Open-loop simulations of the primate saccadic system using burst cell discharge from the superior colliculus

 Saccade-related burst neurons (SRBNs) in the monkey superior colliculus (SC) have been hypothesized to provide the brainstem saccadic burst generator with the dynamic error signal and the movement initiating trigger signal. To test this claim, we performed two sets of open-loop simulations on a burst generator model with the local feedback disconnected using experimentally obtained SRBN activity as both the driving and trigger signal inputs to the model. First, using neural data obtained from cells located near the middle of the rostral to caudal extent of the SC, the internal parameters of the model were optimized by means of a stochastic hill-climbing algorithm to produce an intermediate-sized saccade. The parameter values obtained from the optimization were then fixed and additional simulations were done using the experimental data from rostral collicular neurons (small saccades) and from more caudal neurons (large saccades); the model generated realistic saccades, matching both position and velocity profiles of real saccades to the centers of the movement fields of all these cells. Second, the model was driven by SRBN activity affiliated with interrupted saccades, the resumed eye movements observed following electrical stimulation of the omnipause region. Once again, the model produced eye movements that closely resembled the interrupted saccades produced by such simulations, but minor readjustment of parameters reflecting the weight of the projection of the trigger signal was required. Our study demonstrates that a model of the burst generator produces reasonably realistic saccades when driven with actual samples of SRBN discharges. Received: 25 October 1994/Accepted in revised form: 20 June 1995     

Improved techniques for use of the triploid cell marker in the axolotl, Ambystoma mexicanum

Techniques for using the triploid cell marker for studying cell lineage during the development and regeneration of the axolotl limb are described. Triploid animals possess cells with three nucleoli while diploid animals possess cells with two nucleoli. We have developed a technique for isolating the limb dermis as a sheet of cells for whole-mount analysis of cellular ploidy. Whole-mount tissue preparations as well as paraffin-embedded sectioned tissues were stained specifically for nucleoli with bismuth. Cell counts from a number of triploid and diploid dermal preparations show that (1) diploid dermal cells never possess three nucleoli, (2) the frequency of trinucleolate cells in whole-mount triploid dermal preparations is not 100% but varies between animals from 30 to 76%, (3) within a single triploid animal, the frequency of trinucleolate cells in different dermal preparations is constant. These data establish the usefulness of this technique and emphasize the need for appropriate control cell counts when using the triploid cell marker in the axolotl.     

Head injuries related to the use of baby walkers.

To determine what proportion of head injuries in children under 24 months of age who presented to an emergency department were related to the use of baby walkers, we reviewed the charts of 52 such children. Walkers were involved in 42% of the head injuries in the children under 12 months of age and in none of those in the children aged 12 to 24 months. All walker-related injuries, including skull fractures in three children, involved stairs (p less than 0.001). Questionnaires were also sent to all families with children aged 3 to 18 months attending a private pediatric practice to determine the prevalence of falls involving baby walkers among these children and the factors associated with such falls. Of the 152 responding families 82% reported using or having used a walker. Thirty-six percent of the families reported that their child had a fall while in a walker, with 8.8% of the falls resulting in contact with a doctor. Walker-related falls were directly associated with time spent in the walker (p less than 0.001) and with a previous fall from the walker by an older sibling (p less than 0.03). Since there is no demonstrated benefit of walkers, their use should not be encouraged, and parents should be advised of their potential danger.     

Microvilli in sea urchin eggs : Differences in their formation and type

The elongation of microvilli, which normally occurs upon fertilization in sea urchin eggs, was also observed in unfertilized eggs treated with the enzyme, papain. Cortical granule exocytosis, which is thought to be the source of membrane used in microvillar elongation, does not occur in the papain-treated eggs. It appears, therefore, that there is more than one way in which the egg plasma membrane can increase very quickly and to a great extent. In addition, the kinds of microvilli formed in the two instances appear to be different. Previous work with reaggregating sea urchin cells is also cited to support the suggestion that microvilli can form in different ways and are of different types.     

Mortality and age differences in marriage.

Data from the 1971 census population were used to evaluate the effects of age differences of married partners on mortality rates. Different age groups were isolated to highlight the association between mortality and age of spouse for specific ages of married men and women. Men married to much younger or to older women exhibited a higher mortality rate than men married to women who were only a few years younger than themselves. A similar trend was observed among women married to much younger or much older men compared with those whose spouses were a few years older or of similar ages as themselves. Trends for other age groups (women aged 60-69 years, men below 40, and women below 30) did not exhibit a clear pattern. Although statistical biases within age groups may in part account for the differences in the findings, the trend which emerged from the analysis suggests that lower morbidity is associated with the most common age combinations (husbands same age or slightly older than wives). Other factors could also account for the differences (e.g., selection of healthy partners in 1st marriages, differences in lifestyles between married and single).     

Cytophotometric Evidence of Non-S-Phase Extra-Dna In Human Neuronal Nuclei

After Feulgen staining with acriflavine-Schiff, the DNA content of glial and neuronal nuclei from various sites of the human CNS (pre- and post-central gyrus, cerebellar cortex and spinal cord) were determined by fluorescence cytophotometry. the specimens were obtained from twelve adult human autopsy cases. Glial cell nuclei always revealed a biomodal DNA distribution pattern with a large 2c and a smaller 4c peak. the 4c peak was most prominent in the cerebellum. A few 8c glial nuclei were found. Neuronal cell nuclei disclosed unimodal DNA histograms with hyperdiploid means in the range 2.2–2.5c (1.8–2.9c for the individual populations). Tetraploid 4c DNA values were not observed, neither in Purkinje cells, nor in pyramidal cells. In eleven out of a total of forty-four slides the higher DNA means of neuronal nuclei were found to be statistically significant (P < 0.05) when compared with a population of 2c hepatocytes on the same slide. The results indicate the existence of some ‘extra DNA’ in human neuronal cell nuclei, the biological significance of which has still to be elucidated. It is however, suggested that it may play an important role in the functional activity of the CNS.     

The effect of propylthiouracil (PTU) on heat production in chickens maintained on different levels of nutrition

Using the method of forced feeding on higher and lower levels, two groups of chickens differring in growth rate were obtained. In the chickens of the two groups 72 measurements of heat production were made. Chickens of these groups were compared considering either the same age or similar body weight. It has been found that propythiouracil treatment (120 mg/kg duging 10 consecutive days) decreases heat production in the chickens of both groups. However, this effect was much more pronounced in the birds maintained on the higher level of nutrition and growing more rapidly than in those maintained on the lower level of nutrition and growing slowly.