Central nervous system and computation

Computational systems are useful in neuroscience in many ways. For instance, they may be used to construct maps of brain structure and activation, or to describe brain processes mathematically. Furthermore, they inspired a powerful theory of brain function, in which the brain is viewed as a system characterized by intrinsic computational activities or as a "computational information processor. "Although many neuroscientists believe that neural systems really perform computations, some are more cautious about computationalism or reject it. Thus, does the brain really compute? Answering this question requires getting clear on a definition of computation that is able to draw a line between physical systems that compute and systems that do not, so that we can discern on which side of the line the brain (or parts of it) could fall. In order to shed some light on the role of computational processes in brain function, available neurobiological data will be summarized from the standpoint of a recently proposed taxonomy of notions of computation, with the aim of identifying which brain processes can be considered computational. The emerging picture shows the brain as a very peculiar system, in which genuine computational features act in concert with noncomputational dynamical processes, leading to continuous self-organization and remodeling under the action of external stimuli from the environment and from the rest of the organism.     

Central chemoreceptors

When all peripheral chemoreceptors are denervated, animals continue to show increased ventilation when made to breathe CO2, indicating that receptors within the brain ("central chemoreceptors") are excited by acidity or changes in CO2. No cells have been identified within the brain that are indisputedly chemoreceptors for CO2 or H+, but there is abundant evidence that respiration can be affected by chemical, electrical, and thermal stimuli applied locally to the ventral surface of the medulla. Furthermore, the actions of traditional central chemical respiratory stimuli can be blunted or abolished after inhibition of neural function within this ventrolateral medullary shell (VMS). The VMS is an integrative region for cardiovascular and respiratory function and may be involved in nociception. The distinction between the former two is not always clear, but recent studies using microinjection techniques seem promising for identifying the respiratory substrates. The many recent advances elucidating anatomic connections between the VMS and other brain regions are important but do not directly address the question of the site of central respiratory chemosensitivity. Knowledge of such connections, however, should provide more definitive opportunities for addressing this question.     

Terrestrial invertebrates inhabiting lowland river floodplains of Central Amazonia and Central Europe: a review

1. Amazonian terrestrial invertebrates produce high population densities during favourable periods and may suffer a drastic decrease during occasional floods and droughts. However, the monomodal, predictable flood pulse of the larger Amazonian rivers favours the development of morphological (respiratory organs, wing‐dimorphism), phenological (synchronization of life cycles, univoltine mode of life), physiological (flooding ability, gonad dormancy, alternating number of developmental stages), and behavioural adaptations (migration, temporal diving) with numerous interactions. 2. In lowlands of Central Europe, the flood pulse of large rivers is less predictable than in Central Amazonia and is superimposed by the seasonal light/temperature pulse (summer/winter regime). Some terrestrial invertebrates show physiological resistance against inundation or drought, phenologies fitting the normal annual rhythm of water level fluctuation (quiescence or diapause of eggs or adult invertebrates), high dispersal ability and migration. However, most species survive simply using a `risk strategy', combining high reproduction rates, dispersal and reimmigration following catastrophic events. 3. The diversity of species in terrestrial invertebrates is lower in lowland riverine ecosystems of Central Amazonia and Central Europe compared with the respective uplands because of flood stress in these systems. However, floodplains in Central Amazonia possess a greater number of endemic species in comparison with Central European floodplains because of long periods of fairly stable climatic conditions in comparison with large palaeoclimatic changes in Central Europe.     

Central mechanisms of osmosensation and systemic osmoregulation

Systemic osmoregulation is a vital process whereby changes in plasma osmolality, detected by osmoreceptors, modulate ingestive behaviour, sympathetic outflow and renal function to stabilize the tonicity and volume of the extracellular fluid. Furthermore, changes in the central processing of osmosensory signals are likely to affect the hydro-mineral balance and other related aspects of homeostasis, including thermoregulation and cardiovascular balance. Surprisingly little is known about how the brain orchestrates these responses. Here, recent advances in our understanding of the molecular, cellular and network mechanisms that mediate the central control of osmotic homeostasis in mammals are reviewed.     

Silver Impregnation of Peripheral and Central Axons

A silver impregnation method suitable for peripheral and central nervous system axons is described. Essential features are the use of reagent grade chemicals only, a pretreatment solution to ensure optimal impregnation of different organs from different animals and species, and an unvarying procedure. The results are compared to those obtainable with a number of current impregnation methods and with modern immunocytochemical reactions.     

Central nervous system and mechanisms of hypertension

There are several mechanisms by which the central nervous system participates in the neural and humoral alterations associated with various forms of experimental hypertension. Structures in forebrain with multiple integrative roles in neuroendocrine control of the circulation are involved. Tissue surrounding the anteroventral region of the third cerebral ventricle (AV3V region) is involved physiologically in thirst, sodium homeostasis, osmoreception, secretion of vasopressin and natriuretic factor and sympathetic discharge to blood vessels. Destruction of this tissue prevents or reverses many forms of hypertension. In genetically based spontaneous hypertension, limbic structures such as the central nucleus of the amygdala rather than the AV3V region are the necessary neuroanatomic substrate. Recent evidence suggests that a circumventricular organ in brain stem, the area postrema, is also involved in the mediation of several forms of experimental hypertension. In renin- and nonrenin-dependent forms of renal hypertension, two major factors activate central mechanisms. First, direct central actions of angiotensin, acting through receptors in the subfornical organ and organum vasculosum of the lamina terminalis, increase sympathetic discharge and secretion of vasopressin through mechanisms integrated at the level of the AV3V region. Second, sensory systems originating in the kidney can activate increased sympathetic discharge through complex projection pathways involving forebrain systems. Mineralocorticoid hypertension appears to involve enhanced secretion of vasopressin and central vasopressinergic mechanisms also dependent on the AV3V region. Reciprocal connections between key central areas involved in control of arterial pressure provide the neuroanatomical basis for central nervous system participation in hypertension.     

Central sympathoplegic and norepinephrine-depleting effects of antioxidants

Carbon disulfide (CS2), tetraethyl lead (TEL), tetraethyl tin (TeET), dithiothreitol (DTT), and gossypol acetic acid (GAA) significantly decreased brain norepinephrine (NE) in rats. The central dopamine (DA) increased after ip administration of CS2, TEL, and DTT, but decreased after TeET and GAA. The brain serotonin decreased only after TeET. Two doses of DTT decreased the NE longer than one dose (24 vs 2 hr) but did not increase DA. L-DOPA, given SC with DTT, delayed the decrease in NE by 24 hr. The similar behavioral and autonomic effects of each of these compounds suggest a central sympatholytic effect and an antipsychotic type of sedation and rigidity. A possible mechanism is reversible inhibition of dopamine beta-hydroxylase through the reduction of the copper ion of the enzyme. Each of these reducing agents, together with the boranes previously studied, has similar behavioral and autonomic effects and a common effect on NE concentration, suggesting that the agents act through a physicochemical property rather than by combination with a cellular component. These data have applications to the toxicity of the single agents. They also provide an index of activity, previously lacking, of systemic antioxidant effect.     

Wetlands of Central America

The wetlands of seven Central American countries – Belize, Guatemala, Honduras, El Salvador, Nicaragua, Costa Rica, and Panamá – are reviewed. The region's wetlands are classified into five systems: marine, estuarine, riverine, lacustrine, and palustrine. At a minimum, wetlands cover 40,000 km² (8%) of the land area of Central America. These wetlands support high levels of biological diversity, especially of invertebrates, amphibians, and migratory birds. Because of intensive deforestation and conversion of forest lands to agriculture, many species of birds and mammals that formerly were abundant in upland forests now are restricted to wetland refugia. Annual primary productivity of some Central American wetlands equals or exceeds that of tropical rainforests, and wetlands also provide essential ecosystem services such as maintaining water quality. Population and development pressures formerly restricted to upland areas are expanding rapidly into wetlands, resulting in losses of wetlands at rates comparable to losses of rainforests. Since 1990, all seven Central American countries have become signatories to the Ramsar convention on wetlands of international importance, but integrated planning for management and conservation of wetlands in the region only began in 2002. A specific set of recommendations for wetland inventory, ecological research, and management is provided that would be feasible and effective within the social and cultural framework of the Central American countries.