflex. Together, these results suggest that there is spatial convergence of corticospinal and cutaneous afferent activity and that this convergence is mediated by distinct subpopulations of spinal interneurons.     

Parallel reflex pathways from flexor muscle afferents evoking resetting and flexion enhancement during fictive locomotion and scratch in the cat

Reflex actions of muscle afferents in hindlimb flexor nerves were examined on ipsilateral motoneurone activity recorded in peripheral nerves during midbrain stimulation-evoked fictive locomotion and during fictive scratch in decerebrate cats. Trains of stimuli (15–30 shocks at 200 Hz) were delivered during the flexion phase at intensities sufficient to activate both group I and II afferents (5 times threshold, T ). In many preparations tibialis anterior (TA) nerve stimulation terminated ongoing flexion and reset the locomotor cycle to extension (19/31 experiments) while extensor digitorum longus (EDL) stimulation increased and prolonged the ongoing flexor phase activity (20/33 preparations). The effects of sartorius, iliopsoas and peroneus longus muscle afferent stimulation were qualitatively similar to those of EDL nerve. Resetting to extension was seen only with higher intensity stimulation (5 T ) while ongoing flexor activity was often enhanced at group I intensity (2 T ) stimulation. The effects of flexor nerve stimulation were qualitatively similar during fictive scratch. Reflex reversals were consistently observed in some fictive locomotor preparations. In those cases, EDL stimulation produced a resetting to extension and TA stimulation prolonged the ongoing flexion phase. Occasionally reflex reversals occurred spontaneously during only one of several stimulus presentations. The variable and opposite actions of flexor afferents on the locomotor step cycle indicate the existence of parallel spinal reflex pathways. A hypothetical organization of reflex pathways from flexor muscle afferents to the spinal pattern generator networks with competing actions of group I and group II afferents on the flexor and extensor portions of this central circuitry is proposed.     

Synaptic connections from large afferents of wrist flexor and extensor muscles to synergistic motoneurones in man

 Short-latency excitatory Ia reflex connections were determined between pairs of human wrist flexor and extensor muscles. Spindle Ia afferents were stimulated by either tendon tap or electrical stimulation. The activity of voluntarily activated single motor units was recorded intramuscularly from pairs of wrist flexor or extensor muscles. Cross-correlation between stimuli and the discharge of the motor units provided a measure of the homonymous or heteronymous excitatory input to a motoneurone. Homonymous motoneurone facilitation was generally stronger than that of the heteronymous motoneurones. The principal wrist flexors, flexor carpi radialis (FCR) and flexor carpi ulnaris (FCU), were tightly connected through a bidirectional short-latency reflex pathway. In contrast, the extensor carpi ulnaris (ECU) and the extensor carpi radialis (ECR) did not have similar connections. ECU motoneurones received no short-latency excitatory Ia input from the ECR. ECR motoneurones did receive excitatory Ia input from ECU Ia afferents; however, its latency was delayed by several milliseconds compared with other heteronymous Ia excitatory effects observed. The wrist and finger extensors were linked through heteronymous Ia excitatory reflexes. The reflex connections observed in humans are largely similar to those observed in the cat, with the exception of heteronymous effects from the ECU to the ECR and from the extensor digitorum communis (EDC) to the ECU, which are present only in humans. The differences in the reflex organization of the wrist flexors versus the extensors probably reflects the importance of grasping. Received: 19 August 1996 / Accepted: 6 March 1997