Functional organization of long ascending propriospinal pathways linking lumbo-sacral and cervical segments in the cat

Lamina VII and VIII neurons of the S2 segment bilaterally projecting to the C6 segment of the spinal cord in the cat

Intracellular and extracellular recordings of antidromic action potentials were applied to investigate neurons of the S2 segment projecting to the C6 segment of the cat spinal cord. The cell bodies were located in laminae VII and VIII of the gray matter while axons ascended in lateral funiculi. Thirty-two out of the total 45 neurons were found to project to the C6 segment bilaterally, seven ipsilaterally and six contralaterally. The axonal conduction velocities were in the 42-96 m/s range and in some neurons were significantly lower in distal parts of axons, supposing that some neurons may give off collateral branches to various segments of the spinal cord. It is discussed if the investigated neurons form a part of the propriospinal system or if their cervical projections are only collaterals of long tracts ascending to supraspinal levels. The organisation of the presented connections between spinal enlargements indicates their contribution in complex mechanisms of co-ordination of movements of the limbs.

Vibration-induced finger flexion reflex and inhibitory effect of acupuncture on this reflex in cervical spinal cord injury patients

The vibration-induced finger flexion reflex (VFR) and the inhibitory effect of acupuncture on this reflex were studied in five cervical spinal cord injury patients (C-SCIs). VFR, which is a tonic finger flexion reflex induced by vibratory stimulation on the finger tip, was induced before and after acupuncture was carried out on the same hand. A stainless steel needle was inserted to the Hoku point. As in healthy subjects, VFR was performed and it was significantly inhibited by acupuncture in the C-SCIs; mean maximum VFR was 204.2 +/- S.E. 68.6 g before and 119.8 +/- S.E. 42.2 g after acupuncture. The present results suggest that at least part of the reflex center for VFR is located in the spinal cord and that part of VFR inhibition by acupuncture may be mediated via the spinal cord.

The development of the long descending propriospinal projections in the opossum, Monodelphis domestica

The origin of the long descending propriospinal (LDP) projections have been studied in adult and developing opossums, Monodelphis domestica. This species has been chosen because of the considerable immaturity of the hindlimbs at birth, the postnatal appearance of their motility and the late development of coordination between them and the forelimbs. Neuroanatomical tracing has indicated that some LDP projections form postnatally. The ones present at birth arise from the regions of the cord where they are the most numerous in the adult opossum, presumptive laminae VII and VIII of the brachial enlargement. Subsequently, LDP projections arise from neurons located in adjacent laminae (IV to VI and IX and X) and at more rostral cervical levels. The origin of LDP projections in the adult opossums generally matches that reported for other mammals. These long propriospinal projections are in place well before the behavioral appearance of coordination between the hindlimbs and the forelimbs, but the timing of their synaptogenesis is not yet known.

Unrestrained walking in cats with partial spinal lesions

In four cats with partial spinal lesions, performed at a low thoracic level, involving ventral quadrants and, to a different extent, the dorsolateral funiculi, several parameters of locomotion were analyzed during unrestrained walking at moderate speed (0.3-1.0 m/s). Special attention was paid to the analysis of support patterns and the durations of support phases in step cycles. The operated subjects displayed a much greater variability of support patterns than intact cats as well as changes in the relative duration of some support phases. The most striking difference was an increase in the relative duration of support on two homolateral limbs accompanied by a reduction of support on diagonal limbs. These changes were mainly due to an impairment of fore-hindlimb coordination as shown by an increase in the phase shifts between the movements of diagonal limbs. Other parameters of locomotion were essentially unaltered, except for cats in which the lesion destroyed bilaterally major portions of the dorsolateral funiculi.

Motor unit discharge characteristics and short term synchrony in paraplegic humans

Frequency of firing and regularity of discharge of human motor units, and short term synchrony between pairs of motor units, have been assessed in extensor digitorum communis (EDC) and tibialis anterior (TA) muscles in control subjects and in clinically complete paraplegic subjects. The discharge pattern of TA motor units in paraplegia ranged from extremely regular to very irregular for different motor units whereas in the control population, and in EDC of both groups, there was a narrow, but intermediate, range of regularity. There was little difference in the incidence and degree of short term synchrony (STS) in EDC between paraplegic and normal subjects. In contrast, virtually no STS of motor units was observed in the TA muscles of the paraplegic group whereas control subjects exhibited approximately the same amount of STS in their TA and EDC muscles. It is concluded that the extra burden placed on arm muscles in paraplegia does not change the amount of synchronisation between motor units. Furthermore, section of the spinal cord does not increase STS as predicted from lesions of the reticulospinal tract in cats. This may reflect the coincidental removal of supraspinal synchronising inputs of motoneurons or the reorganisation of synaptic inputs in chronic paraplegia.

Cutaneus trunci muscle reflex of the guinea pig

The cutaneus trunci muscle reflex in guinea pigs was studied with a combination of video analysis, electromyography, lesioning, and light microscopy. The muscle forms a bilateral, subdermal sheet over much of the trunk. Local contractions of the dorsal part of the muscle are produced in response to brief tactile or electrical stimulation of the skin and consist of a twitch centered 1-2 cm rostral of the stimulus site. The reflex receptive field covers most of the thoracic and lumbar dorsal surface. The sensory information is carried via segmental dorsal cutaneous nerves. Receptive fields of adjacent nerves overlap and form rectangular areas perpendicular to the midline, at thoracic levels. Motor innervation projects through the lateral thoracic nerves of the brachial plexus. The motoneurons are located near the cervical thoracic junction (C7-T1). Lesions of the lower thoracic cord indicate that ascending sensory information is carried to the motor nuclei via the ventral half of the lateral funiculus. This pathway conveys information primarily from ipsilateral skin. There is a weaker input from contralateral skin, crossing at segmental levels. Electromyographic responses to brief electrical stimulation of lower thoracic skin occur usually as 10-12 msec bursts at latencies of 10-20 msec, and do not readily habituate or fatigue at stimulus frequencies below 10 Hz. The reflex persists under light pentobarbital anesthesia. This combination of characteristics makes the reflex useful for a variety of physiological and pathophysiological studies.

Acute effects of electromagnetic stimulation of the brain on cortical activity, cortical blood flow, blood pressure and heart rate in the cat: an evaluation of safety

The influence of repeated high intensity electromagnetic stimulation of the brain on cortical activity, cortical blood flow, blood pressure and heart rate has been investigated in the cat, to evaluate the safety of the method. The observations have been made in preparations under propofol anaesthesia before, during and after periods of anoxia. Electromagnetic stimulation of the brain evoked activity in descending motor pathways and was recorded by activity in the median nerve and by muscle twitches. Following repeated series of high intensity stimulation there were no systematic changes in somatosensory evoked potentials or background EEG, nor were there signs of epileptogenic activity during electromagnetic stimulation, before, during or after periods of anoxia. No systematic changes in cortical blood flow, blood pressure or heart rate were observed during electromagnetic stimulation, before or after periods of anoxia. In conclusion, no acute adverse consequences following electromagnetic stimulation in the normal and anoxic cat brain were demonstrated.

Transneuronal transport of wheat germ agglutinin conjugated horseradish peroxidase into last order spinal interneurones projecting to acromio- and spinodeltoideus motoneurones in the cat. 1. Location of labelled interneurones and influence of synaptic activity on the transneuronal transport

Transneuronal transport of wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) was used to define the location of last order spinal interneurones projecting to deltoideus motoneurones in C5-C8 of the cat. Labelled interneurones were found bilaterally from rostral C1 to caudal Th5 and from L3 to the L4/5 border. Ipsilaterally they were located in laminae V-IX, while contralaterally they were confined to lamina VIII except for a few cells in laminae VII and IX. To estimate the degree to which interneuronal activity facilitates the transneuronal transport from deltoideus motoneurones, the numbers of labelled interneurones were compared under different experimental conditions after WGA-HRP injection. The number of labelled last order interneurones was larger in one awake and active cat than in one awake but inactive cat and also larger in six anaesthetized animals in which spinal pathways were stimulated to evoke antidromic and synaptic activation of the interneurones, than in two anaesthetized animals without stimulation. It is concluded that the transneuronal transport of WGA-HRP is considerably facilitated by increased activity in the last order interneurones. An overall tendency was observed for a positive correlation between the number of labelled interneurones and the number of primarily stained deltoideus motoneurones. In order to reach a detectable concentration of WGA-HRP in the last order interneurones a certain number of motoneurones has to be labelled to the extent that they appear homogenously black.

Changeover from alternate to synchronous bilateral pattern of the phrenic bursts entrained by fictive locomotion in the spinal rabbit preparation

Phrenic bursting resulting from locomotor entrainment during fictive locomotion was shown previously in high spinal preparation after nialamide-DOPA administration. The temporal evolution of the bilateral pattern of phrenic vs locomotor activity is considered here. At variance with the bilateral locomotor pattern which is always alternate (fictive stepping), the pattern on both phrenic nerves changes with time after DOPA injection: first alternate, left and right phrenic bursts become synchronous. A study of ipsilateral phrenic-locomotor phase relationships allowed to disclose the way the transition from alternate to synchronous phrenic coupling was achieved: synchronism appeared as resulting from a strong facilitation on the overlapping parts of the bilaterally alternating phrenic bursts; this phase shifting, vs the ipsilateral locomotor pattern, accounts for the transfer of phrenic bilateral coupling.

Parallel effects of joint receptors on motor neurones and intersegmental interneurones in the locust

At the distal end of a mesothoracic tibia of the locust, Schistocerca gregaria, is a chordotonal organ which monitors the position and movement of the tarsus relative to the tibia. It contains approximately 35 receptors that variously encode different spatial and temporal parameters (position, velocity and direction of movement). Some excite intersegmental interneurones that respond phasically or tonically, with directional sensitivity to active or imposed movements of the tarsus. Some of these interneurones are also excited by intrinsic movements of the tarsal segments. Others, besides being excited by tarsal proprioceptive inputs, are also excited by exteroreceptors on the tarsus. When stimulated mechanically or electrically, chordotonal afferents evoke excitatory postsynaptic potentials with a central latency of between 0.9 and 1.4 ms simultaneously in the intersegmental interneurones and in tarsal motor neurones. The central arborizations of the afferents, the intersegmental interneurones and the tarsal motor neurones overlap in certain neuropilar regions of the mesothoracic ganglion. Other afferents cause an inhibition of the motor neurones, with a longer and non-consistent latency suggesting the involvement of other intercalated interneurones. These results indicate that proprioceptive inputs from the tarsal joint receptors are transmitted in parallel and monosynaptically to tarsal motor neurones and to the intersegmental interneurones.

The effects of 4-aminopyridine on the spinal cord: rhythmic discharges recorded from the peripheral nerves

The effects of an intravenous injection (20 mg/kg) of 4-aminopyridine (4-AP) were initially investigated in acute low spinal cats (Th 13), in which L-DOPA had induced fictive locomotion after paralysis. 4-AP first accelerated the locomotor rhythm and could also change markedly the pattern of activation of some muscle nerves. Shortly after, the locomotor activity was replaced by synchronous rhythmic discharges (2.5-8.5 Hz) in flexor and extensor muscle nerves of the same limb girdle. Similar rhythmic activity was recorded after 4-AP alone (5-20 mg/kg) in the acute decerebrate spinal cat. Whilst the mean rate of the rhythmic activity could differ in the two limb girdles, discharges generated in one girdle appeared to be strongly influenced by those generated in the other. After a complete section of the spinal cord (Th13), the activity persisted in both the rostral and caudal segments although the interactions between the two disappeared. The persistence of the rhythmic activity caudal to the section underscores its spinal origin. In the chronic spinal rat, such rhythmic activity could still be induced in the lumbo-sacral cord despite degeneration of descending pathways. It appears that large doses of 4-AP exert potent effects on the spinal cord which can override other patterns of activity and synchronize the electrical activity of many neuronal elements.

Anatomical organization of long ascending propriospinal neurons in the cat spinal cord

Retrograde transport of lectin-HRP conjugate (WGA-HRP) was used to examine the anatomical organization of long ascending propriospinal neurons (LAPNs) projecting to the cervical enlargement (C5-T1) and to the upper part of the cervical cord (C3-4) in cats. Small injections (0.05-1.0 microliter) of dilute (1-4%) WGA-HRP were made into the C5-T1 or C3-4 regions. The field potential evoked from stimulation of the superficial radial nerve served to position the micropipette delivering injections. Small and localized populations of labelled LAPNs were found in the dorsal horn (laminae IV-V), the intermediate zone (dorsal and medial lamina VII), and the ventral horn (ventral lamina VII, laminae VIII and IX). Ventral horn LAPNs projecting to the C5-T1 region were preferentially located in rostral lumbar regions. Ventral LAPNs projecting to the C3-4 region were more caudally situated. No regional differences in distribution of dorsal horn and intermediate zone LAPNs were noted in comparing the results of C3-4 with C5-T1 injection protocols. It is concluded that the caudally located ventral LAPNs may exert their influence on cervical motor output through C3-4 propriospinal interneurons. Other LAPNs are considered to exert their effect more directly, either at the C5-T1 or the C3-4 levels.

Ascending and descending effects of joint afferent discharge on forelimb and hindlimb flexion reflex excitability in decerebrate cats

In decerebrate cats with intact innervation of the fore- and hindlimbs, flexion reflexes are most easily elicited in forelimb muscles when the hindlimb is extended, and hindlimb flexion reflexes are most easily elicited when the forelimbs are extended. After intra-articular injection of local anaesthetic, this modulation of reflex excitability is abolished. Thus, in addition to their known segmental effects, joint afferents also exert significant ascending and descending effects on motoneurone excitability.

Neurobiological bases of rhythmic motor acts in vertebrates

The general principles governing the nervous control of innate motor acts in vertebrates are discussed. Particular consideration is given to the control of locomotion in both mammals and lower vertebrates. One in vitro model of the lamprey central nervous system has been developed. It can be maintained in vitro for several days and the motor pattern underlying locomotion can be elicited in isolated sections of the spinal cord. These findings now allow a detailed analysis of the underlying neural mechanisms. The hypothesis that different parts of the network controlling locomotion can be used in a variety of other motor acts, including learned ones, is reviewed.

Mathematical Models of Central Pattern Generators in Locomotion

As a background for subsequent studies of mathematical models of central pattern generators in locomotion (Stafford & Barnwell, 1985a, b) relevant aspects of the literature on locomotion are reviewed, concepts of locomotion discussed, and extant models considered. Advantages and disadvantages of present models are discussed, and the need for mathematical models is emphasized. It is shown that realistic models of pattern generation in locomotion must take numerous factors into account, including phases of step cycle, muscle sequencing, gait and interlimb coordination, initiation and cessation of locomotion, and many aspects of neuromuscular control and function.

Mathematical models of central pattern generators in locomotion: III. Interlimb model for the cat

Possible neural connective patterns and functions with respect to interlimb coordination are studied theoretically with a mathematical model of the central pattern generating system for cat locomotion. Activities in populations of neurons controlling limb joint flexors and extensors in all four limbs are represented by a system of nonlinear differential equations. Solutions of the system for various parameter values simulate various gaits of the cat. The model is shown to be capable of generating all gaits of the cat and accounting for corresponding phase changes in interlimb coordination. The model also exhibits smooth changes of gait, and smooth initiation and termination of stepping. Further, within each limb, muscle sequencing, step cycle phases, and flexor-extensor interactions can be studied. The model suggests that one of the simplest mechanisms for a central command system to change the gait is via inhibition of specific interlimb propriospinal pathways. In a final section, properties of both proposed single limb and interlimb models are reviewed with specific reference to planning future experimental and theoretical studies.

Descending lumbosacral cord potentials (DLCP) evoked by stimulation of the median nerve

In 22 normal human subjects, descending lumbosacral cord potentials (DLCP) were recorded intrathecally after stimulation of the median nerve at the elbow. The onset of DLCP is very short in latency (mean 12.1 ms) with a prominent sharp early positive peak (mean latency 13.7 ms) followed by a sharp negative peak (mean 17.6 ms). The amplitude of the first part of DLCP varied between 0.6 and 6.7 microV (mean 2.3 microV). The response was recorded most easily when the tip of intrathecal electrode was posterolaterally positioned. The threshold of the response was above or around the excitation threshold of the motor nerve fibers and it could not be produced by pure skin nerve stimulation. It resisted to subtetanic peripheral shocks. Mean peripheral conduction velocity responsible for the response was about 60.8 m/s. Some late and slower deflections appeared in many cases. It was concluded that the DLCP must have originated from the descending and very fast conducting propriospinal pathways located within the anterolateral funiculus which has an oligosynaptic anatomical organization. This response seemed to be the first direct evidence of interlimb reflex action between the arm and leg in man which is important in the coordination of movements and posture.

Disruption of fore- and hindlimb coordination during overground locomotion in cats with bilateral serial hemisection of the spinal cord

In order to investigate if inter-limb propriospinal reflexes participate in coordination of locomotive movements of fore- and hindlimbs, we examined the relations between fore- and hindlimbs during overground locomotion of adult cats with spinal cord lesions. In a group of cats (T-T preparations), the spinal cord was hemisected first at around Th12 and then at intervals of 37-126 days contralaterally at mid-thoracic level, propriospinal tracts being mostly severed in this group. In a second group of cats (C-T preparations), which received hemisections first at around C2 and then at intervals of 21-73 days at mid-thoracic level, propriospinal tracts were left intact at least on one side of the spinal cord. Control observations were also made in intact cats and those with single hemisections at C2 or Th12, or with double unilateral hemisections at Th6 and Th12. Thus, it was found that in both T-T and C-T preparations, step length of the forelimbs was shortened significantly, whereas that of the hindlimbs was significantly lengthened. Furthermore, phase relations between the fore- and hindlimbs were completely lost in these preparations, suggesting that the stepping generator for the forelimbs operates independently of that for the hindlimbs. In other single-hemisected or unilaterally double-hemisected preparations, by contrast, no such changes were observed. The close similarity of the results in T-T and C-T preparations, in spite of different degrees of impairment of propriospinal tracts in them, leads to a conclusion that inter-limb propriospinal reflexes play little role in coordination of locomotive movements of fore- and hindlimbs.

Spinal afferents to lamina IX of the cervical enlargement in the rat studied by the retrograde transport of horseradish peroxidase

Spinal neurons that project to the ventrolateral, dorsolateral and ventromedial portions of lamina IX of the cervical enlargement in the rat were investigated by means of horseradish peroxidase retrograde transport. In the cervical and upper thoracic segments, labeled neurons were observed ipsilaterally in laminae V-VIII and contralaterally in laminae VII-VIII. In the lower thoracic and upper lumbar segments, labeled neurons were seen after HRP injection into the ventrolateral part of lamina IX, and were distributed mainly in the lateral parts of the ipsilateral laminae V-VI.

Depression of activities of dorsal horn convergent neurones by propriospinal mechanisms triggered by noxious inputs; comparison with diffuse noxious inhibitory controls (DNIC)

The ability of heterotopic noxious stimuli to inhibit the activity of dorsal horn convergent neurones was investigated in both intact anesthetized, and spinal unanesthetized rats. Forty-four convergent neurones in lumbar dorsal horn were recognized by their ability to respond to both noxious and non-noxious natural stimuli and by their characteristic responses corresponding to A- and C-fibre activity following electrical stimulation of their cutaneous excitatory receptive fields on the ipsilateral hindpaw. The application of a sustained pinch to the excitatory receptive field resulted in an initial phasic activation of the neurone, which adapted to a stable tonic level of activity (mean 31.8 +/- 2.2 spikes/s). The levels of activity produced in this fashion were not appreciably different between the two types of preparation. In the intact anesthetized rat, the tonic activity produced by the sustained pinch could be strongly depressed by noxious conditioning stimuli applied to various parts of the body for all 10 neurones studied: heating the tail or pinching the contralateral hindpaw, the tail or a forepaw during 30 s each resulted in comparable inhibitions which had mean values in the order of 80% and which were always marked by post-effects lasting for upwards of 30 s. These inhibitory effects have been called Diffuse Noxious Inhibitory Controls (DNIC). In the spinal unanesthetized rat, the tonic activity was depressed to some extent by the same conditioning stimuli, for only 16/34 neurones studied. By comparison with the intact animals these inhibitions were weak, adapted to base-line levels within 30 s and were more marked for conditioning stimuli applied to structures proximal (tail, contralateral hindpaw) to the excitatory receptive field than for stimuli applied more distally (forepaws). The differences between the inhibitions found in the intact and spinal preparations were subsequently confirmed in a series of experiments in which single convergent neurones were studied before and after the pharmacological blocking of the cervical spinal cord in anaesthetized rats. The results in the spinal preparations provide evidence for the existence of some propriospinal modulatory processes, triggered by the onset of noxious stimulation and acting on convergent neurones. These processes appear to be different from those mediating DNIC, which have been shown to involve supraspinal structures, to concern all convergent neurones, to be very potent and associated with long-lasting post-effects whether the conditioning noxious stimuli are applied to parts of the body proximal or distal to the excitatory receptive field.

Interlimb coordination during stepping in the cat: in-phase stepping and gait transitions

The coordination of step cycles between all 4 limbs during in-phase stepping and during transitions to and from alternate stepping was studied in 12 adult cats during repeated overground stepping trials. The temporal spacing of step cycles of the different limbs was determined from analysis of electromyographic (EMG) activity in a single extensor muscle of each limb. Patterns of coordination of the different limbs were established on the basis of the frequency with which phase values separating step cycles were encountered. Steps in which the phasing of step cycles of the two hindlimbs were closer to true in-phase coordination than true alternation (phase between 270 degrees and 90 degrees) and where similar coupling was found in both the preceding and following steps were defined as steady state conditions. Distinct patterns of coordination of forelimb-forelimb and forelimb-hindlimb step cycles were noted under steady state conditions. During stepping sequences which include transitions either to or from alternate stepping, both gradual and abrupt phase changes were found. The changes in both forelimb-forelimb and forelimb-hindlimb phase relationships were more often gradual than abrupt. Where abrupt changes were encountered in the change in phase relationships between one such limb pair the phase change in the other pair was gradual. Changes in hindlimb-hindlimb phase relationships during transitions were nearly always abrupt. It is concluded that the 4 limbs are coordinated during in-phase stepping according to a few patterns, but that the variability about these patterns makes their association with simple neural circuitry rather speculative. The finding that transitions were most often gradual is interpreted in terms of a state-dependent model of interlimb control, in which the type of transition utilized depends on the strength of neural coupling of step cycles of all 4 limbs at the time that the transition is initiated.

The cytoarchitecture of GABAergic neurons in rat spinal cord

Glutamic acid decarboxylase (GAD), the enzyme that synthesizes the transmitter gamma-aminobutyric acid (GABA), has previously been localized within synaptic terminals in rat spinal cord by immunocytochemistry. In the present study, GAD was localized within the somata and dendrites of GABA neurons following colchicine injections into rat lumbar spinal cord. All regions of the spinal gray matter contained GAD-positive somata except the motoneuron pools (lamina IX). GAD-positive somata also were observed in the ependymal layer and in the dorsolateral funiculus. Small GAD-positive somata, averaging 9 X 13 micrometer in size, were located in laminae I-III, and the size of GAD-positive somata increased for cells located in progressively more ventral laminae, reaching a maximum in lamina VII where somal size averaged 12 X 19 micrometer. Lamina I contained two classes of GAD-positive cell bodies; lenticular shaped, intermediate size neurons that were reminiscent of stalked cells, and a smaller cell type that was elongated in the sagittal plane. GAD-positive somata in laminae II and III had the size and position of islet cells. In laminae IV-VI, GAD-positive somal profiles averaged 12 X 17 micrometer in size. Lamina IV neurons were concentrated along laminar edges, while those in laminae V and VI were distributed more homogeneously. In lamina VIII, GAD-positive cell bodies appeared in groups of 3 or 4 and were smaller than those in lamina VII. Lamina X contained GAD-positive somal profiles averaging 12 X 16 micrometer in size. In the ependymal layer, there were two types of cerebrospinal fluid (CSF)-contacting neurons that contained GAD; one spherical and the other elongated. Both types sent extensions into the central canal where these processes expanded into 4-5 micrometer-wide end bulbs. CSF-contacting cells with sizes and shapes similar to the GAD-positive ones were seen to receive synapses in electron micrographs. The widespread distribution of GABA neurons in spinal cord was suggestive of diverse functions for these cells, encompassing conventional synaptic roles and, perhaps, an involvement in hormonally modulated communication via GABAergic, CSF-contacting neurons.

Interlimb reflexes evoked in human arm muscles by ankle displacement

Interlimb reflexes evoked by ankle displacements were studied in arm muscles of 6 normal subjects. EMGs from grastrocnemius (G), tibialis anterior (TA), biceps brachii (BB), and triceps brachii (TB) were amplified, rectified and low-pass filtered before recording. Averaging and Wiener filtering were used to detect changes in tonic EMG activity evoked by dorsiflexing or plantarflexing displacements of the ankle. A consistent pattern of response was observed in all subjects. In the leg muscles, the responses to stretch were consistent with previous reports. In the arm muscles, the response of TB was dominant. Dorsiflexing displacements of the ankle evoked a small excitation followed by a more marked decrease in TB activity but had no effect on BB. In contrast, plantarflexing displacements of the ankle resulted in a large, early period of excitation followed by a decreased level of activity in TB. A similar but smaller pattern of activity was observed in BB. It is notable that the TB responses to displacement were sizable, often modulating the tonic EMG activity by as much as 80%. Interlimb reflexes evoked by ankle displacement were larger and of shorter latency than those evoked by cutaneous electrical stimulation of the foot reported previously. This suggests that proprioceptive afferents may have stronger and more direct interlimb reflex connections than cutaneous afferents and may therefore play an important role in the coordination of movement.

Propriospinal fibers interconnecting the spinal enlargements in some quadrupedal reptiles

The cells of origin, course and site of termination of long propriospinal fibers interconnecting the intumescences have been studied with the aid of the horseradish peroxidase technique, as well as with anterograde degeneration techniques, in some quadrupedal reptiles (the lizards Tupinambis nigropunctatus and Varanus exanthematicus, and the turtles Testudo hermanni and Pseudemys scripta elegans). The anterograde degeneration findings suggest that long descending propriospinal fibers from the cervical intumescence are distributed bilaterally to the ventral gray of the lumbar enlargement, including the lateral motoneuron column. Long ascending fibers from the lumbar to the cervical intumescence are distributed, also bilaterally, to the ventromedial part of area VII--VIII. The cells of origin of long descending propriospinal fibers wre found in the medial part of area VII--VIII in the cervical intumescence, particularly contralateral to the injection side. The cells of origin of long ascending propriospinal fibers were found in the lumbar intumescence, also particularly contralateral to the injection side, in the ventromedial part of area VII--VIII. It seems likely that in the reptiles studied the long propriospinal fibers interconnecting the spinal enlargements are in large part organized as crossed connections. The demonstration of long propriospinal fibers in lizards and turtles--i.e. quadrupedal reptiles that move their limbs in a particular diagonal pattern--suggests that these pathways are of great importance for the coordination of forelimb and hindlimb movements.

Responses of long descending propriospinal neurons to natural and electrical types of stimuli in cat

Long descending propriospinal (LDP) neurons (antidromically identified) having cell bodies of origin in the cervical enlargement and projecting axons at least as far as the L2 segment were studied. Extracellular recording of responses to natural and electrical stimuli was done in high-spinal cats. (1) A receptive field for natural stimuli was found for 123 LDP neurons. An additional 108 LDP cells were not activated by the natural stimuli used, but some of these fired spike potentials in response to electrical stimulation of peripheral nerves of the forelimb. There was no distinction between neurons activated and those not activated by natural stimuli on the basis of location or conduction velocity. (2) The most effective natural stimuli were mechanical manipulation of the skin (both low and high threshold), movement of joints of the paw, and pressure to the deep tissues, especially to the extensor side of the arm. These modalities of stimuli were most often excitatory, but could be inhibitory as well. (3) On the basis of modality, 4 subgroups of LDP cells were identified: those which were responsive only to mechanical-cutaneous, joint-movement, or deep-pressure stimuli, and those which responded to several of these modalities of stimuli, the multimodal group. These subgroups could not be distinguished on the basis of conduction velocity. (4) The receptive fields varied in size from small (one digit) to large (all of a forelimb). For single LDP cells they included ones with single and/or multimodal input from one or both forelimbs and various combinations of excitation and/or inhibition. However, those in the dorsal horn had only ipsilateral receptive fields, mainly of the mechanical-cutaneous type. Cells with bilateral receptive fields were mainly located medially in the ventral gray in laminae VII and VIII. (5) A comparison of the location of the subtypes of LDP cells revealed that neurons activated by mechanical-cutaneous stimuli were in laminae I and IV-VIII; whereas deep-pressure and multimodal activated neurons were almost exclusively in laminae VII and VIII. (6) LDP cells receiving input from deep-pressure receptors of the paw probably relay position or weight-bearing information about the forelimbs to the lumbosacral spinal cord. This arrangement suggests that LDP neruons function in interlimb coordination and would be active during locomotion. They probably participate also in other reflexes elicited by cutaneous and deep stimuli.

The location of spinal neurons with long descending axons (long descending propriospinal tract neurons) in the cat: a study with the horseradish peroxidase technique

The distribution spinal neurons with long descending axons was studied in the cat by means of retrograde transport of horseradish peroxidase. Labeled neurons appeared bilaterally in the cervical and the thoracic cord following injections in the lumbosacral cord. In some cases hemisections were made rostrally and contralaterally to the injections in an attempt to determine whether or not the axons crossed. Neurons with uncrossed descending axons were located in laminae I, V, VII and VIII. Lamina I neurons were present in all the spinal segments. In lamina V labeled neurons were distributed mainly laterally in the cervical cord but medially and laterally in the thoracic cord. In the upper cervical and the thoracic cord laminae VII and VIII neurons were distributed very densely along the lateral cord, accounting for 30 and 40 of the total labeled neurons, respectively. In the cervical enlargement they were located in the middle part of lamina VII and in lamina VIII, accounting for about 25% of the total labeled neurons. Neurons with crossed descending axons were found in laminae V, VII and VIII, in the medial part of lamina VII including the intermediomedial nucleus of the thoracic levels and close to the central canal. Lamina V neurons were very small in number. The largest collections of labeled neurons were present in the medial part of laminae VII and VIII. They accounted for about 45% to 55% and 37% of the total in the cervical and the thoracic cord. These neurons may function as the long spinal reflex paths for forelimb-hindlimb synergies and the intercalated paths between the supraspinal descending tracts and the spinal motor centers.

Cells of origin of propriospinal fibers and of fibers ascending to supraspinal levels. A HRP study in cat and rhesus monkey

In the spinal cord of cat and rhesus monkey the cells of origin of long and short propriospinal fibers and those of the spinal fibers ascending to supraspinal levels were identified by means of retrograde HRP labeling after large unilateral HRP-injections, i.e. in the spinal white and grey matter at different levels, in the pons and in the dorsal column nuclei. The findings indicate the existence of the following arrangement. Long ascending supraspinal fibers arise mainly from neurons in the dorsal grey (laminae I-IV and the medial parts of laminae V and VI) as well as from neurons in the medial part of the ventral grey (lamina VIII), in Clarke's column and in the spinal border cell area. Some neurons in the dorsal grey projects to the dorsal column nuclei, which in turn distribute fibers back to the spinal cord. Long propriospinal fibers mainly derived from neurons in the medial part of the ventral grey (lamina VIII), while short propriospinal fibers are characteristically derived from neurons in the intermediate zone (lateral halves of laminae V and VI and lamina VII). The neurons located laterally in laminae V-VII distribute fibers mainly ipsilaterally, while those located medially in lamina VII distribute them to some degree bilaterally. The findings in cats with transections of either the dorsal or the ventral halves of the spinal white matter (both above and below the injected segment), show that the fibers from the dorsal grey and the lateral parts of laminae V-VII travel mainly through the dorsal half of the white matter, while those from the medial part of lamina VII and from lamina VIII travel mainly through the ventral half.

Reversal of sign of long spinal reflexes dependent on the phase of the step cycle in the high decerebrate cat

Reversal of the sign of long ascending and descending spinal reflexes dependent on the phase of the step cycle has been shown in the high decerebrate cat. Electrical stimuli were applied to the skin over the dorsum of the metacarpals and metatarsals and the effects were recorded in the corresponding hindlimb or forelimb of the same side. Stimulation generally evoked an increase in activity of flexor or extensor muscles shortly before, and during, activity of the muscle. Inhibition of a muscle occurred when its activity overlapped with the activity of an antagonist muscle. From the latencies it was concluded that at least the early components of the long reflex responses are due to spinal mechanisms. In the stimulated limb the activity of flexor and extensor muscles was also modulated in a phase dependent manner. No consistent long spinal responses to skin stimulation were obtained in high spinal cats during stepping. Electrical stimulation of cutaneous nerve trunks in decerebrate preparations during stepping gave variable though phase dependent long spinal reflex effects.

Coordination of movements of the kindlimbs and forelimbs in different forms of locomotion in normal and decerebrate cats

The coupling of movements of the hindlimbs and forelimbs has been analysed in intact cats stepping overground and on a treadmill and during swimming, and in decerebrate cats stepping on a treadmill, immersed in water('swimming') and stepping suspended in the air. In the different preparations, and under different types of locomotion, two basic patterns of coupling have been observed. Both concern the hindlimb and forelimb of the same side of the body. The first pattern is found in the pacing gait where flexion of the forelimb precedes extension of the hindlimb, measured at the elbow and knee, respectively. The second pattern is typically found in the trot where flexion of the forelimb follows extension of the hindlimb. In decerebrate cats both patterns of coupling remain after bilateral deafferentation of the hindlimbs. In the alternate form of locomotion these patterns of coupling occurs symmetrically on both sides. In the rotatory and transverse gallop (examples of the in-phase form of locomotion) the coupling is asymmetrical: on one side it is comparable to pacing (forelimb flexion precedes hindlimb extension), and on the other side to trotting (forelimb flexion follows extension). These basic patterns of interlimb coordination simplify considerably the problem of neural control of the limbs in locomotion. Obersations of EMGs during the alternative forms of locomotion show that in the pacing type of coupling the extensor EMGs of forelimb and hindlimb overlap, with the hindlimb leading the forelimb by about 10% of a step cycle, while in the trotting type of coupling the forelimb flexor EMGs overlap the hindlimb extensor EMGs, the forelimb flexors leading the hindlimb extensors by about 10% of a step cycle. During acceleration the transition between the two forms of EMG occurs within one or two step cycles, and at some intermediate velocities the EMG coupling springs back and forth between the two different forms. These results further support the hypotesis of two basic forms of interlimb coupling in which long propriospinal pathways probably play a role.

Movements of the forelimbs of the cat during stepping on a treadmill

In normal cats stepping on a treadmill an analysis has been made of movements of the scapula, shoulder, elbow, wrist and digits. The scapula is capable of making large and complicated movements over the rib cage. In locomotion they may be resolved into a cyclical, similunar movement of the glenoid cavity in the parasagittal plane of the cat's body. The movements of the scapula are rather constant over a wide range of velocities. They most resemble those of the hip which also shows a delay in the onset of extension and no yield phase. It is suggested that movements of the scapula have a high priority in the execution of locomotion in the forelimbs. The movements at the shoulder tend to parallel those of the elbow. The terms of palmar flexion and dorsiflexion are retained for the movements at the wrist and digits. The rapid phase of palmar flexion corresponds with the flexion phase at the elbow, and the phase of dorsiflexion with the first extension phase at the elbow. Palmar flexion during the second and third extension phases of the more proximal joints would appear to be initiated by contact of the foot with the ground. The sequences of movement at the wrist and digits allow a comparison of the roles of flexor and extensor muscles of the forearm during locomotion and the flexion reflex, which is compatible with the functional and anatomical organization of cervicothoracic segments. With exception of extension at the scapula the movements of the elbow provide an index of the onsets of flexion and extension at the other joints of the forelimb. Modifications are suggested for the Phillippson step cycle of both forelimbs and hindlimbs to include the delayed onset of extension at the scapula and hip and the particular movements of the wrist, digits and toes.