Showing papers in "Proceedings of The Royal Society B: Biological Sciences in 1984"
TL;DR: The equations, which were initially developed to explain isolated triggered bursts, provide one of the simplest models of the more general phenomenon of oscillatory burst discharge.
Abstract: We describe a modification to our recent model of the action potential which introduces two additional equilibrium points. By using stability analysis we show that one of these equilibrium points is a saddle point from which there are two separatrices which divide the phase plane into two regions. In one region all phase paths approach a limit cycle and in the other all phase paths approach a stable equilibrium point. A consequence of this is that a short depolarizing current pulse will change an initially silent model neuron into one that fires repetitively. Addition of a third equation limits this firing to either an isolated burst or a depolarizing afterpotential. When steady depolarizing current was applied to this model it resulted in periodic bursting. The equations, which were initially developed to explain isolated triggered bursts, therefore provide one of the simplest models of the more general phenomenon of oscillatory burst discharge.
1,631 citations
TL;DR: The total daily flux of photosynthetically fixed carbon in light- and shade-adapted phenotypes of the symbiotic coral, Stylophora pistillata, was quantified and the ‘growth rate’ method emerged as superior to the conventional in vitro and in vivo methods.
Abstract: The total daily flux of photosynthetically fixed carbon in light- and shade-adapted phenotypes of the symbiotic coral, Stylophora pistillata, was quantified. Light adapted corals fixed four times as much carbon and respired twice as much as shade corals. Specific growth rates of zooxanthellae in situ were estimated from average daily mitotic indices and from ammonium uptake rates (nitrate uptake or nitrate reductase activity could not be demonstrated). Specific growth rates were very low, demonstrating that of the total net carbon fixed daily, only a small fraction (less than 5%) goes into zooxanthellae cell growth. The balance of the net fixed carbon (more than 95%) is translocated to the host. New and conventional methods of measuring total daily translocation were compared. The `growth rate' method, which does not employ $^{14}$C, emerged as superior to the conventional in vitro and in vivo methods. The contribution of translocated carbon to animal maintenance respiration (CZAR) was 143% in light corals and 58% in shade corals. Thus, translocation in the former could supply not only the total daily carbon needed for respiration but also a fraction of the carbon needed for growth. Whereas light-adapted corals released only 6%, shade-adapted corals released almost half of their total fixed carbon as dissolved or particulate organic material. This much higher throughput of organic carbon may possibly benefit the heterotrophic microbial community in shade environments.
497 citations
TL;DR: The acetylcholine-synthesizing neurons of the rabbit retina were selectively stained by intraocular injection of the fluorescent dye 4, 6-diamidino-2-phenylindole (DAP1) and found to form a single morphological population.
Abstract: The acetylcholine-synthesizing neurons of the rabbit retina were selectively stained by intraocular injection of the fluorescent dye 4, 6-diamidino-2-phenylindole (DAP1). Retinas were then isolated from the eye, fixed for 10-30 min with 4% paraformaldehyde, and mounted flat on the stage of a fluorescence microscope. The acetylcholine-synthesizing cells were penetrated under visual control by microelectrodes filled with lucifer yellow CH. When the dye was electrophoretically injected into the cells, complete filling of their dendrites often occurred. Cells were successfully injected as long as one month after fixation of the tissue. Complete or nearly complete filling of 281 cells was accomplished, at retinal locations systematically covering the retinal surface. The cells stained with DAPI were found to form a single morphological population. They have two to seven primary dendrites, which branch repeatedly within a narrow plane and form a round or slightly oval dendritic tree. The branching becomes very fine for the distal one third of the dendritic tree, and the dendrites there are studded with small swellings. The distal dendritic tree lies mainly within one of the two thin strata of the inner plexiform layer where acetylcholine is present. The shape and size of the dendritic tree are continuously graded across the retina ; the dendritic tree is narrower and the branching denser in the central retina, wider and sparser in the periphery. From knowledge of the population density and the shape of the neurons, one can reconstruct the array of dendrites that exists within the inner plexiform layer. The overlap of the dendritic fields is an order of magnitude greater than of any other retinal neuron previously described. Because the cells not only overlap widely but branch quite profusely, a very dense plexus of cholinergic dendrites is created.
373 citations
TL;DR: Good quantitative agreement was found when the microspectrophoto-metrically measured absorbance spectra were used to predict the behavioural sensitivity of individual animals to long wavelengths and suggests that the behavioural variation arises from variation in the retinal photopigments.
Abstract: The squirrel monkey (Saimiri sciureus) exhibits a polymorphism of colour vision: some animals are dichromatic, some trichromatic, and within each of these classes there are subtypes that resemble the protan and deutan variants of human colour vision. For each of ten individual monkeys we have obtained (i) behavioural measurements of colour vision and (ii) microspectrophotometric measurements of retinal photopigments. The behavioural tests, carried out in Santa Barbara, included wavelength discrimination, Rayleigh matches, and increment sensitivity at 540 and 640 nm. The microspectrophotometric measurements were made in London, using samples of fresh retinal tissue and a modified Liebman microspectrophotometer: the absorbance spectra for single retinal cells were obtained by passing a monochromatic measuring beam through the outer segments of individual rods and cones. The two types of data, behavioural and microspectrophotometric, were obtained independently and were handed to a third party before being interchanged between experimenters. From all ten animals, a rod pigment was recorded with $\lambda \_{\max}$ (wavelength of peak absorbance) close to 500 nm. In several animals, receptors were found that contained a short-wave pigment (mean $\lambda \_{\max}$ = 433.5 nm): these violet-sensitive receptors were rare, as in man and other primate species. In the middle- to long-wave part of the spectrum, there appear to be at least three possible Saimiri photopigments (with $\lambda \_{\max}$ values at about 537, 550 and 565 nm) and individual animals draw either one or two pigments from this set, giving dichromatic or trichromatic colour vision. Thus, those animals that behaviourally resembled human protanopes exhibited only one pigment in the red--green range, with $\lambda \_{\max}$ = 537 nm; other behaviourally dichromatic animals had single pigments lying at longer wavelengths and these were the animals that behaviourally had higher sensitivity to long wavelengths. Four of the monkeys were behaviourally judged to be trichromatic. None of the latter animals exhibited the two widely separated pigments (close to 535 and 567 nm) that are found in the middle- and long-wave cones of macaque monkeys. But the spread of $\lambda \_{\max}$ values for individual cones was greater in the trichromatic squirrel monkeys than in the dichromats; than in the case of three, behaviourally deuteranomalous, trichromats there was clear evidence that the distribution of $\lambda \_{\max}$ values was bimodal, suggesting photopigments at approximately 552 and 565 nm. The fourth, behaviourally protanomalous, trichromat exhibited a spread of individual $\lambda _{\max}$ values that ranged between 530 and 550 nm. Good quantitative agreement was found when the microspectrophotometrically measured absorbance spectra were used to predict the behavioural sensitivity of individual animals to long wavelengths. The concordance of the two sets of measurements places beyond question the existence of a polymorphism of colour vision in Saimiri sciureus and suggests that the behavioural variation arises from variation in the retinal photopigments. Heterozygous advantage may explain the polymorphism.
350 citations
TL;DR: Four physiologically identified neurons in the A laminae of the cat’s dorsal lateral geniculate nucleus were filled with horseradish peroxidase and studied using the electron microscope, and major fine structural differences observed between X- and Y-cells were almost entirely related to the retinal afferents.
Abstract: Four physiologically identified neurons in the A laminae of the cat's dorsal lateral geniculate nucleus were filled with horseradish peroxidase and studied using the electron microscope. Two were X-cells and two were Y-cells. Each had electrophysiological properties appropriate for its X- Or Y-cell class, and each also had an axon that projected into the optic radiation, indicative of a geniculocortical relay cell. Representative samples from about 10% of each neuron's entire dendritic arbor (proximal and distal) were taken to obtain an estimate of the types and distributions of synapses contacting these arbors. One X-cell had a cytoplasmic laminar body, but there were no other significant cytological differences seen among the neurons. Common to each of the neurons were the following synaptic features: (i) retinal terminals (r.l.p.) were mostly or entirely restricted to proximal dendrites or dendritic appendages (less than 100 microns from the soma). These terminals constituted about 15-25% of the synapses on the proximal dendrites. (ii) Terminals with flattened or pleomorphic synaptic vesicles (f. terminals) were predominant on the proximal dendrites (30-55% of the total synapses for that region) and were mainly located near the retinal terminals. A smaller percentage (10-20%) were also distributed onto the distal dendrites. (iii) Small terminals with round synaptic vesicles (r.s.d.), many presumably having a cortical origin, predominated (60-80%) on distal dendrites (greater than 100 microns), but also formed a large proportion (40-70%) of the synapses on the intermediate (50-150 microns) dendrites. Total synaptic contacts for one X-cell and one Y-cell were estimated at about 4000 and 5000, respectively. The major fine structural differences observed between X- and Y-cells were almost entirely related to the retinal afferents. First, the retinal synapses for X-cells were mostly made on to dendritic appendages (spines, etc.), whereas Y-cells had most of their retinal synapses onto the shafts of primary and proximal secondary dendrites (that is, near branch points. Second, the retinal terminals that contacted X-cell dendrites nearly always formed triadic arrangements that included nearby f. terminals, but those on Y-cells rarely did so. Finally, the main type of f. terminals associated with X-cells were morphologically different from most of those associated with the Y-cells, and this also related directly to the triadic arrangements; that is, f. terminals in the triadic arrangements were morphologically distinguishable from f. terminals that did not participate in triadic arrangements.(ABSTRACT TRUNCATED AT 400 WORDS)
277 citations
TL;DR: It is shown that light-adapted Stylophora pistillata can acquire all of their basal metabolic carbon through photosynthesis and translocation, but that shade- Adapted St Sylophora colonies growing in shade acquire slightly less than half.
Abstract: Photoadaptation by photosynthetic organisms to lowered light intensities occurs in part through changes in pigment concentrations and in characteristics of the photosynthetic response curve. We have characterized photoadaptive responses of light- and shade-adapted colonies of the reef coral Stylophora pistillata , which possesses symbiotic algae (zooxanthellae) and grows naturally under a variety of light intensities in the highly cavernous reefs of the Red Sea. Shade-adapted corals have significantly more chlorophyll per individual zooxanthella cell than light-adapted corals (2.98 compared to 12.97 pg chlorophyll a per cell), but not a significantly different number of cells per unit area (1.00 × 10 6 cells per square centimetre), with the result that the mass of chlorophyll per unit area is greater for shade-adapted corals than for light-adapted corals. Tissue nitrogen content per unit area is significantly lower ( p p > 0.05) in shade forms. These biomass characteristics are concomitant with a variety of functional responses to natural light intensities. Rate of photosynthesis at saturating light intensities is the same per unit area in both forms (20.2 µgO 2 cm -2 h -1 for shade specimens; 18.8 for light specimens); but it is significantly different when measured by amount of chlorophyll (1.6 µg O 2 (chl a ) -1 h -1 for shade specimens compared with 5.0 for light specimens). The initial slope of the P: I curve, α , is significantly higher for shade specimens by area (0.21 for shade corals compared with 0.12 for light corals), but significantly lower for shade specimens by amount of chlorophyll a (0.01 for specimens from shade compared to 0.04 for specimens growing in the light). I k (the point at which maximum production begins) is significantly lower for shade specimens (138 µmol m -2 s -1 for shade compared to 273 for light), and likewise I c (the compensation point at which net coral photosynthesis = 0) is also significantly less for shade specimens (30 µmol m -2 s -1 for shade compared to 141 for light). The average nocturnal respiration rate is significantly higher for specimens growing in the light (13.9 µg O 2 cm -2 h -1 for light specimens compared to 7.6 for shade specimens). Corals in intense sunlight respire at almost twice the rate of shade corals, probably in response to their higher total gross production. Owing to higher production rates and lower respiration rates, integrated P c (gross)/ R c (24 h) ratios are greater for shade-adapted specimens either in direct sunlight (1.76 P/R for shade specimens in the light compared to 1.10 for light specimens in the light), or in the shade (0.43 for shade specimens in the cave compared to 0.10 for light specimen in the cave). By using previously defined equations and biomass assumptions, it can be shown that light-adapted Stylophora pistillata can acquire all of their basal metabolic carbon through photosynthesis and translocation, but that shade-adapted Stylophora colonies growing in shade acquire slightly less than half. These results also show that if there were no photoadaptive response, shade-adapted specimens would acquire less than 4 % of their carbon from photosynthesis
243 citations
TL;DR: In this paper, it was shown that the optic flow field arising from motion relative to a visually textured plane may be characterized by eight parameters that depend on the observer's linear and angular velocity and the coordinate vector of the plane.
Abstract: It is shown that the optic flow field arising from motion relative to a visually textured plane may be characterized by eight parameters that depend on the observer's linear and angular velocity and the coordinate vector of the plane. These three vectors are not, however, uniquely determined by the values of the eight parameters. First, the optic flow field does not supply independent values for the observer's speed and distance from the plane; it only gives the ratio of these two quantities. But more unexpectedly, the equations relating the observer's linear velocity and the plane's coordinate vector to the eight parameters are still satisfied if the two vectors are interchanged or reversed in direction, or both. So in addition to the veridical interpretation of the optic flow field there exist three spurious interpretations to be considered and if possible excluded. This purpose is served by the condition that an interpretation can be seriously entertained only if it attributes every image element to a light source in the observer's field of view. This condition immediately eliminates one of the spurious interpretations, and exhibits the other two as mutually inconsistent: one of them is tenable only if all the visible sources lie on the forward half of the plane (relative to the observer's linear velocity); the other only if they all lie on the backward half-plane. If the sources are distributed over both halves of the plane, only the veridical interpretation survives. Its computation involves solving a 3 $\times $ 3 eigenvalue problem derived from the flow field. If the upper two eigenvalues coincide, the observer must be moving directly towards the plane; if the lower two eigenvalues coincide, his motion must be directly away from it; in both cases the spurious interpretation merges with the veridical one. If all three eigenvalues are equal, it may be inferred that either the observer's linear velocity vanishes or the plane is infinitely distant.
216 citations
TL;DR: It is proposed that the dense plexus of cholinergic dendrites and the transient nature of acetylcholine release combine to create the local subunit that enables detection of motion within regions smaller than those ganglion cells’ receptive fields.
Abstract: Rabbit retinas were incubated in vitro under conditions known to maintain their physiological function. The acetylcholine stores of the cholinergic amacrine cells were labelled by incubation in the presence of [3H]choline. The tissue was then mounted in a fast-flow superfusion chamber, and the release of [3H]acetylcholine under various conditions was measured by liquid cation exchange or high-voltage electrophoresis. When the retina was stimulated by flashing light, the rate of appearance of radioactive acetylcholine in the superfusate increased, with a latency shorter than the resolution of the system. The rate of release of acetylcholine remained elevated as long as the light was flashing, and returned rapidly to baseline when the light was extinguished. A one minute stimulation with steady light caused a burst of acetylcholine release following stimulus onset and a second, smaller, burst following stimulus cessation. In the presence of 2-amino-4-phosphonobutyrate (APB), an agent known to eliminate selectively the transmission of ON responses to the proximal retina, steady light caused acetylcholine release only at stimulus cessation. Other retinas were labelled with [3H]choline, then incubated for 10-80 min in the presence of flashing light (to promote acetylcholine release) and either control medium or medium containing 100 micron APB (to prevent release from cells activated by stimulus onset). These retinas were quick-frozen, freeze-dried and radioautographed on dry emulsion. In retinas incubated under control conditions [3H]acetylcholine was initially present within two bands within the inner plexiform layer. The two bands became fainter together as the tissue's [3H]acetylcholine was released. APB selectively retarded the depletion of [3H]acetylcholine from the band nearest the ganglion cell layer. We conclude that the displaced cholinergic amacrine cells release acetylcholine at the transient when light appears, and the conventionally placed cholinergic amacrine cells release acetylcholine at the transient when light is extinguished. The retinal ganglion cells that receive a light-driven cholinergic input are distinguished from those that do not by a great sensitivity to slow stimulus motion. It is proposed that the dense plexus of cholinergic dendrites and the transient nature of acetylcholine release combine to create the local subunit that enables detection of motion within regions smaller than those ganglion cells' receptive fields.
199 citations
TL;DR: It is concluded that topically applied DAPI selectively labels the acetylcholine-synthesizing neurons of the ganglion cell layer, indicating that they are the same population previously stained by neurofibrillar methods in the peripheral rabbit retina.
Abstract: The fluorescent DNA stain 4,6,diamidino-2-phenylindole (DAPI) was applied to the cut axons of the rabbit optic tract, from which it was retrogradely transported to the retinal ganglion cell bodies. The labelled retinas were isolated from the eye and maintained in vitro in the presence of [3H]choline. They were then quick-frozen, freeze-dried, vacuum-embedded, and radioautographed on dry emulsion for identification of the acetylcholine-synthesizing cells. Inspection of the radioautographs by fluorescence microscopy showed the two labels not to co-exist: the cells that contained the transported fluorescence did not contain radioactive acetylcholine. In other animals the optic nerve was sectioned, causing retrograde degeneration of a large fraction of the ganglion cells. A population of small, round neurons in the ganglion cell layer was spared. These retinas synthesized [3H]acetylcholine at the same rate as control tissues; and radioautography showed an identical distribution of the acetylcholine-synthesizing cells. We conclude that the acetylcholine-synthesizing neurons of the ganglion cell layer are displaced amacrine cells. When DAPI was injected intraocularly instead of being applied to the optic tract, a regular mosaic of neurons in the ganglion cell layer was selectively stained, and two bands of fluorescence were observed in the inner plexiform layer, at the level where two bands of radioactive acetylcholine were observed in radioautographs. Quantitative analysis showed that the DAPI-stained cells were the same size as those that survive optic nerve section. Like the acetylcholine-synthesizing cells, they appear to be displaced amacrines; when wheatgerm agglutinin labelled by Evans blue was applied to the optic tract and DAPI was injected intraocularly, the red fluorescence of Evans blue and the blue fluorescence of DAPI accumulated in different cells. When DAPI was injected intraocularly and radioautography for acetylcholine was carried out, the cells brightly labelled by DAPI were found to have synthesized acetylcholine. We conclude that topically applied DAPI selectively labels the acetylcholine-synthesizing neurons of the ganglion cell layer. The distribution of the acetylcholine-synthesizing cells was established by counting the DAPI-labelled cells in whole-mounts.(ABSTRACT TRUNCATED AT 400 WORDS)
186 citations
TL;DR: The results show that the coronate amacrines in the ganglion cell layer are type b starburst cells, and that those DAPI-labelled neurones in the inner nuclear layer with a coronate-like nuclear morphology are type a star Burst cells.
Abstract: Acetylcholine-synthesizing cells in the rabbit retina are symmetrically distributed about the inner plexiform layer: one population of cholinergic amacrines has cell bodies in the inner nuclear layer and an equivalent population of displaced amacrines has cell bodies in the ganglion cell layer. It has been suggested that the morphological correlates of the acetylcholine-synthesizing cells are either coronate amacrine cells or starburst amacrine cells. Coronate cells have a characteristic nuclear morphology and can be selectively labelled by neurofibrillar methods or with the fluorescent dye 4$^{\prime}$,6-diamidino-2-phenyl-indole (DAPI). Starburst cells have a characteristic dendritic morphology but have only been described from Golgi-stained retinae. This paper bridges the gap between the previous studies. DAPI-labelled coronate cells were impaled with a micropipette under microscopic control and filled with Lucifer yellow by iontophoresis. The results show that the coronate amacrines in the ganglion cell layer are type b starburst cells, and that those DAPI-labelled neurones in the inner nuclear layer with a coronate-like nuclear morphology are type a starburst cells. At a given eccentricity the dendritic field diameter of type a starburst cells is about 1.13 times larger than that of type b starburst cells. The dendritic field coverage of coronate (type b starburst) cells increases linearly with decreasing coronate cell density and ranges from 25 on the peak visual streak to 70 + in the superior periphery.
184 citations
TL;DR: Colony morphology, rates of production and respiration, translocation of carbon from symbiotic algae to host, and the daily contribution of carbon fixed by zooxanthellae to animal respiration demands (CZAR) in phenotypes of Stylophora pistillata were compared.
Abstract: Colony morphology, rates of production and respiration, translocation of carbon from symbiotic algae to host, and the daily contribution of carbon fixed by zooxanthellae to animal respiration demands (CZAR) in phenotypes of Stylophora pistillata from 3 and 35 m were compared. Corals from 35 m showed an increase in branch density, a decrease in zooxanthellae density, and an increase in chlorophyll a per algal cell when compared to colonies from 3m. These changes are explained as adaptations to limited photosynthetically active radiation at the deeper depth. Photosynthetic efficiency was higher at 35 m, as evidenced by a production rate 25% that at 3 m, but with light only about 8% that of shallow water irradiance. Respiration of deeper corals decreased by a half. A depth-specific respiratory decline was displayed by both the algae and the animal fractions. Decreased coral animal respiration appears to be a direct function of decreased photosynthetically fixed carbon availability, and to be an immediate response to daily carbon input. Decreased carbon availability to the host animal at 35 m was a consequence of both decreased net carbon fixation and decreased percentage of net fixed carbon translocated to the host. The daily CZAR at 35 m was less than half that at 3m. Mean CZAR at 35 m was 78%, suggesting that deeper corals have an obligate requirement for heterotrophically obtained carbon. By contrast, corals from 3m, which displayed a mean CZAR of 157%, appeared to be photo trophic with respect to carbon required for respiration. Altered trophic strategies with depth were confirmed by daily carbon budgets calculated for average size corals from both depths. Multiple correlation tests of all parameters confirmed the utility of expressing production and respiration measures in terms of unit surface area. However, significant correlations with other normalizing parameters were found, and their usefulness discussed.
TL;DR: DiFrancesco & Noble’s (1984) equations have been modified to apply to the mammalian sino-atrial node and successfully reproduce action potential and pacemaker activity in the node.
Abstract: DiFrancesco & Noble9s (1984) equations (Phil. Trans. R. Soc. Lond. B (in the press.)) have been modified to apply to the mammalian sino-atrial node. The modifications are based on recent experimental work. The modified equations successfully reproduce action potential and pacemaker activity in the node. Slightly different versions have been developed for peripheral regions that show a maximum diastolic potential near -- 75 mV and for central regions that do not hyperpolarize beyond -- 60 to -- 65 mV. Variations in extracellular potassium influence the frequency of pace-maker activity in the s.a. node model very much less than they do in the Purkinje fibre model. This corresponds well to the experimental observation that the node is less sensitive to external [K] than are Purkinje fibres. Activation of the Na-K exchange pump in the model by increasing intracellular sodium can suppress pacemaker activity. This phenomenon may contribute to the mechanism of overdrive suppression.
TL;DR: Empirical studies show the predictions of this computation of an instantaneous two-dimensional velocity field from the changing image to be consistent with human motion perception, and an additional constraint of smoothness of the velocity field is formulated, based on the physical assumption that surfaces are generally smooth.
Abstract: The organization of movement in the changing retinal image provides a valuable source of information for analysing the environment in terms of objects, their motion in space, and their three-dimensional structure. A description of this movement is not provided to our visual system directly, however; it must be inferred from the pattern of changing intensity that reaches the eye. This paper examines the problem of motion measurement, which we formulate as the computation of an instantaneous two-dimensional velocity field from the changing image. Initial measurements of motion take place at the location of significant intensity changes. These measurements provide only one component of local velocity, and must be integrated to compute the two-dimensional velocity field. A fundamental problem for this integration stage is that the velocity field is not determined uniquely from information available in the changing image. We formulate an additional constraint of smoothness of the velocity field, based on the physical assumption that surfaces are generally smooth, which allows the computation of a unique velocity field. A theoretical analysis of the conditions under which this computation yields the correct velocity field suggests that the solution is physically plausible. Empirical studies show the predictions of this computation to be consistent with human motion perception.
TL;DR: The specific absorption coefficient for chlorophyll a (kc) was measured in zooxanthellae from light- and shade-adapted colonies of the hermatypic coral, Stylophora pistillata from the Red Sea, to compare the quantum yields of photosynthesis in these corals.
Abstract: The specific absorption coefficient for chlorophyll a (k$\_{\text{c}}$) was measured in zooxanthellae from light- and shade-adapted colonies of the hermatypic coral, Stylophora pistillata from the Red Sea These data, together with measurements of photosynthetic rates and irradiance, were used to compare the quantum yields of photosynthesis in these corals Quantum yields varied from 010 CO$\_{2}$ per quantum at low light to less than 0001 CO$_{2}$ per quantum at maximal irradiances Shade-adapted corals had higher pigment content, which allowed them to have twice as much light harvesting capability as light-adapted corals In all cases, however, the quantum yield of the light-adapted corals was higher by a factor of about 15
TL;DR: Baboons, contrary to cats, have descending tonic inhibition of transmission in the reciprocal la-inhibitory pathway, and the phylogenetic flexibility of I a-connections is discussed and contrasted with their ontogenetic stability.
Abstract: The pattern of Ia-connections to motor nuclei of 17 hindlimb muscles (or groups of muscles) has been investigated in baboons by intracellular recording of Ia-e.ps.p.s evoked in motoneurons from different muscle nerves. The amplitudes are normalized to 70 m V resting potential and compared with similarly normalized Ia-e.ps.p.s in cats. As in the cat, Ia-excitation is drawn from a restricted number of muscles and the homonymous effect is usually dominating. Heteronymous connections to many motor nuclei are different in the two species. For example, hip extensors are generally more Ia-isolated from each other in baboons than in cats, and also knee flexors have fewer Ia-interconnections than in cats. A unidirectional Ia-synergism between some hip extensors and knee flexors in cats has changed to a bidirectional one in baboons, with a tendency to lateralization of the connections. Among ankle extensors, soleus has smaller heteronymous Ia-connections from its synergic ankle extensors than in cats. In baboons, plantaris is heteronymously Ia-excited from gastrocnemius-soleus but not from the intrinsic plantar muscles; whereas in casts there exists a considerable Ia-projection from the intrinsic plantar muscles but not from gastrocnemius-soleus. There is a corresponding difference in the insertion of the plantaris tendon, which shows that this muscle acts as toe extensor in cats but as ankle extensor in baboons. For most of the motor nuclei, the homonymous as well as the total aggregate of Ia-e.ps.p.s is smaller in the baboon than in the cat; but the amplitude range between different motor nuclei is larger in the baboon. Reciprocal Ia-i.ps.p.s are evoked only after spinal transection or when brain function is depressed. It is postulated that baboons, contrary to cats, have descending tonic inhibition of transmission in the reciprocal Ia-inhibitory pathway. The phylogenetic flexibility of Ia-connections is discussed and contrasted with their ontogenetic stability.
TL;DR: In this paper, the chlorophyll fluorescence oscillation described in barley protoplasts has been quantified and the mechanism of the changes in energy and redox states that can contribute additively but in differing proportions to the fluorescence was discussed.
Abstract: During the chlorophyll fluorescence oscillation described in barley protoplasts (Quick & Horton, Proc. R. Soc. Lond. B 220, 361-370, 1984) the components which contribute to quenching have been quantified. Quenching due to oxidized Q (q$\_{\text{Q}}$) was measured either by DCMU addition or by light doubling and indicated an oscillation in the redox state of Q which was antiparallel to the rate of oxygen evolution but was approximately 15 s out of phase at a variety of light intensities and temperatures. An oscillation in the extent of energy-dependent quenching, q$\_{\text{e}}$, was observed in strong but not weak light. These results are discussed in terms of the mechanism of the changes in energy and redox states that can contribute additively but in differing proportions to the fluorescence oscillation.
TL;DR: Evidence is presented to demonstrate that propulsive force is generated both on the upstroke and the downstroke of the green turtle’s foreflippers, and that the force is produced largely by hydrodynamic lift generated as the flippers move through the water at relatively modest angles of attack.
Abstract: The swimming actions and performances of young green turtles (Chelonia mydas) and freshwater chelonians of similar size (Mauremys caspica, Chrysemys scripta elegans and Kinosternon subrubrum) were compared to evaluate the advantages conferred by the hypertrophied flapping forelimbs and streamlined form of the marine species. Evidence is presented to demonstrate that propulsive force is generated both on the upstroke and the downstroke of the green turtle9s foreflippers, and that the force is produced largely by hydrodynamic lift generated as the flippers move through the water at relatively modest angles of attack. The green turtle9s forelimbs therefore act like wings rather than as oars or paddles. Specimens of Chelonia mydas can generate twice the propulsive force, and swam six times as fast as Mauremys caspica. Much of this advantage stems from the efficiency of the swimming action (much less negative thrust is generated by C. mydas than by M. caspica) together with the streamlined shape, which also permits effective gliding. The swimming performance of young green turtles is similar in efficiency to that of streamlined fish such as mackerel.
TL;DR: Anion-selective channels, with very large single unit conductances, are present in the cell membrane of rat cultured Schwann cells measured with the patch-clamp technique, and showed a slight permeability towards small cations the major permeability was to anions.
Abstract: Anion-selective channels, with very large single unit conductances, are present in the cell membrane of rat cultured Schwann cells measured with the patch-clamp technique. In inside-out membrane patches, channels with a conductance of about 450 pS (in symmetrical 150 mM NaCl) were observed. These channels did not become active until several minutes after the cytoplasmic surface had been exposed to the bathing medium, suggesting that these channels may normally be kept in an inactive state by some as yet unknown internal factor. The channel opened over a relatively small potential range ( — 10 mV to + 20 mV) and closed rapidly at more positive and more negative potentials with voltage-dependent kinetics. Although the channels showed a slight permeability towards small cations the major permeability was to anions. The order of permeability was: I > Br > Cl > methyl SO 4 > SO 4 > acetate = isethionate. Aspartate and glutamate were not detectably permeant. The physiological role of these channels remains unknown.
TL;DR: Ten species of shark belonging to three families were recorded from Aldabra Atoll and stomach contents of the more abundant species indicate that fish are the most important item in the diet, except for Nebrius concolor which feeds principally on octopus.
Abstract: Ten species of shark belonging to three families were recorded from Aldabra Atoll. Carcharhinus melanopterus and Negaprion acutidens were the most abundant species in the lagoon, while Carcharhinus albimarginatus was the most common shark outside the reef. Twelve hundred sharks of six species were tagged and individual recapture rates varied from 15 to 34%. Some specimens of C. melanopterus were caught up to seven times. All five species for which recapture data were available are restricted in their movements at Aldabra. C. melanopterus in particular is very localized, normally remaining in an area of a few square kilometres. Length increment data obtained from tagging demonstrated a slow growth rate for C. melanopterus, averaging 3.5 cm a$^{-1}$, with no detectable difference between the growth rates of small and large individuals. Limited data for juvenile Negaprion and C. albimarginatus indicated average growth rates of 12.5 and 8.8 cm a$^{-1}$ respectively. Population densities calculated for several areas in the lagoon varied from 19 to 198 C. melanopterus per square kilometre. It is suggested that C. melanopterus may be food-limited at Aldabra owing to the intensity of intra- and inter-species competition. C. melanopterus and Negaprion have restricted and almost identical reproductive cycles at Aldabra. C. melanopterus females mature at 110 cm total length and breed every second year giving birth to about four pups after a 10-11 month gestation period. Stomach contents of the more abundant species indicate that fish are the most important item in the diet, except for Nebrius concolor which feeds principally on octopus.
TL;DR: Xenopus laevis oocytes injected with poly(A)+ mRNA extracted from rat brain became sensitive to serotonin, glutamate, kainate, acetylcholine and γ-aminobutyrate, and it appears that mRNA from rat head contains translationally active messengers which code for various neurotransmitter receptors.
Abstract: Xenopus laevis oocytes injected with poly (A)$^{+}$ mRNA extracted from rat brain became sensitive to serotonin, glutamate, kainate, acetylcholine and $\gamma $-aminobutyrate. Application of these substances to mRNA-injected oocytes elicited membrane currents. The glutamate- and acetylcholine-induced currents usually showed oscillations, while the kainate current was smooth. The current oscillations during glutamate application reversed direction at about the chloride equilibrium potential (--24 mV), but the reversal potential for the kainate current was close to 0 mV. The current-voltage relation for the glutamate-induced current oscillations showed strong rectification at hyperpolarized potentials, while that for the kainate current was nearly linear. In some oocytes, glutamate elicited smooth membrane currents, with oscillations either absent, or appearing after a delay. The reversal potential of this component was close to 0 mV, and was clearly different from that of the oscillatory component. The appearance of glutamate and kainate sensitivity in the oocyte membrane is due to the translation of the foreign messenger RNA, and not to activation of the oocytes9 own genome, because oocytes still become sensitive when transcription is prevented by enucleation or by treatment with actinomycin D. It appears that mRNA from rat brain contains translationally active messengers which code for various neurotransmitter receptors. When this mRNA is injected into Xenopus oocytes, the messengers are translated and receptors are inserted into the oocyte membrane, where they form functionally active receptor-channel complexes.
TL;DR: The abnormally reduced size of the uncrossed component appears earlier than the abnormal segregation of the retinogeniculate terminals, suggesting that the primary action of the albino gene upon central visual pathways is prechiasmatic.
Abstract: The retinogeniculate pathways of normal and albino ferrets have been studied with horseradish peroxidase and tritiated proline used as axonal markers. The uncrossed retinogeniculate projection of adult albino ferrets is abnormally small and occupies only a fraction of the geniculate area normally occupied by uncrossed afferents. The crossed pathway is correspondingly expanded, occupying almost the entire nucleus. The geniculate laminae in the albino ferret are abnormal, showing abnormal fusions between layers receiving crossed input and abnormal discontinuities next to the small cell islands receiving uncrossed afferents. In early development, retinofugal fibres can be labelled within the optic tracts on the 28th intrauterine day and a few crossed fibres can be traced into the lateral geniculate nucleus. At this stage, the uncrossed component is extremely small in normal and albino animals and cannot be traced beyond the tract. By day 32 retinal fibres are invading the lateral geniculate nucleus bilaterally, the invasion by the crossed component being significantly more advanced than that by the uncrossed component. The uncrossed pathway of the albinos is already abnormal in terms of its size, in terms of the position it occupies in the optic tract, and in terms of its limited invasion of the lateral geniculate nucleus. The abnormally reduced size of the uncrossed component appears earlier than the abnormal segregation of the retinogeniculate terminals, suggesting that the primary action of the albino gene upon central visual pathways is prechiasmatic. At postnatal stages (41 days after conception and older) the normal, gradual withdrawal of the uncrossed fibres from the monocular segment, and the separation of crossed from uncrossed retinogeniculate terminal arbors is significantly delayed in the albinos. The uncrossed retinogeniculate terminals are abnormally sparse initially and become distributed in an abnormal, interrupted pattern as development proceeds. The abnormal pattern of geniculate lamination appears to be secondary to the abnormal distribution of retinogeniculate afferents.
TL;DR: Although sponges do have mechanisms to ingest particles such as food, they appear to be unable to consume bacterial symbionts, probably because their identity is masked by capsular sheaths.
Abstract: This study considered whether marine sponges are selective particle feeders, and whether they are capable of distinguishing between sponge bacterial symbionts and other bacteria. Four species of marine sponges (Aplysina aerophoba, A. cavernicola, Pericharax heteroraphis and Spongia sp.) were fed in situ with tritium-labelled bacteria, either symbionts or other bacteria isolated from seawater. A large proportion of bacterial symbionts passed through the sponge and were expelled in the exhalant current, whereas seawater bacteria disappeared from the incubation water and were retained within the sponge. The seawater bacteria were observed in choanocytes and occasionally in pinacocytes after 30-40 min, whereas symbiont bacteria were rarely observed. Although sponges do have mechanisms to ingest particles such as food, they appear to be unable to consume bacterial symbionts, probably because their identity is masked by capsular sheaths. A specific mechanism for recognition and rejection of self-particulate matter is proposed for sponge epidermal cells.
TL;DR: The classical monoamine neurotransmitters, acetylcholine and the catecholamines, and the large number of neuropeptides, may be chemical signals used for a different type of chemically addressed form of information transmission between neurons in c.n.s. characterized by less precise spatial connections, a slower time course and a far richer diversity of chemical signals than used in classical neurotransmission.
Abstract: The classical monoamine neurotransmitters, acetylcholine and the catecholamines, are used by only a small proportion of synapses in mammalian c.n.s. The amino acids GABA (y-aminobutyric acid) and L-glutamate may be the principal inhibitory and excitatory neurotransmitters used for fast point-to-point transmission in the c.n.s. The monoamines and the large number of neuropeptides (over 30) now known to exist in c.n.s. may be chemical signals used for a different type of chemically addressed form of information transmission between neurons in c.n.s. characterized by less precise spatial connections, a slower time course and a far richer diversity of chemical signals than used in classical synaptic neurotransmission. In this context the brain can be viewed as a neuroendocrine secretory organ of great complexity.
TL;DR: A theoretical analysis of the passive electrical properties of normal myelinated nerve suggests that the function of the voltage-dependent potassium channels in theinternodal axolemma under the myelin sheath is to permit the generation of an internodal resting potential.
Abstract: A theoretical analysis of the passive electrical properties of normal myelinated nerve suggests that the function of the voltage-dependent potassium channels in the internodal axolemma under the myelin sheath is to permit the generation of an internodal resting potential. Calculation shows that if this internodal potential were not present, the nodal potential would be reduced (by electrotonic short-circuiting) thus impairing the security of conduction. This impairment is particularly pronounced with smaller diameter fibres.
TL;DR: Symbiotic anemones that were not fed zooplankton for a month, to reduce the internal pools of excretory ammonium, showed a slight ability to deplete the medium of nitrate.
Abstract: The tropical sea anemone, Aiptasia pulchella, harbours symbiotic dinoflagellates (zooxanthellae). Animals in their natural habitat in Hawaii and those maintained in the laboratory in Los Angeles took up ammonium from nutrient enriched seawater. That the uptake experiment was done in the dark did not influence uptake although prolonged (19 h) dark treatment caused the animals to release ammonium. Aposymbiotic anemones (lacking zooxanthellae) typically excreted ammonium in the light. Freshly isolated zooxanthellae were also able to deplete enriched seawater of ammonium, indicating that the capacity for uptake by the symbiosis is due to the zooxanthellae. Uptake rates of freshly isolated zooxanthellae exceeded those of zooxanthellae in symbiosis. There was evidence that uptake of ammonium follows diffusion kinetics rather than the Michaelis--Menten model, since there was no saturation component. Neither symbiotic A. pulchella nor their isolated algae were able to remove nitrate from enriched seawater, even when pretreated for 24 h or one month with 5 or 10 $\mu $M nitrate. Symbiotic anemones that were not fed zooplankton for a month, to reduce the internal pools of excretory ammonium, showed a slight ability to deplete the medium of nitrate. Enzymes involved in assimilation of dissolved inorganic nitrogen were assayed in cell-free zooxanthellae extracts. Neither nitrate reductase nor glutamate synthase activity was found. There was glutamate dehydrogenase activity, with a K$\_{\text{m}}$ for ammonium of 4--5 mM; K$\_{\text{m}}$ for $\alpha $-ketoglutarate of 5--7 mM. The zooxanthellae also showed glutamine synthetase activity with a K$_{\text{m}}$ for glutamate of 12.5 $\mu $M but exhibiting negative cooperativity with ammonium.
TL;DR: Evidence, supported by the knowledge of sponge evolution and of the sparse bacterial populations in hexactinellid sponges, suggests that the symbioses may have commenced during the Precambrian, before the evolution of the extant sponge orders and classes.
Abstract: Many marine sponge species contain bacterial symbionts within the intercellular matrix. However, direct evidence on when in the course of evolution these symbioses might have begun is not available. Sponge-specific bacterial strains (296) were isolated from 36 sponge species from the three major sponge classes collected from seven widely separate geographical regions. Strains resembling these sponge-specific ones were not found among 202 ambient water strains. A considerable proportion (40%) of the sponge-specific strains showed common antigens in agglutination reactions against immune sera prepared from seven of them, whereas none of the non-specific bacterial strains (94) and none of the seawater bacteria (34) agglutinated in these sera. Furthermore, the sponge-specific bacteria did not agglutinate with five antisera prepared from control, non-specific, bacterial strains. The establishment of symbioses is a rare and conservative event and immunological cross-reactions are usually considered to be highly specific. This evidence, supported by the knowledge of sponge evolution and of the sparse bacterial populations in hexactinellid sponges, suggests that the symbioses may have commenced during the Precambrian, before the evolution of the extant sponge orders and classes.
TL;DR: It is proposed that the regulation of algal cell division in the natural symbiosis is principally mediated through relatively small and temporary changes in the pH of the perialgal vacuole, as well as the relation between the timing of symbiont and host cell division.
Abstract: The relative amount of symbiotic algae and animal tissue in the European strain of green hydra was altered by changes in illumination and feeding regimes. This indicates that the host can regulate the algal population to different sizes depending on external conditions. For animals maintained in continuous illumination, 12 h light: 12 h dark, and continous darkness, each with thrice-weekly feeding, a highly significant regression of algal volume per digestive cell on digestive cell volume was demonstrated, suggesting that the space available for the algae may be one factor that determines the population size of the algal symbionts. Seven strains of Chlorella originally symbiotic with other invertebrates formed stable associations with the European strain of green hydra; this included one strain (NC64A) which released very little maltose at pH 4-5. Althought the relative amounts of algal and host biomass of these experimental associations were very similar under standard culture conditions, large numbers of cells of strain NC64A were regularly expelled from the host. This suggests that the ability of the host to control the growth rate of its symbionts is related to the alga's capacity for maltose release. The latter characteristic is also correlated with a sensitivity of growth to acid conditions. Of the five cultured strains of symbiotic Chlorella examined, only the two strains that released substantial amounts of maltose at pH 4-5 failed to grow at pH 4.0 and pH 4.5. It is proposed that the regulation of algal cell division in the natural symbiosis is principally mediated through relatively small and temporary changes in the pH of the perialgal vacuole. At more acid values, photosynthetically fixed carbon is primarily directed towards maltose release and little or no algal growth occurs. At higher pH values, maltose release declines sharply and the carbon becomes primarily directed towards symbiont growth. Such a relatively simple hypothetical model, involving stimulation of symbiont growth by temporary alkalinization of the perialgal vacuole, can explain the observed responses to change in environmental conditions, as well as the relation between the timing of symbiont and host cell division.
TL;DR: The evolutionary origins and relationships of deer are briefly reviewed, with special reference to the species currently living wild or feral in Britain this paper, with emphasis on changing patterns of distribution and the climatic and vegetational conditions associated with fossil occurrences.
Abstract: The evolutionary origins and relationships of deer are briefly reviewed, with special reference to the species currently living wild or feral in Britain. Two species (red and roe) are native to this country; a third (fallow) was previously native but has been re-introduced from continental Europe; and three taxa (sika, muntjac and Chinese water-deer) have been introduced from eastern Asia. The Pleistocene history of red, roe and fallow deer in Europe is traced in detail, with emphasis on changing patterns of distribution and the climatic and vegetational conditions associated with fossil occurrences. Comparison of Pleistocene habitats with those occupied at the present day helps in an understanding of the ecological flexibility and rangelimiting factors of each species. The taxonomic status and habitats of the Asian species in Britain, as compared to their countries of origin, are briefly reviewed, and the dates of introduction into Britain of the Asian species and of fallow deer are discussed.
TL;DR: Three time-dependent membrane currents which could contribute to pacemaker depolarization are found to occur in the relevant potential range: decay of the potassium current, iK, and activation of the inward current, it, and of the slow inward current.
Abstract: The membrane currents underlying the pacemaker depolarization have been investigated in rabbit s.a. node preparations using the two-microelectrode voltage clamp technique. Many of the experimental results have been simulated using a computer model of s.a. node electrical activity. Changes of three time-dependent membrane currents which could contribute to pacemaker depolarization are found to occur in the relevant potential range: decay of the potassium current, iK, and activation of the inward current, if, and of the slow inward current, isi. The contribution of if activation to the pacemaker depolarization ranges from nil to an appreciable part depending on the preparation; when Cs (1 mM) blocks if, it nevertheless does not prevent pacemaking. In the model, holding the if activation variable at zero slows but does not stop pacemaking; doubling if conductance and shifting its activation curve by 15 mV in the positive direction causes a 15% faster rate of pacemaking. The slow time course of re-availability of isi must be allowed for when determining the isi threshold. A voltage clamp protocol designed to mimic as closely as possible an action potential followed by a pacemaker depolarization gives an estimate of isi threshold at the potential level of the last third of the pacemaker depolarization. This has been confirmed in experiments in which the voltage clamp was switched on at different points in the pacemaker depolarization. In the computer simulation, 'blocking' isi depolarizes the membrane to the zero current level (close to the potential reached at the end of a pacemaker depolarization) and stops the generation of action potentials. The decay of iK contributes to the pacemaker depolarization; with both our own model and that of K. Yanagihara, A. Noma and H. Irisawa, Jap. J. Physiol. 30, 841-857 (1980) 'blocking' iK decay abolishes pacemaker activity. Computations of extracellular K+ concentration changes compared with iK decay in a cylindrical model allow re-assessment of the interpretation of K+ concentration measurements during pacemaking made by J. Maylie, M. Morad and J. Weiss, J. Physiol., Lond. 311, 167-178 (1981).(ABSTRACT TRUNCATED AT 400 WORDS)
TL;DR: Construction of a schematic eye indicates that the eye of Spheniscus humboldti is aquatic in design and minimum pupil size would seem insufficient to allow the pupil to function as a stenopaic aperture and increase depth of focus sufficiently to overcome the eye’s aerial myopia.
Abstract: Construction of a schematic eye indicates that the eye of Spheniscus humboldti is aquatic in design. The lens has a power of 100 dioptres (D) while (in air) the cornea has a power of 29 D. In air, the eye is myopic (approximately 28 D) but in water it is emmetropic. Minimum pupil size would seem insufficient to allow the pupil to function as a stenopaic aperture and increase depth of focus sufficiently to overcome the eye's aerial myopia. Entry into water reduces maximum image brightness by approximately three times. In air, the maximum width of the retinal binocular field is 45 degrees and this occurs approximately 10 degrees above the line of the bill. The bill intrudes into the retinal field and binocular field width in the plane containing the bill and the optic axes is 28 degrees. The vertical extent of the binocular field is 125 degrees. In the plane containing the optic axes the cyclopean field equals 282 degrees and the optic axes diverge by 116 degrees. In this plane the mean uniocular field is 155 degrees with the temporal hemifield approximately 11 degrees larger than the nasal hemifield. Entry into water reduces the widths of the visual fields such that maximum binocular field width is only 17 degrees and the vertical extent is reduced to about 80 degrees. Binocular vision is lost in the plane of the bill, and the uniocular retinal field is reduced by 32 degrees and the cyclopean field by 36 degrees.