Showing papers in "Philosophical Transactions of the Royal Society B in 1982"

Specific impairments of planning

Abstract: An information-processing model is outlined that predicts that performance on non-routine tasks can be impaired independently of performance on routine tasks. The model is related to views on frontal lobe functions, particularly those of Luria. Two methods of obtaining more rigorous tests of the model are discussed. One makes use of ideas from artificial intelligence to derive a task heavily loaded on planning abilities. A group of patients with left anterior lesions has a specific deficit on the task. Subsidiary investigations support the inference that this is a planning impairment.

A memory system in the monkey

Abstract: A neural model is presented, based largely on evidence from studies in monkeys, postulating that coded representations of stimuli are stored in the higher-order sensory (i.e. association) areas of the cortex whenever stimulus activation of these areas also triggers a cortico-limbo-thalamo-cortical circuit. This circuit, which could act as either an imprinting or rehearsal mechanism, may actually consist of two parallel circuits, one involving the amygdala and the dorsomedial nucleus of the thalamus, and the other the hippocampus and the anterior nuclei. The stimulus representation stored in cortex by action of these circuits is seen as mediating three different memory processes: recognition, which occurs when the stored representation is reactivated via the original sensory pathway; recall, when it is reactivated via any other pathway; and association, when it activates other stored representations (sensory, affective, spatial, motor) via the outputs of the higher-order sensory areas to the relevant structures.

Neural systems control of spatial orienting.

Abstract: A peripheral visual cue in an empty field (1) often summons head or eyes, or both, (2) improves efficiency at the cued position while attention is directed to it, even without overt movements, and (3) reduces processing efficiency at the cued position once attention is withdrawn. We have studied the time course and the effects of mid-brain and cortical damage on these components of orienting. The facilitation arises from shifts in covert attention. In cases of mid-brain degeneration due to progressive supranuclear palsy, saccadic movements were abolished, while covert orienting still occurs. However, covert orienting was found to be delayed in directions in which eye movements were most affected, suggesting a role for mid-brain pathways in covert orienting. Parietal lesions can cause massive loss in detection contralateral to the lesion. This is especially true when attention has been directed to the opposite side. These findings relate aspects of covert orienting of attention to neural control systems.

On the Stochastic Properties of Bursts of Single Ion Channel Openings and of Clusters of Bursts

Abstract: Characteristics of observed bursts of single channel openings were derived recently for two particular ion channel mechanisms. In this paper these methods are generalized so that the observable characteristics of bursts can be calculated directly for any mechanism that has transition probabilities that are independent of time as long as the process is at equilibrium or is maintained in a steady state by an energy supply. General expressions are given for the distributions of the open time, the number of openings per burst, the total open time per burst, the gaps within and between bursts, and so on. With the aid of these general results a single computer program can be written that will provide numerical values for such distributions for any postulated mechanism, given only the transition rates between the various states. The results are illustrated by a numerical example of a mechanism in which two agonist molecules can bind sequentially, and either singly or doubly occupied receptor ion channels may open. The analogous theory is also given for the case where bursts of channel openings are grouped into clusters; many of the results bear a close analogy with those found for simple bursts.

The Brain of the Honeybee Apis Mellifera. I. The Connections and Spatial Organization of the Mushroom Bodies

Abstract: The mushroom bodies of the bee are paired neuropils in the dorsal part of the brain. Each is composed of the arborizations of over 17 10 $^4$ small interneurons of similar architecture called Kenyon cells. Golgi staining demonstrates that these neurons can be divided into five groups distinguished on the basis of their dendritic specializations and geometry. The mushroom body neuropils each consist of a pair of cup-shaped structures, the calyces, connected by two short fused stalks, the pedunculus, to two lobes, the $\alpha$ - and $\beta$ -lobes. Each calyx is formed from three concentric neuropil zones, the basal ring, the collar and the lip. The calyces are organized in a polar fashion; within the calyces each of the five categories of Kenyon cell has a distribution limited to particular polar contours. The dendritic volumes of neighbouring Kenyon cells arborizing within each individual contour are greatly overlapped. Fibres from groups of neighbouring cells within a calycal contour are gathered into bundles that project into the pedunculus, each fibre dividing to enter both the $\alpha$ - and $\beta$ -lobes. The pedunculus and the lobes are conspicuously layered. Kenyon cells with neighbouring dendritic fields within the same calycal contour occupy a single layer in the pedunculus and lobes. Thus the two-polar organization of the calyces is transformed into a Cartesian map within the pedunculus, which continues into the $\alpha$ - and $\beta$ -lobes. The calyx receives input fibres from both the antennal lobes and the optic neuropils. The branching patterns of these cells reflect the polar organization of the calyces as their terminals are restricted to one or more of the three gross compartments of the calycal neuropil. The course of these tracts and the morphologies of the fibres that they contain are described. Cells considered to represent outputs from the mushroom bodies arborize in the pedunculus and $\alpha$ - and $\beta$ -lobes. Generally the arborizations of the output neurons reflect the layered organization of these neuropils. Fibres from the two lobes run to the anterior median and lateral protocerebral neuropil, and the anterior optic tubercle. Additionally there is an extensive network of feedback interneurons that inter-connect the $\alpha$ - and $\beta$ -lobes with the ipsi- and contralateral calyces. Many individual neurons have branches in both the $\alpha$ - and the $\beta$ -lobes and in the pedunculus. The pathways and geometries of the fibres subserving the two lobes are described. The hypothesis of Vowles (1955) that the individual lobes represent a separation of sensory and motor output areas is shown to be incorrect. The anatomy of the bee's mushroom bodies suggests that they process second-order antennal and fourth- and higher-order visual information. The feedback pathways are discussed as possible means of creating long-lasting after-effects which may be important in complex timing processes and possibly the formation of short-term memory.

Some cognitive effects of frontal-lobe lesions in man.

Abstract: The study of patients undergoing unilateral frontal-lobe excisions for the relief of focal epilepsy has revealed specific cognitive disorders that appear against a background of normal functioning on many intellectual, perceptual and memory tasks. Lesions that invade the frontal eye field cause subtle impairments of voluntary oculomotor control, which reveal themselves as an inability to suppress an initial glance at a potentially distracting stimulus. After frontal lobectomy in either hemisphere, deficits are found quite consistently on motor-differentiation tasks (Konorski 1972) in which the subject must learn to produce different responses to different, randomly presented, environmental signals. More directly related to the concept of planning are those sequential tasks in which the subject is free to choose his own order of responding, but must not make the same response twice. Here the left frontal lobe plays the major role, a finding consistent with the notion of left-hemisphere dominance for the programming of voluntary actions. In contrast, the right frontal lobe appears to be more critically involved in monitoring the temporal sequence of externally ordered events, although the verbal or non-verbal nature of the stimuli remains a relevant factor.

Retinal Ganglion Cells: A Functional Interpretation of Dendritic Morphology

Abstract: The electrical properties of the different anatomical types of retinal ganglion cells in the cat were calculated on the basis of passive cable theory from measurements made on histological material. Standard values for the electrical parameters were assumed (R $\_i$ = 70 $\Omega$ cm, C $\_m$ = 2 $\mu$ F cm $^{-2}$ , R $\_m$ = 2500 $\Omega$ cm $^2$ ). We conclude that these neurons need not be equipotential despite their small dimensions, mainly because of their extensive branching. The interactions between excitation and inhibition when the inhibitory battery is near the resting potential can be strongly nonlinear in these cells. To characterize the different types of ganglion cells in terms of this property we introduce F, the factor by which the soma depolarization induced by a given excitatory input is reduced by inhibition. In this framework we analyse some of the integrative properties of an arbitrary passive dendritic tree and we then derive the functional properties that are characteristic for the various types of ganglion cells. Our main results are: (i) Nonlinear saturation at the synapses may be made effectively smaller by spreading the same (conductance) input among several subunits on the dendritic field. Subunits are defined as regions of the dendritic field that are somewhat isolated from each other and roughly equipotential within. (ii) Shunting inhibition can specifically veto an excitatory input, if it is located on the direct path to the soma. The F values can then be very high even when the excitatory inputs are much larger than the inhibitory, as long as the absolute value of inhibition is not too small. Inhibition more distal than excitation is much less effective. (iii) Specific branching patterns coupled with suitable distribution of synapses are potentially able to support complex information processing operations on the incoming excitatory and inhibitory signals. The quantitative analysis of the morphology of cat retinal ganglion cells leads to the following specific conclusions: (i) None of the cells examined satisfies Rall's equivalent cylinder condition. The dendritic tree cannot be satisfactorily approximated by a non-tapering cylinder. (ii) Under the assumption of a passive membrane, the dendritic architecture of the different types of retinal ganglion cells reflects characteristically different electrical properties, which are likely to be relevant for their physiological function and their information processing role: (a) $\alpha$ cells have spatially inhomogeneous electrical properties, with many subunits. Within each subunit nonlinear effects may take place; between subunits good linear summation is expected. F values are relatively low. (b) $\beta$ cells at small eccentricities have rather homogeneous electrical properties. Even distal inputs are weighted rather uniformly. Electrical inhomogeneities of the $\alpha$ type appear for $\beta$ cells at larger eccentricities. F values are low. (c) $\gamma$ -like cells have few subunits, each with high input resistance underlying nonlinear saturation effects possibly related to a sluggish character. F values are high: inhibition of the shunting type can interact in a strongly nonlinear way with excitatory conductance inputs. (d) $\delta$ -like cells show many subunits with a high input resistance, covering well the dendritic area. Within each subunit inhibition on the direct path to the soma can specifically veto a more distal excitation. It is conjectured that such a synaptic organization superimposed on the $\delta$ cell morphology underlies directional selectivity to motion. (iii) Most of our data refer to steady-state properties. They probably apply, however, to all light evoked signals, since transient inputs with time to peak of 30 ms or more can be treated in terms of steady-state properties of the ganglion cells studied. (iv) All our results are affected only slightly by varying the parameter values within reasonable ranges. If, however, the membrane resistance were very high, all ganglion cells would approach equipotentiality. For R $\_m$ = 8000 $\Omega$ cm $^2$ subunits essentially disappear in all types of ganglion cells (for steady state inputs). Our results concerning nonlinear interaction of excitation and inhibition (F values) would, however, remain valid even for much larger values of R $\_m$ and for any value of R $\_i$ larger than 30-50 $\Omega$ cm. The critical requirement is that peak inhibitory conductance changes must be sufficiently large (around 5 x 10 $^{-8}$ S) with an equilibrium potential close to the resting potential. Underestimation of the diameters of the dendritic branches may affect these conclusions (F could be significantly lower).

Open-State Substructure of Single Chloride Channels from Torpedo Electroplax

Abstract: Chloride channels from Torpedo californica electroplax were inserted into planar phospholipid membranes, and single-channel currents were studied at high time-resolution. The open channel fluctuates rapidly between three substates, with conductances of 18.5, 9.4 and 0 pS in 150 mM Cl-. Under various ionic conditions the three substates are always equally spaced in conductance; at various voltages leading to different probabilities of observing the three substates, the substate frequencies are always binomially distributed. The conclusion emerges that the conducting of unit of Cl- channel is composed of two identical Cl- diffusion pathways, each with a voltage-dependent gate.

The Flight of Petrels and Albatrosses (Procellariiformes), Observed in South Georgia and its Vicinity

Abstract: Nine procellariiform species, covering a range of body mass exceeding 200: 1, were studied during a visit to Bird Island, South Georgia, with the British Antarctic Survey, in the 1979-1980 field season. Speed measurements were made by ornithodolite of birds slope-soaring over land, birds flying over the sea but observed from land, and birds observed from a ship. In the second group, which showed the least anomalies, lift coefficients corresponding to mean airspeeds were about 1 for albatrosses, decreasing to about 0.3 for the smallest petrels. All species increased speed when flying against the wind. The small species proceeded by flap-gliding, while the large ones flapped infrequently, and only in light winds. The small species flew lower than the larger ones, but this may be related to the fact that most of the observations were of birds flying into wind. The albatrosses ( Diomedea, Phoebetria ) and giant petrels ( Macronectes ) were found to have a ‘shoulder lock’, consisting of a tendon sheet associated with the pectoralis muscle, which restrained the wing from elevation above the horizontal. This arrangement was not seen in the smaller species, and was interpreted as an adaptation reducing the energy cost of gliding flight. The main soaring method in the large species appeared to be slope-soaring along waves. Windward ‘pullups’ suggestive of the classical ‘dynamic soaring’ technique were seen in large and medium-sized species. However, the calculated strength of the wind gradient would have been insufficient to maintain airspeed to the heights observed, and it was concluded that most of the energy for the pullups must come from kinetic energy, acquired by gliding along a wave in slope lift. Gliding downwind through the wind gradient should significantly increase the glide ratio, but this was not observed. Slope-soaring along moving waves in zero wind was recorded. The data were used to derive estimates of the average speeds that the different species should be able to maintain on foraging expeditions. Estimates of the rate of energy consumption were also made, taking into account the greater dependence on flapping in the smaller species, and on soaring in the larger ones. The distance travelled in consuming fuel equivalent to a given fraction of the body mass would seem to be very strongly dependent on mass. Comparison of the largest species ( Diomedea exulans ) with the smallest ( Oceanites oceanicus ) suggests that ‘range’, defined in this way, varies as the 0.60 power of the mass, although the relation is more complex than a simple power function.

Ecology of the bacteria of the sulphur cycle with special reference to anoxic-oxic interface environments

Abstract: H $\_2$ S is produced as a main end-product of anaerobic mineralization in anoxic, sulphate-rich environments by a diverse population of sulphate-reducing bacteria. The sulphate reducers can carry out an almost complete oxidation of detrital organic matter to CO $\_2$ . The H $\_2$ S consequently becomes an important electron carrier from the anoxic to the oxic world. Thiobacilli and other colourless sulphur bacteria have the potential to oxidize the H $\_2$ S at the oxic-anoxic interface in sediments or stratified waters, but their role is still poorly understood. A comparison of sulphide oxidation processes in the chemoclines of the Black Sea, the Solar Lake and in a Beggiatoa mat indicated that depth scales and retention times of coexisting O $\_2$ and H $\_2$ S regulate the bacterial involvement in the sulphide oxidation. The H $\_2$ S specialists, Beggiatoa and Thiovulum, are optimally adapted to compete with the autocatalytic oxidation of H $\_2$ S by O $\_2$ . Microelectrode measurements show retention times of O $\_2$ -H $\_2$ S in the bacterial mats or veils of less than 1 s. In photic chemoclines of stratified waters or sulfureta, the phototrophic sulphur bacteria or cyanobacteria interact with the sulphide oxidation at the O $\_2$ -H $\_2$ S interface. Short cycles between H $\_2$ S and intermediate oxidation products, S $^0$ or S $\_2$ O $^{2-}\_3$ , are created. The bacteria of the sulfuretum are highly adapted to the diurnal rhythm of light, O $\_2$ and H $\_2$ S.

Strategies of mixed substrate utilization in microorganisms

Abstract: In natural and man-made environments microorganisms often grow in the presence of a diversity of functionally similar substrates. The pattern of utilization of these mixed substrates is generally dependent upon their concentration. When substrates are present in high (not growth-limiting) concentrations, sequential utilization and diauxic growth is often observed and the substrate that supports the highest growth rate is utilized preferentially from the mixture. When the substrate concentrations are growth-limiting, simultaneous utilization of the various compounds present in the mixture appears to be the general response. Recent studies on mixed substrate utilization in both batch and continuous cultures have thrown light on the strategies of the control mechanisms that, in microbes, govern the utilization of the various substrates. But perhaps more importantly these studies have indicated the possible significance of mixed substrate utilization in microbial competition in nutrient-limited natural ecosystems.

The Neuroanatomy of an Amphibian Embryo Spinal Cord

Abstract: Horseradish peroxidase has been used to stain spinal cord neurons in late embryos of the clawed toad (Xenopus laevis). It has shown clearly the soma, dendrites and axonal projections of spinal sensory, motor and interneurons. On the basis of light microscopy we describe nine differentiated spinal cord neuron classes. These include the Rohon-Beard cells and extramedullary cells which are both primary sensory neurons, one class of motoneurons that innervate the segmental myotomes, two classes of interneurons with decussating axons, three classes of interneurons with ipsilateral axons and a previously undescribed class of ciliated ependymal cells with axons projecting ipsilaterally to the brain. We believe that all differentiated neuron classes are described and that this anatomical account is the most complete for any vertebrate spinal cord.

A Guide to the Neuroanatomy of Locust Suboesophageal and Thoracic Ganglia

Abstract: The organization of the thoracic and suboesophageal ganglia in the locust is presented to provide a framework into which details of individual neurons can be inserted as information becomes available. Three species were examined, Chortoicetes terminifera (Walker), Schistocerca gregaria (Forskal) and Locusta migratoria migratorioides (Reiche and Fairmaire). The basic plan of the ganglia is similar in all three species. Series of selected sections in transverse, horizontal and sagittal planes are illustrated to show the arrangement of the main nerve fibre tracts and areas of neuropil, and these are described briefly. A guide is given to prominent features that assist in the interpretation of sections in each plane. In the simpler mesothoracic and prothoracic ganglia nine longitudinal tracts are present in each half of the neuromere, and six dorsal and four ventral transverse tracts (commissures) link the two halves. Four vertical or oblique tracts are conspicuous, the T-tract, ring tract, C-tract and I-tract. Major roots of each peripheral nerve useful as landmarks are numbered from anterior to posterior. Two regions of fine fibrous neuropil are prominent, the ventral association centre and an area associated with the ring tract, a little above it. In the metathoracic ganglion three abdominal neuromeres are fused posteriorly to the true metathoracic neuromere. All four neuromeres show modification of the basic framework chiefly in the arrangement of the ventral commissures and the degree of development of the ventral association centre. In the suboesophageal ganglion three neuromeres, mandibular, maxillary and labial, are fused together from anterior to posterior. They show increasing modification of the basic plan anteriorly. Additional anterior longitudinal tracts are present, which connect with the brain, the dorsal commissures are much reduced and compressed, particularly in the mandibular neuromere, and the ventral commissures of all three neuromeres differ considerably from those of the thoracic ganglia.

Biochemistry of the chemolithotrophic oxidation of inorganic sulphur.

Abstract: A historical review is presented of the elucidation of the mechanisms of oxidation of inorganic sulphur compounds and of electron transport in the thiobacilli. A unitary mechanism, consistent with current knowledge, is proposed. The significance of polythionates is discussed. The relations between oxidation mechanisms, substrate-level and electron transport-dependent phosphorylation, energy-dependent NAD+ reduction and efficiency of growth are assessed in order to evaluate the efficiency of energy conservation in different species. The unresolved problems are identified for the benefit of those planning further assaults on the last redoubts of the thiobacilli.

Left-Hemisphere Control of Oral and Brachial Movements and Their Relation to Communication

Abstract: The left cerebral hemisphere of the brain in man is known to be involved in both vocal and manual communication, as evidenced by speech and manual sign language disorders after left-hemisphere pathology. The left hemisphere also has important motor control functions in certain kinds of non-communicative oral and manual movements. Several tasks requiring oral and manual-branchial movements were presented for reproduction to patients with unilateral restricted lesions of the left or right hemisphere. Patients with left-hemisphere lesions were impaired relative to patients with equivalent right-hemisphere damage. thus confirming and extending earlier reports on the effects of more widespread left-hemisphere damage. Within the left hemisphere, frontal and parietal lesions had the most severe effect, with lesions in the central, occipital or temporal areas producing no or minimal impairment. The left frontal area was paramount in the control of oral movements, and the parietal lobe in control of hand movements, but both regions were required for the more demanding oral and manual tasks. Oral and manual control systems appeared more separable in the frontal than in the parietal region, suggesting that the latter may play a general programming role that is then enacted through the left frontal region. Speech-disordered (aphasic) patients were inferior to non-aphasic patients on oral and manual tasks. There was a close association between aphasia and the presence of oral or manual defects in left frontal and parietal lobe lesions, but not in temporal-lobe lesions. In addition, a speech task requiring reproduction of a single syllable closely paralleled the reproduction of a non-verbal oral movement, in that both depended critically on the left anterior region; aphasic patients with parietal or temporal lesions were relatively unaffected on either task. However, on reproduction of multisyllabic familiar phrases, there was no association with oral motor deficits. Aphasic patients with anterior and parietal lesions could reproduce these phrases relatively well, but patients with temporal-lobe lesions (and lesser impairment on motor tasks), had great difficulty. It is suggested that oral and manual apraxia, as well as aphasia, may be a manifestation of a basic motor selection problem in lesions of frontal and parietal lobes, but that the temporal region has some important acoustic-motor function in speech. When this temporal system is intact, it can, at least with overlearned material, bypass defective frontal or parietal oral motor systems.

Nitrification in forested ecosystems

Abstract: Nitrification, the microbial oxidation of NH $^+\_4$ -N, plays a key role in the cycling of N in forested and other terrestrial ecosystems. Solution losses of nitrate and gaseous losses of N $\_2$ and nitrous oxides are important vectors of N loss from many forested systems and are directly or indirectly controlled by the activity of the nitrifiers. These losses can also have important consequences for downstream ecosystems, groundwater quality, and atmospheric concentrations of ozone. Relative nitrification (the proportion of the total mineral N that is nitrate at the end of an incubation period) provides an independent means of evaluating the general importance of site factors thought to regulate nitrification in situ. Regression of relative nitrification against soil pH, C:N, and percentage N, with the use of data from previously published studies, suggest that although these factor may be important regulators of nitrification in particular sites, they are not good predictors of nitrification across a wide range of sites. Reasons for their low predictive ability may include limitations of current measurement techniques or the capacity of nitrifiers to adapt to relatively extreme conditions.

Neuropsychological Studies of Object Recognition

Abstract: It is well established that disorders of visual perception are associated with lesions in the right hemisphere. Performances on tasks as disparate as the identification of objects from unusual views of objects drawn so as to overlap, of fragmented letters, of familiar faces, and of anomalous features in drawings, have been shown to be impaired in patients with focal right posterior lesions. A series of investigations are reviewed, directed towards analysing the basis of these deficits. Explanations in terms of primary visual impairment can be rejected, as can an account in terms of faulty figure-ground organization. It is argued that a wide variety of such perceptual deficits--all of which are concerned with meaningful visual stimuli--can be encompassed by the notion of faulty perceptual categorization at an early post-sensory stage of object recognition. Moreover, there is evidence suggesting that some of these various perceptual deficits can be mutually dissociated. The concept of perceptual categorization is discussed in the wider context of tentative model of object recognition.

Co-Metabolism [and Discussion]

Abstract: There have been numerous instances reported when potentially recalcitrant compounds have been modified by microorganisms or completely mineralized by mixed communities of organisms; an example is pesticide biodegradation. Both situations rely upon the ability of microorganisms to transform compounds that they cannot utilize as sole sources of carbon and energy. This phenomenon of co-oxidation or co-metabolism has been fraught with confusion for many years as a result of the ambiguous use of terms and definitions. A redefinition of co-metabolism is proposed in an attempt to alleviate the problem: Co-metabolism - the transformation of a non-growth substrate in the obligate presence of a growth substrate or another transformable compound. The term `non-growth substrate' describes compounds that are unable to support cell replication as opposed to an increase in biomass. This definition was devised primarily as a result of non-growth substrate metabolism studies with methaneutilizing bacteria. These studies are described in the text. The possible impact of endogenous polymer reserves on co-metabolic events is discussed. A number of examples where non-growth substrate metabolism is of environmental importance are presented, in particular the potential role of methane-oxidizing bacteria in the removal of CO from the environment. The evolutionary significance, if any, of fortuitous metabolism or co-metabolism is discussed, as are potential applications of these phenomena.

Optic Aphasia: A Process of Interaction Between Vision and Language

Abstract: A neurological syndrome, called in the literature ether optic aphasia or visual anomia, is defined in principle as the inability to name visually presented objects, together with the preservation of both the ability to identify them by sight correctly and to name them when they are presented in another sensory modality. This syndrome was first described by Freund in 1889, but since then its existence has been continually questioned. When it is accepted, the most common interpretation of it is in terms of an anatomical visuo-verbal disconnection. However, the precise level of the psychological process impaired remains unspecified. The purpose of this paper is threefold. First, evidence is reported showing that a verbal impairment specific to visually presented objects can be observed, as well as analogous syndromes (e.g. tactile aphasia). Secondly, a particular kind of visuo-verbal impairment is defined and called optic aphasia, to distinguish it from other possible cases of visuo-verbal impairments. This syndrome is defined by the specification of the level of the particular psychological process supposed to be impaired, i.e. a disturbance between visual semantics and verbal semantics, both of which operate normally. Thirdly, three hypotheses concerning the operation of the semantic system in normal subjects are derived from the evidence coming from this syndrome.

Biochemistry of dissimilatory sulphate reduction.

Abstract: Extensive information is available on the enzymology of respiratory sulphate reduction and the structure of electron transfer proteins isolated from the sulphate-reducing bacteria; however, it has not yet been possible to delineate satisfactorily the function of these electron transfer proteins in terms of the enzymes involved in respiratory sulphate reduction. New information about differences in pyrophosphate metabolism by Desulfovibrio and Desulfotomaculum, cellular localizations of electron transfer proteins and enzymes, and the concepts of vectorial electron transfer plus hydrogen cycling suggest that previous data on the function of electron transfer proteins must be re-evaluated and new experimental approaches designed before the problem is resolved. New information on the enzymology of lactate dehydrogenase, pyruvate dehydrogenase, adenylyl sulphate reductase, bisulphite reductase and hydrogenase is presented and discussed in the context of enzyme localization and specifically for electron transfer proteins. The function of cytochrome c3 (Mr = 13000) in the mechanism of the periplasmic hydrogenase and the role of the new [3Fe-3S] non-haem iron centres in electron transfer is emphasized.

Surface-associated growth.

Abstract: In natural ecosystems, microbial activity is often associated with the presence of a surface, particularly in low-nutrient environments. The chemostat allows the study of such low-nutrient environments together with the precise control of other growth parameters. By using this system, enrichment cultures with inocula from two different river sources have been made. A more diverse community attached itself to surfaces placed in the chemostat when the cultures were carbon-limited than when the limiting nutrient was nitrogen. Further studies on a pseudomonad isolated from the carbon-limited enrichment cultures have shown that surface-associated organisms grow at approximately twice the rate of the same organism in the free surrounding medium. A hypothesis to explain this phenomenon based on the chemiosmotic theory is discussed.

A Computational Theory of Visual Surface Interpolation

Abstract: Computational theories of structure-from-motion and stereo vision only specify the computation of three-dimensional surface information at special points in the image. Yet the visual perception is clearly of complete surfaces. To account for this a computational theory of the interpolation of surfaces from visual information is presented. The problem is constrained by the fact that the surface must agree with the information from stereo or motion correspondence, and not vary radically between these points. Using the image irradiance equation, an explicit form of this surface consistency constraint can be derived. To determine which of two possible surfaces is more consistent with the surface consistency constraint, one must be able to compare the two surfaces. To do this, a functional from the space of possible functions to the real numbers is required. In this way, the surface most consistent with the visual information will be that which minimizes the functional. To ensure that the functional has a unique minimal surface, conditions on the form of the functional are derived. In particular, if the functional is a complete semi-norm that satisfies the parallelogram law, or the space of functions is a semi-Hilbert space and the functional is a semi-inner product, then there is a unique (to within possibly an element of the null space of the functional) surface that is most consistent with the visual information. It can be shown, based on the above conditions plus a condition of rotational symmetry, that there is a vector space of possible functionals that measure surface consistency, this vector space being spanned by the functional of quadratic variation and the functional of square Laplacian. Arguments based on the null spaces of the respective functionals are used to justify the choice of the quadratic variation as the optimal functional. Possible refinements to the theory, concerning the role of discontinuities in depth and the effects of applying the interpolation process to scenes containing more than one object, are discussed.

The morphology of the bipolar cells, amacrine cells and ganglion cells in the retina of the turtle Pseudemys Scripta Elegans

Abstract: The morphology of the neurons that contribute to the inner plexiform layer of the retina of the turtle Pseudemys scripta elegans has been studied by light microscopy of whole-mount material stained by the method of Golgi. Cells have been distinguished on the basis of criteria that include dendritic branching patterns, dendritic morphology, dendritic tree sizes and stratification of processes in the inner plexiform layer. Many of the neurons have dendritic trees oriented parallel to and a few exhibit an orthogonal orientation with the linear visual streak present in the retina of this species. The neurons of the turtle retina have been compared, where possible, with the neurons of the lizard retina as described by Cajal. The findings are discussed in relation to other vertebrate retinas, and correlations are made with recent electrophysiological recordings of the turtle retina. Comments are made with regard to the significance of orientation of neurons relative to the linear visual streak.

Chloride-bicarbonate exchange in red blood cells: physiology of transport and chemical modification of binding sites

Abstract: About 80% of the CO2 formed by metabolism is transported from tissues to lungs as bicarbonate ions in the water phases of red cells and plasma. The catalysed hydration of CO2 to bicarbonate takes place in the erythrocytes but most of the bicarbonate thus formed must be exchanged with extracellular chloride to make full use of the carbon dioxide transporting capacity of the blood. The anion transport capacity of the red cell membrane is among the largest ionic transport capacities of any biological membrane. Exchange diffusion of chloride and bicarbonate is nevertheless a rate-limiting step for the transfer of CO2 from tissues to lungs. Measurements of chloride and bicarbonate self-exchange form the basis for calculations that demonstrate that the ionic exchange processes cannot run to complete equilibration at capillary transit times less than 0.5 s. The anion exchange diffusion is mediated by a large transmembrane protein constituting almost 30% of the total membrane protein. The kinetics of exchange diffusion must depend on conformational changes of the protein molecule, associated with the binding and subsequent translocation of the transported anion. We have characterized the nature of anion-binding sites facing the extracellular medium by acid-base titration of the transport function and modification of the transport protein in situ with group-specific amino acid reagents. Anion binding and translocation depend on the integrity and the degree of protonation of two sets of exofacial groups with apparent pK values of 12 and 5, respectively. From the chemical reactivities towards amino acid reagents it appears that the groups whose pK = 12 are guanidino groups of arginyl residues, while the groups whose pK = 5 are likely to be carboxylates of glutamic or aspartic acid. Our studies suggest that the characteristics of anion recognition sites in water-soluble proteins and in the integral transport proteins are closely related.

Anion-dependent cation transport in erythrocytes.

Abstract: A selective survey of the literature reveals at least three major anion-dependent cation transport systems, defined as Na+ + Cl-, K+ + Cl- and Na+ + K+ + Cl- respectively. In human red cells, kinetic data on the fraction of K+ and Na+ influx inhibitable by bumetanide are presented to indicate an Na+:K+ stoichiometry of 1:2. For LK sheep red cells the large Cl- -dependent K+ leak induced by swelling is shown to share many characteristics with that induced by N-ethylmaleimide (NEM) treatment. NEM has complex effects, both inhibiting and then activating Cl- -dependent K+ fluxes dependent on NEM concentration. The alloantibody anti-L can prevent the action of NEM. In human red cells NEM induces a large Cl- -dependent specific K+ flux, which shows saturation kinetics. Its anion preference is Cl- greater than Br- greater than SCN- greater than I- greater than NO3- greater than MeSO4-. This transport pathway is not inhibited by oligomycin or SITS, although phloretin and high concentrations of furosemide and bumetanide (over 0.3 mM) do inhibit. Quinine (0.5 mM) is also an inhibitor. It is concluded that at least two distinct Cl- -dependent transport pathways for K+ are inducible in mammalian red cells, although the evidence for their separation is not absolute.

Nitrate Leaching to Groundwater

Abstract: Groundwater provides over 30% of developed supplies of potable water in Britain. The outcrops of the important aquifers form extensive tracts of agricultural land. Groundwater resources largely originate as rainfall that infiltrates this land. During the 1970s, growing concern about rising, or elevated, groundwater nitrate concentrations, in relation to current drinking water standards, stimulated a major national research effort on the extent of diffuse pollution resulting from agricultural land-use practices. The results presented derive from intensive and continuing studies of a number of small groundwater catchments in eastern England. It is in this predominantly arable region that the groundwater nitrate problem is most widespread and severe. The distribution of nitrate in the unsaturated and saturated zones of the aquifers concerned is summarized. These data have important implications for the water-supply industry, but their interpretation is discussed primarily in relation to what can be deduced about both the recent and long-term histories of leaching from the more permeable agricultural soils.

Form and distribution of sensory terminals in cat hindlimb muscle spindles.

Abstract: The sensory innervation of cat hindlimb muscle spindles was studied by reconstruction, electron microscopy, and examination of teased, silver preparations to ascertain the form of the terminals and their distribution to bag $\_1$ (b $\_1$ ), bag $\_2$ (b $\_2$ ), and chain (c) muscle fibres. Reconstructions were made of two primary endings, one S $\_1$ secondary ending, and the branching of four primary and six secondary axons. For the silver analysis spindles were teased from 14 different hindlimb muscles, the largest samples being from tenuissimus, peroneus brevis, p. longus, p. tertius, superficial lumbrical, extensor digitorum longus, and soleus. Among 310 spindles examined, 40 lacked a b $\_1$ fibre. These were all portions of tandem spindles in which the b $\_2$ fibre was continuous from one capsule, where it was accompanied by b $\_1$ and c fibres, to another, in which it was accompanied by c fibres only. These have been designated 'b $\_2$ c spindle units' as distinct from 'b $\_1$ b $\_2$ c spindle units'. Counts of myonuclei in the primary regions of four b $\_1$ b $\_2$ c spindle units revealed 52-106 in the nuclear bags, b $\_1$ bags averaging 68, b $\_2$ bags 80. The average number in a myotube region was nine (range 6-12), and in a c fibre 24 (range 11-38). The reconstructions showed a close association between nucleation and innervation, but no constant relation between number of myonuclei and terminal contact area. They also showed that the largest cross-sectional areas of each bag fibre corresponded with the sites of S $\_1$ secondary ending innervation. Approximately 70-90% of the cross-sectional area of each bag fibre was occupied by myonuclei in the bag region, 30-50% in the myotube regions, and 10% in the region of secondary innervation. In b $\_2$ c spindle units the equatorial nucleation of the b $\_2$ fibre usually resembled that of a c fibre. The intramuscular diameter of Ia axons supplying b $\_1$ b $\_2$ c spindle units (mean 7.5 $\mu m$ , range 3.4-12.8 $\mu m$ , n = 213) was generally thicker than that of Ia axons supplying b $\_2$ c spindle units (mean 5.1 $\mu m$ , range 2.2-9.0 $\mu m$ , n = 37). The distribution of terminals by the first-order branches (usually two) of Ia axons to b $\_1$ , b $\_2$ , and c fibres was exclusively from heminodes, and was either segregated (b $\_1$ fibres supplied separately from b $\_2$ and c fibres, thereby resulting in separation of dynamic and static inputs) or mixed. Mixing was restricted most frequently to the dynamic input, and usually resulted from b $\_1$ fibres sharing a supply of terminals with a few c fibres. Distribution of terminals was usually segregated in tenuissimus and mixed in superficial lumbrical muscles, but in most muscles neither type of distribution predominated. Among 270 b $\_1$ b $\_2$ c spindle units, 32 had more than one b $\_1$ fibre, and 12 had primary endings formed by two Ia axons. Primary terminal systems supplied to bag fibres consisted of a middle portion, in which the terminals were arranged mainly as regular transverse bands, and portions at each end, in which they were disposed irregularly. In the silver analysis, b $\_1$ terminal systems in 151 b $\_1$ b $\_2$ c primary endings were distinguished from those supplied to b $\_2$ fibres by having more of their total length occupied by irregular portions (on average 57%, as compared with 33% in b $\_2$ fibres), and more bands per unit length in the middle. In the two reconstructed primaries the b $\_1$ fibres received 33 and 37% of the total terminal contact area, the b $\_2$ fibres 25 and 24%, and the c fibres 5-12% individually, 42 and 39% collectively. Primary endings supplied to b $\_2$ c spindle units were mostly irregular in appearance. The polar position of 351 secondary endings was S $\_1$ 253, S $\_2$ 79, S $\_3$ 15, S $\_4$ 3, S $\_5$ 1; 67.8% were distributed to b $\_1$ b $\_2$ c fibres (mostly as S $\_1$ endings), 20.8% to b $\_2$ c fibres, 6.3% to c fibres, and 5.1% to b $\_1$ c fibres. The intramuscular diameter of II axons terminating as S $\_1$ endings was generally greater (mean 3.9 $\mu m$ ) than that of II axons terminating as S $\_2$ -S $\_5$ endings (mean 2.9 $\mu m$ ); 75% of b $\_1$ b $\_2$ c II axons had diameters that fell within the lower part of the b $\_1$ b $\_2$ c Ia diameter range. Most II axons had two first-order branches; the distribution of terminals by the first-order branches of b $\_1$ b $\_2$ c II axons was usually mixed. Terminals were derived either exclusively from heminodes (as in most S $\_1$ endings) or from both heminodes and penultimate nodes. The mean length of 313 secondary endings was 348 $\mu m$ (range 138-716 $\mu m$ ) as compared with a mean length of 359 $\mu m$ (range 242-608 $\mu m$ ) for 151 primaries. In 83 b $\_1$ b $\_2$ c S $\_1$ endings the innervated portions of the bag fibres represented, on average, 42% (b $\_1$ ) and 51% (b $\_2$ ) of the total length of the ending. In 64% of the endings the b $\_2$ fibre received more terminals than the b $\_1$ . In the reconstructed S $\_1$ ending the b $\_1$ fibre received 8% of the total terminal contact area, the b $\_2$ fibre 17%, and the c fibres 16-22% individually, 75% collectively. Some muscles had fewer secondaries than others; the Ia:II ratio ranged from 1:1.2 (superficial lumbrical) to 1:1.8 (peroneus longus). The constant features of spindle sensory innervation that emerge from this study (e.g. the dense primary innervation of the b $\_1$ fibre) are discussed in the context of spindle development and in terms of their functional significance. The data on the branching of spindle afferents is related to the work of others on pacemakers and the manner in which nerve impulses are generated from the endings and propagated into the axons. We suggest that transduction may occur by a deformation of the sensory terminal owing to increased tension in the basal lamina. It is supposed that the permeability of the Na $^+$ channels is affected by an intracellular messenger (probably Ca $^{2+}$ ) released from a bound state by increase in cytoskeletal tension. Reasons are given as to why the afferent innervating b $_2$ c spindle units should be regarded as primary. The probable functional significance of these units is discussed, and some correlations are made between the function of certain muscles and the characteristics of their spindle populations.

Cytoplasmic structure and contractility: the solation-contraction coupling hypothesis

Abstract: We have briefly described our studies of cytoskeletal and contractile elements in intact cells, in cell extracts, and in mixtures of purified proteins. Changes in the concentration of calcium and of protons have been found to modulate both gelation and contraction in all of these preparations. The distribution of calcium, protons and actin has been studied in intact amoeboid cells. Using these results, we have refined our working model of the relation of cytoskeletal and contractile proteins: the solation-contraction coupling hypothesis. The model is also supported by quantitative analysis of the rates of contraction in a soluble extract of Dictyostelium discoideum amoebae allowed to gel in a capillary and stimulated by the addition of calcium ions at one end. A plausible interpretation of the most prominent cytological features of amoeboid locomotion is obtained by application of the principles of our model. In addition, we propose that the solation-contraction coupling hypothesis may be useful in further study of a variety of motile phenomena observed in many types of cells.

Experiments on the central pattern generator for swimming in amphibian embryos.

Abstract: The central nervous system of paralysed Xenopus laevis embryos can generate a motor output pattern suitable for swimming locomotion By recording motor root activity in paralysed embryos with transected nervous systems we have shown that: (a) the spinal cord is capable of swimming pattern generation; (b) swimming pattern generator capability in the hindbrain and spinal cord is distributed; (c) caudal hindbrain is necessary for sustained swimming output after discrete stimulation By recording similarly from embryos whose central nervous system was divided longitudinally into left and right sides, we have shown that: (a) each side can generate rhythmic motor output with cycle periods like those in swimming; (b) during this activity cycle period increases within an episode, and there is the usual rostrocaudal delay found in swimming; (c) this activity is influenced by sensory stimuli in the same way as swimming activity; (d) normal phase coupling of the left and right sides can be established by the ventral commissure in the spinal cord We conclude that interactions between the antagonistic (left and right) motor systems are not necessary for swimming rhythm generation and present a model for swimming pattern generation where autonomous rhythm generators on each side of the nervous system drive the motoneurons Alternation is achieved by reciprocal inhibition, and activity is initiated and maintained by tonic excitation from the hindbrain

Anaerobic oxidation of sulphur compounds as electron donors for bacterial photosynthesis

Abstract: Most phototrophic bacteria use reduced inorganic sulphur compounds as electron donors during anoxygenic photosynthesis. Principally, sulphide is oxidized via sulphite to sulphate. Elemental sulphur may appear as intermediary storage product (inside: Chromatium, Thiocapsa; outside: Chlorobium, Ectothiorhodospira; not in: Rhodopseudomonas sulfidophila). Adenosine phosphosulphate is an intermediate in sulphite oxidation by Chromatium, Thiocapsa and Chlorobium. Thiosulphate undergoes splitting to sulphide (or elemental sulphur) and sulphite, or is oxidized to tetrathionate. Sulphide may be oxidized to elemental sulphur by cytochrome c or to thiosulphate (perhaps sulphite?) by flavocytochrome c, or to sulphite by a reverse (sirohaem) sulphite reductase. The latter enzyme also oxidizes polysulphides and probably elemental sulphur. Sulphite is either oxidized by APS reductase to form adenosine phosphosulphate - from which sulphate is released by ADP sulphurylase - or by sulphite: acceptor oxidoreductase directly to sulphate. The electron acceptor of most of these oxidative enzymes are cytochromes or non-haem iron-sulphur proteins. The pathways of photolithotrophic sulphur oxidation in Chlorobiaceae, Chromatiaceae and Rhodospirillaceae are separately compiled under evaluation of the presently available data.