Cutaneous dermatomes for initiation of three forms of the scratch reflex in the spinal turtle

The turtle spinal cord produces three forms of the hindlimb scratch reflex. Each scratch form is initiated in response to gentle mechanical stimulation of a distinct set of sites in the periphery, termed the receptive field for that scratch form. The turtle spinal cord consists of 8 cervical segments (C1-C8), 10 dorsal segments (D1-D10), 2 sacral segments (S1, S2), and about 16 caudal segments (Ca1-Ca16). First, we recorded cutaneous afferents in peripheral nerves to reveal the tactile dermatomes of segments D3-D8. These segments innervate regions of the body between the forelimb and hindlimb, directly lateral to their spinal cord segments. Adjacent segments innervate adjacent and partially overlapping regions of the periphery. Second, we used successive spinal cord transections combined with either a) behavioral analysis in turtles with limb movements or b) electroneurographic recordings in immobilized turtles, and mapped the zone of remaining sensibility after each transection to measure the borders of dermatomes D2-Ca2. This technique revealed that adjacent dermatomes are innervated by non-adjacent spinal segments in regions near the hip. Segments D8 and Ca1 innervate adjacent and partially overlapping regions ventral to the hip. There is a similar discontinuity in the innervation of the shell and skin dorsal to the hip. These discontinuities correlate with the innervation of the hindlimb skin by segments D8-Ca1. The rostral scratch receptive field is innervated by sensory afferents entering spinal segments D3-D6; the pocket scratch receptive field is innervated by D6-D8; the caudal scratch receptive field is innervated by S2, Ca1, and more caudal segments. The rostral-pocket transition zone is innervated mainly by one segment, D6; the ventral part of the caudal-pocket transition zone is innervated by two non-adjacent segments, D8 and Ca1. Thus the motor pattern blends elicited by stimulation of sites within the rostral-pocket transition zone must be produced in response to a very different distribution of sensory inputs than the blends elicited by stimulation of sites within the caudal-pocket transition zone.     

Glutamate antagonists applied to midbody spinal cord segments reduce the excitability of the fictive rostral scratch reflex in the turtle

Glutamate antagonists applied to the cutaneous-processing region of the rostral scratch circuit in turtles reduced the excitability of the rostral scratch reflex. Segments D3-D6 (D3 = 3rd postcervical) of the midbody spinal cord receive cutaneous afferents from the rostral scratch receptive field and perform the initial integration of this cutaneous sensory input. These cutaneous-processing segments are located anterior to the rostral scratch motor pattern generator that resides mainly in segments D7-D10 located in and near the hindlimb enlargement. We prepared 1 or 2 of the midbody segments for bath application of glutamate antagonists in preparations with a complete transection of the spinal cord anterior to segment D3. Each preparation was immobilized by neuromuscular blockade and fictive scratch motor output was recorded from hindlimb muscle nerves. Application of the NMDA N-methyl-D-aspartate) antagonist APV (D-2-amino-5-phosphonovaleric acid, 50 microM) to a midbody segment significantly reduced the motor burst frequency of rostral scratch responses evoked by 3-Hz electrical stimulation of a site in that segment's dermatome. These data suggest that NMDA receptors contribute to cutaneous processing in the rostral scratch circuit. Application of APV to a midbody segment also reduced the magnitude of temporal summation in the scratch circuit in response to electrical stimuli delivered to the shell at 4- to 5-s intervals. Temporal summation was monitored at the level of hindlimb motor output as well as at the level of unit activity from 'long-afterdischarge' neurons in the midbody segments. Our observations are consistent with the hypothesis that NMDA receptors contribute to the prolonged activation of 'long-afterdischarge' neurons and the multisecond storage of excitation in the scratch reflex pathway.     

N-methyl-D-aspartate antagonist applied to the spinal cord hindlimb enlargement reduces the amplitude of flexion reflex in the turtle

APV (D(-)-2-amino-5-phosphonovalerate), an NMDA (N-methyl-D-aspartate) antagonist, was applied in situ onto segments of the hindlimb enlargement of the turtle spinal cord. APV reduced the response amplitude of the flexion reflex. In contrast, APV did not alter the responsiveness of the rostral scratch reflex. Afferents for the flexion reflex enter the spinal cord via the dorsal roots of the middle segment of the hindlimb enlargement; afferents for the rostral scratch reflex enter the spinal cord via dorsal roots located anterior to the hindlimb enlargement. The results are consistent with the hypothesis that sensory interneuron NMDA receptors, synaptically activated either directly or indirectly by nearby cutaneous afferent axons, play a role in the spinal cord processing of cutaneous information.     

Motor pattern deletions and modular organization of turtle spinal cord

The turtle spinal cord contains a central pattern generator (CPG) that produces rhythmic hindlimb motor patterns during a rostral scratch. This review describes evidence in support of the hypothesis that the turtle rostral scratch CPG has a modular structure similar to that described in the Unit-Burst-Generator hypothesis for cat locomotion by Grillner. During normal rostral scratch in turtle, activity bursts rhythmically alternate with quiescence for each motor neuron pool; agonist activity rhythmically alternates with antagonist activity at each degree of freedom, e.g., hip, knee; and a transition from knee flexor to knee extensor motor neuron activity occurs midway during each hip flexor motor neuron burst. Hip extensor deletions, knee flexor deletions, and knee extensor deletions are motor pattern variations of rostral scratch. During each of these variations, agonist activity is rhythmic; antagonist activity and agonist quiescence are absent. Several classes of evidence during both normal and variation motor patterns support a modular organization of the turtle rostral scratch CPG: electroneurographic recordings from axons of motor neurons, intracellular recordings of synaptic potentials in motor neurons, and extracellular unit recordings from spinal interneurons. These data support the hypotheses that the knee extensor module is different from the hip extensor module and that the knee flexor module is different from the hip flexor module. Potential mechanisms for rhythmogenesis include reciprocal connections between agonist and antagonist modules at each degree of freedom, and agonist module rhythmogenesis. Additional tests of the modular hypothesis for turtle rostral scratch include unit recordings from knee-related interneurons during normal rostral scratch, as well as during knee-related deletions.     

Blends of rostral and caudal scratch reflex motor patterns elicited by simultaneous stimulation of two sites in the spinal turtle

Simultaneous tactile stimulation of 2 sites on the body surface of a spinal turtle elicits complex blends of the scratch forms and motor patterns associated with each site. Our previous work has utilized 1-site stimulation to elicit distinct forms of the scratch reflex in the spinal turtle (Mortin et al., 1985; Robertson et al., 1985). Using this paradigm, stimulation of a site on the shell bridge anterior to the hindlimb elicits a rostral scratch reflex in which the dorsum of the foot rubs against the stimulated site; stimulation of a site near the tail elicits a caudal scratch reflex in which the heel or side of the foot rubs against the stimulated site (Mortin et al., 1985). During each scratch cycle, the monoarticular knee extensor muscle is active when the limb rubs against the stimulated site, and there is rhythmic alternation between hip protractor and hip retractor muscle activity (Robertson et al., 1985). In a rostral scratch, the monoarticular knee extensor muscle is active during the latter portion of hip protractor muscle activity; in a caudal scratch, the monoarticular knee extensor muscle is active near the end of hip retractor muscle activity. Pure-form motor patterns that are similar to those recorded from these muscles during movement can be recorded from the corresponding nerves in a spinal turtle immobilized with a neuromuscular blocking agent (Robertson et al., 1985). In this paper, we describe blend responses to simultaneous stimulation of 2 sites, one in the rostral scratch and the other in the caudal scratch receptive field. During these blends, the responding hindlimb rubs against both stimulated sites in one continuous movement sequence.(ABSTRACT TRUNCATED AT 250 WORDS)     

Propriospinal projections to the ventral horn of the rostral and caudal hindlimb enlargement in turtles

In limbed vertebrates, the capacity to generate rhythmic motor patterns for locomotion and scratching is distributed over spinal cord segments of the limb enlargement (e.g., lumbosacral segments), but within this region, rostral segments are more rhythmogenic than caudal segments. The underlying reasons for this rostrocaudal asymmetry are not clear. One possibility is that rostral and caudal segments receive distinct sets of propriospinal projections. To test this hypothesis, I injected horseradish peroxidase (HRP) into the ventral horn unilaterally in a rostral or caudal segment of the turtle hindlimb enlargement. I quantitatively assessed the distributions of retrogradely labeled neurons in six hindlimb enlargement and pre-enlargement segments. The cross-sectional distribution did not depend on which segment was injected. Ipsilateral labeling occurred predominantly in the deep dorsal horn, the lateral part of the intermediate zone, and the dorsal two-thirds of the ventral horn, while contralateral labeling occurred mainly in the medial part of the ventral horn and the lateral part of the intermediate zone. This cross-sectional distribution is similar to what has been seen in mammals. The rostrocaudal distribution of labeled cells, however, depended on which segment was injected. Rostral injections gave rise to rostrally skewed distributions, dominated by descending propriospinal neurons. Caudal injections gave rise to caudally skewed distributions, dominated by ascending propriospinal neurons. Thus, rostral segments of the hindlimb enlargement received more propriospinal inputs from immediately rostral than immediately caudal segments, while the reverse was true for inputs to caudal segments. This anatomical asymmetry may contribute to known functional asymmetries within the enlargement.     

Interruptions of fictive scratch motor rhythms by activation of cutaneous flexion reflex afferents in the turtle

A low-spinal immobilized turtle displays a fictive scratch reflex in hindlimb muscle nerves in response to mechanical stimulation of specific regions of the shell (Robertson et al., 1985). There are 3 forms of the scratch reflex: the rostral, the pocket, and the caudal; each exhibits rhythmic activation of hindlimb motor neurons. Cutaneous stimulation of the distal hindlimb elicits a fictive flexion reflex that exhibits tonic excitation of hip protractor (flexor) motor neurons and tonic inhibition of knee extensor motor neurons (Stein et al., 1982). In the present study, we describe the motor pattern blends that resulted from transient activation of either the ipsilateral or the contralateral flexion reflex pathway during ongoing scratch motor patterns. Two types of blends were observed: (1) insertions of a flexion reflex synergy into an interrupted scratch cycle and (2) deletions of parts of a scratch cycle. Associated with each type of motor pattern blend was a permanent reset of the ongoing scratch rhythm. The sign of the reset (phase-advance or phase-delay) could be predicted for all forms of the scratch based on the location of the foot stimulus (ipsi- or contralateral) and its timing relative to the hip protractor/retractor cycle. The timing of knee extensor activity within the hip cycle is different for each form of the scratch (Robertson et al., 1985); thus, the sign of the reset cannot be predicted from the timing of the stimulus relative to the knee extensor cycle. These results indicate the importance of the hip rhythm in determining the overall timing of the scratch reflex.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sonic motor pathways in piranhas with a reassessment of phylogenetic patterns of sonic mechanisms among teleosts

Sound production has evolved independently a number of times among teleost fishes. In most cases, sound is generated by fast contracting muscles that vibrate the swim bladder by way of their direct attachment (intrinsic muscles) or indirectly by way of other skeletal elements (extrinsic muscles). This study focuses on the red and black piranha, Pygocentrus nattereri and Serrasalmus rhombeus (superorder Ostariophysi, Order Characiformes), that have extrinsic swim bladder sonic muscles innervated by the third and fourth spinal nerves. This innervation pattern diverges from that found in most teleosts, including the closely related catfishes (Ostariophysi, Siluriformes), where sonic muscles are innervated by ventral occipital nerve roots that arise just caudal to the vagus nerve. Here, we tested the hypothesis that piranhas would also differ from most other teleosts in the location of their sonic motor neurons. Following biotin labeling of branches of the third and fourth spinal nerves that innervate the sonic muscles in the red and black piranha, sonic motor neurons were identified amongst other non-sonic motor neurons in the central part of the spinal cord, slightly ventrolateral to the central canal. To our knowledge, this is the first example of sonic motor neurons positioned entirely within the spinal cord. In the other species so far studied, sonic motor neurons form well-defined nuclei that extend from far caudal levels of the medulla into the rostral spinal cord and are located either within the ventral motor column or near the midline, close to or just ventrolateral to the fourth ventricle and central canal. A piranha-like pattern may be more widespread among characiforms and is likely present in other teleost orders, e.g., Sciaenidae (drumfishes), that also have sonic muscles innervated by spinal nerves.     

Noradrenergic innervation of the rat spinal cord caudal to a complete spinal cord transection: Effects of olfactory ensheathing glia

Transplantation of olfactory bulb-derived olfactory ensheathing glia (OEG) combined with step training improves hindlimb locomotion in adult rats with a complete spinal cord transection. Spinal cord injury studies use the presence of noradrenergic (NA) axons caudal to the injury site as evidence of axonal regeneration and we previously found more NA axons just caudal to the transection in OEG- than media-injected spinal rats. We therefore hypothesized that OEG transplantation promotes descending coeruleospinal regeneration that contributes to the recovery of hindlimb locomotion. Now we report that NA axons are present throughout the caudal stump of both media- and OEG-injected spinal rats and they enter the spinal cord from the periphery via dorsal and ventral roots and along large penetrating blood vessels. These results indicate that the presence of NA fibers in the caudal spinal cord is not a reliable indicator of coeruleospinal regeneration. We then asked if NA axons appose cholinergic neurons associated with motor functions, i.e., central canal cluster and partition cells (active during fictive locomotion) and somatic motor neurons (SMNs). We found more NA varicosities adjacent to central canal cluster cells, partition cells, and SMNs in the lumbar enlargement of OEG- than media-injected rats. As non-synaptic release of NA is common in the spinal cord, more associations between NA varicosities and motor-associated cholinergic neurons in the lumbar spinal cord may contribute to the improved treadmill stepping observed in OEG-injected spinal rats. This effect could be mediated through direct association with SMNs and/or indirectly via cholinergic interneurons.     

The respective contribution of lumbar segments to the generation of locomotion in the isolated spinal cord of newborn rat

Various studies on isolated neonatal rat spinal cord have pointed to the predominant role played by the rostral lumbar area in the generation of locomotor activity. In the present study, the role of the various regions of the lumbar spinal cord in locomotor genesis was further examined using compartmentalization and transections of the cord. We report that the synaptic drive received by caudal motoneurons following N-methyl-d-l-aspartate (NMA)/5-HT superfusion on the entire lumbar cord is different from that triggered by the same compounds specifically applied on the rostral segments. These differences appear to be due to the direct action of NMA/5-HT on motoneuron membrane potential, rather than on premotoneuronal input activation. In order to assess the possible participation of the caudal lumbar segments in locomotor rhythm generation, the segments were over-stimulated with high concentrations of NMA or K+. We find that significant variations in motor cycle period occurred during the over-activation of the rostral segments. Over-activation of caudal segments only si+gnificantly increased the caudal ventral roots burst amplitude. We find that low 5-HT concentrations were unable to induce fictive locomotion under our experimental conditions. When a hemi-transection of the cord was performed between the L2-L3 segments, rhythmic bursting in the ipsilateral L5 disappeared while rhythmicity persisted on the contralateral side. Sectioning of the remaining L2-L3 side totally suppressed rhythmic activity in both L5 ventral roots. These results show that the thoracolumbar part of the cord constitutes the key area for locomotor pattern generation.     

Spinal evoked potential in the monkey

Computer-averaged evoked potential responses (EPs) to stimulation of the sciatic nerve and cervical spinal cord were recorded from the dura and skin over the cauda equina and spinal cord in seven monkeys, three with chronic spinal cord lesions. Sciatic EPs consisted of predominantly negative triphasic propagated potentials recorded at all spinal levels and greatest in amplitude over the cauda equina and caudal spinal cord. The conduction velocity of this EP was faster over the cauda equina and rostral spinal cord than over caudal cord segments. Triphasic potentials were succeeded by small negative potentials over the cauda equina and larger negative potentials over the lumbar enlargement. Sciatic EPs over the upper lumbar and thoracic cord were more sensitive to asphyxia than the initial triphasic potentials recorded over cauda equina and caudal cord but resisted changes from increasing the rate of stimulation up to 100 per second. Propagated thoracic EPs were preceded by nonpropagated potentials. The longer latency negative potentials occurring locally over the cauda equina and lower lumbar enlargement were abolished at levels of asphyxia and were attenuated at rates of stimulation that did not affect the preceding triphasic potentials. Following complete spinal cord transection, nonpropagated sciatic EPs were recorded in leads rostral to the section. In preparations with chronic partial cord hemisection involving dorsal and lateral quadrants, ipsilateral sciatic EPs had increased latency, reduced amplitude, and poor definition in the vicinity of and rostral to the lesion. Direct cervical cord stimulation elicited caudally propagated potentials which were followed by large, broad potentials over the lumbar enlargement.     

In vitro motor program for the rostral scratch reflex generated by the turtle spinal cord

Tactile stimulation applied to the turtle shell bridge elicits a rostral scratch reflex in the spinal turtle. The triphasic rostral scratch motor program produced in vivo can also be also produced by an in vitro turtle shell and spinal cord preparation. The in vitro production of this scratch program provides support for the generalization that coordinated motor programs can be generated by the spinal cord. This in vitro preparation can be utilized to examine the pharmacological sensitivities of the spinal cord neurons generating the scratch motor program.     

Morphology of central terminations of intra-axonally stained, large, myelinated primary afferent fibers from facial skin in the rat

Horseradish peroxidase was intra-axonally injected into functionally identified primary afferent fibers within the rat spinal trigeminal tract in order to study the morphology of their central terminations. They were physiologically determined to be large, myelinated, cutaneous primary afferents by means of electrical and mechanical stimulation of their receptive fields. Ninety-three axons that innervated vibrissa follicles, guard hair follicles, and slowly adapting receptors were stained for distances of 4-12 mm at the levels of the main sensory nucleus, spinal trigeminal nucleus, and rostral cervical spinal cord. The collaterals of single axons from these receptors formed terminal arbors in the outer part of the spinal trigeminal nucleus rostral to and near the level of the obex (rostral type collaterals). In the rostral part of the subnucleus caudalis (Vc) they were confined to lamina V (caudalis type collaterals) and in the caudal part of Vc and in cervical segments they were confined to lamina III/IV (spinal-dorsal-horn-type collaterals). There were no transitional forms between the rostral and caudalis types, but there was a transitional form between the caudalis and spinal dorsal horn types. This transitional form was distributed in laminae III/IV and V. The terminal arbors of the rostral type of collaterals formed an interrupted, rostrocaudally oriented column like those seen in the lumbar dorsal horn, but the column shifted down to lamina V near the obex, and more caudally, gradually shifted upward to lamina III. Major morphological differences were not observed among the three different functional types of collaterals with respect to the rostrocaudal distribution of collaterals, and the shape and location of collaterals. The differential laminar distribution of collateral arbors of single axons along the rostrocaudal axis distinguishes the spinal trigeminal nucleus from the spinal dorsal horn where functional types of mechanoreceptive afferents form continuous or interrupted sagittal columns of terminal arbors that do not shift dorsoventrally within segments.     

Cutaneous projections from the rat hindlimb foot to the substantia gelatinosa of the spinal cord studied by transganglionic transport of WGA-HRP conjugate

The somatotopic organization of cutaneous afferent fibers from the hindlimb foot in the substantia gelatinosa of the spinal cord was investigated in adult Sprague Dawley rats following intracutaneous injections of wheat germ agglutinin-horseradish peroxidase (WGA-HRP). The different parts of the foot were found to project in a precise manner to the medial two-thirds of the substantia gelatinosa of the spinal cord segments L3-L5. The projections from the digits were arranged in a consecutive row rostrocaudally, with the most medial digit in caudal L3 and the most lateral digit in caudal L4 or rostral L5. The plantar skin was found to project both rostral and caudal to the projection of the digits. The dorsal foot skin projects lateral to the digits, neighboured by the medial edge of the foot rostrally and the lateral edge caudally. Within the projection of a particular digit, the plantar skin was found medially and the dorsal skin laterally. A certain degree of overlap for the projections from the different foot skin areas was found.     

Forelimb locomotor generators and quadrupedal locomotion in the neonatal rat

The spinal localization of the forelimb locomotor generators and their interactions with other spinal segments were investigated on in vitro brainstem-spinal cord preparations of new-born rats. Superfusion of the cervicothoracic cord (C1-T4) with high K+/low Mg2+ artificial cerebrospinal fluid (aCSF) evoked rhythmic motor root activity that was limited to low cervical (C7, C8) and high thoracic (T1) spinal levels. This activity consisted of synchronous, homolateral bursts and a typical alternating bilateral pattern. Rhythmic activity with similar locomotor-like characteristics could be induced with either serotonin (5-HT, 5 microm), N-methyl-d-aspartate (NMDA, 5 microm), kainate (10 microm) or a "cocktail" of 5-HT (5 microm) and NMDA (5 microm). During 5-HT/NMDA perfusion of the cervicothoracic cord, induced bursting was no longer restricted to C7-T1 levels, but also occurred at cervical C3-C5 levels and with C5-C8 homolateral alternation. Spinal transections between C6 and C7 cervical segments did not abolish rhythmic activity in C7-T1, but suppressed locomotor-like rhythmicity at C3-C5 levels. Reduced regions comprising the C7-C8 or C8-T1 segments maintained rhythmicity. Superfusion of the whole cord with 5-HT/NMDA induced ventral root bursting with similar frequencies at all recorded segments (cervical, thoracic and lumbar). After isolation, the T3-T10 cord was unable to sustain any rhythmic activity while cervical and lumbar segmental levels continued to burst, albeit at different frequencies. We also found that the faster caudal and the slower rostral locomotor generators interact to produce coordinated locomotor-like activity in all segments of the intact spinal cord. In conclusion, C7-T1 spinal levels display a strong motor rhythmogenic ability; with the lumbar generators, they contribute to coordinated rhythmic activity along the entire spinal cord of a quadrupedal locomoting mammal.     

Neuromuscular patterns of stereotypic hindlimb behaviors in the first two postnatal months. III. Scratching and the paw-shake response in kittens

Neuromuscular patterns of scratching and the paw-shake response were studied in normal kittens from birth to postnatal day 60. Onset of both behaviors coincided with the development of secure weight-bearing posture and occurred on postnatal day 21 for scratching and postnatal day 26 for paw shaking. At onset, cycle periods for scratching (5-6 Hz) and paw shaking (8-10 Hz) were similar to that for adult cats, and EMG patterns were adult-like. The scratch cycle consisted of reciprocal flexor and extensor bursts of equal duration, while the shake cycle consisted of coactive knee extensor and ankle flexor bursts alternately active with ankle extensor bursts. The lack of scratching and paw shaking during the first 3 postnatal weeks and the adult-like EMG patterns at onset are consistent with the hypothesis that pattern-generating circuits within lumbosacral segments are available early in development but inhibited by the rostral neuraxis until postural control is sufficient to accommodate the response. To eliminate rostral inputs, including descending input critical for postural control, kittens were spinalized at the T12 level, and onset of paw shaking was accelerated. In kittens spinalized at birth, paw-shake onset occurred on postnatal day 14, while in kittens spinalized on postnatal day 14, onset occurred 48 h after spinalization. In all spinal kittens, however, knee extensor activity was disrupted and not normal by postnatal day 60. Mature neuromuscular patterns for scratching and paw shaking are available at onset of the behavior during normal development. Spinalization hastens the onset of paw shaking but the normal neuromuscular synergy is disrupted as well as the temporal structure of the multi-cycle response. Disruptions following spinalization may be due to altered development of spinal pattern generators or aberrant feedback from atypical hindlimb motions due to a retardation of hindlimb growth and an alteration of muscle contractile properties in spinal kittens.     

Restitution of Functional Neural Connections in Chick Embryos Assessedin Vitroafter Spinal Cord Transectionin Ovo

Functional neural reconnection is not common after spinal cord transection in the CNS of adult higher vertebrates but has been demonstrated in embryonic avian and neonatal mammalian CNS. Chick brainstem spinal cord preparations from nontransected controls and embryos transected at the cervical level on embryonic days (E) 8, 9, or 10in ovowere assessedin vitrobetween E12 and E20 for their ability to produce and maintain episodic motor activity (EMA) using electrophysiological, voltage sensitive dye and anatomical tract-tracing techniques. After 3 to 4 days recovery, cycle-by-cycle coupling of EMA between segments separated by a transection was absent or inconsistent, although otherwise normal bouts of locally stimulated and spontaneous EMA were routinely observed restricted to segments of a cord separated by a transection site. After 5–7 days recoveryin ovothe cross-transection coordination during bouts of EMA approached that of nontransected controls. The delay between the initiation of EMA in cervical segments to its initiation in lumbosacral segments caudal to a transection was an indicator of reconnection strength. The delay shortened from 0.5 to a few seconds after 3 days of recovery to around 150 ms (i.e., normal) after 5 days of recovery. We conclude that the reconnection of spinal central pattern generators for EMA across the transection was served mainly by axons which established connections with local circuits after extending 1–3 segments through a transection. Propriospinal axons that originated within 1–3 segments rostral to the transection then served to serially initiate EMA in distal caudal segments.     

Spinomesencephalic tract: projections from the lumbosacral spinal cord of the rat, cat, and monkey

Anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase was used to determine the terminal domain of the projection from the lumbosacral spinal cord to the midbrain in the rat, cat, and monkey. Results have shown that several midbrain regions receiving afferent input from this level of the spinal cord are common to the three species examined. Structures innervated by this projection were located throughout the full rostrocaudal extent of the midbrain. The strongest projections were to the intercollicular region and caudal midbrain contralateral to injection sites in the spinal cord. Terminal labeling in the rostral midbrain, except that observed in the nucleus of Darkschewitsch, was substantially less than that observed at more caudal midbrain levels. Structures receiving the strongest input from the spinal cord included the central gray, nucleus cuneiformis, the deep and intermediate layers of the superior colliculus, and the intercollicular nucleus. Other structures receiving afferent input from the lumbosacral spinal cord included the anterior and posterior pretectal nuclei, red nucleus, Edinger-Westphal nucleus, interstitial nucleus of Cajal, and the mesencephalic reticular formation. It is concluded that the spinal projection to the midbrain is a multicomponent projection consisting of several pathways terminating in discrete midbrain regions. Considering the diverse functions associated with midbrain regions receiving spinal input and the response and receptive field properties of cells belonging to this pathway, the results of the present study are discussed in relation to the potential role of the spinomesencephalic tract in somatic, visceral, and motor function.     

Hindlimb movement in the cat induced by amplitude-modulated stimulation using extra-spinal electrodes

Hindlimb movement in the cat induced by electrical stimulation with an amplitude-modulated waveform of the dorsal surface of the L5-S1 spinal cord or the L5-S1 dorsal/ventral roots was investigated before and after acute spinal cord transection at the T13-L1 level. Stimulation of the spinal cord or dorsal/ventral root at the same spinal segment induced similar movements including coordinated multi-joint flexion or extension. The induced movements changed from flexion to extension when the stimulation was moved from rostral (L5) to caudal (S1) spinal segments. Stimulation of a dorsal or ventral root on one side induced only ipsilateral hindlimb movement. However, stimulation on the dorsal surface of the spinal cord along the midline or across the spinal cord induced bilateral movements. The extension induced by stimulation of L7 dorsal root produced the largest ground reaction force that was strong enough to support body weight. Dorsal root stimulation induced a larger ground reaction force than ventral root stimulation and produced a more graded recruitment curve. Stepping at different speeds could be generated by combined stimulation of the rostral (L5) and the caudal (L6/L7) spinal segments with an appropriate timing between the different stimulation channels. Acute transection of the spinal cord did not change the responses indicating that the induced movements did not require the involvement of the supraspinal locomotor centers. The methods and the stimulation strategy developed in this study might be utilized to restore locomotor function after spinal cord injury.     

Development of catecholaminergic systems in the spinal cord of the dogfish Scyliorhinus canicula (Elasmobranchs)

The development of catecholamine-synthesizing cells and fibers in the spinal cord of dogfish (Scyliorhinus canicula L.) was studied by means of immunohistochemistry using antibodies against tyrosine hydroxylase (TH). The only TH-immunoreactive (TH-ir) cells already present in the spinal cord of stage 26 embryos were of cerebrospinal fluid-contacting (CSF-c) type. These cells were the first catecholaminergic neurons of the dogfish CNS. The number of these TH-ir cells increased very considerably in later embryos and adult dogfish. In later embryos (stage 33; prehatching), faintly TH-ir non-CSF-contacting neurons were observed in the ventral horn throughout most of the spinal cord. In adult dogfish, some non-CSF-contacting TH-ir cells were observed ventral or lateral to the central canal. In the rostral spinal cord, the catecholaminergic neurons observed in dorsal regions were continuous with caudal rhombencephalic populations. Numerous TH-ir fibers were observed in the spinal cord of later embryos and in adults, both intrinsic and descending from the brain, innervating many regions of the cord including the dorsal and ventral horns. In addition, some TH-ir fibers innervated the marginal nucleus of the spinal cord. The early appearance of catecholaminergic cells and fibers in the embryonic spinal cord of the dogfish, and the large number of these elements observed in adults, suggests an important role for catecholamines through development and adulthood in sensory and motor functions.