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R Baker

Publications and source records attributed to R Baker.

At least 415 records · Page 23Linked to original sources
PubMed1988

Synaptic organization of the tectal-facial pathways in the cat. I. Synaptic potentials following collicular stimulation.

1. The synaptic pathways underlying tectal influence over pinna movements were studied using an acute electrophysiological approach. Under pentobarbital anesthesia, postsynaptic potentials were recorded intracellularly in antidromically identified, cat facial motoneurons following electrical stimulation of the superior colliculus. How collicular topography is reflected in these synaptic potentials was examined using multiple stimulation sites. The pathways responsible for tectally evoked synaptic potentials were studied by making acute brain stem lesions and by intra-axonal horseradish peroxidase (HRP) staining. 2. Monosynaptic excitatory potentials (EPSPs) with latencies ranging from 0.7 to 1.1 ms and amplitudes that were always less than 1 mV were recorded in motoneurons following stimulation of the contralateral superior colliculus. Larger disynaptic EPSPs ranging in latency from 1.2 to 2.0 ms were recorded both in isolation and in association with monosynaptic EPSPs. In addition, disynaptic inhibitory synaptic potentials (IPSPs) with latencies ranging from 1.5 to 2.5 ms were observed, often in combination with monosynaptic EPSPs. Both disynaptic EPSPs and IPSPs were graded, augmented by multiple stimuli and found in all categories of motoneurons. 3. Stimulation of the ipsilateral superior colliculus produced nearly the same spectrum of potentials and latencies as did contralateral tectal stimulation. Occlusion between ipsi- and contralaterally evoked IPSPs suggests there might be a common element in the inhibitory disynaptic pathways. 4. More discrete populations of facial motoneurons were investigated. Specifically, motoneurons innervating the platysma and orbicularis oculi muscles, the intrinsic ear muscles, and muscles that move the vibrissae all displayed tectally elicited mono- and di-synaptic potentials. Collicular input was not restricted to motoneurons involved in orienting the pinnae. 5. The presence, polarity, and amplitude of the synaptic potentials evoked in individual facial motoneurons exhibited variations that were related to the site of stimulation in either the ipsi- or contralateral colliculus. These variations are compatible with the idea that the collicular input to facial motoneurons is topographically organized. 6. Acute lesions at the level of the superior olive indicated that the pathway producing the contralateral monosynaptic EPSPs runs, near the midline, ipsilateral to the target facial nucleus, whereas the contralateral disynaptic and the ipsilateral mono- and disynaptic pathways lie further lateral.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1987

Morphology and distribution of serotoninergic and oculomotor internuclear neurons in the cat midbrain.

Serotoninergic fibers have been reported in both the abducens and facial nuclei of the cat. Furthermore, serotoninergic dorsal raphe and oculomotor internuclear neurons occupy similar locations in the periaqueductal gray overlying the oculomotor and trochlear motor nuclei. To resolve the issue of whether these two populations of neurons overlap, serotoninergic fibers were assayed in the abducens and facial nucleus; then the morphologies and distributions of identified serotoninergic neurons and oculomotor internuclear neurons were determined. Both the abducens and facial nuclei contained varicosities labelled with antibody to serotonin, but a much higher density of immunoreactive fibers was present in the latter, especially in its medial aspect. Distinct synaptic profiles labelled with antibodies to serotonin were observed in both nuclei. In both cases, terminal profiles contained numerous small, predominantly spheroidal, synaptic vesicles as well as a few, large, dense-core vesicles. These profiles made synaptic contacts onto dendritic and, in the facial nucleus, somatic profiles that occasionally displayed asymmetric, postsynaptic, membrane densifications. Following injection of horseradish peroxidase into either the abducens or facial nuclei, double-label immunohistochemical techniques demonstrated that the serotoninergic and oculomotor internuclear neurons form two distinct cell populations. The immunoreactive serotoninergic cells were distributed within the dorsal raphe nucleus, predominantly caudal to the retrogradely labelled oculomotor internuclear neurons. The latter were located in the oculomotor nucleus along its dorsal border and in the adjacent supraoculomotor area. Intracellular injection of horseradish peroxidase revealed that oculomotor internuclear neurons have multipolar somata with up to ten long, tapering dendrites that bifurcate approximately five times. Their dendritic fields were generally contained within the nucleus and adjacent supraoculomotor area. In contrast, putative serotoninergic neurons were often spindle-shaped and exhibited far fewer primary dendrites. Many of these long, narrow, sparsely branched dendrites crossed the midline and extended to the surface of the cerebral aqueduct. In the vicinity of the aqueduct they branched repeatedly to form a dendritic thicket. The axons of the intracellularly stained serotoninergic neurons emerged either from the somata or the end of a process with dendritic morphology, and in some cases they produced axon collaterals within the periaqueductal gray. Thus the oculomotor internuclear and serotoninergic populations differ in both distribution and morphology.(ABSTRACT TRUNCATED AT 400 WORDS)

Abducens Nerve↗
PubMed1987

Extra- and intracellular HRP analysis of the organization of extraocular motoneurons and internuclear neurons in the guinea pig and rabbit.

The distribution of extraocular motoneurons and abducens and oculomotor internuclear neurons was determined in guinea pigs by injecting horseradish peroxidase (HRP) into individual extraocular muscles, the abducens nucleus, the oculomotor nucleus, and the cerebellum. Motoneurons in the oculomotor nucleus innervated the ipsilateral inferior rectus, inferior oblique, medial rectus, and the contralateral superior rectus and levator palpebrae muscles. Most motoneurons of the trochlear nucleus projected to the contralateral superior oblique muscle although a small number innervated the ipsilateral superior oblique. The abducens and accessory abducens nuclei innervated the ipsilateral rectus and retractor bulbi muscles, respectively. The somata of abducens internuclear neurons formed a cap around the lateral and ventral aspects of the abducens nucleus. The axons of these internuclear neurons terminated in the medial rectus subdivision of the contralateral oculomotor nucleus. At least two classes of guinea pig oculomotor internuclear interneurons exist. One group, located primarily ventral to the oculomotor nucleus, innervated the abducens nucleus and surrounding regions. The second group, lying mainly in the dorsal midline area of the oculomotor nucleus, projected to the cerebellum. Intracellular staining with HRP demonstrated similar soma-dendritic organization for oculomotor and trochlear motoneurons of both guinea pigs and rabbits. Dendrites of oculomotor motoneurons radiated symmetrically from the soma to cover approximately one-third of the entire nucleus, and each motoneuron sent at least one dendrite into the central gray overlying the oculomotor nucleus. In both species, a small percentage of oculomotor motoneurons possessed axon collaterals that terminated both within and outside of the nucleus. The dendrites of trochlear motoneurons extended into the medial longitudinal fasciculus and the reticular formation lateral to the nucleus. Our data on the topography of motoneurons and internuclear neurons in the guinea pig and soma-dendritic organization of motoneurons in the guinea pig and rabbit show that these species share common organizational and morphological features. In addition, comparison of these data with those from other mammals reveals that dendritic complexity (number of dendrites per motoneuron) of extraocular motoneurons exhibits a systematic increase with animal size.

Animals↗
PubMed1987

Synthesis and characterization of all four isomers of the muscarinic agonist 2'-methylspiro[1-azabicyclo[2.2.2]octane-3,4'-[1,3]dioxolane].

The four separated isomers of the muscarinic agonist 1, previously known as AF30, have been synthesized by a route that has allowed the absolute stereochemistry of each isomer to be assigned. With the chirality of (-)-camphanic acid known, X-ray analysis of the more crystalline intermediate diastereomeric camphanate 5A allowed the absolute stereochemistry at the quinuclidine chiral center to be determined. Each diastereomer was separately transformed into the spirodioxolane with concomitant introduction of the second chiral center. Chromatographic separation followed by a second crystal structure determination revealed the absolute stereochemistry of all four isomers of 1. Detailed biological evaluation of each isomer indicated that while the 3(R),2'(S) isomer was the most active in binding studies, it was the 3(R),2'(R) isomer that displayed the largest functional selectivity between ganglion (M-1 site) and heart (M-2 site). With the same internal chiral standard, the absolute configuration of the more active enantiomer of 3 was shown to be S, confirming earlier literature reports.

Animals↗
PubMed1987

Motoneuronal innervation and mechanical properties of extraocular muscles in the catfish, (Ictalurus punctatus).

Mechanical characteristics and electrical activity were studied in the extraocular muscles of the catfish, Ictalurus punctatus. The contractile properties were determined by stimulation of the individual muscle nerve branches to lateral and medial rectii and the superior and inferior oblique muscles. The speed of contraction was higher than in most other fish muscle, with a twitch contraction time of about 12 ms and a tetanus fusion frequency of 150-170 Hz in all four eye muscles. The fatigue resistance was also high. These properties were the same in fully innervated and partially innervated muscle, largely irrespective of what part of the muscle that was activated. Although different fibre types are known to exist in fish extraocular muscle, it was not possible to obtain functional separation of the mechanical force profile even in the lateral rectus with two distinct motoneuronal innervations. We suggest that polyneuronal innervation of the muscle fibres produces the mechanical responses. Since EMG activity during spontaneous eye movements was similar in the global and the orbital parts of the muscle, all types of fibres in fish extraocular muscle are probably recruited for all types of eye movements.

Animals↗
PubMed1987

Hereditary bovine syndactyly: diagnosis in bovine fetuses.

Diagnostic guidelines were established for progeny testing of hereditary bovine syndactyly. Through the use of superovulation and embryo transfer, 139 fetuses were recovered at 50 to 77 days gestation. Normal (+/+, +/sy) and syndactylous (sy/sy) anatomy of Holstein fetuses was defined, and the accuracy of macroscopic versus microscopic limb diagnosis was assessed. Chondrification and ossification differences between normal (+/+, +/sy) and syndactylous (sy/sy) fetuses were only age-related. Normal (+/+, +/sy) fetal limbs differed from normal (+/+, +/sy) adult bovine limbs in two ways. Fetal metacarpal and metatarsal III and IV bones were not fused, and fetal metacarpal and metatarsal II and V bones often extended up to three-fourths the length of metacarpal and metatarsal III and IV bones. In syndactylous (sy/sy) fetuses, synostosis asymmetries occurred within and between fetal limbs, and between fetuses, representing variable gene expressivity. Synostosis pattern within limbs did not correspond with those of the adult bovine; the second phalangeal pair was synostotic most frequently in the fetus, followed by the first, and then the third pair. Synostosis patterns between fetal limbs agreed with those of the adult; there was a right-left and front-rear limb gradient. Partial synostoses occurred sporadically in all three paired phalanges. Those of the first and third pair always involved the tip closest to the second phalangeal pair. A unique example of variable gene expressivity occurred in one syndactylous fetus. Both front limbs were syndactylous, while both rear limbs were normal grossly. Microscopically the right rear limb was normal while the left rear limb consisted of closely apposed phalangeal blastemata without coalescence.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1986

Organization of the efferent vestibular nuclei and nerves of the toadfish, Opsanus tau.

The efferent vestibular nuclei and nerves were studied in the toadfish, Opsanus tau, with morphological and electrophysiological techniques. The origin and course of the efferent vestibular nerves was extensively documented. One major morphological observation was that the efferent nerves comprise a peripheral network that is anatomically distinct, and separable by dissection from the primary afferents innervated by each end organ. These anatomically distinct nerves are likely to be a major asset in physiological studies of efferent vestibular function. The retrograde transport of horseradish peroxidase (HRP) from each of the nerves innervating the vestibular and lateral line organs was used to delineate the subgroups of efferent neurons projecting to these end organs. The efferent vestibular nuclei are located in the posterior medulla in and around the median longitudinal fasciculi (MLF). We divided the nuclei cytoarchitecturally into lateral, medial, and dorsal subdivisions. The lateral cells had bilateral dendritic trees while the dorsal cells had ipsilateral, unilateral dendritic trees. There was a higher proportion of lateral cells that innervated the canal organs and the utricle while the dorsal cells tended to innervate the other organs. The total number of cells obtained by summing those from separate nerve label was twice the total cell count present in the nuclei. Indirectly, this indicates that some cells project to more than one end organ. Efferent neurons were penetrated with glass microelectrodes, and their end organs and patterns of connectivity with other end organs were investigated by stimulating various vestibular nerves. Posterior semicircular canal efferent cells are electrically coupled to each other and could be activated electrically or chemically by stimulating other ipsilateral or contralateral vestibular nerves. It is suggested that electrical coupling might be responsible for the uniform behavior of these cells under certain conditions. Morphological and physiological experiments suggested that the semicircular canals are innervated by their own, exclusive populations of efferent neurons while other end organs may share efferent innervation. Single cells were injected intracellularly with HRP and their morphology was studied and characterized by light microscopy. Intracellular label confirmed the morphological features demonstrated by retrograde transport of HRP and also revealed that some cells had central axon collaterals that terminated within the MLF. These morphological and physiological results provide a basis for understanding the behavior of efferent vestibular neurons in the alert animal.

Acoustic Maculae↗
PubMed1986

Effect of norepinephrine and angiotensin II on intrarenal oxygen availability.

Oxygen availability (O2a) in the cortex, outer medulla, inner medulla, and papilla of the left kidney was measured in 12 unanesthetized rabbits implanted with O2 sensitive electrodes. Each was injected with a single i.v. bolus of either norepinephrine (NE) or angiotensin II (AII) in equipressor doses, and intrarenal O2a was continuously recorded. The NE response consisted of parallel and proportional intrarenal O2a decreases and overshoots in all zones; AII response consisted of progressively smaller O2a decreases from outer to inner zones during maximum response and smaller increases during maximum overshoot. These response differences may be important under various physiologic or pathologic conditions.

Angiotensin II↗
PubMed1986

Histochemical localization of acetylcholinesterase in relation to motor neurons and internuclear neurons of the cat abducens nucleus.

Three distinct patterns of AChE localization have been observed in relation to cat abducens motor neurons and internuclear neurons labelled by retrograde transport of horseradish peroxidase. First, AChE was localized predominantly within cisternae of granular endoplasmic reticulum and agranular reticulum of motor neuron somata, dendrites and axons, but was absent from internuclear neurons. AChE was also associated with saccules of the Golgi apparatus in the motor neurons, but was was absent from all other cytoplasmic organelles. Second, AChE was observed on the soma-dendritic and axonal surface membrane of the motor neurons, particularly at sites of apposition of synaptic endings of all morphological types, but was usually absent from the surface membranes of internuclear neurons. Third, AChE was associated both extracellularly and intracellularly with certain synaptic endings that contained spheroidal synaptic vesicles and that contacted both motor neurons and internuclear neurons. A similar pattern of staining of synaptic endings was observed at the neuromuscular junctions in the lateral rectus muscle. Axotomy of the VIth nerve resulted in loss of intracellular AChE associated with the Golgi apparatus and extracellular AChE on the somatic surface membrane of the motor neurons. The patterned localization of AChE contrasted with the localization of butyrylcholinesterase, which was associated predominantly with astrocytes. The findings suggest different roles of AChE as a function of the different patterns of localization.

Abducens Nerve↗
PubMed1986

Androgen and estrogen metabolism: relationship to obesity.

Adipose tissue contains both aromatase and 17 beta-steroid dehydrogenase activity. Therefore, to see whether there was a relationship between obesity and certain parameters of androgen and estrogen metabolism we infused 88 women, mean age 51.1 +/- 0.3 years and mean weight 140 +/- 3 lbs, with 3H-testosterone (T)/14C-estradiol (E2) and 3H-androstenedione (A)/14C-estrone (E1) on separate occasions. Blood samples were obtained during the infusion and all urine was collected for 4 days following the start of the infusion. The blood samples were analyzed for radioactivity as A, T, E1, E2, and dihydrotestosterone (DHT) and the urines were analyzed for radioactivity as E1 and E2. From these data we calculated the percent of A converted to T ([rho]A,TBB = percent of A infused measured as T in the blood), [rho]T,ABB [rho]E1,E2BB, and [rho]E2,E1BB. We also measured the ratio of radioactivity as 3H-DHT to radioactivity as 3H-A (CRA,DHT) and 3H-T (CRT,DHT) during the respective androgen infusions. From the ratio of 3H/14C as estrone or estradiol in the urine following 3H-A or 3H-T infusions, we calculated the percent of A or T that was aromatized to E1 or E2 ([rho]A,E1BM; [rho]T,E2BM). When the data from these women were related to weight or Quetelet's Index (QI = wt/ht2) by unweighted linear regression, the only values that were significantly correlated with weight and QI were [rho]T,E2BM and [rho]A,E1BM; for all other [rho] and CR values there was no correlation with weight or QI. We conclude that peripheral aromatization is positively correlated with adiposity but androgen interconversions and estrogen interconversions are not related to adiposity.

17-Hydroxysteroid Dehydrogenases↗
PubMed1986

Behavior of neurons in the abducens nucleus of the alert cat--I. Motoneurons.

The activity of 53 antidromically identified abducens motoneurons was analyzed in alert cats during spontaneous and vestibular induced eye movements. Conduction velocities ranged from 13 to 70 m/s and all motoneurons increased their discharge rates with successive eye positions in the abducting direction. Motoneurons were recruited from -19 degrees to +7 degrees. Within the oculomotor range frequency saturation was never observed for any cell. The slope of rate-position (k) relationships ranged from 2 to 17.7 spikes/s/deg (n = 40, mean 8.7 +/- 2.5). Regression analysis showed that the rate-position plots could be fit by straight lines but in most cases exponential curves produced slightly better statistical fits. Steeper slopes suggest that successively larger increases in k are required for the lateral rectus muscle to maintain more eccentric fixations in the on direction. Interspike intervals for a constant eye position exhibited low variability (less than 3.5%) for fixations shorter than 1 s. Over longer periods, variability increased in proportion to the duration of the fixation in exponential-like fashion up to 14%. Abducens motoneurons showed considerable variability in frequency during repeated fixations of the same eye position. Discharge rates were found to depend upon both the direction of the previous eye movement and, more importantly, the animal's level of alertness. The rate-position regression lines for fixation periods after saccades in the on direction significantly differed in slopes (100%) and thresholds (20%) from those in the off direction. The observed static hysteresis in abducens motoneuron behavior was in opposite direction to that previously described for the mechanical properties of the lateral rectus. This suggests both neural and mechanical factors are significantly involved in determining final eye position. The animal's level of alertness was evaluated in this study by counting the number of saccadic movements/s occurring in "alert" (1 +/- 0.2 saccades/s), and "drowsy" (0.5 +/- 0.2 saccades/s) circumstances. Comparison of the rate-position regression lines between the two conditions showed a significant decrease in slopes (100%) and elevation of thresholds (70%). Discharge rate of abducens motoneurons increased abruptly 8.9 +/- 2.8 ms prior to saccades in the horizontal on direction, and decreased 14.8 +/- 4.05 m before saccades in the off direction. During purely vertical saccades the firing frequency of abducens motoneurons did not change. Burst frequency did not saturate during saccades, but increased with saccadic velocity in a linear fashion.(ABSTRACT TRUNCATED AT 400 WORDS)

Abducens Nerve↗
PubMed1986

Behavior of neurons in the abducens nucleus of the alert cat--II. Internuclear neurons.

The activity of 43 antidromically identified abducens internuclear neurons with conduction velocities ranging from 14 to 54 m/s was analyzed in alert cats during spontaneous and vestibular induced eye movements. The discharge rate of internuclear neurons significantly increased with successive adducting positions of the contralateral eye. Slopes of rate-position (k) relationships ranged from 3.1 to 17.9 spikes/deg (mean 12.01 +/- 3.1). Threshold ranged from -19 degrees to +3 degrees. Frequency saturation was never observed for any internuclear neuron within the oculomotor range. Although straight lines were selected to illustrate the rate-position relationships, exponential curves always provided the best statistical fit demonstrating that an enhancement in frequency potentiation (k) must accompany more eccentric fixations in the on direction. Internuclear neurons showed a low variability in firing rate (less than 3.0%) for fixations less than 1 s. Variability increased with both longer and repeated fixations of the same eye position. Discharge rates were found to depend upon both the direction of the preceding eye movement and the animal's level of alertness. Separate regression lines of rate-position relations following saccades in the on and off directions differed significantly in slope (100%), but not threshold. The observed static hysteresis in an identified non-motoneuron shows this property to be in a central neural circuit prior to the extraocular motoneuron. The slopes (k) of rate-position plots for all internuclear neurons decreased significantly (100%) when level of alertness changed from "alert" (1 +/- 0.2 saccades/s) to "drowsy" (0.5 +/- 0.2 saccades/s). Thresholds, however, were not significantly altered. Discharge rate of abducens internuclear neurons increased abruptly 10.4 +/- 2.5 ms preceding saccades in the on direction, and decreased 20.5 +/- 7.8 ms before saccades in the off direction. Internuclear neuronal activity was not affected by pure vertical saccades. During on direction saccades, firing frequency did not saturate, but increased with velocity in a linear fashion. Exponential functions often fit the data better due to the difference in slopes of rate-velocity plots for on vs off direction saccades. Slopes (rs) of rate-velocity regression lines during spontaneous saccades ranged from 0.99 to 4.10 spikes/s/deg/s (mean 2.16 +/- 0.93). During saccades in the off direction activity always decreased, but it seldom ceased. Rate-velocity regression lines measured during the fast phase of vestibular nystagmus (rsv = 2.09 +/- 0.88) showed no significant differences from rs slopes in 82% of the cases.(ABSTRACT TRUNCATED AT 400 WORDS)

Abducens Nerve↗
PubMed1986

Steroid and gonadotropin levels in women during the peri-menopausal years.

We obtained blood samples from 88 women 45-58 yr old who were having cyclic menses every 1-2 mth (37 women, 133 samples) or were amenorrheic for greater than 3 mth (51 women, 310 samples). Samples were obtained at intervals of 3-4 mth and analyzed for estrogens, androgens and gonadotropins using radioimmunoassay techniques. There was a gradual decline in the concentrations of estrone (E1), estradiol (E2), estrone sulfate (E1SO4) and progesterone (P) as the time from the last menses increased. A relatively stable concentration was reached in 12 mth for E1, E2, and E1SO4 and in 2 mth for P. The concentrations of testosterone, dihydrotestosterone, androstenedione, dehydroepiandrosterone and dehydroepiandrosterone sulfate remained relatively constant as the time from the last menses increased. There was no apparent difference in the mean values of any of these hormones for any time interval from the last menses. The concentrations of both luteinizing hormone (LH) and follicle stimulating (FSH) were noted to increase initially but they appeared to become stable after 12 mth for FSH and after only 6 mth for LH. Using only the measurements made on the initial blood samples obtained in all patients, we found significant correlations between FSH concentrations and the concentrations of E1, E2 and E1SO4 for women who were less than 3 mth from a menses as well as those whose last menses had occurred 3 or more mth previously. The correlations were generally not significant for LH in either groups of women.(ABSTRACT TRUNCATED AT 250 WORDS)

Androgens↗