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PubMed2001

An investigation into mechanisms of reflex reinforcement by the Jendrassik manoeuvre.

Tendon jerk and H-reflexes are both potentiated by the Jendrassik manoeuvre, but the mechanism of potentiation remains uncertain. We investigated several possibilities in human subjects. Evidence for fusimotor activation during the Jendrassik manoeuvre was sought by recording the tendon jerk reflex as surface EMG in triceps surae after the muscles had been conditioned to leave their spindles in a slack, insensitive state. Interposing a Jendrassik manoeuvre between conditioning and the test reflex should have increased reflex amplitude by restoring spindle sensitivity, but this was not the case. In humans, a close synergist of the triceps surae is the quadriceps. A possible presynaptic disinhibitory mechanism was investigated by testing the effect of a Jendrassik manoeuvre on facilitation of the soleus H-reflex produced by a quadriceps afferent volley. The Jendrassik manoeuvre failed to increase facilitation, contrary to what would be expected if it reduced the level of tonic presynaptic inhibition; the assumption being that the inhibition acts on both homonymous and synergist afferent terminals. The Jendrassik manoeuvre did not increase the level of ongoing EMG in the soleus during a weak voluntary contraction, indicating that it does not operate by direct facilitation of motoneurones. There was found to be less potentiation of soleus tendon jerk and H-reflexes by the Jendrassik manoeuvre under conditions when spindles in the soleus were likely to have a high resting discharge rate. A remaining possibility is discussed: that the Jendrassik manoeuvre operates by modulation of oligosynaptic pathways that may contribute to the largely monosynaptic reflex response. These experiments demonstrate, with new, more sensitive methods than previously used, that neither is the fusimotor system involved in reinforcement nor are direct excitatory or presynaptic disinhibitory effects on motoneurones. While this confirms the previously prevailing view, none of the lingering uncertainties associated with the methods used now remains.

Adult↗

PubMed1990

Monosynaptic and dorsal root reflexes during locomotion in normal and thalamic cats.

1. In normal and thalamic walking cats electrical stimulation of muscle nerves via chronically implanted electrodes produced electromyographic (EMG) and neurographic responses that were modulated in amplitude depending on the phase of the step cycle. These responses were examined for possible indications of effects of primary afferent depolarization (PAD) during stepping. 2. Monosynaptic reflexes (MSRs) produced by stimulating the lateral gastrocnemius (LG) and medial gastrocnemius (MG) nerves were recorded as EMGs in MG or LG muscles during treadmill locomotion in normal cats. These heteronymous MSR responses were greatest during the stance (extensor) phase. 3. In the same animals, after decerebration, similar modulation of the heteronymous ankle extensor MSRs occurred during spontaneous locomotion with the use of the same stimulus and recording sites. 4. In both normal and thalamic cats the amplitude of neurogram responses recorded from LG or MG nerve after stimulation of the other muscle nerve varied with phase of stepping but did not parallel the variations of the MSR measured as EMG amplitude in the same muscle. The nerve responses were largest during the flexion phase of the step cycle and had a calculated central latency of 0.6-1.0 ms. These are interpreted as arising from antidromic activity in large-caliber afferent nerve fibers (i.e., dorsal root reflexes). 5. Spontaneous antidromic activity in severed L7 dorsal rootlet fibers to triceps surae was observed in the thalamic cats during episodes of locomotion and was closely correlated with flexion phase EMG activity in semitendinosus, a bifunctional muscle. 6. In decerebrate cats, dorsal root reflexes (DRRs) in severed filaments of L4-L7 dorsal roots were produced by stimulation of saphenous and posterior tibial nerves. These DRRs were always smaller during locomotion than during rest and were smallest during the flexion phase. 7. The short-latency antidromic activity produced in muscle nerves by stimulating heteronymous muscle nerves thus appears to be a DRR produced in Group I terminal arborizations that are depolarized close to threshold during the flexion phase. Such PAD could account for changes in the MSR that do not always parallel the levels of recruitment of the motor pools as manifest by background EMG amplitude.

Animals↗

PubMed1997

Medium-latency stretch reflexes of foot and leg muscles analysed by cooling the lower limb in standing humans.

1. In standing subjects, an ankle-dorsiflexing perturbation of the supporting surface evokes a short-latency response (SLR) and a medium-latency response (MLR) to stretch in both soleus (Sol) and flexor digitorum brevis (FDB) muscles. The SLR is the counterpart of the monosynaptic reflex, whilst the MLR might be either mediated by Ia fibres, the delay being due to a long-loop central circuit, or by fibres of slower conduction velocity. Since small afferents are slowed more than large ones by low temperature, a greater latency increment for the MLR than the SLR induced by cooling of the limb would point to a peripheral origin of the MLR. 2. In nine subjects, one limb was cooled by circulating water in a tube wrapped around it for about 120 min. Perturbations were delivered to the same limb prior to and during cooling, and after rewarming. EMG was recorded by surface electrodes from the Sol and FDB muscles. 3. The mean increase in latency of MLRs was significantly greater than that of SLRs in both muscles. On average, the Sol SLR increased from 42.4 to 47.0 ms and the Sol MLR from 72.0 to 82.3 ms. The FDB SLR increased from 58.1 to 66.5 ms and the FDB MLR from 94.9 to 110.5 ms. The mean difference (MLR minus SLR) increased from 29.6 to 35.2 ms for Sol, and from 36.8 to 43.9 ms for FDB at the end of cooling. After 30 min of rewarming, the responses of both muscles recovered towards control values. 4. The greater latency increment of the MLRs than of the SLRs favours the hypothesis of a slower conduction velocity of the responsible afferent fibres. The most likely candidate fibres are the spindle group II afferents.

Adult↗

PubMed1988

Do muscle afferents contribute to the cervical response evoked by electrical stimulation of the median nerve in man?

The possible contribution of low threshold muscle afferents to the postsynaptic component (N13) of the cervical response evoked by electrical stimulation of the median nerve (MN) was investigated in normal subjects. Electroneurographic (ENG) and electromyographic (EMG) correlates of the reflex motoneuronal discharge (RMND) were recorded simultaneously. A. No reflex activity could be elicited by stimulation of the MN at the wrist, at least in the resting subjects, while well developed ENG (P2 efferent volley) and EMG (H reflex) monosynaptic responses occurred following stimulation of the MN at the elbow at suitable strengths. In neither case could a surface correlate of interneuronal activity evoked by muscle afferents be demonstrated. B. Recruitment curves showed that at stimulus intensities above maximal for the H reflex both P2 and H responses started to decrease until they completely disappeared, while N13 showed further enhancement. C. Subthreshold conditioning stimulation of the MN enhanced both P2 and H responses, while vibratory muscle stimuli provoked a clearcut suppression of these two responses. In contrast, N13 was completely unaffected by either manoeuvre. D. No cervical evoked activity could be detected following tendon tapping of the anterior forearm muscles in spite of the appearance of well developed cortical responses and the ENG and EMG correlates of the T reflex. E. Conditioning volleys elicited by tendon taps of the anterior forearm muscles suppressed both P2 and H responses following stimulation of the MN at the elbow without affecting the related N13 component. Conditioning supramaximal stimulation of the MN at the wrist suppressed the N13 component of the cervical response evoked by stimulation of the MN at the elbow without affecting the related reflex responses. No component chronologically related to the RMND could be recorded at the posterior neck region during suppression of N13, thus ruling out the possibility that failure to detect the RMND (as well as its interneuronal concomitants) with cervical electrodes is due to a masking effect of the N13 component. G. Conditioning tendon taps of anterior forearm muscles provoked a clearcut reduction of the primary cortical response to finger stimulation without affecting the postsynaptic component of the related cervical response. It is concluded that neither segmental (motoneuronal or interneuronal in origin) nor ascending postsynaptic impulses generated in the spinal cord by stimulation of low threshold muscle afferents contribute to N13, the latter being probably due to activation of both short and long axoned spinal neurons by cutaneous afferents.

Adult↗

PubMed2001

Modulation of spinal excitability during observation of hand actions in humans.

There is growing evidence that observation of actions performed by other individuals activates observer's cortical motor areas. This matching of observed actions on the observer's motor repertoire could be at the basis of action recognition. Here we investigated if action observation, in addition to cortical motor areas, involves also low level motor structures mimicking the observed actions as if they were performed by the observer. Spinal cord excitability was tested by eliciting the H-reflex in a finger flexor muscle (flexor digitorum superficialis) in humans looking at goal-directed hand actions presented on a TV screen. We found that, in the absence of any detectable muscle activity, there was in the observers a significant modulation of the monosynaptic reflex size, specifically related to the different phases of the observed movement. The recorded H-reflex rapidly increased in size during hand opening, it was depressed during hand closing and quickly recovered during object lifting. This modulation pattern is, however, opposite to that occurring when the recorded muscles are actually executing the observed action [Lemon et al. (1995) J. Neurosci., 15, 6145-56]. Considering that, when investigated at cortical level the modulation pattern of corticospinal excitability replicates the observed movements [Fadiga et al. (1995) J. Neurophysiol., 73, 2608-2611], this spinal 'inverted mirror' behaviour might be finalised to prevent the overt replica of the seen action.

Adult↗

PubMed1992

[Segmental reflexes after sciatic nerve damage in rats given thyroxine].

Amplitude of the L5 segmental reflex responses 20 min. before and after intraperitoneal introduction of obsidan, clopheline and finoptine was recorded in experiments on spinalized and thyroxinized rats which 3 weeks before have endured transection of the sciatic nerve. It is demonstrated that obsidan does not change the amplitude of the monosynaptic reflex responses of the ventral root restored under the effect of thyroxine at the operated side, while clopheline and finoptine mainly reduce it.

Animals↗

PubMed1989

Tonic suppression of reflex transmission in low spinal cats.

Transmission in various spinal hindlimb reflex pathways arising from muscle, cutaneous and joint afferents, was investigated in acute low spinal (Th10) cats before and after lesions at more caudal levels in the lumbar segments. Lesions of the ipsilateral dorsolateral funiculus (DLF) resulted in increased mono- and polysynaptic ventral root discharges. With conditioning of monosynaptic reflexes it was demonstrated that the DLF lesion enhanced transmission in reflex pathways from group II and III muscle afferents. The DLF lesion also resulted in increased dorsal root potentials from cutaneous, joint and group III muscle afferents. These reflex enhancements could not be obtained with lesions rostral to L2, but developed with lesions in L2 and to some extent in L3, but no further effect was obtained by adding lesions caudal to L3. Ventral extension of the DLF lesion gave hardly any increase of reflex transmission. It is postulated that the investigated reflex pathways may be tonically inhibited in the acute low spinal state by propriospinal neurones with cell bodies in the L2-L3 segments and with axons descending in the dorsolateral funiculus.

Animals↗

PubMed1980

The effects of substance P and baclofen on motoneurones of isolated spinal cord of the newborn rat.

The effects of substance P (SP) and baclofen were studied in the isolated spinal cord of newborn rats. Potential changes generated in motoneurones were recorded extracellularly from the ventral root (L3-L5). When SP (8 X 10(-5) M) was introduced into the bath, the depolarization of motoneurones began with a delay of 1.1 s. A large part of this delay can be explained as a time needed for SP to reach the site of action on spinal neurones. When the preparation was perfused with artificial cerebrospinal fluid (CSF) containing low Ca (0.1 mM) and high Mg (1.6-3.5 mM), the spinal reflexes induced by dorsal root stimulation and recorded from the corresponding ventral root were completely abolished. The depolarizing action of SP (10(-7) M) on the motoneurones was potentiated in the low-Ca medium, suggesting that SP acts directly on the motoneurones. Baclofen at 10(-6) M depressed the monosynaptic reflex by about 75%. The SP-induced depolarization of motoneurones was greatly depressed by baclofen in both normal and 0.1 mM-Ca mediums. The effects of baclofen (10(-6) M) on the responses to various depolarizing agents were compared with that on the response to SP in artificial CSF containing 0.1 mM-Ca and 1.6-2 mM-Mg. The SP response was reduced by about 80%, whereas the responses to acetylcholine and glycine were not appreciably affected, and those to L-glutamate, GABA and noradrenaline were depressed by 10-22% by baclofen. The results suggest that baclofen blocks transmission at certain primary afferent synapses by antagonizing the action of SP that is released as a transmitter.

Action Potentials↗

PubMed1998

Effects of R-84760, a selective kappa-opioid receptor agonist, on nociceptive reflex in isolated neonatal rat spinal cord.

We tested the effects of (3 R)-3-(1-pyrrolidinylmethyl)-4-[(1S)-5,6-dichloro-1-indancarbony l]-2,3,5,6-tetrahydro-1,4-thiazine hydrochloride (R-84760), a selective kappa-opioid receptor agonist, on the slow ventral root potential in the isolated spinal cord of neonatal rats. R-84760 at 10 nM decreased the slow ventral root potential to 35% of the control, leaving the monosynaptic reflex unaffected. The depressant effect of R-84760 progressed slowly for 60 min to the maximum and recovered slightly after removal of the drug from the perfusing solution. This contrasts with [D-Ala2, MePhe4, Gly-ol5]enkephalin (DAMGO) or [MeTyr1, MeArg7, D-Leu-NHEt8]dynorphin A-(1-8) (E-2078) which attained their maximum depressant effect within 15 min with recovery immediately after washout. Reversibility of the R-84760 effect was observed in vivo in antinociceptive tests in mice. R-84760 reduced the depolarization induced by substance P or L-glutamate in the normal solution, but not in the presence of tetrodotoxin at 0.3 microM. Naloxone inhibited the effect of R-84760 at a higher concentration (1 microM) than that (0.1 microM) needed to antagonize the effect of DAMGO. In contrast, R-84760 was more sensitive to nor-binaltorphimine than was DAMGO. The results show that R-84760 selectively inhibits the nociceptive response presynaptically through kappa-opioid receptors and that the inhibitory effect is characteristic, with long duration, in the neonatal rat spinal cord.

Analgesics↗

PubMed2005

Zinc modulates primary afferent fiber-evoked responses of ventral roots in neonatal rat spinal cord in vitro.

Zinc ions (Zn(2+)) are known to modulate the functions of a variety of channels, receptors and transporters. We examined the effects of Zn(2+) on the reflex potentials evoked by electrical stimulation and responses to depolarizing agents in the isolated spinal cord of the neonatal rat in vitro. Zn(2+) at low concentrations (0.5-2 microM) inhibited, but at high concentrations (5 and 10 microM) augmented, a slow depolarizing component (slow ventral root potential). Zn(2+) had no effect on fast components (monosynaptic reflex potential; fast polysynaptic reflex potential). Unlike Zn(2+), strychnine (5 microM), a glycine receptor antagonist, and (S),9(R)-(-)-bicuculline methobromide (10 microM), a GABA(A) receptor antagonist, potentiated both fast polysynaptic reflex potential and slow ventral root potential. Zn(2+) (5 microM) did not affect depolarizing responses to glutamate and N-methyl-D-aspartate. Zn(2+) enhanced the substance P-evoked depolarization in the absence of tetrodotoxin (0.3 microM) but not in its presence. The dorsal root potential was inhibited by (S),9(R)-(-)-bicuculline methobromide (10 microM) but not by Zn(2+) (5 microM). The Zn(2+)-potentiated slow ventral root potential was inhibited by the N-methyl-D-aspartate receptor antagonists, ketamine (10 microM) and DL-2-amino-5-phosphaonovaleric acid (50 microM) but not by P2X receptor antagonists, pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (30 microM) and 2',3'-O-(2,4,6-trinitrophenyl)ATP (10 microM). Ketamine (10 microM) and DL-2-amino-5-phosphaonovaleric acid (50 microM) almost abolished spontaneous activities increased by Zn(2+). It is concluded that Zn(2+) potentiated slow ventral root potential induced by primary afferent stimulation, which was mediated by the activation of N-methyl-D-aspartate receptors but not by activation of P2X receptors or blockade of glycinergic and GABAergic inhibition. Zn(2+) does not seem to directly affect N-methyl-D-aspartate receptors. The release of glutamate from interneurons may play an important role in Zn(2+)-induced potentiation of slow ventral root potential in the spinal cord of the neonatal rat.

Animals↗

PubMedDate not supplied

Presynaptic inhibition in humans: a comparison between normal and spastic patients.

During the last 40 years, several studies in man have been devoted to the pathophysiological mechanisms underlying spasticity. Spasticity is characterised by a velocity dependent increase in muscle tone. Many spinal pathways control stretch reflex excitability and a malfunction in any one of them could theoretically produce the exaggeration of the stretch reflex. Delwaide showed that the vibration-induced inhibition of Ia fibres is reduced in spastic patients. However, the relation between a decrease in presynaptic Ia inhibition and the pathophysiology of spasticity has been recently questioned since it was argued that homosynaptic depression (or post-activation depression) also contributes to the vibratory-induced depression of monosynaptic reflexes. This paper is thus devoted to a review of the methods recently developed to study selectively presynaptic Ia inhibition in man and to a reevaluation of the relations between modifications in presynaptic Ia inhibition and spasticity in hemiplegic and spinal spastic patients.

Extremities↗

PubMed1990

Plasticity of recurrent inhibitory reflexes in cat spinal motoneurons following peripheral nerve injury.

Chronic axotomy of a peripheral motor nerve in cat causes a gradual reduction in the number of intramedullary axon collaterals originating from the axotomized motoneurons (Havton and Kellerth 1984, 1989). This axon collateral elimination would be expected to reduce the amount of recurrent inhibitory reflex actions mediated by these cells. The aim of the present study was to investigate the recurrent inhibition originating from axotomized motoneurons and, also, to see whether the elimination of axon collaterals from the axotomized neurons might induce secondary compensatory changes in the recurrent inhibitory pathways originating from synergistic non-lesioned motoneurons. The results, which were obtained by means of intracellular recordings and monosynaptic reflex testing, indicate that postoperative enhancement of reflex actions may take place in the recurrent inhibitory pathways persisting in the axotomized motoneurons as well as in those originating from synergistic nonlesioned motoneurons. It is suggested that the site of compensatory enhancement is at the synaptic reflex contacts between the motoraxon collaterals and the inhibitory Renshaw interneurons.

Animals↗

PubMed1994

Stretch reflex responses following mechanical stimulation in power- and endurance-trained athletes.

The basic stretch reflex responses evoked by the mechanical stimuli were investigated among different athletic groups. The tests were performed by a special ankle ergometer, which created dorsiflexion velocities from 25 deg x s-1 to 115 deg x s-1 around the ankle joint. The EMG activity of soleus (SO), gastrocnemius (GA) and tibialis anterior (TA) muscles was recorded with surface electrodes. In all subjects higher angular velocities caused higher stretch reflex responses. The endurance-trained athletes were more sensitive to the used stimuli than the power-trained athletes, because of their shorter latency times (43 +/- 7 ms vs 45 +/- 7 ms, p < 0.001 for the SO muscle and 40 +/- 8 ms vs 43 +/- 6 ms, p < 0.001 for the GA muscle), longer duration of action potentials and shorter recovery times of the monosynaptic reflex. Thus, the neural control of reflex movements in skeletal muscles was dependent on the training background of the subjects. Additionally, the findings may be explained by differences in the motor unit composition as well as by possible differences in the muscle spindle density of the two subject groups.

Action Potentials↗

PubMed1985

Effects of catecholamines on spinal motoneurones and spinal reflex discharges in the isolated spinal cord of the newborn rat.

The effects of catecholamines on spinal motoneurones and spinal reflex discharges were investigated in the isolated spinal cord of newborn rat. Noradrenaline (NA), adrenaline (Adr), dopamine (DA) and isoproterenol (Iso) caused depolarization of the motoneurones in a dose-dependent manner. The depolarizing action persisted in Ca2+-deficient Krebs solution. The order of potency was Adr greater than NA greater than DA much greater than Iso. The effects of NA and Adr on the monosynaptic reflex discharge varied; depression, potentiation or depression followed by potentiation. The polysynaptic reflex discharge was consistently depressed. DA depressed both the mono- and polysynaptic reflex discharges in all the preparations. Tyramine and adamantanamine induced a response similar to that to DA rather than to NA. Depolarization of the motoneurones and the effects on the spinal reflex discharges induced by all the catecholamines were decreased by phentolamine or phenoxybenzamine but not by propranolol or haloperidol. It is suggested that the endogenous catecholamines, mainly DA, depolarize the motoneurones and depress the mono- and polysynaptic reflex discharges through an alpha-adrenoceptor in the spinal cord of the newborn rat.

Animals↗

PubMed1988

Effect of a tachykinin antagonist on a nociceptive reflex in the isolated spinal cord-tail preparation of the newborn rat.

1. The pharmacological profile of Spantide, [D-Arg1, D-Trp7,9, Leu11] substance P, as a substance P (SP) antagonist was examined in isolated spinal cords of newborn rats. Potential changes were recorded extracellularly from a lumbar ventral root (L1-L5). Application of SP to the perfusion bath with a brief pressure pulse of 0.05-0.8 s duration produced a dose-dependent depolarization of the ventral root. Spantide in concentrations of 2-16 microM depressed the depolarizing responses of the ventral root to SP in a concentration-dependent manner. The log dose-response curve of SP was shifted to the right in the presence of 16 microM-Spantide by log 5. The responses to neurokinin A (NKA) and bombesin were similarly depressed by 16 microM-Spantide whereas the responses to noradrenaline, gamma-aminobutyric acid (GABA), neurotensin and thyrotrophin-releasing hormone were not affected by 16 microM-Spantide. 2. In an isolated spinal cord-tail preparation of the newborn rat, brief pulse injection of capsaicin into the perfusion solution of the tail induced a depolarizing response in a lumbar ventral root (L3-L5). This response probably represents a nociceptive C fibre reflex. 3. The capsaicin-induced nociceptive reflex was markedly depressed by 16 microM-Spantide and the reflex recovered its original amplitude and shape 30-60 min after removal of Spantide. 4. The capsaicin-induced nociceptive reflex was depressed by morphine (2 microM) and dynorphin (1-13) (0.2 microM), and these effects were reversed by 1 microM-naloxone. 5. In an isolated spinal cord preparation of the newborn rat, stimulation of a dorsal root with single or double shocks induced depolarizing responses of slow time course in both ipsilateral and contralateral ventral roots of the same segment. These slow depolarizing responses were also depressed by 16 microM-Spantide. In contrast the monosynaptic reflex was not affected by 16 microM-Spantide. 6. The present results suggest that SP and NKA are involved as neurotransmitters in the capsaicin-induced nociceptive reflex in the isolated spinal cord-tail preparation of the newborn rat.

Analgesics↗

PubMed1994

EMG study on the effect of ageing on the human masseteric jaw-jerk reflex.

The effect of age on the masseteric jaw-jerk reflex was investigated in 22 young (11 males and 11 females with mean age 23.2 years) and 22 older dentate subjects (11 males and 11 females with mean age 61.3 years). Electromyographic (EMG) recordings were obtained, after chin taps, from the relaxed masseter muscle of the preferred chewing side, by use of a computerised recording and analysis system. With increasing age the occurrence of the reflex was reduced, the latency was increased, while the amplitude was decreased. Those findings are probably related to the general age related changes in the muscular tissue, the sense organs, the peripheral nerves and especially the central nervous system. Increased biological variance was also observed in the older subjects, as in most aspects of performance in the latent years. Furthermore, the effects of ageing were generally similar in men and women. The age-related decrement in the monosynaptic reflex response is indicative of a generalised decline in the motor performance of the stomatognathic system and the decreased ability of the older dental patient to easily adapt to any dramatic changes in the sensory input.

Adult↗

PubMed1994

Pattern of heteronymous recurrent inhibition in the human lower limb.

Changes in the firing probability of motor units belonging to leg and thigh muscles were used to describe the pattern of distribution of recurrent inhibition evoked by motor discharges from various motor nuclei in the human lower limb. Discharges of units in soleus, gastrocnemius medialis, peroneus brevis, tibialis anterior, quadriceps and biceps femoris were investigated following a conditioning stimulation which evoked either a monosynaptic reflex in quadriceps, triceps surae or peroneal motor neurones, or an antidromic motor volley in one of the following nerves: inferior soleus, gastrocnemius medialis, superficial peroneal, deep peroneal, or femoral nerve. In many motor unit-nerve combinations a trough in the post-stimulus time histogram, indicating an inhibition, appeared immediately after the heteronymous Ia excitation. This inhibition is thought to be Renshaw in origin, because it appeared and increased with the conditioning motor discharge, was independent of the conditioning stimulus intensity per se and had a long duration. These recurrent connections were widely distributed with a pattern very similar to that described for heteronymous monosynaptic Ia excitation. In particular Renshaw coupling between muscles operating at different joints seems to be the rule in the human lower limb.

Adult↗

PubMed1994

An unsupervised neural network model for the development of reflex co-ordination.

In this paper, we present a model for the development of connections between muscle afferents and motoneurones in the human spinal cord. The model consists of a limb with six muscles, one motoneurone pool, one pooled (Ia-like) afferent for each muscle and a central programme generator. The weights of the connections between the afferents and the motoneurone pools are adapted during centrally induced movements of the limb. The connections between the afferents and the motoneurone pools adapt in a hebbian way, using only local information present at the synapses. This neural network is tested in two examples of a limb with two degrees of freedom and six muscles. Despite the simplifications, the model predicts the pattern of autogenic and heterogenic monosynaptic reflexes quite realistically.

Humans↗