Biomedical subjects

J D Steeves

Publications and source records attributed to J D Steeves.

At least 55 records · Page 3Linked to original sources

PubMed1991

Funicular organization of avian brainstem-spinal projections.

The purpose of this study was to determine which reticulospinal projections need to be preserved to allow voluntary walking and to differentiate between those pathways descending within the ventrolateral funiculus versus the ventromedial funiculus. Retrogradely transported tracers (True Blue, Fast Blue, Diamidino Yellow dihydrochloride, fluorescein-conjugated dextran-amines) were used alone as discrete funicular injections (4-5 microliters) into the lumbar cord (L1), or in conjunction with a more rostral subtotal lesion of the low thoracic cord, to determine the trajectories of brainstem-spinal projections in adult ducks and geese. No difference was found between the species. The major components of the ventromedial funiculus include projections from the medullary reticular formation, pontine reticular formation, raphe obscurus and pallidus, lateral vestibular nucleus, and interstitial nucleus, and to a minor extent from the locus coeruleus, lateral hypothalamus, and nucleus periventricularis hypothalami. The components of the ventrolateral funiculus (VLF) include projections from the nucleus of the solitary tract, nucleus alatus, pontomedullary reticular formation, raphe pallidus, raphe magnus, locus coeruleus, subcoeruleus, lateral vestibular, and descending vestibular nuclei. The principal descending projections within the dorsolateral funiculus (DLF) arose from the red nucleus, the paraventricular nucleus, locus coeruleus, subcoeruleus, dorsal division of the caudal medullary reticular formation, and raphe magnus. The functional implications of the distribution of these descending pathways are discussed with regard to locomotion. Since birds were able to walk despite bilateral lesion of the DLF or VMF but were unable to walk following a bilateral lesion of the VLF, this suggests that medullary reticulospinal pathways coursing within the VLF are essential for the provision of locomotor drive.

Amidines↗

PubMed1991

Avian locomotion activated by brainstem infusion of neurotransmitter agonists and antagonists. I. Acetylcholine excitatory amino acids and substance P.

Previous studies have demonstrated that focal electrical stimulation of regions within the brainstem of a decerebrate bird will elicit all the normal patterns of avian locomotion. However, electrical stimulation can activate a variety of neuronal elements within the radius of effective current spread, including axons of passage traversing the stimulation point. To restrict activation to neuronal cell bodies within the immediate vicinity, we have utilized direct intracerebral injection of neurotransmitters, their agonists and antagonists, into identified brainstem locomotor regions. To undertake these studies, birds (geese or ducks) were placed in a stereotaxic frame and decerebrated under halothane anesthesia. After completion of surgery, several discrete locomotor regions were first identified with electrical microstimulation. Acetylcholine (ACh) and excitatory amino acid (EAA) agonists and antagonists, as well as Substance P were then slowly infused into each brainstem region. Any change in locomotor behavior was recorded by electromyographic techniques. When injected into a variety of sites, carbachol (an ACh nicotinic (AChN) and muscarinic (AChM) agonist) and pilocarpine (an AChM agonist) evoked locomotion, whereas atropine (an AChM antagonist) blocked locomotion. N-methyl-D-aspartate NMDA), but not glutamate, also elicited locomotion or reduced the current intensity threshold for electrically-evoked locomotion. The NMDA-induced locomotion evoked locomotion. The NMDA-induced locomotion could be blocked by the injection of glutamic acid diethyl ester (GDEE, an EAA antagonist) or D-2-amino-5-phosphonopentanoic acid (AP5) into the same site. Finally. Substance P also evoked locomotion. The above observations strongly suggest that brainstem electrically-stimulated locomotion in decerebrate birds is not due to activation of fibers traversing a brainstem locomotor region, but instead, is due to the activation of receptors located on neuronal cell bodies, dendrites or presynaptic terminals in the immediate vicinity of the micropipette tip. After correlating our findings with similar lamprey and mammalian studies, the comparable discoveries serve to underscore the suggestion that the neuroanatomical substrates underlying the brainstem control of locomotion appear to be highly conserved in all vertebrates.

Acetylcholine↗

PubMed1991

Avian locomotion activated by brainstem infusion of neurotransmitter agonists and antagonists. II. gamma-Aminobutyric acid.

The companion article (Sholomenko et al. 1991) described the brainstem locomoter regions in the bird where direct intracerebral injection of a number of putative excitatory neurochemicals, including cholinergic agonists, N-methyl-d-aspartate (NMDA) and Substance P, evoke locomotion. Using the same experimental protocol, this study focuses on the locomotor effects following discrete brainstem injections of the inhibitory neurotransmitter, gamma-aminobutyric acid (GABA), and some of GABA agonists and antagonists. Brainstem regions that were electrically and chemically stimulated included the ventromedial medullary reticular formation, the pontobulbar locomotor strip of the dorsolateral pons and medulla, the pontine reticular formation, and the mesencephalic reticular formation. Locomotion was evoked after the injection of the GABA antagonists picrotoxin (a GABAA receptor antagonist) and bicuculline (GABAA antagonist) into several brainstem locomoter regions. Brainstem stimulated locomotion (both chemically and electrically induced) could be transiently blocked by intracerebral infusion of GABA and irreversibly blocked by muscimol (GABAA agonist). Our avian results are similar to those described for mammals and provide support for the suggestion that motor circuitry, at least at brainstem levels, is similar in all vertebrates.

Animals↗

PubMed1991

Locomotor activities in the decerebrate bird without phasic afferent input.

We examined whether forelimb and hindlimb phasic afferent input is a prerequisite for the production of avian locomotor patterns. We eliminated phasic afferent feedback through paralysis of a decerebrate animal. The term "fictive" has been used to describe the neural activity associated with spontaneous or evoked motor output during neuromuscular paralysis. We observed that a paralysed decerebrate bird is capable of producing similar locomotor activity patterns as an unparalysed preparation, regardless of whether the "fictive" locomotion is generated spontaneously, or in response to focal electrical and/or neurochemical stimulation of discrete brainstem locomotor regions. Not all aspects of "fictive" locomotor patterns were identical to the locomotion elicited prior to paralysis. The stimulus current threshold necessary to evoke hindlimb locomotion increased from 69 +/- 22 mu A (mean +/- S.D.) prior to paralysis to 185 +/- 87 mu A for "fictive" stepping. For wing activity, the threshold increased from 84 +/- 46 mu A during wing flapping to 228 +/- 148 mu A for "fictive" flight. In addition, the frequency of "fictive" efferent locomotor activity from the leg nerve (1.04 +/- 0.44 Hz) decreased relative to the frequency of leg activity prior to paralysis (1.55 +/- 0.70 Hz). Similarly, the frequency of wing activity decreased from 2.73 +/- 0.73 Hz before paralysis to 1.8 +/- 0.69 Hz after paralysis. Finally flexor burst duration remained constant during treadmill and "fictive" walking while the extensor burst duration was markedly increased during "fictive" walking. Thus, the relative contributions of leg flexor activity to the overall step cycle (burst proportion = burst duration/cycle duration) decreased during evoked "fictive" stepping, while the burst proportion of the leg extensor increased. Afferent feedback therefore appears to modulate leg extensor burst duration more than leg flexor duration. For the wings, the burst proportion of the major wing depressors remained constant before and after paralysis.

Afferent Pathways↗

PubMed1990

Stimulation of the brainstem reticular formation evokes locomotor activity in embryonic chicken (in ovo).

This study was designed to examine the period of embryonic chick development during which descending brainstem-spinal projections, originating from defined avian brainstem locomotor regions, become functionally active. Locomotor activity was examined using a new in ovo preparation for the focal electrical stimulation of embryonic brainstem locomotor regions. Embryos or hatchlings were anesthetized and mounted in a stereotaxic apparatus. Leg and wing muscle electromyographic (EMG) recordings were used to monitor any brainstem-stimulated motor activity. At present, we have been successful in demonstrating coordinated brainstem-evoked locomotion in embryos as early as embryonic day 15. The patterns of evoked locomotor activity were similar to locomotion evoked in hatchling chicks and were of 4 types: (1) alternating hindlimb movements ('stepping'), (2) synchronous (in-phase) hindlimb movements ('hatching'), (3) synchronous wing movements ('flapping'), and (4) simultaneous 'stepping' and 'flapping'. The cycle durations of evoked embryonic hindlimb movements are shorter than those observed for hatchling chicks. The present results are the first direct demonstration of functional connections between descending supraspinal neurons and spinal locomotor circuits at such an early stage of embryonic development. With modifications in technique, it may be possible to demonstrate functional connections at even earlier stages of embryonic development.

Animals↗

PubMed1990

Origins of descending spinal pathways in prehensile birds: do parrots have a homologue to the corticospinal tract of mammals?

In mammals, the supraspinal descending projections that influence distal limb muscles are the rubrospinal and corticospinal tracts. The former, which is found in other vertebrates, shows greater somatotopy in mammals that are 'dextrous' (e.g. monkeys) than those that are not (e.g. opossums). Similarly, the corticospinal tract, which is found only in mammals, has more extensive connections (i.e. direct corticomotoneural) in mammals that are dextrous than in mammals that are not. Descending spinal pathways have been described in 'non-dextrous' avian species (chickens, ducks, geese and pigeons), and the purpose of this study was to determine if there are any differences in the origins of descending projections to the spinal cord in 'dextrous' or prehensile parrots (sulphur-crested cockatoo, Cacatua galerita, and eastern rosella, Platycerus eximius). True Blue or wheat germ agglutinin-horseradish peroxidase was injected into the lumbar or cervical spinal cord. The distribution of retrogradely labelled cells was similar to that previously reported for non-prehensile birds. We found no evidence of any direct spinal projections from the telencephalon (including any pathway homologous to the corticospinal tract of mammals), nor any specialized anatomical organization of the descending pathways that could account for the pedal dexterity of these species.

Animals↗

PubMed1988

Origins of brainstem-spinal projections in the duck and goose.

The purpose of this study was to determine the location of neuronal cell bodies with projections to the cervical or lumbar spinal cord in the adult duck and goose. Bilateral or unilateral injections (5-10 microliter) of the retrograde tracer dye True Blue (TB:5%) were made into the high cervical or high lumbar levels of the spinal cord. Similar results were obtained in both species. First, we found no evidence of retrogradely labelled cells in the telencephalon. In the brainstem, the distribution of TB cells was similar to those previously reported for the pigeon; however, the present study now demonstrates that some of these descending pathways project as far as the lumbar cord. We also discovered that there is a topographical representation of spinal projecting neurons within the avian medullary-pontine reticular formation.

Animals↗

PubMed1987

Distribution of phenylethanolamine-N-methyltransferase (PNMT)-immunoreactive neurons in the avian brain.

The distribution of neurons immunoreactive to tyrosine hydroxylase and phenylethanolamine-N-methyltransferase (PNMT) were described in adjacent sections of the avian medulla oblongata. PNMT-positive neurons were found in two bilaterally symmetrical columns in the ventrolateral and dorsomedial medulla. Within the ventrolateral column, PNMT cells were centered in and around the lateral paragigantocellular and lateral reticular nuclei. In the dorsomedial medulla, PNMT neurons were concentrated within and around the nucleus of the tractus solitarius. The distribution of PNMT-immunoreactive neurons in the avian medulla is similar to those observed in mammals, except there appears to be a greater number of PNMT-positive cells in the bird.

Adrenergic Fibers↗

PubMed1987

Stimulation of the pontomedullary reticular formation initiates locomotion in decerebrate birds.

The initiation and control of spinal locomotor mechanisms by supraspinal brainstem centers has been studied in many vertebrate species. However there is very little information regarding brainstem-spinal pathways involved in avian locomotion. To undertake these studies, birds (geese or ducks) were placed in a stereotaxic frame and decerebrated under halothane anesthesia. After anesthesia was discontinued, discrete brainstem regions were electrically stimulated. Locomotion could be evoked from several areas within the pontomedullary reticular formation. The stimulus threshold for evoking walking ranged from 25-50 microA at 30-80 Hz (pulse duration 0.5 ms). At slightly higher current strengths (50-100 microA) wing flapping ('flying') was initiated. Histological examination of the stimulation sites indicated that the predominant locomotor areas were restricted to the ventromedial gigantocellular reticular formation and a more dorsolateral location within the parvocellular reticular formation. To ascertain whether neurons in these regions of the avian brainstem project directly to the spinal cord, a retrograde fluorescent tracer dye, True blue, was injected into either the cervical or low thoracic spinal cord. Comparing the positions of the retrogradely labeled neurons and the effective locomotor stimulation sites indicated a high degree of overlap. The present results, when compared with previous studies, suggest strong similarities in descending brainstem control of locomotion for all vertebrates.

Animals↗

PubMed1987

Effects of selective spinal cord lesions on hind limb locomotion in birds.

In birds, a variety of subtotal thoracic spinal cord lesions were made to determine the spinal cord pathways essential for avian hind limb locomotion (bipedal walking). The various surgical disruptions included: section of the dorsal half of the cord, a hemisection, section of the dorsal cord and ventromedial funiculi, section of the dorsal cord ventrolateral funiculi, section of the entire thoracic cord except for one ventrolateral quadrant, section of the ventral half of the cord, and complete transection of the thoracic cord. The study compared the locomotion following these lesions in both chronic surviving and acutely decerebrated, brain stem-stimulated birds. Behavioral assessments and electromyographic recording techniques were used to evaluate locomotor activity. Our results showed that transectioning pathways within the dorsal cord did not hinder the activation and maintenance of self-supported walking in either preparation. However, sparing the spinal cord pathways within either the ventromedial or ventrolateral funiculi of the thoracic spinal cord was essential for the activation of self-supported walking in both preparations. When our findings are integrated with previous studies, medullary reticulospinal pathways (projecting through the ventral funiculi) are strongly implicated as a common descending projection for the activation of spinal cord locomotor networks and the initiation of locomotion. Similar findings have been found in quadrapedal mammals and, as a complement, birds may make an excellent model for the study of bipedal locomotion.

Animals↗

PubMed1986

Resetting of resultant stiffness in ankle flexor and extensor muscles in the decerebrate cat.

Flexor (tibialis anterior, TA, and extensor digitorum longus, EDL) and extensor (soleus, SOL) muscles in the decerebrate cat were subjected to length changes and the force responses were measured. Resultant muscular stiffness, which arises from the mechanical reaction of muscle fibers contracting prior to the length change and from a change in force due to reflex action, was calculated by dividing the changes in force by the corresponding length changes. As shown previously in the premammillary preparation, resultant stiffness was usually higher in SOL than in TA or EDL. Following an intercollicular transection in some preparations, resultant stiffness increased markedly for TA but not substantially for SOL. During continuous electrical stimulation in the magnocellular red nucleus in premammillary preparations, resultant stiffness of SOL decreased for a wide range of forces while EDL responses were unaffected. These results show that reflex gain is not determined solely by the level of motoneuronal excitability but also by a descending control from the brainstem, and that the lower resultant stiffness in flexors compared to extensors in the decerebrate cat is set by this control system and not by inherent differences in the strength of autogenetic reflex pathways for the two muscles.

Animals↗

PubMed1984

Autoradiographic demonstration of the projections from the mesencephalic locomotor region.

An autoradiographic tracing technique was used to examine the projections of the classically defined mesencephalic locomotor region (MRL). Injections of [3H]proline and [3H]leucine were made into sites in the caudal mesencephalon which can be stimulated to produce locomotion. The injection sites were confined to the cuneiform nucleus (stereotaxic coordinates P2.0, L4.0, H-1.0). Descending projections were primarily ipsilateral to the gigantocellular and magnocellular reticular formation of the pons and medulla, the dorsal tegmental reticular nucleus, and the nucleus raphe magnus. Some sparse contralateral projections were also observed within the magnocellular and gigantocellular reticular formation. Direct axonal connections with the spinal cord were not consistently observed. Ascending projections were observed to the subthalamic nucleus, caudal hypothalamic nuclei, the centrum medianum nucleus of the thalamus, the ventral tegmental area of Tsai, the superior colliculus, and the periaqueductal gray region. The ascending projections were also ipsilateral, with sparse contralateral labeling confined to areas which received ipsilateral projections. Projections to the contralateral cuneiform nucleus were also consistently observed. The results, when compared to those of another study, suggest that the classical MLR is anatomically distinct from the more medial sites in the mesencephalon which can also induce locomotion.

Animals↗

PubMed1984

Characterization of a reduced-eye mutant of the grasshopper, Melanoplus sanguinipes.

A reduced-eye (re) mutant grasshopper of Melanoplus sanguinipes has been characterized by small flat compound eyes lacking facets, no lateral ocelli and only a remnant of the median ocellus. The re grasshoppers walk, jump, fly and feed in a normal manner, but do not respond to visual and auditory stimuli, suggesting they may be blind and deaf. Extracellular recordings from the ventral nerve cord of re mutants verified the lack of neural activity in response to visual and auditory inputs, yet the mutants detected mechanical and tactile stimuli. Electroretinograms implied that a visual deficit may be within the photoreceptors of the compound eye. Histological examination of the compound eyes and ocelli indicated that the cells of the mutant compound eye incompletely differentiate. The optic lamina underlying the retina is missing, as is the outer optic chiasma. The medulla and lobula of the mutant optic lobe are present, however, the neuropil of the medulla lacks the characteristic axonal projection patterns of wild-type grasshoppers. The re grasshopper also lacks all ocellar nerves. Ocellar nerves are normally formed from processes of second order ocellar neurons (SONs), suggesting that if the mutant SONs are present within the protocerebrum, their morphology is drastically altered. Comparison of embryos and juvenile nymphs supports the suggestion that the alterations in the re visual system are the result of abnormal differentiation during development. Even though there is clear evidence of morphological alterations in second and third order optic lobe interneurons, one higher order visual interneuron of the midbrain, the descending contralateral movement detector (DCMD), has the same morphology as the DCMD in a wild-type brain. In this instance, the complete deprivation of the primary sensory input does not appear to alter cellular development.

Animals↗

PubMed1983

Neuronal cell death in grasshopper embryos: variable patterns in different species, clutches, and clones.

Previous studies showed that cell death plays an important role in adjusting the segment-specific number of ganglionic neurones during grasshopper embryogenesis (Bate, Goodman & Spitzer, 1979; Goodman & Bate, 1981). In every segment, the single midline precursor 3 (MP3) divides once to produce two progeny. In some segments, one or both of these two progeny die; there is a general pattern of cell death of the MP3 progeny across the thoracic and abdominal segments. In the present study we examined the pattern of cell survival versus death of the MP3 progeny in 472 embryos from four different species, from the genetically related offspring within different clutches of the same species and from the genetically identical offspring within isogenic clones of the same species. We find variability in the pattern of cell survival versus death amongst embryos of the same species, clutch and clone, suggesting a significant epigenetic influence on this pattern. However, our results also show significant differences in the pattern of cell death between different genera and species, and between different clones and clutches within a single species, suggesting a genetic influence on this pattern as well.

Animals↗

PubMed1982

Modulation of stretch reflexes during locomotion in the mesencephalic cat.

1. A cat preparation was used to study the modulation of stretch reflexes during locomotion. The brain stem was transected and locomotion was induced by electrical stimulation of the mesencephalic locomotor region below the level of transection. Three legs walked normally on a treadmill, while the fourth leg, which was denervated except for the soleus muscle, was held fixed. Brief length perturbations were applied to the soleus muscle at various phases of the stepping cycle.2. The stretch reflex in this muscle was deeply modulated during the step cycle, and reached its peak at or before the peak in soleus e.m.g. activity associated with the locomotion. A similar variation was observed when the sciatic nerve was stimulated electrically at a strength which elicited reflex activity (H wave), but did not directly elicit motor nerve activity (M wave). Variation in the reflex during electrical stimulation could not be accounted for by cyclic variation in fusimotor activity or in the afferent volley, but must be due to post-synaptic changes in alpha-motoneurones or in the presynaptic inputs to them.3. Large changes were also observed in intrinsic muscle stiffness during the step cycle. The maximum stiffness occurred near the time the limb would normally strike the ground during locomotion. A high stiffness would be useful in reducing the amount that the limb would yield under the weight of the body during the extension phase of the step cycle. The variation of the stretch reflex in parallel with stiffness suggests that reflexes could assist in this load compensation. The variation is not consistent with the idea that the stretch reflex is used to compensate for changes in intrinsic muscle properties, so that the total system behaves more like a spring of constant stiffness than does muscle alone.

Animals↗

PubMed1980

Effect of noradrenaline and 5-hydroxytryptamine depletion on locomotion in the cat.

It has recently been hypothesized that stimulation of the mesencephalic locomotor region (MLR) can give rise to locomotion in mesencephalic cats due to activation of descending monoaminergic pathways to the spinal cord. This notion is based on the findings that monoamine agonists and precursors can induce hindlimb stepping in acute low spinal animals, and on the similarities between the effects of the noradrenaline (NA) precursor, L-DOPA, and stimulation of the MLR. The hypothesis that the descending monoamine systems comprise the only pathways which control the initiation of locomotion has been tested in the present study. NA was depleted from the CNS using intraspinal and intraventricular injections of 6-hydroxydopamine and i.v. injections of the NA synthesis inhibitor, alpha-methyltyrosine. Depletion of 5-hydroxytryptamine (5-HT) was achieved using intraventricular injections of 5,6-dihydroxytryptamine and i.p. p-chlorophenylalanine. These treatments did not abolish evoked locomotion in spite of substantial depletion of NA and 5-HT in the spinal cord and brain stem (maximal depletions of NA up to 14% of control in lumbar cord and 16% of control in pons; maximal depletions of 5-HT up to 19% of control in sacral cord and 25% of control in medulla). Combined depletion of NA and 5-HT did not abolish evoked locomotion in mesencephalic cats, although the treated animals displayed pronounced ataxia prior to decerebration. Depletion of NA or 5-HT alone did not alter locomotion in otherwise intact animals. A previous report that phenoxybenzamine antagonizes the effects of MLR stimulation was not confirmed. The results therefore do not support the hypothesis that descending pathways containing monoamines are essential for locomotion evoked by brain stem stimulation.

5,7-Dihydroxytryptamine↗

PubMed1980

Localization of a descending pathway in the spinal cord which is necessary for controlled treadmill locomotion.

The spinal cord pathways which are important for controlled treadmill locomotion evoked by stimulation of the mesencephalic locomotor region (MLR) were investigated in cats subjected to subtotal spinal cord lesions at the C1-C2 level. Locomotion could be evoked following bilateral lesions of the dorsal columns, the dorsolateral funiculi, and the ventromedial funiculi, and after combined lesions of the dorsolateral and ventromedial funiculi, but not after bilateral lesions of the ventrolateral quadrant. Unilateral lesions of the ventrolateral quadrant abolished locomotion in the limbs is ipsilateral to the lesion. It is suggested that MLR stimulation may give rise to locomotion by activation of pontine and medullary reticulospinal pathways projecting through the ventrolateral quadrant.

Animals↗

PubMed1980

Triggering of locust jump by multimodal inhibitory interneurons.

1. The locust jump is triggered by a sudden inhibition of activity in hindleg flexor tibiae motoneurons following cocontraction of the hindleg flexor and extensor tibiae muscles. The main result of this investigation was the identification of two interneurons (one for each hindleg) that monosynaptically inhibit flexor tibiae motoneurons and whose properties are all consistent with them being the trigger interneurons for initiating a jump. 2. These interneurons receive strong excitatory input from many sensory modalities (visual, auditory, tactile, and proprioceptive). Because of their multimodal response characteristics, we designated them M-neurons. A particularly strong excitatory input to each M-neuron is from both descending contralateral movement detector (DCMD) interneurons. 3. The threshold for spike initiation in the M-neurons is high (approximately 14 mV). As a consequence, input from any one sensory modality alone rarely initiates action potentials. 4. Each M-neuron is depolarized by sensory input from leg proprioceptors. We propose that proprioceptive feedback during the cocontraction phase depolarizes the M-neurons to decrease their threshold, thus enabling extrinsic sensory stimuli to generate action potentials in both M-neurons and in so doing trigger a jump. The function of the proprioceptive gating of inhibitory transmission from the various sensory systems to the flexor motoneurons (via the M-neurons) is to ensure the development of a strong isometric contraction of the extensor tibiae muscle, and thus a powerful jump in response to external stimuli. 5. Insofar as the initiation of the locust jump depends on sensory convergence onto large identified interneurons, this behavior is similar to ballistic movements in some other animals such as the crayfish tail flip and the startle response in fish. The unique feature of the locust jump is that the trigger interneurons initiate the jump only after a preceding phase (cocontraction) has been accomplished.

Animals↗