Color, composition, and thermal environment of Kuiper Belt object (486958) Arrokoth.

The outer Solar System object (486958) Arrokoth (provisional designation 2014 MU69) has been largely undisturbed since its formation. We studied its surface composition using data collected by the New Horizons spacecraft. Methanol ice is present along with organic material, which may have formed through irradiation of simple molecules. Water ice was not detected. This composition indicates hydrogenation of carbon monoxide-rich ice and/or energetic processing of methane condensed on water ice grains in the cold, outer edge of the early Solar System. There are only small regional variations in color and spectra across the surface, which suggests that Arrokoth formed from a homogeneous or well-mixed reservoir of solids. Microwave thermal emission from the winter night side is consistent with a mean brightness temperature of 29 ± 5 kelvin.

The formation of Charon's red poles from seasonally cold-trapped volatiles.

A unique feature of Pluto's large satellite Charon is its dark red northern polar cap. Similar colours on Pluto's surface have been attributed to tholin-like organic macromolecules produced by energetic radiation processing of hydrocarbons. The polar location on Charon implicates the temperature extremes that result from Charon's high obliquity and long seasons in the production of this material. The escape of Pluto's atmosphere provides a potential feedstock for a complex chemistry. Gas from Pluto that is transiently cold-trapped and processed at Charon's winter pole was proposed as an explanation for the dark coloration on the basis of an image of Charon's northern hemisphere, but not modelled quantitatively. Here we report images of the southern hemisphere illuminated by Pluto-shine and also images taken during the approach phase that show the northern polar cap over a range of longitudes. We model the surface thermal environment on Charon and the supply and temporary cold-trapping of material escaping from Pluto, as well as the photolytic processing of this material into more complex and less volatile molecules while cold-trapped. The model results are consistent with the proposed mechanism for producing the observed colour pattern on Charon.

The small satellites of Pluto as observed by New Horizons.

The New Horizons mission has provided resolved measurements of Pluto's moons Styx, Nix, Kerberos, and Hydra. All four are small, with equivalent spherical diameters of ~40 kilometers for Nix and Hydra and ~10 kilometers for Styx and Kerberos. They are also highly elongated, with maximum to minimum axis ratios of ~2. All four moons have high albedos (~50 to 90%) suggestive of a water-ice surface composition. Crater densities on Nix and Hydra imply surface ages of at least 4 billion years. The small moons rotate much faster than synchronous, with rotational poles clustered nearly orthogonal to the common pole directions of Pluto and Charon. These results reinforce the hypothesis that the small moons formed in the aftermath of a collision that produced the Pluto-Charon binary.

Surface compositions across Pluto and Charon.

The New Horizons spacecraft mapped colors and infrared spectra across the encounter hemispheres of Pluto and Charon. The volatile methane, carbon monoxide, and nitrogen ices that dominate Pluto's surface have complicated spatial distributions resulting from sublimation, condensation, and glacial flow acting over seasonal and geological time scales. Pluto's water ice "bedrock" was also mapped, with isolated outcrops occurring in a variety of settings. Pluto's surface exhibits complex regional color diversity associated with its distinct provinces. Charon's color pattern is simpler, dominated by neutral low latitudes and a reddish northern polar region. Charon's near-infrared spectra reveal highly localized areas with strong ammonia absorption tied to small craters with relatively fresh-appearing impact ejecta.

The Pluto system: Initial results from its exploration by New Horizons.

The Pluto system was recently explored by NASA's New Horizons spacecraft, making closest approach on 14 July 2015. Pluto's surface displays diverse landforms, terrain ages, albedos, colors, and composition gradients. Evidence is found for a water-ice crust, geologically young surface units, surface ice convection, wind streaks, volatile transport, and glacial flow. Pluto's atmosphere is highly extended, with trace hydrocarbons, a global haze layer, and a surface pressure near 10 microbars. Pluto's diverse surface geology and long-term activity raise fundamental questions about how small planets remain active many billions of years after formation. Pluto's large moon Charon displays tectonics and evidence for a heterogeneous crustal composition; its north pole displays puzzling dark terrain. Small satellites Hydra and Nix have higher albedos than expected.

The relationship of the subarachnoid injection of blood and blood fractions with cardiac rate change and arrhythmias.

The relationship of subarachnoid hemorrhage and cardiac arrhythmias was studied utilizing a Sprague-Dawley rat model. A total of 30 male animals were divided into five groups and given subarachnoid injections of either blood, blood fractions, or control substances. Blood pressure, intracranial pressure, serum electrolytes, arterial blood gases, hypothalamic multiple unit activity and an electrocardiogram were concurrently monitored. Cardiac arrhythmias were graded on a 0 to 4 + objective scale. Control parameter values were similar for all animals. Arrhythmias, hypotension, and decreased hypothalamic multiple unit activity were seen with infusion of whole blood and packed red blood cells. Packed red blood cells were statistically demonstrated to have the most potent arrhythmogenic effect. Cardiac histopathology revealed myocardial contraction band lesions most predominant in the packed red blood cell group. In addition, significant QT interval prolongation was observed after subarachnoid injection of either whole blood or packed red blood cells. These findings indicate that packed red blood cells, or a component thereof, may play an important role in the etiology of immediate (i.e. acute) post subarachnoid hemorrhage induced cardiac arrhythmias.

Morphometric analysis of rabbit skeletal muscle and peripheral nerve tissue after electrical stimulation.

Transcutaneous medium frequency electrical stimulation is used extensively to improve muscle strength in people who encounter difficulty in improving strength voluntarily. The purpose of this study was to describe some morphometric effects of electrical stimulation applied to rabbit skeletal muscle and peripheral nerve tissue (N = 5 control and 7 experimental rabbits). Intermittent electrical current (4000 Hz pulse modulated at 50 Hz) was applied transcutaneously to adult female rabbit thigh muscle 3 times/week for 3 months. Muscle ATPase histochemical staining, followed by morphometric analysis, demonstrated that type IIB fibers in stimulated muscles (sartorius and vastus lateralis) had larger cross-sectional areas in comparison to nonstimulated muscle fibers. Type IIA fibers of the stimulated sartorius muscle also were hypertrophied in comparison to nonstimulated muscle fibers. The percentage distribution of muscle fiber types did not change significantly as a result of stimulation. The femoral nerves of these rabbits were fixed and stained. Morphometric analysis did not detect any significant change in myelin cross-sectional area or thickness. Also, nerve axoplasmic cross-sectional area in stimulated femoral nerves was not significantly different from controls. These data suggest that electrical stimulation can increase the size of skeletal muscle fibers if applied consistently for an extended period of time. Further research is needed to further characterize this effect and to determine whether the same effect can be observed in humans after prolonged stimulation. J Orthop Sports Phys Ther 1990;11(7):313-320.

The renal vascular system of the monkey: a gross anatomical description.

This study was conducted on two species of monkeys, Macaca fascicularis and Macaca mulatta, to describe their gross renal vascular morphology. After death, twelve monkeys were perfused with isotonic saline to flush their vascular systems. The monkeys were then perfused either with latex or methyl methacrylate, or both, one into the arterial and the other into the venous system. The results indicated that there were six to eight arterial segments in the monkey kidney, each supplied by a segmental artery. The anterior segments were named apical, upper, middle and lower, while the posterior segments were named posterior-apical, superior, intermediate and inferior. The branching patterns of both the anterior and posterior segmental arteries were classified into one of four types: magistral, cruciate, bifurcating or quadripartite. The renal vein generally collects blood from three or four large intrarenal veins. Peripheral to this, veins accompanied arteries and were given their corresponding names. Despite this juxtaposition of veins and arteries, and the resulting convention in naming vessels, the intrarenal venous system was organised into three regions. Each region was arranged around an anterior and a posterior large intrarenal vein. The various segments of the kidney, as defined by the arterial system, were united by the arcuate veins, which anastomose throughout the corticomedullary region and drain into the large intrarenal veins mentioned above. The gross renal vascular system of the monkey was compared to, and contrasted with, human and canine renal vascular anatomy.

The renal microvasculature of the monkey: an anatomical investigation.

Twelve monkeys, Macaca fascicularis and Macaca mulatta, were investigated to study their renal microvasculature. After death, all monkeys were perfused with heparinised isotonic saline to flush their vascular systems. The kidneys were then perfused with silicone rubber and examined. The silicone rubber injections allowed description of afferent arterioles and several efferent vascular patterns observed in the subcapsular, midcortical, and inner cortical regions. The medullary vasculature was particularly interesting in that no particular vascular zonation was observable. Silicone rubber injections indicated the existence of vascular bundles that run parallel to one another from outer medulla nearly to the papillary tip. Branching of descending vasa recta into capillaries, or precapillary vessels, occurs frequently and at all levels of the medulla. Ascending vasa recta are formed from the interbundle capillary plexus and from the plexus at the papillary tip. They ascend primarily within vascular bundles to the corticomedullary junction where these vessels may empty into collecting veins or arcuate veins. In addition, many ascending vasa recta penetrate into the cortex where they drain into the proximal third of interlobular veins. The venous drainage of the cortex appears to be regional, in that the area surrounding an interlobular vein generally drains into it directly via venules or small veins. The arterial and venous morphology of the monkey kidney may be important to the monkey's ability to concentrate urine despite the virtual absence of an inner medullary zone. The potential physiological significance of the monkey's microvasculature is discussed extensively and compared with various other mammals.

A correlation between multiple unit activity in the hypothalamus and electrocardiographic changes during a subarachnoid hemorrhage.

A sudden onset and short latency of cardiovascular responses that may follow a subarachnoid hemorrhage (SAH) in the vicinity of the circle of Willis implicate neural mechanisms. To investigate this, multi unit activity (MUA) was recorded from the posterolateral hypothalamus and electrocardiogram was recorded from lead II before and during SAH in rats. A temporal correlation between MUA, heart rate and rhythm changes was observed. Following SAH, transient depression of MUA (approximately 5 s or less) did not affect heart rate; a more sustained suppression preceded bradycardia and arrhythmias. Bradycardia, premature atrial contractions and premature ventricular contractions occurred under two conditions: when MUA was suppressed; and during bursts in MUA when interburst phases were suppressed. Further evidence that bradycardia may be the result of suppression of brain potentials was obtained by failure of bilateral vagotomy, atropine and isoproterenol to reverse bradycardia generated after SAH under the ventral aspect of the brain in the vicinity of circle of Willis. However, bilateral vagotomy reversed bradycardia after SAH in the cisterna magna indicating an activation of central parasympathetic neurons. The results indicate that different neural mechanisms underlie bradycardia generated from the above mentioned two sites, namely, ventral brain and cisterna magna.

Central neural control of blood pressure and cardiac arrhythmias during subarachnoid hemorrhage in rats.

Sudden death may follow subarachnoid hemorrhage which indicates involvement of neural mechanisms connected with the cardiovascular system. Since various regions of the brain mediate blood pressure and heart rate changes, these parameters and heart rhythm could be affected due to a subarachnoid hemorrhage near the circle of Willis which surrounds the hypothalamus, the highest center for autonomic control. To investigate this in the control group, intracranial pressure, blood pressure, and electrocardiogram were measured before and after a simulated subarachnoid hemorrhage; blood pressure and electrocardiogram were measured following midcollicular lesions in the decerebrate group both before and after a subarachnoid hemorrhage. The results demonstrate that an increase in systemic arterial blood pressure and premature ventricular contractions (with respect to unlesioned group, p less than 0.04) are mediated by forebrain areas and require the integrity of neuroanatomical connections with structures that are caudal to the midbrain. Since bradycardia and other electrocardiographic abnormalities could still be produced after midcollicular lesioning it is suggested that they can be mediated via the brainstem only without involvement of more rostral areas and may occur due to increased intracranial pressure.

An investigation of potential vascular connections between the kidney and the adrenal gland.

This study was conducted on two species of monkeys, Macaca fascicularis and Macaca mulatta, to determine if there were vascular connections between the kidney and other abdominal structures such as the adrenal glands. Microfil vascular perfusions, followed by microscopic observations and dissections, were utilized to investigate the existence of these potential connections. Highly anastomotic renal capsular vessels were always observed on the outer surface of the renal capsule. However, these capsular vessels did not make connections with the subcapsular capillary plexus in the majority of monkeys studied. Vascular connections between the adrenal gland and kidney were not observed. It was concluded that, although the region between the adrenal gland and kidney was rich in vasculature, it did not appear to play an anatomical role in anastomosing the extrarenal and intrarenal circulations.

A small animal model for electrocardiographic abnormalities observed after an experimental subarachnoid hemorrhage.

The male Sprague-Dawley rat was successfully used to produce increases in arterial blood pressure and electrocardiographic abnormalities similar to those seen in patients after a subarachnoid hemorrhage. The model employs a less expensive, easily available laboratory animal than other existing models. Whole heparinized blood is introduced in the area of the circle of Willis to simulate a subarachnoid hemorrhage. This model ensures a constant body-blood volume and minimizes damage to brain tissue as no blood vessel has to be punctured. The volume of blood required to produce bradycardia and other electrocardiographic abnormalities can be controlled and calculated for each animal. The model produced a sudden onset and progression of cardiovascular symptoms. Blood apparently is specific for producing electrocardiographic abnormalities as compared to 0.9% saline, Dextran 40 and Gentran 75 when introduced in the area of the circle of Willis. This specificity was not maintained when blood and saline were introduced into the cisterna magna as electrocardiographic abnormalities were observed with both agents.

Morphological changes accompanying physiological changes in atrial stretch receptors.

A shunt connecting the descending aorta and posterior vena cava was established surgically in six mongrel dogs. All animals subsequently developed hemodynamic and clinical signs of congestive heart failure at which time the shunt was closed. Six to seven weeks after ligation of the shunt, activity of the left atrial stretch receptors was recorded and found to be similar to that of sham-operated dogs. Methylene blue-stained sections of left atrial endocardium indicated that approximately one-third of the receptors were normal in histological appearance. The remaining two-thirds were fragmented and degenerate. Evidence is presented which indicates that structural changes in these receptors parallel their changes in physiological activity.

Changes in the sensitivity of left atrial receptors following reversal of heart failure.

Recordings were made from left atrial type B receptors in six mongrel dogs after recovery from the cardiac effects of a chronic AV fistula. All animals showed hemodynamic and clinical signs of congestive heart failure after 44.5 +/- 3.6 days with a patent Dacron shunt between the aorta and inferior vena cava below the level of the renal arteries. The stimulus-response curves of the left atrial stretch receptors (change in spikes/cardiac cycle or in spikes/minute vs. change in left atrial pressure) after 45.2 +/- 7.2 days of shunt closure were similar to those seen in sham-operated dogs from a previous study. However, the slope of the stimulus-response curve of the dogs in which the AV fistula was closed was significantly greater than the slope of the curve from the AV fistula dogs with heart failure. Radiographs indicated that after shunt closure, cardiac dilatation had regressed. This study incidates that a decrease in the sensitivity of left atrial receptors in dogs with congestive heart failure is a reversible phenomenon and that the initial depression is most likely related to the concomitant cardiac dilatation that accompanies the failure state.

The mechanism of adaptation of left atrial stretch receptors in dogs with chronic congestive heart failure.

Chronic congestive heart failure (CHF) was induced in dogs by the construction of an aorto-caval fistula below the level of the renal arteries. Aorto-caval fistula dogs showed signs of CHF which included ascites, hind limb edema, and pulmonary congestion. Ventricular catheterization indicated a significantly higher left ventricular end diastolic pressure and lower maximum velocity of left ventricular pressure development/left ventricular end diastolic pressure in CHF dogs when compared to sham-operated controls. Heart weight/body weight ratios were significantly higher in CHF dogs. Electrophysiological recordings from medullated left atrial type B receptors from the cervical vagus indicated a depressed sensitivity of these receptors in CHF dogs when compared to sham-operated control dogs. For any given change in left atrial pressure, the discharge of left atrial receptors was significantly reduced in CHF dogs compared with sham-operated controls. The mechanism for this depressed sensitivity was investigated. Sonomicrometry of the left atrial appendage indicated a decreased compliance of the left atrial appendage in the dogs with chronic CHF. In addition, microscope examination of the complex unencapsulated receptor endings taken from the left atrial endocardium indicated a marked alteration in receptor morphology. A loss of the end arborization was the most typical finding. It is concluded that chronic CHF brought about by an aorto-caval fistula results in a depressed left atrial stretch receptor response and that both decreased left atrial compliance and structural alterations in the receptor endings may account for this depressed sensitivity.