Interneuronal relay in spinal pathways from proprioceptors

A survey of spinal dorsal horn neurones encoding the spatial organization of withdrawal reflexes in the rat

The withdrawal reflex pathways to hindlimb muscles have an elaborate spatial organization in the rat. In short, the distribution of sensitivity within the cutaneous receptive field of a single muscle has a spatial pattern that is a mirror image of the spatial pattern of the withdrawal of the skin surface ensuing on contraction in the respective muscle. In the present study, a search for neurones encoding the specific spatial input-output relationship of withdrawal reflexes to single muscles was made in the lumbosacral spinal cord in halothane/nitrous oxide-anaesthetized rats. The cutaneous receptive fields of 147 dorsal horn neurones in the L4-5 segments receiving a nociceptive input and a convergent input from A and C fibres from the hindpaw were studied. The spatial pattern of the response amplitude within the receptive fields of 118 neurones was quantitatively compared with those of withdrawal reflexes to single muscles. Response patterns exhibiting a high similarity to those of withdrawal reflexes to single muscles were found in 27 neurones located in the deep dorsal horn. Twenty-six of these belonged to class 2 (responding to tactile and nociceptive input) and one belonged to class 3 (responding only to nociceptive input). None of the neurones tested (n = 20) with reflex-like response patterns could be antidromically driven from the upper cervical cord, suggesting that they were spinal interneurones. With some overlap, putative interneurones of the withdrawal reflexes to the plantar flexors of the digits, the plantar flexors of the ankle, the pronators, the dorsiflexors of the ankle, and a flexor of the knee, were found in succession in a mediolateral direction.(ABSTRACT TRUNCATED AT 250 WORDS)

Reciprocal inhibition between wrist flexors and extensors in man: a new set of interneurones?

1. Interneurones mediating reciprocal inhibition between wrist flexors and extensors in man are characterized using both Renshaw cells and transarticular group I afferent activation. 2. Renshaw cells were activated by reflex discharges evoked by a tendon tap. The tendon tap was applied to the tendon of the muscles from which the Ia fibres responsible for the reciprocal inhibition originated. Contrary to what was observed both in the cat hindlimb and in human elbow muscles, this Renshaw cell activation never resulted in a long depression of the reciprocal inhibition between wrist flexors and extensors. 3. Convergence from group I elbow muscle afferents and antagonistic group I afferents onto interneurones mediating reciprocal inhibition between wrist muscles was revealed in post-stimulus time histogram (PSTH) experiments using the technique of spatial facilitation. 4. The characteristics of the interneurones mediating reciprocal inhibition between wrist flexors and extensors could therefore be summarized as follows: (a) they are fed by antagonistic group I afferents and group I afferents originating from both flexor and extensor elbow muscles; (b) they are not inhibited by Renshaw cells; (c) they are not excited by low threshold cutaneous afferents; and (d) they are probably interposed in a disynaptic pathway. 5. It is therefore concluded that interneurones mediating reciprocal inhibition between wrist flexors and extensors in man differ both from Ia interneurones and from interneurones interposed in the Ib reflex pathways and these characteristics are related to the complex circumduction movements developed in the wrist.

A commentary on the segmental motor system of the turtle: implications for the study of its cellular mechanisms and interactions

A commentary is provided on the segmental motor system of the turtle Pseudemys (Trachemys) scripta elegans with an emphasis on neuronal, neuromuscular, and muscular mechanisms that control the development of force under normal, fatiguing, and pathophysiological conditions. For the central neuronal component of the segmental motor system, it has recently been shown that intracellular analysis of the firing properties of motoneurons and interneurons can be undertaken for relatively long periods of time in in vitro slices of the lumbosacral spinal cord of the adult turtle. In other less reduced in vitro preparations, analyses are available on complex motor behaviors generated by the isolated spinal cord. These behaviors of spinal neuronal networks are analogous in key aspects to those generated by the isolated in vivo cord, and by the cord in intact preparations. These results suggest that the neuronal components of the segmental motor system can not be studied from the cellular/molecular level of analysis in in vitro slice preparations to the systems level in conscious, freely moving animals. The in vitro approach can also be used for the analysis of cellular mechanisms in suprasegmental brain structures, which contribute to the control of voluntary movement. For the peripheral neuromuscular component of the segmental motor system, information is now available on muscle fiber types and selected aspects of sensory innervation, and it is feasible to study the mechanical and biochemical properties of motor units. As such, the turtle presents a valuable model for exploring interrelations between the neuronal and mechanical components of the segmental motor system of the generalized tetrapod. A prominent feature of these recent developments is the extent to which they have been driven by findings that have emphasized an evolutionary conservation of motor-control mechanisms extending from ion channels, at the cellular level, to the control of multijointed movements at the systems level of analysis.

Dorsal spinocerebellar tract neuronal activity in the intact chronic cat

The ability to electrophysiologically identify the axonal projections of lumbar neurons recorded in chronic unanesthetized intact awake animals is a formidable but essential requirement toward understanding ascending sensory transmission under naturally occurring conditions. Chronic immobilization procedures previously introduced by Morales et al. (1981) for intracellular studies of motoneurons are modified and then integrated with procedures for antidromic cellular identification and extracellular recording of upper (or lower) dorsal lumbar spinocerebellar tract (DSCT) neuronal activity, in conjunction with behavioral state recording and drug microiontophoresis. These implant procedures provide up to 6 months of stable recording conditions and, when combined with other techniques, allow individual DSCT neurons to be monitored over multiple cycles of sleep and wakefulness, following the induction into and recovery from barbiturate anesthesia and/or during the juxtacellular microiontophoretic ejection of inhibitory or excitatory amino acid neurotransmitters. The combination of such techniques allows a comprehensive examination of synaptic transmission through the DSCT and other lumbar sensory pathways in the intact normally respiring cat and its modulation during the general anesthetic state. These techniques permit investigations of the supraspinal controls impinging on lumbar sensory tract neurons during wakefulness and other behavioral states such as active sleep.

Dopaminergic control of transmission from group II muscle afferents to spinal neurones in the cat and guinea-pig

The effects of dopamine and its agonists on transmission from muscle afferents to spinal neurones were investigated in the cat and guinea-pig spinal cord, by measuring the drug effects on the amplitude of monosynaptic field potentials evoked by electrical stimulation of group I and group II muscle afferents. Local iontophoretic application of dopamine, the dopamine D1/D5 agonist SKF-38393 and the D2/D3/D4 agonist quinpirole all depressed the group II field potentials evoked at the base of the dorsal horn. Group II field potentials in the intermediate zone were depressed by dopamine to a similar degree as the dorsal horn field potentials, whereas the dopamine agonists were without effect upon them. The intermediate zone field potentials evoked by group I muscle afferents were not depressed by any of the drugs. The dopamine-evoked depression of the group II-evoked field potentials in the dorsal horn in the guinea-pig spinal cord was reduced by the simultaneous application of haloperidol. The results demonstrate that dopamine receptors mediate the depression of transmission from group II muscle afferents to interneurones in the dorsal horn, but not to neurones in the intermediate zone of the spinal cord.

Contacts between serotoninergic fibres and dorsal horn spinocerebellar tract neurons in the cat and rat: a confocal microscopic study

Contacts between serotoninergic nerve fibres and dorsal horn dorsal spinocerebellar tract neurons were analysed in order to investigate the morphological basis of actions of serotonin upon dorsal spinocerebellar tract neurons. In a series of experiments dorsal spinocerebellar tract neurons were labelled with intracellularly injected rhodamine-dextran in the cat. The neurons were monosynaptically excited by group II muscle afferents and cutaneous afferents and were identified by antidromic activation following stimuli applied in the cerebellum. In the second series of experiments dorsal spinocerebellar tract neurons were labelled by retrograde transport of Fluorogold injected into the cerebellum in the rat. In both series, serotoninergic fibres were labelled by using a specific anti-serotonin antiserum and were revealed by immunofluorescence. Appositions between the serotoninergic fibres and the cells were inspected with a dual channel confocal microscope. The merged images obtained with the two channels of the microscope were viewed in single optical planes 2 microns apart and in rotated three-dimensional reconstructions. Serotoninergic nerve fibres were found in apposition to cell bodies of all feline dorsal spinocerebellar tract neurons (n = 7) and of 75% of rat dorsal spinocerebellar tract neurons (n = 90). The numbers of putative contacts on cell bodies varied between less than 100 and nearly 300 (mean 160) in the cat and between about five and 30 in the rat. Contacts with dendrites of feline neurons were seen on 96% of 72 dendrites within 300 microns from soma and on 91% of 23 dendrites at distances of 300-500 microns. The number of such contacts varied from less than five to 150 on a single dendrite within these ranges of distances. Their total number within 100 microns from the soma was comparable or exceeded the number of contacts on the soma.

Group Ia non-reciprocal inhibition from wrist extensor to flexor motoneurones in humans

Interneurones mediating disynaptic inhibition from extensor to flexor carpi radialis muscles were characterized by pharmacological stimulation of Renshaw cells. It is, indeed, known that only Ia interneurones are blocked by recurrent inhibition. Renshaw cell potentiation, induced by intravenous administration of 2 g levo-acetylcarnitine, blocked Ia reciprocal inhibition from triceps to biceps muscles but not disynaptic inhibition from extensor to flexor carpi radialis muscles. It is concluded that the interneurones mediating this latter inhibition are not Ia interneurones. This kind of inhibition could be an example of a Ia non-reciprocal inhibitory pathway.

The isolated mammalian spinal cord

This review considers: spinal cord slices; isolated spinal cord sagitally or transversely hemisected; whole spinal cord; respiration control--[brain-stem spinal cord; brain-stem spinal cord with attached lungs]; nociception--[spinal cord with tail]; fictive locomotion--[spinal cord with one hind limb; spinal cord with two hind limbs]. Much of the functional circuitry of the CNS can be studied in the isolated spinal cord with the additional advantage that the isolated spinal cord can be perfused with known concentrations of ions, neurotransmitters, agonists, antagonists, and anaesthetics. These can be washed away, the circuitry allowed to recover and other drugs or different concentrations applied. Future preparations including the complete spinal cord, the two hind limbs, and a sagittal section of the complete brain will allow greater understanding of the multiple sensory and motor pathways and their interactions in the CNS.

Long-lasting inhibition of the human soleus H reflex pathway after passive movement

Human soleus H reflexes are attenuated during passive pedalling movements. This depression occurs within 70 ms of movement onset. We hypothesized that the reflex gain would return to control values with a similar brevity following movement. However, H reflexes sampled following a slow (10 rpm) passive pedalling movement of a single leg remained below control values for the duration of a 200 ms collection period, for all four pedal positions tested. The extent of the attenuation after movement was position dependent in a manner similar to that observed during movement. This position effect was more precisely defined by sampling reflexes 200 ms post-movement at 10 pedal crank positions. Also, the full course of reflex recovery was investigated by sampling up to 8 s post-movement at four pedal positions. Reflex gain remained reduced 1-4 s post-movement, in a position dependent manner. There was a subsequent facilitation of the reflex. Thus, following a locomotor-like movement there is sustained attenuation of the soleus H reflex. The early post-movement period is likely the continued expression of movement-induced reflex inhibition while the later period may arise from descending influences consequent to the termination of movement. Presynaptic inhibition is implicated, as reflexes still showed the gain modulation when sampled while soleus was tonically contracted, both following and during the passive movement.

Development of the longitudinal projection patterns of lumbar primary sensory afferents in the chicken embryo

The literature on the anatomical organization of primary sensory afferents, though extensive, contains relatively little information about the longitudinal extent of the central collateral projections. Our understanding of intersegmental sensorimotor integration in the spinal cord and of the developmental mechanisms that establish its underlying circuitry could be significantly enhanced by a more complete description of these projections. To address this issue from a developmental perspective, we labeled the central projections of lumbar primary afferents in fixed preparations of the chicken embryo with the lipophilic tracer DiI. At late embryonic stages, the afferent projections had the following characteristics: Primary afferents originating from a single lumbar dorsal root ganglion bifurcated to project longitudinally in the dorsal funiculus or Lissauer's tract. Dorsal funiculus axons extended up to seven segments caudally and to at least ten segments rostrally, whereas axons in Lissauer's tract extended up to seven segments in each direction. Collaterals branched off the longitudinal axons over a range of about seven segments in each direction. Within this range, collaterals to specific terminal fields exhibited more restricted ranges. The development of these longitudinal patterns during earlier embryonic stages was followed from the time the afferents first reached the neural tube on day 4 of embryogenesis. The longitudinal axons lengthened as a single bundle up to day 10, with medial axons consistently longer than lateral axons. After day 10, the longitudinal axons were segregated into the dorsal funiculus and Lissauer's tract. Collaterals sprouted after about 2 days of longitudinal axon growth, by which time the axons had extended several segments in each direction. The segmental range over which collaterals were present reached a maximum of 20 segments at day 10. Collaterals to the different terminal areas differed in their segmental ranges already by this time. After day 10, the total segmental range of collaterals decreased to the stable level of about seven segments in each direction, which is characteristic of late-stage embryos.

The relationship between the kinematics of passive movement, the stretch of extensor muscles of the leg and the change induced in the gain of the soleus H reflex in humans

The gain of the H reflex attenuates during passive stepping and pedalling movements of the leg. We hypothesized that the kinematics of the movement indirectly reflect the receptor origin of this attenuation. In the first experiment, H reflexes were evoked in soleus at 26 points in the cycle of slow, passive pedalling movement of the leg and at 13 points with the leg static (the ankle was always immobilized). Maximum inhibition occurred as the leg moved through its most flexed position (P < 0.05). Inhibition observed in the static leg was also strongest at this position (P < 0.05). The increase in inhibition was gradual during flexion movement, with rapid reversal of this increase during extension. In the second experiment, the length of stretch of the vasti muscles was modelled. Variable pedal crank lengths and revolutions per minute (rpm) altered leg joint displacements and angular velocities. Equivalent rates of stretch of the vasti, achieved through different combinations of joint displacements and velocities, elicited equivalent attenuations of mean reflex magnitudes in the flexed leg. Reflex gain exponentially related to rate of stretch (R2 = 0.98 P < 0.01). The results imply that gain attenuation of this spinal sensorimotor path arises from spindle discharge in heteronymous extensor muscles of knee and/or hip, concomitant with movement.

Influence of aging on leg muscle reflex responses to stance perturbation

The effect of age on latency and amplitude of leg muscle responses to stance perturbations was studied in 75 control subjects. They stood upright on a platform and were displaced by toe-up (upward tilt) and toe-down (downward tilt) platform rotations. Perturbations were induced during free and supported stance (holding on to a stable structure). Surface electromyograms (EMG) of the soleus (Sol) and tibialis anterior (TA) were recorded and latency and area of responses were measured. Body sway variables during stance with open or closed eyes were also recorded. Upward tilt evoked a short-latency response (SLR) in Sol and a long-latency response (LLR) in TA. Downward tilt evoked a medium-latency response (MLR) in TA and a LLR in Sol. This pattern of EMG responses was similar in both young and elderly subjects, although there were some differences in latency and amplitude. There was a significant relationship between latency of all responses and age. Slope of the regression lines of TA LLR, TA MLR, and Sol LLR was steeper than that of Sol SLR. Area of Sol SLR was unrelated to age, but a positive trend was identified in the other responses, significant for TA LLR. Under supported-stance condition, amplitude of TA MLR, TA LLR, and Sol LLR was decreased to a similar extent in both young and elderly subjects. There was a weak relationship between age and most body sway variables. A significant relationship was found between most sway variables and latency of Sol SLR and LLR, chiefly with eyes closed. Neither TA MLR nor LLR were significantly correlated with sway variables, but a trend was present for TA MLR with eyes closed.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurobiology of muscle fatigue. Advances and issues

Throughout this epilogue, we have emphasized that rapid advances in understanding of neural and muscular aspects of fatigue have occurred since the 1980 London Symposium. However, in each instance of progress, from the single muscle fiber to the forebrain, the application of more precise techniques have raised important new questions. Neuroscientists and muscle physiologists have expanded opportunities for rigorous study of a topic of major scientific and social importance.

Overview: potential role of segmental motor circuitry in muscle fatigue

This chapter reviews several mechanisms that the CNS may use to mitigate muscle fatigue, including intrinsic motoneuron properties and feedback systems. The emphasis is on the effects of sensory inputs on spinal cord interneurons including: Renshaw cells; Ib inhibitory interneurons; interneurons mediating presynaptic inhibition; Ia inhibitory interneurons; and interneuronal networks constituting central pattern generators for locomotion. This exercise brings out how little is known about the operation of these circuits in dealing with muscle fatigue.

Flexibility of lower limb reflex responses to painful cutaneous stimulation in standing humans: evidence of load-dependent modulation

1. In six human subjects standing without support, the reflex response of the tibialis anterior muscle (TA) was elicited by painful electrical stimulation (500 Hz, 20 ms) of the anterior sole of the foot and analysed by post-stimulus averages of rectified electromyography. The threshold intensity for the reflex response was very close to the subjective pain sensation (mean value, 1.05 times). Estimation of the afferent conduction velocity gave a mean value of 26.5 m s-1, suggesting that a contribution from A delta fibres was necessary to evoke the reflex response. The TA reflex response was then used as the pain test reflex. 2. Changes in the TA excitatory pain reflex response (elicited at 1.2 times the pain threshold) were investigated while the subjects maintained different postures in upright stance. Standing on the ipsilateral leg produced a significant decrease in the reflex response with respect to its value in symmetrical stance (standing on both legs), whereas a significant facilitation was observed when the subject was standing on the contralateral leg. A parallel depression of the response in both limbs was present when the subject maintained an upright stance with the lower limbs abducted. Thus, it was apparent that the TA pain responses decreased as the supporting function of the leg increased. 3. A significant inverse correlation between the load to which the limb was subjected and the size of the reflex response was observed in all subjects. We propose that the load to which the limb is subjected, measured from peripheral mechanoreceptors, is used as a measure of the current supporting function of the limb, on the basis of which the reflex is regulated.

Central control of disynaptic reciprocal inhibition in humans

The disynaptic pathway from muscle spindle Ia afferents to motoneurones of the antagonist muscle is one of the best studied pathways in the spinal cord. Early animal studies--mainly in the cat--have provided a detailed knowledge of the pathway itself and of the integration of segmental and supraspinal convergence at the interneuronal level. Although this knowledge was used to formulate hypotheses on the function of the pathway during natural movements, the reduced animal preparation limited the possibilities of testing these ideas. However, such information has more recently been obtained from human subjects by using indirect electrophysiological techniques. In most of these experiments the disynaptic Ia inhibition was demonstrated as a short-latency depression of a monosynaptic test reflex (H-reflex) following a conditioning stimulation of the antagonist nerve. Changes in the size of this depression during voluntary tasks were then taken as evidence of a central regulation of the pathway. It has for example been demonstrated in this way that the brain regulates the Ia inhibitory interneurones in parallel with their corresponding motoneurones during extension-flexion movements, but not during co-contraction of antagonistic muscles. The importance of the central control of the pathway has also been emphasized by the finding of a disordered regulation of its activity in patients with lesions of the brain. This may possibly contribute to the inappropriate co-contraction of antagonistic muscles observed in some of these patients. It seems reasonable to expect that this kind of experiment in the future may contribute significantly to the knowledge of the central control of spinal motor mechanisms.

Waveform parameters of recurrent inhibitory postsynaptic potentials in cat motoneurons during time-varying activation patterns

A considerable number of theoretical and experimental studies have been undertaken to establish quantitative relationships between the time course of postsynaptic potentials in a neuron and the change in firing probability thereby induced. Depending on background synaptic noise level, the time course of the postsynaptic potential per se as well as its time derivative are both of importance in varying proportion. We have recently begun to study recurrent inhibitory potentials in cat hindlimb motoneurons during rhythmically varying rates of stimulation of motor axons. The amplitude-rate relationship exhibits hysteresis in that amplitudes are usually larger during augmenting than decrementing rates in the cycle. We here report results on the other important variable, that is the slope of recurrent inhibitory potential development, which need not a priori be correlated with amplitude. We found that the slope has a relation to stimulus rate similar to amplitude, so that both parameters are correlated. In pentobarbitone anaesthetized or decerebrate cats, intracellular recordings were obtained from hindlimb skeleto-motoneurons. Various hindlimb muscle nerves were prepared for electrical stimulation to elicit recurrent inhibitory potentials, with dorsal roots cut. Test stimulus patterns consisted of repetitive pulse trains whose rates varied, at modulation frequencies between 0.1 and 1.0 Hz, in one of two waveforms: triangular or sinusoidal. Modulation depths were either "full", with rates varying between a minimum of less than 10 and a maximum of around 50 pulses per s. Or they were about "half" this depth, with mean rates shifted into a "low", "medium" or "high" rate region. Recurrent inhibitory potentials were averaged with respect to stimuli occurring during different phases of the stimulation cycle. Most often when, throughout the cycle, the amplitude changed in a consistent way, so did the slopes of the inhibitory potentials. That is, when the amplitudes rhythmically declined with increasing and recovered with decreasing stimulus rate, the rate of hyperpolarization followed the same pattern. With prominent hysteresis in amplitude, a corresponding hysteresis appeared in slopes. Hence, amplitude and slopes were correlated, occasionally showing a hysteresis among themselves. To a certain extent, these results can be explained by Renshaw cell behaviour, the contribution of the Renshaw cell-motoneuron synapse being unknown and difficult to assess experimentally. For the inhibitory effect of Renshaw cells on motoneurons (and reciprocal Ia inhibitory interneurons), both its magnitude and its time course probably play an important role in determining the efficacy of counteracting local excitatory inputs. The change in slope of inhibitory potentials, and likely its underlying conductance, during cyclic motoneuron activation can be presumed to significantly contribute to the temporal pattern of discharge of motoneurons, in particular in relation to the prevention of synchronization leading to enhanced tremor.

Development of spinal reflex pathways from muscle afferents to motoneurones in chick embryos devoid of descending inputs

1. The synaptic connections of reflex pathways between hindlimb muscle afferents and motoneurones were investigated in chicken embryos. Descending inputs to the lumbar spinal cord were eliminated by removing two to four segments of the thoracic spinal cord at embryonic day 2 (E2; gap operation). Intracellular recordings from motoneurones innervating the lateral gastrocnemius (LG) muscle were performed in isolated spinal cord preparations of normal and gap-operated embryos at E14-E15. 2. In both normal and gap-operated embryos, homonymous and synergistic muscle nerve stimulation evoked EPSPs in LG motoneurones at a short and fixed latency, suggesting that they were evoked monosynaptically. EPSPs from synergistic muscle afferents were much smaller than those from homonymous muscle afferents. The volleys from the antagonistic muscle nerve evoked IPSPs at a longer central latency than found for EPSPs in both embryos. 3. The maximal amplitudes of homonymous and synergistic EPSPs in gap-operated embryos were 1.3 and 1.7 times, respectively, larger than in normal embryos. Homonymous EPSPs were observed in all LG motoneurones examined, but synergistic EPSPs were more common in gap-operated than in normal embryos. 4. Antagonistic IPSPs were more common in motoneurones of gap-operated embryos than in those of normal embryos. Homonymous and synergistic muscle nerve stimulation also elicited IPSPs in LG motoneurones in both normal and gap-operated embryos. IPSPs evoked both by homonymous and by synergistic muscle nerve stimulation were more common in gap-operated than in normal embryos. 5. The spatial pattern of reflex pathways from hindlimb muscle afferents to motoneurones in chick embryos devoid of both supraspinal and long propriospinal inputs to the lumbar spinal cord is similar to that in normal embryos. However, both mono- and polysynaptic connections in these pathways are enhanced by the blockade of descending projections.

The role of stretch reflex threshold regulation in normal and impaired motor control

Some hypotheses suggest that stretch reflex threshold regulation may be an essential element of motor control. Disturbances in this mechanism may lead to motor dysfunction. We investigated this possibility by comparing stretch reflex threshold regulation in 11 spastic hemiparetic and 6 normal subjects. Subjects sat with their arms fully supported in a forearm and hand mold attached to a manipulandum mounted on and controlled by a torque motor. They remained completely passive while their elbow was extended from 30 degrees flexion through an arc of 100 degrees. Displacement and velocity of the forearm were measured as well as EMG signals from 2 elbow flexors and 2 elbow extensors, when the elbow flexors were stretched at each of 7 velocities. Velocities ranged from 8 to 160 degrees/s for hemiparetic subjects and from 32 to 300 degrees/s for normal subjects. Phase diagrams (velocity versus angle) were plotted and the threshold angles (lambda) for muscle activation at each velocity of stretch were used to determine the static stretch reflex threshold (lambda) and the slope (mu) of the relationship between the lambda s and velocity. Our main findings were that static and dynamic stretch reflex thresholds were decreased in spastic hemiparetic compared to normal subjects and that the thresholds depended on velocity. The static threshold value correlated with the severity of clinically measured spasticity. In addition, the range of regulation of lambda was decreased in the patients compared to normal. This may explain some of the problems of force and position regulation as well as hypertonus (and weakness) common to these patients.

Coordination between head and hindlimb motions during the cat scratch response

Coordination between motions of the head and the hindlimb paw ipsilateral to the stimulated pinna were assessed during the scratch cycle in freely moving cats. Motor patterns were determined by electromyographic (EMG) recordings made from epimysial-patch electrodes surgically implanted on the biventer cervicis (BC), complexus (CM), obliquus capitis inferior (OC), and splenius (SP) muscles and by fine-wire EMG electrodes implanted in two ankle muscles, medial gastrocnemius (MG), and tibialis anterior (TA). To assess head motions during the three phases of the scratch cycle (precontact, contact, postcontact), several responses were filmed, and in some cats an in vivo force transducer was implanted on an ankle extensor muscle (MG or plantaris, PL) to determine the tension profile during the scratch cycle. During the scratch cycle, the head's trajectory was usually characterized by a small oscillation in which the head was pushed away during paw contact (as hindlimb joints extended) and then repositioned during the noncontact phases (as hindlimb joints flexed). Neck muscle activity did not occur during all responses or during all cycles of a single multicycle scratch response, and when it occurred, neck muscle EMG was characterized as phasic (a single burst during the cycle) or tonic (low-level activity during the entire cycle). Neck muscles ipsilateral (i) to the scratching limb exhibited phasic bursts more than contralateral (c) muscles, and phasic activity was most frequently observed in the iBC, iSP, iOC, and cOC muscles. The cOC was reciprocally active with the ipsilateral muscles, and its burst coincided with the postcontact phase and the ankle flexor (TA) burst. The ipsilateral muscles (iOC, iSP, iBC) were active during paw contact, and the termination of all three bursts occurred synchronously just after peak tension of the ankle extensor was reached. The iBC was active before the onset of paw contact and may have been responsible for repositioning the head, along with the cOC, during the precontact phase. The iOC became active after the onset of paw contact (22 ms) and was recruited more often when the peak extensor tendon force was high (10-16 N). The iSP, in contrast, was active during the contact phase of most scratch cycles examined and its recruitment appeared to be unrelated to tendon forces. Our data suggest that phasic neck muscle activity is not obligatory during the cat scratch response, but is related to certain conditions such as a higher than average tendon force of an ankle extensor during contact and the need to reposition the head during the noncontact phases of the cycle.(ABSTRACT TRUNCATED AT 400 WORDS)

Sensorimotor transformation in a spinal motor system

To use sensory information from the skin to guide motor behaviour the central nervous system must transform sensory coordinates into movement coordinates. As yet, the basic principles of this crucial neural computation are unclear. One motor system suitable as a model for the study of such transformations is the spinal withdrawal reflex system. The spatial organization of the cutaneous input to these reflexes has been characterized, and we now introduce a novel method of motion analysis permitting a quantitative analysis of the spatial input-output relationship in this motor system. For each muscle studied, a "mirror-image" relationship was found between the spatial distribution of reflex gain for cutaneous input and the pattern of cutaneous unloading ensuing on contraction. Thus, there is an "imprint" of the movement pattern on this motor system permitting effective sensorimotor transformation. This imprint may indicate the presence of a learning process which utilizes the sensory feedback ensuing on muscle contraction.

Selective cortical control of information flow through different intraspinal collaterals of the same muscle afferent fiber

We have analyzed in the anesthetized cat the effects of electrical stimulation of the cerebral cortex on the intraspinal threshold of two collaterals belonging to the same muscle spindle or tendon organ afferent fiber. The results obtained provide, for the first time, direct evidence showing that the motor cortex is able to modify, in a highly selective manner, the synaptic effectiveness of individual collaterals of the same primary afferent fiber. This presynaptic control could function as a mechanism that allows funneling of information to specific groups of spinal neurons in the presence of extensive intraspinal branching of the afferent fibers.

Shaping static elbow torque-angle relationships by spinal cord circuits: a theoretical study

Static torque-angle relationships (invariant characteristics) as measured by Feldman [Feldman A. G. (1980) Neuroscience 5, 81-90] at the human elbow joint for constant descending excitatory drive have a monotonic convex shape determining joint angle-dependent stiffness. In contrast, for constant activation of elbow flexors, the torque increases, peaks and decreases again with increasing angle because of related moment arm alterations [Hasan Z. and Enoka R. M. (1985) Expl Brain Res. 59, 441-450]. Conversion of such constant-excitation torque-angle shapes into an invariant characteristic might result from action of the stretch reflex which adds excitation with increasing joint angle. To test whether a simple linear model of the stretch reflex could convert constant excitation torque-angle relationships into invariant characteristics, the following assumptions were made. (1) Muscle fibre length increases linearly with joint angle. (2) Reflex muscle excitation (electromyogram) is linearly related to muscle (fibre) length. With these assumptions, invariant characteristic shape cannot be derived from constant excitation torque-angle relationships because it would be sigmoid at low and nearly straight at large joint angles, whilst real flexor invariant characteristics are more convex at large than small angles. It is suggested that recurrent inhibition via Renshaw cells contributes to bend the invariant characteristics into their right shape. Renshaw cells show a nonlinear saturating dependence on motor axon input rate and amount of excitation, i.e. number of active axon collateral synapses. These relationships can contribute to shape motoneuron output so as to yield convex invariant characteristics. Whilst it is not quite clear whether the gain of recurrent inhibition from and to skeleto-motoneurons is high enough to co-determine the invariant characteristic shape significantly, recurrent inhibition of Ia inhibitory interneurons mediating reciprocal inhibition between antagonists is supposed to be quite strong and may influence joint stiffness by interacting with reciprocal inhibition. The arguments presented here extend those of Feldman and co-workers concerning the role of recurrent inhibition and in addition provide a possible explanation for the functional role of mutual inhibition between Renshaw cells. Together with reflex feedback, recurrent inhibition thus contributes to fine-regulate force output and joint stiffness. To account for this cooperation and to make another step towards a general theory of spinal cord circuits, major traits of a new concept are briefly outlined.

Interneurones in pathways from group II muscle afferents in sacral segments of the feline spinal cord

1. Properties of dorsal horn interneurones that process information from group II muscle afferents in the sacral segments of the spinal cord have been investigated in the cat using both intracellular and extracellular recording. 2. The interneurones were excited by group II muscle afferents and cutaneous afferents but not by group I muscle afferents. They were most effectively excited by group II afferents of the posterior biceps, semitendinosus, triceps surae and quadriceps muscle nerves and by cutaneous afferents running in the cutaneous femoris, pudendal and sural nerves. The earliest synaptic actions were evoked monosynaptically and were very tightly locked to the stimuli. 3. EPSPs evoked monosynaptically by group II muscle afferents and cutaneous afferents of the most effective nerves were often cut short by disynaptic IPSPs. As a consequence of this negative feedback the EPSPs gave rise to single or double spike potentials and only a minority of interneurones responded with repetitive discharges. However, the neurones that did respond repetitively did so at a very high frequency of discharges (0.8-1.2 ms intervals between the first 2-3 spikes). 4. Sacral dorsal horn group II interneurones do not appear to act directly upon motoneurones because: (i) these interneurones are located outside the area within which last order interneurones have previously been found and (ii) the latencies of PSPs evoked in motoneurones by stimulation of the posterior biceps and semitendinosus, cutaneous femoris and pudendal nerves (i.e. the main nerves providing input to sacral interneurones) are compatible with a tri- but not with a disynaptic coupling. Spatial facilitation of EPSPs and IPSPs following synchronous stimulation of group II and cutaneous afferents of these nerves shows, however, that sacral interneurones may induce excitation or inhibition of motoneurones via other interneurones. 5. Comparison of the properties of group II interneurones in the sacral segments with those of previously studied group II interneurones in the midlumbar segments leads to the conclusion that these two populations of neurones are specialized for the processing of information from different muscles and skin areas. In addition, equivalents of only one of the two subpopulations of midlumbar interneurones have been found at the level of the pudendal nucleus: neurones with input from group II but not from group I muscle afferents. Neurones integrating information from group I and II muscle afferents and in direct contact with motoneurones thus seem to be scarce in the sacral segments.(ABSTRACT TRUNCATED AT 400 WORDS)

Heteronymous recurrent inhibition from gastrocnemius muscle to soleus motoneurones in humans

Presence of heteronymous recurrent inhibition in motoneurones (Mns) innervating the soleus muscle (Sol) was investigated following electrical stimulation of the nerve of gastrocnemius medialis muscle (GM). Sub-threshold electrical stimulation for alpha Mns produced short-lasting inhibition of the Sol, reflecting non-reciprocal group I inhibition. After increasing the intensity of stimulation above the motor threshold, a short-latency, long-lasting inhibition appeared superimposed on the group I inhibition. Its amount increased with the size of the conditioning motor response and after acute administration of L-acetylcarnitine. It is concluded that this long-lasting inhibition of the Sol Mns is due to the heteronymous activity of the GM-coupled Renshaw cells.

Transmission in a locomotor-related group Ib pathway from hindlimb extensor muscles in the cat

It has been previously shown that phasic stimulation of group I afferents from ankle and knee extensor muscles may entrain and/or reset the intrinsic locomotor rhythm; these afferents are thus acting on motoneurones through the spinal rhythm generators. It was also concluded that the major part of these effects originates from Golgi tendon organ Ib afferents. Transmission in this pathway to lumbar motoneurones has now been investigated during fictive locomotion in spinal cats injected with nialamide and L-DOPA, and in decerebrate cats with stimulation of the mesencephalic locomotor region. In spinal cats injected with nialamide and L-DOPA, it was possible to evoke long-latency, long-lasting reflexes upon stimulation of high threshold afferents before spontaneous fictive locomotion commenced. During that period, stimulation of ankle and knee extensor group I afferents evoked oligosynaptic excitation of extensor motoneurones, rather than the "classical" Ib inhibition. Furthermore, a premotoneuronal convergence (spatial facilitation) between this group I excitation and the crossed extensor reflex was established. During fictive locomotion, in both preparations, the transmissions in these groups I pathway was phasically modulated within the step cycle. During the flexor phase, the group I input cut the depolarised (active) phase in flexor motoneurones and evoked EPSPs in extensor motoneurones; during the extensor phase the group I input evoked smaller EPSPs in extensor motoneurones and had virtually no effect on flexor motoneurones. The above results suggest that the group I input from extensor muscles is transmitted through the spinal rhythm generator and more particularly, through the extensor "half-centre". The locomotor-related group I excitation had a central latency of 3.5-4.0 ms. The excitation from ankle extensors to ankle extensors remained after a spinal transection at the caudal part of L6 segment; the interneurones must therefore be located in the L7 and S1 spinal segments. Candidate interneurones for mediating these actions were recorded extracellularly in lamina VII of the 7th lumbar segment. Responses to different peripheral nerve stimulation (high threshold afferents and group I afferents bilaterally) were in concordance with the convergence studies in motoneurones. The interneurones were rhythmically active in the appropriate phases of the fictive locomotor cycle, as predicted by their response patterns. The synaptic input to, and the projection of these candidate interneurones must be fully identified before their possible role as components of the spinal locomotor network can be evaluated.

Presynaptic modulation of spinal reflexes

Recent evidence suggests that independent sets of interneurons mediate presynaptic inhibition of primary and secondary muscle spindles and of tendon organ afferents. There is also evidence that the information which flows through different intraspinal collaterals of a single muscle spindle or tendon organ afferent fiber is selectively affected by electrical stimulation of the motor cortex. These studies suggest that presynaptic inhibition plays an important role in the selection of the sensory signals required for the execution of a specific motor task.

Axonal projections and synaptic connections of C5 segment expiratory interneurones in the cat

1. Respiratory interneurones in the C4-C6 segments of the spinal cord have only recently been described; until now their projections and connections were not known. We investigated expiratory interneurones in the C5 spinal segment, using antidromic mapping to trace their projections and spike-triggered averaging to test their synaptic connections with phrenic motoneurones. 2. A total of seventy expiratory interneurones were recorded in nineteen cats anaesthetized with pentobarbitone, paralysed and ventilated. The interneurones were found scattered dorsomedial to the phrenic motor nucleus, with discharge patterns of a constant (66%), augmenting (24%) or decrementing (10%) type. 3. Interneurone axons were found in the ipsilateral ventrolateral funiculus using antidromic activation at thresholds < 20 microA. The axons of eighteen of thirty-three interneurones tested (55%) were found to extend to the rostral part of the C6 segment, seventeen of thirty-three (52%) to the caudal part of the C6 segment and ten of nineteen (53%) to the rostral part of the C7 segment. 4. Axon collaterals for thirteen of thirty-three interneurones (39%) were found in the ipsilateral half of the C6 segment, with their endings near the phrenic motor nucleus. In three cases two collaterals were found. None of the interneurones had projections in the contralateral halves of the C5 or C6 segments. 5. In a separate group of thirty-four expiratory interneurones, antidromic mapping was used to find an axon collateral in the C6 segment prior to spike-triggered averaging. Eleven of these interneurones had collaterals (32%) and were subsequently tested for synaptic connections to thirty-two phrenic motoneurones. In three separate instances (9%), inhibitory postsynaptic potentials were observed. Amplitudes, fall times and half-amplitude widths of the inhibitory postsynaptic potentials were 6.7, 10.4 and 10.6 microV; 0.3, 0.5 and 0.7 ms and 0.6, 1.6 and 3.3 ms respectively. 6. We conclude: (i) there is a population of expiratory interneurones in the C5 segment, located predominantly dorsomedial to the phrenic motor nucleus; (ii) at least one-half of these interneurones have ipsilateral intersegmental projections to the C6 segment and (iii) although synaptic connections from expiratory interneurones in the C5 segment to phrenic motoneurones in the C6 segment may be rare, the observed inhibitory postsynaptic potentials had fall times and latencies commensurate with monosynaptic connections.

The coactivation command for antagonist muscles involving Ib interneurons in mammalian motor control systems: an electrophysiologically testable model

The hypothesis of a weighted combination of independent reciprocal (R) and co-activation (C) commands to agonist and antagonist motoneurons (MNs) underlying movement is considered. In contrast to the R command, C command does not influence the equilibrium position of the joint. This constraint together with experimental data on descending and segmental afferent pathways to MNs forms the basis of the neuronal model for the C command. In the model, descending systems issue identical signals to agonist and antagonist MNs. To prevent shifts in the equilibrium position, these signals are adjusted by interneurons, in proprioceptive pathways compensating the asymmetry of muscle action.

A relay for input from group II muscle afferents in sacral segments of the cat spinal cord

1. A neuronal relay for input from group II afferents of hindlimb muscle nerves has been found in the previously little explored sacral segments of the cat spinal cord. 2. Electrical stimulation of group II muscle afferents of a number of nerves evoked negative potentials on the surface (cord dorsum potentials) and population postsynaptic potentials (field potentials) within the sacral segments. The largest potentials were evoked by stimulation of the posterior biceps-semitendinosus and triceps surae nerves which evoke much smaller potentials in other segments. Group II afferents of other nerves, notably those which have their main relay within the middle lumbar segments, were much less effective. 3. The sites at which cord dorsum and field potentials evoked by group II muscle afferents were recorded varied in relation to the external topography of the L7-S2 spinal segments but were consistent in their location relative to the pudendal motor nucleus (Onuf's nucleus). Potentials evoked by group II afferents of the posterior biceps and semitendinosus nerves peaked at a level corresponding to the rostral half of Onuf's nucleus and potentials evoked by afferents of the gastrocnemius nerves peaked just rostral to this nucleus. The largest field potentials (of 0.5-1.0 mV) were recorded within the dorsal horn. Field potentials in the intermediate zone were much smaller (< 0.3 mV) and were seen less frequently. 4. Evidence was obtained that the dorsal horn field potentials are to a great extent evoked monosynaptically by the fast conducting fraction of group II muscle afferents: (i) they were evoked at short latencies (2.4-2.7 ms from the stimulus; 1.3-1.7 ms from group I components of afferent volleys and 0.5-0.7 ms from group II components of these volleys), (ii) the conduction times of impulses in the fastest conducting fraction of group II afferents, between the sacral segments (where these impulses were induced by intraspinal stimuli) and the peripheral nerves, were only about 0.5 ms shorter than the latencies of field potentials recorded at the site of intraspinal stimulation and evoked by stimulation of the same peripheral nerves and, (iii) the field potentials followed repetitive stimuli without temporal facilitation. 5. Negative cord dorsum and field potentials were also evoked by small stretches of the semitendinosus and triceps surae muscles. Although they were smaller than potentials evoked by electrical stimulation of sensory fibres and appeared at longer latencies, their presence is consistent with a contribution of muscle spindle afferents to the actions of group II muscle afferents within the sacral segments.(ABSTRACT TRUNCATED AT 400 WORDS)

Gating of transmission to motoneurones by stimuli applied in the locus coeruleus and raphe nuclei of the cat

1. Neuronal systems activated by stimulation in the region of the locus coeruleus/subcoeruleus (LC/SC) and raphe nuclei have previously been shown to depress transmission from group II muscle afferents in regions of the midlumbar spinal segments in which premotor interneurones are located. The aim of the present investigation was to determine the extent to which such depression is paralleled by depression of the reflex actions of group II afferents on motoneurones. 2. The effects of short trains of conditioning electrical stimuli applied within the LC/SC and raphe nuclei were examined on postsynaptic potentials (PSPs) evoked by group I and group II muscle afferents in hindlimb motoneurones. The effects were examined over a wide range of conditioning-test intervals but particular emphasis was placed on the effects produced at long intervals (> 100 ms) since such effects are more likely to be mediated by the descending noradrenergic and serotonergic neurones of the LC/SC and raphe nuclei which are of slow conduction velocity. In addition, conditioning stimuli alone evoked PSPs in motoneurones (with latencies of 7-15 ms and a duration of 50-80 ms) and effects evoked at short conditioning-test intervals might therefore have been secondary to changes in motoneurone membrane properties. 3. At conditioning-test intervals between 100 and 350 ms synaptic actions of group II origin were strongly and consistently depressed. Both EPSPs and IPSPs were affected, two-thirds of those tested being reduced in amplitude by 50% or more. A similar depression was exerted on PSPs evoked from the quadriceps and deep peroneal nerves mediated predominantly by interneurones located in the midlumbar segments and on PSPs evoked from the hamstring and triceps surae nerves mediated by interneurones located in more caudal segments. It is thus concluded that neuronal systems activated by stimuli applied in the LC/SC and raphe nuclei are capable of gating transmission in all those interneuronal pathways which mediate the reflex actions of group II afferents on motoneurones in anaesthetized animals.

Interactions between pathways controlling posture and gait at the level of spinal interneurones in the cat

The properties of three interneuronal populations controlling posture and locomotion are briefly reviewed. These are interneurones mediating reciprocal inhibition of antagonistic muscles and interneurones in pathways from secondary muscle spindle afferents to ipsilateral and contralateral motoneurones, respectively. It will be shown that these interneurones subserve a variety of movements, with functionally specialized subpopulations being selected under different conditions. Mechanisms for gating the activity of these neurones appear to be specific for each of them but to act in concert. Interneurones which are active during locomotion and postural reactions are distributed over many segments of the spinal cord and over several of Rexed's laminae, both in the intermediate zone and in the ventral horn (Berkinblit et al., 1978; Bayev et al., 1979; Schor et al., 1986; Yates et al., 1989). The location of neurones discharging during neck and labyrinthine reflexes is illustrated in Fig. 1A and B but indications that neurones with an even wider distribution contribute to locomotion, scratching and the related postural reactions have been provided by neuronal markers which preferentially label active neurones (WGA-HRP; see Noga et al., 1987) or neurones with active genetic transcription (c-fos; I. Barajon, personal communication; Dai et al., 1991). Such a wide distribution indicates a high degree of non-homogeneity, since neurones of different functional types are usually located in different laminae. It has been demonstrated that some of these neurones may be particularly important for setting up the rhythm of muscle contractions specific for different gaits or scratching, as part of their "pattern generators" (see, e.g., Grillner, 1981). Other neurones may be primarily involved in initiation of these movements or in postural adjustments combined with them. A considerable proportion of neurones mediating these movements are nevertheless likely to be used not in one particular type of movement but in a variety of movements, and contribute to postural reactions and locomotion as well as to various segmental reflexes and centrally initiated movements; they are likely to operate as last order (premotor) interneurones of several spinal pathways to motoneurones. One of the indications that this is the case is the overlap between the areas of location of interneurones active during postural reactions, locomotion, or scratching and the areas of location of premotor interneurones (Fig. 1C,D). The latter were labelled by loading motoneurones with WGA-HRP and by its subsequent retrograde transneuronal transport (see Harrison et al., 1986).(ABSTRACT TRUNCATED AT 400 WORDS)

Locomotor-like rotation of either hip or knee inhibits soleus H reflexes in humans

Human soleus H reflexes are depressed with passive movement of the leg. We investigated the limb segment origin of this inhibition. In the first experiment, H reflexes were evoked in four subjects during (1) passive pedaling movement of the test leg at 60 rpm; (2 and 3) pedaling-like flexion and extension of the hip and the knee of the test leg separately; and (4) stationary controls. In the second experiment, with the test leg stationary, the same series of movements occurred in the opposite leg. Rotation of the hip or the knee of the test leg significantly reduced mean reflex amplitudes (p < 0.01) to levels similar to those for whole-leg movement (mean H reflexes: stationary, 71%; test leg pedaling movement, 10%; knee rotation, 15%; hip rotation, 13% [all data are given as percentages of Mmax]). The angle of the stationary joint did not significantly affect the results. Rotation of the contralateral hip significantly reduced mean reflex magnitudes. Rotation of the contralateral knee had a similar effect in three of the four subjects. We infer that a delimited field of receptors induces the movement conditioning of both the ipsilateral and contralateral spinal paths. It appears that somatosensory receptor discharge from movement of the hip or knee of either leg induces inhibition as the foundation for the modulation of H reflexes observed during human movement.

Distribution of glycinergic terminals on lumbar motoneurons of the adult cat: an ultrastructural study

The distribution of glycine-like immunoreactivity on cat lumbar motoneurons was examined in electron microscopy, using pre-embedding immunocytochemistry. In the dorsolateral portion of the ventral horn, numerous labeled axon terminals were presynaptic to somatic and dendritic profiles of alpha-motoneurons. Most of the glycinergic boutons contained pleomorphic vesicles and showed symmetrical contacts. On the somatic and proximal dendritic compartments, glycinergic terminals accounted for, respectively, 24.6 and 26.6% of the total number of terminals. There were very few glycinergic terminals on gamma-motoneurons. Immunoreactive axons, dendrites and cell bodies were also observed near the motoneurons. These results support the view that glycine plays a major role in the inhibition of alpha-motoneurons and suggest that inhibitory mechanisms occur on the soma as well as on dendrites.

Automatic control of limb movement and posture

Studies are reviewed that address the problem of the variables controlled by the central nervous system in the maintenance of body posture and limb movement against disturbing forces. The role of global variables of control, which take into account the dynamic state of the limb, is discussed. Neural substrates that are involved in the distributed control of kinematic and dynamic parameters are also considered.

Control of functional systems in the brainstem and spinal cord

Progress has been made in the identification of cells, circuits, and networks involved in certain important subcortical functional systems, including swallowing, chewing, posture and locomotion, and in the shared mechanisms for selecting the network for specific motor tasks, including a role for excitatory amino acids for network activation, the shaping of the network by inhibitory control, and the selection of inputs and modulation of outputs by monoamines and other agents.