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J I Simpson

Publications and source records attributed to J I Simpson.

At least 19 recordsLinked to original sources
PubMed1996

Effect of nitroglycerin on spinal cord ischemia after thoracic aortic cross-clamping.

BACKGROUND: Thoracic aortic cross-clamping with the use of sodium nitroprusside (SNP) has been shown to cause a decrease in spinal cord perfusion pressure and an increased incidence of paraplegia. Nitroglycerin is frequently used in this setting. This study investigated the effects of nitroglycerin and SNP on spinal cord ischemia. METHODS: Three-groups of 8 mongrel dogs underwent thoracic aortic cross-clamping for 45 minutes. Proximal pressure was maintained between 95 and 100 mm Hg with SNP, nitroglycerin, or phlebotomy. All animals were neurologically evaluated 24 hours later by a blinded observer, and the findings were confirmed by histopathologic study. Statistical analysis (p value of less than 0.05) of measured hemodynamic data was by analysis of variance and of Tarlov scores, the Mann-Whitney U test. RESULTS: Distal aortic pressures (p < 0.001), Tarlov scores, and spinal cord perfusion pressures (p < 0.01 and p < 0.05 for SNP group and nitroglycerin group, respectively) were significantly higher in the phlebotomy group compared with the SNP and NTG groups. Cerebrospinal fluid pressures were significantly lower in the phlebotomy group compared with the SNP group (p < 0.001). CONCLUSIONS: The use of either NTG or SNP was associated with a high incidence of paraplegia. Nitroglycerin appears to be no safer than SNP when used during thoracic aortic cross-clamping.

Animals↗
PubMed1995

Anatomical compartments in the white matter of the rabbit flocculus.

The white matter of the rabbit flocculus is subdivided into five compartments by narrow sheets of densely staining acetylcholinesterase-positive fibers. The most lateral compartment is continuous with the C2 compartment of the paraflocculus and contains the posterior interposed nucleus. The other four compartments are numbered from lateral to medial as floccular compartments 1, 2, 3, and 4 (FC1-4). FC1-3 continue across the posterolateral fissure into the adjacent folium (folium p) of the ventral paraflocculus. FC4 is present only in the rostral flocculus. In the caudal flocculus FC1 and FC3 abut dorsal to FC2. Fibers of FC1-4 can be traced into the lateral cerebellar nucleus and the floccular peduncle. The presence of acetylcholinesterase in the deep stratum of the molecular layer of the flocculus and ventral paraflocculus distinguishes them from the dorsal paraflocculus. The topographical relations to the flocculus and the floccular peduncle with group y and the cerebellar nuclei are discussed.

Acetylcholinesterase↗
PubMed1995

Zonal organization of the climbing fiber projection to the flocculus and nodulus of the rabbit: a combined axonal tracing and acetylcholinesterase histochemical study.

The localization and termination of olivocerebellar fibers in the flocculus and nodulus of the rabbit were studied with anterograde axonal transport methods [wheatgerm agglutinin-horseradish peroxidase (WGA-HRP) and tritiated leucine] and correlated with the compartments in the white matter of these lobules delineated with acetylcholinesterase histochemistry (Tan et al. J. Comp. Neurol., 1995, this issue). Olivocerebellar fibers originating from the caudal dorsal cap travel through floccular compartments FC2 and FC4 to terminate as climbing fibers in floccular zones FZII and FZIV. Fibers from the rostral dorsal cap and the ventrolateral outgrowth traverse compartments FC1 and FC3, which are interleaved with compartments FC2 and FC4, and terminate in zones FZI and FZIII. Fibers from the rostral pole of the medial accessory olive traverse the C2 compartment and terminate in the C2 zone. FZI-III extend into the adjoining folium (folium p) of the ventral paraflocculus. The C2 zone continues across folium p into other folia of the ventral paraflocculus and into the dorsal paraflocculus. Four compartments and five zones were distinguished in the nodulus. Medial compartment XC1 contains olivocerebellar fibers from the caudal dorsal cap and subnucleus beta that terminate in the XZI zone. Olivocerebellar fibers from the rostral dorsal cap and the ventrolateral outgrowth occupy XC2 and terminate in XZII. The XC4 compartment contains fibers from both the caudal dorsal cap and from the rostral dorsal cap and the ventrolateral outgrowth. The latter terminate in a central portion of the XZIV zone. The dorsomedial cell column projects to the XZIII zone, which is present only in the dorsal part of the nodulus. The rostral medial accessory olive projects to the XZV zone, which occupies the lateral border of the nodulus. These results confirm and extend the conclusions of Katayama and Nisimaru ([1988] Neurosci. Res. 5:424-438) on the zonal pattern in the olivo-nodular projection in the rabbit. Additional observations were made on the presence of a lateral A zone (Buisseret-Delmas [1988] Neurosci. Res. 5:475-493) in the hemisphere of lobules VI and VII. Retrograde labeling of the nucleo-olivary tract of Legendre and Courville ([1987] Neuroscience 21:877-891) was observed after WGA-HRP injections into the inferior olive including the rostral dorsal cap and the ventrolateral outgrowth. The anatomical and functional implications of these observations are discussed.

Acetylcholinesterase↗
PubMed1995

Isoflurane versus sodium nitroprusside for the control of proximal hypertension during thoracic aortic cross-clamping: effects on spinal cord ischemia.

OBJECTIVE: This study was designed to compare the effects of isoflurane and nitroprusside on spinal cord ischemia when they are used to control proximal hypertension during thoracic aortic cross-clamping (TACC). DESIGN: Prospective, randomized, blinded experimental study. SETTING: Laboratory and animal research facility. PARTICIPANTS: Adult mongrel dogs. INTERVENTIONS: Two groups of eight dogs had TACC for 45 minutes. Proximal aortic, distal aortic, and cerebrospinal pressure was calculated as the distal mean pressure minus the CSF pressure. Group 1 received nitroprusside and group 2 received isoflurane to control proximal hypertension during cross-clamping. The dogs were neurologically evaluated 24 and 48 hours later by an observer blinded as to the study group. Spinal cord segments were obtained for histopathologic examination. MEASUREMENTS AND MAIN RESULTS: Distal perfusion pressure and spinal cord perfusion pressure were significantly higher in the isoflurane group (p < .005). At 24 hours, seven of eight dogs in group 1 had severe neurologic injury (ie, paraplegia), with the eight having mild neurologic injury. This is in contrast to group 2, where 6 of 8 dogs had either minimal or no injury, one had mild injury, and one had severe injury. Similar results were observed at 48 hours (p < .005). CONCLUSIONS: Isoflurane, when used to control proximal hypertension during TACC, produces a higher spinal cord perfusion pressure and is associated with a lower incidence of neurologic injury than nitroprusside in this canine model.

Animals↗
PubMed1995

Dynamics of abducens nucleus neurons in the awake rabbit.

1. We recorded abducens neurons, identified by electrical stimulation as internuclear neurons or motoneurons, in awake rabbits. The relationship of firing rate to eye movement was determined from responses during stable fixations, sinusoidal rotation in the light (0.05-0.8 Hz), and triangular optokinetic stimulation at 0.1 Hz. 2. All abducens neurons were excited during temporal movement of the ipsilateral eye. Temporal and nasal saccades were associated with bursts or pauses, respectively, in the firing rate. 3. Motoneurons and internuclear neurons are qualitatively indistinguishable. There was no significant quantitative difference between the phase and sensitivity of the two groups for 0.2-Hz sinusoidal rotation in the light. 4. On the basis of the response to stable eye positions, we determined static eye position sensitivity of the abducens neuron pool to be 8.2 +/- 2.5 (SD) spikes.s-1/0, with a static hysteresis of 8.9 spikes/s (1.14 +/- 0.37 degrees). 5. We determined apparent eye position sensitivity (k) and apparent eye velocity sensitivity (r) from the responses to sinusoidal rotation in the light. k increases and r decreases with stimulus frequency, which indicates that the simplest transfer function mediating conversion of abducens nucleus (VI) firing rate to eye position (E) has two poles and one zero. 6. The VI-->E relationship has an "amplitude nonlinearity," manifest as a tendency for k, r, and firing rate phase lead to decrease as eye movement amplitude increases at a fixed frequency. On a percentage basis, phase is less affected than are the sensitivities. The nonlinearity becomes less pronounced for stimulus amplitudes > 2.5 degrees, and consequently a linear model of the VI-->E transformation remains useful, provided that consideration is restricted to the appropriate range of stimulus/response amplitudes. 7. We determined time constants of the linear two-pole, one-zero transfer function from the variation of r/k versus stimulus frequency. The pole time constants were T1 = 3.4 s and T2 = 0.28 s, and the zero time constant (Tz) = 1.6 s. The magnitude of Tz was corroborated by measuring the time constant of the exponential decay in firing rate after step changes in eye position. This transient method yielded a Tz of 1.1 s. 8. The time constants of the VI-->E transfer function are roughly 10 times larger than those reported for the rhesus macaque. The difference is attributable to the reported 10-fold lower stiffness of the rabbit oculomotor plant, which may in turn relate to rabbits postulated lower degree of activation of extraocular muscles at any given position.(ABSTRACT TRUNCATED AT 400 WORDS)

Abducens Nerve↗
PubMed1995

Dynamics of rabbit vestibular nucleus neurons and the influence of the flocculus.

1. We recorded single vestibular nucleus neurons shown by electrical stimulation to receive floccular inhibition [flocculus receiving neurons (FRNs)] and/or to project toward midbrain motoneuronal pools [midbrain projecting neurons (MPNs)] in awake, head-fixed rabbits during compensatory eye movements. Stimuli included head rotation in the light, head rotation in the dark, and rotation of an optokinetic drum about the animal. We employed sinusoidal and triangular position profiles in the 0.05- to 0.8-Hz frequency band. We also examined transient responses to step changes in eye position. 2. We found identified vestibular nucleus cells (i.e., FRN/non-MPNs, FRN/MPNs, and non-FRN/MPNs) in the parvocellular and magnocellular portions of the medial vestibular nucleus, at the rostrocaudal level of the dorsal acoustic stria. 3. All identified vestibular nucleus neurons were excited during ipsilateral (relative to side of recording) head rotation and contralateral eye rotation. 4. The neuronal firing rates could be related to eye position and its time derivatives, and that relationship could be approximated by a two-pole, one-zero linear transfer function. As with abducens neurons, a more detailed approximation requires inclusion of two nonlinearities-a hysteresis and a variable sensitivity term that increases as eye movement amplitude decreases. 5. When the vestibuloocular reflex is suppressed by a conflicting full-field visual stimulus [visual vestibular conflict condition (VVC)], vestibular nucleus neuron modulation is largely suppressed. The remaining modulation is motoric in nature, because it can be related to the residual eye movements. Cells with "sensory vestibular signals," i.e., cells whose modulation during VVC correlates better with head rotation than eye movement, were not encountered. 6. We examined the dependence of firing rate parameters on stimulus modality. All neurons exhibited increased phase lead with respect to abducens nucleus neurons during stimuli involving head rotation. This finding could indicate that vestibular-derived inputs are inhomogeneously distributed on premotor neurons and that the studied premotor population receives a stronger vestibular input than another premotor group, not recorded in the current experiments. 7. FRNs and non-FRNs were similar in their qualitative response to the fast phases, the applicability of the two-pole, one-zero transfer function, hysteresis, and the amplitude nonlinearity. 8. FRNs differed from non-FRNs in having a phase advanced firing rate at all stimulus frequencies during visual and vestibular stimuli. The phase difference suggests that one role of the rabbit flocculus is to regulate phase of the net premotor signal.

Animals↗
PubMed1995

Phase relations of Purkinje cells in the rabbit flocculus during compensatory eye movements.

1. Purkinje cells in the rabbit flocculus that respond best to rotation about the vertical axis (VA) project to flocculus-receiving neurons (FRNs) in the medial vestibular nucleus. During sinusoidal rotation, the phase of FRNs leads that of medial vestibular nucleus neurons not receiving floccular inhibition (non-FRNs). If the FRN phase lead is produced by signals from the flocculus, then the Purkinje cells should functionally lead the FRNs. In the present study we recorded from VA Purkinje cells in the flocculi of awake, pigmented rabbits during compensatory eye movements to determine whether Purkinje cells have the appropriate firing rate phases to explain the phase-leading characteristics of the FRNs. 2. Awake rabbits were sinusoidally rotated about the VA in the light and the dark at 0.05-0.8 Hz with different amplitudes. The phase of the simple spike (SS) modulation in reference to eye and head position was calculated by determining the eye position sensitivity and the eye velocity sensitivity using multivariate linear regression and Fourier analysis. The phase of the SS modulation in reference to head position was compared with the phase of the FRN modulation, which was obtained in prior experiments with the same stimulus paradigms. 3. The SS activity of nearly all of the 88 recorded floccular VA Purkinje cells increased with contralateral head rotation. During rotation in the light, the SS modulation showed a phase lead in reference to contralateral head position that increased with increasing frequency (median 56.9 degrees at 0.05 Hz, 78.6 degrees at 0.8 Hz). The SS modulation led the FRN modulation significantly at all frequencies. The difference of medians was greatest (19.2 degrees) at 0.05 Hz and progressively decreased with increasing frequency (all Ps < 0.005, Wilcoxon rank-sum test). 4. During rotation in the dark, the SS modulation had a greater phase lead in reference to head position than in the light (median 110.3 degrees at 0.05 Hz, 86.6 degrees at 0.8 Hz). The phase of the SS modulation in the dark led that of the FRNs significantly at all frequencies (difference of medians varied from 24.2 degrees at 0.05 Hz to 9.1 degrees at 0.8 Hz; all Ps < 0.005). 5. The complex spike (CS) activity of all VA Purkinje cells increased with ipsilateral head rotation in the light. Fourier analysis of the cross-correlogram of the CS and SS activity showed that the phase lag of the CS modulation in reference to the SS modulation at 0.05 Hz in the light was not significantly different from that at 0.8 Hz (median 199.7 degrees at 0.05 Hz, 198.3 degrees at 0.8 Hz), even though the phases of the SS modulation at these two frequencies were significantly different (P < 0.001). These data indicate that the average temporal reciprocity between CS and SS modulation is fixed across the range of frequencies used in the present study. 6. The CS activity of most Purkinje cells did not modulate during rotation in the dark. Of 124 cases (each case consisting of the CS and SS data of a VA Purkinje cell obtained at 1 particular frequency) examined over the frequency range of 0.05-0.8 Hz, 17 cases (14%) showed CS modulation. In the majority (15 of 17) of these cases, the CS activity increased with contralateral head rotation; these modulations occurred predominantly at the higher stimulus velocities. 7. On the basis of the finding that FRNs of the medial vestibular nucleus lead non-FRNs, we predicted that floccular VA Purkinje cells would in turn lead FRNs. This prediction is confirmed in the present study. The data are therefore consistent with the hypothesis that the phase-leading characteristics of FRN modulation could come about by summation of VA Purkinje cell activity with that of cells whose phase would otherwise be identical to that of non-FRNs. The floccular SS output appears to increase the phase lead of the net preoculomotor signal, which is in part composed of the FRN and non-FRN signals.

Action Potentials↗
PubMed1995

Temporal relations of the complex spike activity of Purkinje cell pairs in the vestibulocerebellum of rabbits.

Parasagittal zones in the vestibulocerebellum contain Purkinje cells whose complex spike (CS) activity is modulated in response to rotational optokinetic stimulation (OKS) about either the vertical axis (VA) or a horizontal axis (HA) that is approximately perpendicular to the ipsilateral anterior canal. In rabbits, there are two VA zones in both the ventral nodulus and flocculus, two HA zones in the flocculus, and one HA zone in the ventral nodulus. We investigated the temporal relationship of the CS activity of Purkinje cell pairs in the same or different zones of the vestibulocerebellum in ketamine-anesthetized pigmented rabbits. A synchronous temporal relationship was defined as the tendency of the CS of each Purkinje cell to fire within, at most, 2 msec of one another. Generally, neurons in the same zone showed a tendency to exhibit CS synchrony. Of 82 pairs consisting of two Purkinje cells in the same zone (e.g., two nodulus HA cells), 33 were synchronous. In contrast, none of 26 pairs consisting of two neurons in functionally different zones (e.g., a VA cell paired with an HA cell), showed CS synchrony. Pairs consisting of neurons in spatially separated VA zones in the ventral nodulus also showed a tendency to be synchronously related (6/16), as did pairs consisting of a nodulus VA cell and a flocculus VA cell (3/14). The CS synchrony was higher during OKS in the preferred direction than during spontaneous activity. This is the first demonstration that CS synchrony in the vestibulocerebellum can be manipulated with a natural sensory stimulus.

Animals↗
PubMed1994

Projections of individual Purkinje cells of identified zones in the flocculus to the vestibular and cerebellar nuclei in the rabbit.

The rabbit flocculus can be divided into five zones (zones 1, 2, 3, 4, and C2) with the use of acetylcholinesterase histochemistry. The projections of individual Purkinje cells in these zones to the vestibular and cerebellar nuclei were studied by using biocytin as an anterograde tracer. The zones were physiologically identified in terms of the Purkinje cell complex spike modulation occurring in response to optokinetic stimulation. In zones 1 and 3 neurons respond best to rotation about a horizontal axis that is close to perpendicular to the ipsilateral anterior semicircular canal, whereas in zones 2 and 4 neurons respond best to rotation about the vertical axis. Complex spike activity in zone C2 is unresponsive to optokinetic stimulation. Collectively, Purkinje cells of zone 1 projected to the ventral dentate nucleus, dorsal group y, and superior vestibular nucleus; Purkinje cells of zones 2 and 4 projected to the magnocellular and parvicellular parts of the medial vestibular nucleus; Purkinje cells of zone 3 projected to dorsal group y, ventral group y, and the superior vestibular nucleus; and Purkinje cells of zone C2 projected to the interposed posterior nucleus and dorsal group y. Some of the labeled Purkinje cell axons branched and innervated two nuclei. Branching axons from zone 1 either innervated both the ventral dentate nucleus and the superior vestibular nucleus or both dorsal group y and the superior vestibular nucleus. Branching axons from zones 2 and 4 innervated both the magnocellular and the parvicellular parts of the medial vestibular nucleus. Branching axons from zone 3 innervated both dorsal group y and the superior vestibular nucleus, or both ventral group y and the superior vestibular nucleus. Branching axons from zone C2 innervated both the interposed posterior nucleus and dorsal group y. Some of the target nuclei of the floccular Purkinje cell axons (e.g., dorsal group y and interposed posterior nucleus) project to the part of the inferior olive that, in turn, projects to the corresponding floccular zone, thus completing a closed pathway consisting of the inferior olive, the cerebellar cortex, and the cerebellar and vestibular nuclei. Other target nuclei (e.g., superior vestibular nucleus and medial vestibular nucleus) do not project back to the olivary subnuclei that innervate the flocculus and are part of an open olivofloccular pathway. An individual Purkinje cell thus can innervate a nucleus in the closed pathway as well as a nucleus in the open pathway.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1994

Projections of individual Purkinje cells of identified zones in the ventral nodulus to the vestibular and cerebellar nuclei in the rabbit.

The projections of Purkinje cells from zones in the ventral nodulus of pigmented rabbits were studied with the use of extracellularly injected biocytin as an anterograde tracer. The zones were physiologically identified according to the complex spike modulation of Purkinje cells in response to optokinetic stimulation. Purkinje cells in the most medial zone do not respond to optokinetic stimulation; they project to the fastigial nucleus, the perifastigial white matter, the periinterposed white matter, and the medial vestibular nucleus. In the adjacent zone, Purkinje cells respond best to optokinetic stimulation about the vertical axis; they project to the periinterposed white matter and the medial vestibular nucleus. Purkinje cells in the next zone respond best to optokinetic stimulation about an axis approximately perpendicular to the ipsilateral anterior canal; they project to the periinterposed white matter, dorsal group y, the superior vestibular nucleus, and the medial vestibular nucleus. In the most lateral zone, Purkinje cells respond best to optokinetic stimulation about the vertical axis; they project to the periinterposed white matter, dorsal group y, and the medial vestibular nucleus. The majority of axons gave off collaterals and innervated more than one nucleus. Often, three or four different areas received terminals from a single Purkinje cell axon. The zonal projection pattern of the ventral nodulus is compared to that of the flocculus, which, with respect to the visual climbing fiber afferents, has similar zones.

Animals↗
PubMed1994

The rostral dorsal cap and ventrolateral outgrowth of the rabbit inferior olive receive a GABAergic input from dorsal group Y and the ventral dentate nucleus.

The dorsal cap and ventrolateral outgrowth of the inferior olive are involved in the control of eye movements. The caudal dorsal cap is predominantly involved in the horizontal optokinetic reflex; it receives most of its GABAergic input from the nucleus prepositus hypoglossi. In the present study, we determined the source of a major inhibitory input to the rostral dorsal cap and the ventrolateral outgrowth, which are the olivary subnuclei mainly involved in the "vertical" optokinetic reflexes. We studied these subnuclei in the rabbit with the use of retrograde tracing of horseradish peroxidase and anterograde tracing of wheat germ agglutinin-coupled horseradish peroxidase combined with postembedding immunocytochemistry. The ventral dentate nucleus of the cerebellum and dorsal group y project contralaterally to the rostral dorsal cap and ventrolateral outgrowth; this projection is entirely GABAergic. The terminals of this input form predominantly symmetric synapses with extraglomerular and intraglomerular dendrites; the remaining terminals are axosomatic. In addition, the dorsal cap and ventrolateral outgrowth contain significantly more crest synapses than any other olivary subnucleus. The terminals that form these crest synapses are derived from dorsal group y and/or the ventral dentate nucleus. None of the terminals in the dorsal cap or ventrolateral outgrowth was glycinergic.

Animals↗
PubMed1994

Functional and anatomic organization of three-dimensional eye movements in rabbit cerebellar flocculus.

1. The three-dimensional, binocular eye movements evoked by electrical microstimulation of the cerebellar flocculus of alert, pigmented rabbits were recorded using the scleral search coil technique. The components of these eye movements were obtained in reference to an orthogonal coordinate system consisting of a vertical axis and two horizontal axes at 45 degrees and 135 degrees azimuth. The azimuth coordinate was taken to increase to both sides from the 0 degrees reference in the direction of the nose. 2. Eye movements were evoked most readily by stimulation (0.2-ms pulses at 200 Hz for 1 s, intensity < or = 20 microA) at loci in the deep granular layer and the white matter. They consisted of slow (5-20 deg/s) movements. The responses were either binocular, with the eye ipsilateral to the stimulated flocculus usually having the larger amplitude, or were monocular, in which case they were restricted to the ipsilateral eye. 3. The evoked responses were classified according to the combination of the largest measured component of rotation for the two eyes and its sense of rotation (clockwise, CW, or counterclockwise, CCW). Seventy-eight percent of the evoked eye movements could be placed in one of two classes. For one of these classes the largest response component was a short-latency abduction of the ipsilateral eye about its vertical axis (19%), whereas for the other class (59%), the largest response component was a short-latency CCW rotation of the ipsilateral (left) eye about its 135 degrees axis. This response was frequently (50%) accompanied by a smaller short-latency CW rotation of the contralateral (right) eye about its 45 degrees axis. 4. The two main classes of three-dimensional eye movements are associated differentially with anatomically distinguishable compartments that are revealed by acetylcholinesterase histochemistry. Of the five anatomically distinguishable compartments in the floccular white matter, three are predominant. The middle of these three compartments is associated with the vertical axis class of movements, whereas the two adjacent compartments are associated with the 135 degrees class of eye movements. The eye movement relation of the other two, smaller compartments, was not determined. 5. The spatial orientation of the rotation axes of the two main classes of evoked eye movements closely corresponds to that of the preferred axes of the visual climbing fiber input to the flocculus. This suggests that both are organized in a similar coordinate system.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1994

Intrathecal magnesium sulfate protects the spinal cord from ischemic injury during thoracic aortic cross-clamping.

BACKGROUND: Paraplegia is a known complication after surgery on the descending thoracic aorta. Thoracic aortic cross-clamping causes an increase in proximal aortic and cerebrospinal fluid pressures. Sodium nitroprusside, though effectively decreasing proximal aortic pressure, has been implicated in worsening the incidence of paraplegia by further increasing cerebrospinal fluid pressure and decreasing distal blood pressure, thereby reducing spinal cord perfusion pressure. Intravenous administration of magnesium sulfate has been shown to offer some spinal cord protection when used with mild hypothermia. This study investigated the effect of intrathecal magnesium on the prevention of paraplegia when sodium nitroprusside is used to control proximal hypertension during thoracic aortic cross-clamping in a dog model of spinal cord ischemia. METHODS: Two groups of eight dogs underwent thoracic aortic cross-clamping via a small thoracotomy incision for 45 min. Proximal, distal, and central venous pressures and cerebrospinal fluid pressures were monitored. Temperature was maintained at 36 degrees C. Sodium nitroprusside was used to control proximal hypertension. The control group received no magnesium sulfate, and a second group received 3 mg/kg intrathecal magnesium sulfate before thoracic aortic cross-clamping. The dogs were neurologically evaluated 24 h later by an observer blinded to the dogs' group. Spinal cord segments were obtained for histologic examination. RESULTS: Proximal mean arterial pressure, cerebrospinal fluid pressure, spinal cord perfusion pressure, and central venous pressure were not statistically different between the two groups. Neurologic outcome, however, was statistically different between the groups. None of the eight dogs in the magnesium group had any measurable neurologic injury, in contrast to the control group, in which seven of the eight dogs had severe neurologic injury (P < 0.005). Post mortem histologic data supported these findings. CONCLUSIONS: Intrathecal magnesium can prevent spinal cord injury despite markedly negative spinal cord perfusion pressure during thoracic aortic cross-clamping in a canine model of spinal cord ischemia.

Animals↗
PubMed1993

Three-dimensional organization of optokinetic responses in the rabbit.

1. Three-dimensional rotations of both eyes were measured in alert rabbits during optokinetic stimulation about axes lying in the horizontal plane or about an earth-vertical axis, with either one or both eyes viewing the stimulus. Optokinetic stimulus speed was 2 degrees /s, either continuous or alternating in polarity (triangular stimulus). In addition to the gains of the responses, the orientations of the response axes relative to the stimulus axes were determined. 2. In comparison to the response to constant-speed optokinetic stimulation about the vertical axis, the response to constant-speed optokinetic stimulation about horizontal axes was characterized by the lack of a speed buildup. In many cases, slow phase tracking was good as long as the eye was within the central oculomotor range but deteriorated when eye deviation became more eccentric and fast phases failed to be generated. These features suggest that the optokinetic reflex about horizontal axes functions as a position-control system, rather than as a velocity-control system. 3. Binocular optokinetic stimulation at constant speed (2 degrees/s) about the roll axis (0 degrees azimuth horizontal axis) elicited disconjugate responses. Although the gain of the response was not significantly different in the two eyes (0.38 for downward and 0.44 for upward stimulation), the response axes of the two eyes differed by as much as 51 degrees. 4. Monocular, horizontal axis optokinetic stimulation at constant speed elicited responses that were grossly dissociated between the two eyes. The magnitude of the responses was anisotropic in that it varied with the azimuthal orientation of the stimulus axis; the maximum gain for each eye (0.41 for the seeing and 0.33 for the covered eye) was at 135 degrees azimuth for each eye. The axis orientation and direction (sense of rotation) of the optokinetic stimulus eliciting the maximal response for each eye coincided with the optic flow normally associated with the maximal excitation of the corresponding ipsilateral anterior canal. 5. Binocular, triangular optokinetic stimulation with small excursions (+/- 10 degrees), which avoided the saturation problems of constant-speed stimulation, elicited adequate responses without systematic directional asymmetries. Gain was approximately 0.9 for all stimulus axis orientations in the horizontal plane. 6. During monocular stimulation with triangular stimuli, the response of the seeing eye showed a gain of approximately 0.5 for all orientations of the stimulus axis. In contrast, the covered eye showed anisotropic responses, with a maximum gain of approximately 0.5 during stimulation of the seeing eye about its 45 degree axis.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1991

The oxygen and nitrous oxide indices of flammability of endotracheal tubes determined by laser ignition.

This study determines the flammability of polyvinylchloride (PVC), red rubber (RR), and silicone (Si) endotracheal tubes in oxygen- and nitrous-oxide-enriched atmospheres. Flammability is measured by using the oxygen and nitrous oxide indices of flammability with laser ignition. The laser-ignited oxygen (O2) index of flammability of the endotracheal tubes is: PVC, 0.25; RR, 0.19; Si, 0.20. The laser-ignited nitrous oxide (N2O) index of flammability of the endotracheal tubes is: PVC, 0.45; RR, 0.37; and Si, 0.41. These results are similar to the previously reported O2 and N2O indices of flammability with propane-torch ignition. This study validates the concept that the indices of flammability are useful measures of endotracheal tube flammability and are independent of the ignition source.

Fires↗