Fast and slow photoreceptors — a comparative study of the functional diversity of coding and conductances in the Diptera

1. From a comparison of the photoresponses and membrane properties of photoreceptors from 20 species of Diptera, we conclude that coding in the time domain is matched to the dictates of visual ecology. This matching involves the dynamics of phototransduction and the use of an appropriate mix of potassium conductances to tune the photoreceptor membrane.
2. Rapidly flying, manoeuvrable diurnal Diptera from several families have fast photoreceptors, with corner frequencies (the frequency at which signal power falls by a half) of between 50 and 107 Hz. The ponderous and predominantly nocturnal tipulids have slow photoreceptors with fully light adapted corner frequencies of 16 to 19 Hz.
3. Dark adapted fast photoreceptors have a lower gain (as indicated by lower noise levels), a lower sensitivity, and light adapt more rapidly than dark adapted slow photoreceptors. Fast cells also have much lower input resistances and shorter time constants.
4. Fast photoreceptors rectify more strongly in the steady state because of a weakly inactivating delayed rectifier potassium conductance with fast and slow components of activation. Slow photoreceptors rectify less strongly in the steady state because their membrane properties are dominated by strongly inactivating outward currents with reversal potentials in the range — 80 to -90 mV.
5. The differences between potassium conductances match the differing functional requirements of fast and slow photoreceptors. The non-inactivating delayed rectifier promotes the rapid response of fast cells by reducing the membrane time constant. This is an expensive strategy, involving large conductances and currents. Slowly flying nocturnal insects do not require a high speed of response. The potassium conductances in their slow photoreceptors inactivate to avoid costly and unnecessary ion fluxes.
6. Both the dynamics of the photoresponse and photoreceptor membrane properties exhibit sexual dimorphism. Light adapted photoreceptors in the enlarged male dorsal eye of Bibio markii have a corner frequency of 42 Hz, compared with 27 Hz for cells in the smaller female eye. This difference in frequency response correlates with the male's higher spatial acuity and is accompanied by consistent differences in potassium conductance activation rate. We conclude that the divison between fast and slow cells is the product of cellular constraints, metabolic costs and the requirements of coding efficiency at different light levels and retinal image velocities.

     

Thermal sensitivity of mitochondrial function in the Antarctic Notothenioid Lepidonotothen nudifrons

The thermal sensitivity of mitochondrial function was investigated in the stenothermal Antarctic fish Lepidonotothen nudifrons. State 3 respiration increases with increasing temperature between 0 °C and 18 °C with a Q ₁₀ of 2.43–2.63. State 4 respiration in the presence of oligomycin, an inhibitor of mitochondrial ATP synthase, quantifies the leakage of protons through the inner mitochondrial membrane, which causes oxygen consumption without concomitant ATP production. This parameter shows an unusually high Q ₁₀ of 4.21 ± 0.42 (0–18 °C), which indicates that proton leakage does not depend merely on ion diffusion but is an enzyme-catalysed process. The differential thermal sensitivity of oxidative phosphorylation (=state 3) and proton leakage (=state 4 in the presence of oligomycin) leads to progressive uncoupling of the mitochondria and decreased efficiency of oxidative phosphorylation under in vivo conditions if the body temperature of L. nudifrons increases. Accepted: 2 September 1999     

An integrative study of the temperature dependence of whole animal and muscle performance during jumping and swimming in the frog Rana temporaria

The aims of this study were: (1) to analyze individual variation in frog locomotor performance, (2) to compare the thermal sensitivity of jumping and swimming, and (3) to contrast whole animal versus muscle fiber performance at different temperatures. The jumping and swimming performance of Rana temporaria was analyzed at 5, 10, 15 and 20 °C. Muscle fiber bundles were isolated from lateral gastrocnemius and subjected to the length and activation patterns thought to occur in vivo. As temperature increased, locomotor performance in R. temporaria improved with a Q ₁₀ of 1.2 for both jump take-off velocity and mean swimming velocity. The slope of the relationship between performance and temperature (TE) was similar for both locomotor parameters and was described by the equation z-scores of locomotor performance = 0.127 × TE − 1.585. Although some frogs performed better than others relative performance was affected by locomotor type and temperature. Locomotor performance improved with temperature as the power required during take-off and the mean muscle power output increased with Q ₁₀ values of 1.7 and 1.6 respectively. The mean muscle power output during take-off was only 34% of the calculated requirements for the whole animal, suggesting the involvement of elastic strain energy storage mechanisms. Accepted: 2 September 1999     

The function of water channels in Chara: The temperature dependence of water and solute flows provides evidence for composite membrane transport and for a slippage of small organic solutes across water channels

Using the cell pressure probe, the effects of temperature on hydraulic conductivity (Lp; osmotic water permeability), solute permeability (permeability coefficient, P_s), and reflection coefficients (σ_s) were measured on internodes of Chara corallina, Klein ex Willd., em R.D.W.. For the first time, complete sets of transport coefficients were obtained in the range between 10 and 35 °C which provided evidence about pathways of water and solutes as they move across the plasma membrane (water channel and bilayer arrays). Test solutes used to check for the selectivity of water channels were monohydric alcohols of different molecular size and shape (ethanol, n-propanol, iso-propanol, and tert-butanol) and heavy water (HDO). Within the limits of accuracy, Q₁₀ values for Lp and for the diffusive water permeability (P_d) were identical (Q₁₀ for Lp = 1.29 ± 0.17 (± SD; n = 15 cells) and Q₁₀ for P_d = 1.25 ± 0.16 (n = 5 cells)). The Q₁₀ values were equivalent to activation energies of E_a = 16.8 ± 6.4 and 16.6 ± 10.0 kJ · mol⁻¹, respectively, which is similar to that of self-diffusion or of viscous flow of water. The Q₁₀ values and activation energies for P_s of the alcohols were significantly larger (ethanol: Q₁₀ = 1.68 ± 0.16, E_a = 37.1 ± 5.9 kJ · mol⁻¹; n-propanol: Q₁₀ =  1.75 ± 0.40, E_a = 43.1 ± 15.3 kJ · mol⁻¹; iso-propanol: Q₁₀ = 2.12 ± 0.42, E_a =  52.2 ± 14.6 kJ · mol⁻¹; tert-butanol: Q₁₀ = 2.13 ± 0.56, E_a = 51.6 ± 17.1 kJ · mol⁻¹; ±SD; n = 5 to 6 cells). Effects of temperature on reflection coefficients were most pronounced. With increasing temperature, σ_s values of the alcohols decreased and those of HDO increased. The data indicate that water and solutes use different pathways when crossing the membrane. Ordinary and isotopic water use water channels and the other test solutes use the bilayer array (composite transport model of membrane). Changes in σ_s values with temperature were found to be a sensitive measure for the open/closed state of water channels. The decrease of σ_s with temperature was theoretically predicted from the temperature dependence of P_s and Lp. Differences between predicted and measured values of σ_s allowed estimation of the bypass flow (slippage) of solutes through water channels which did not completely exclude test solutes. The permeability of channels depended on the structure and size of test solutes. It is concluded that water channels are much less selective than is usually thought. Since water channels represent single-file or no-pass pores, solutes drag along considerable amounts of water as they diffuse across channels. This results in low overall values of σ_s. The σ_s of HDO was extremely low. Its response to temperature was opposite to that for the σ_s of the alcohols. This suggested a stronger effect of temperature on the hydraulic (osmotic) than on the diffusive water flow across individual water channels, i.e. a differential sensitivity of different mechanisms to temperature. Received: 10 October 1996 / Accepted: 2 December 1996     

Heart rate variability in exercising humans: effect of water immersion

Power spectrum analysis of heart-rate variability was made in seven men [mean age 22 (SEM 1) years] in head-out water immersion (W) and in air (A, control) at rest and during steady-state cycling to maximal intensity (maximum oxygen uptake, V˙O_2max). At rest W resulted in a trebled increase in the total power (P < 0.05), coupled with minimal changes in the power (as a percentage of the total) of the high frequency peak (HF, centred at 0.26 Hz; 18% vs 28%) and of the low frequency peak (LF, 0.1 Hz; 24% vs 32%). A third peak at about 0.03 Hz (very low frequency, VLF) represented the remaining power both in W and A. These changes as a whole indicated that immersion caused a vagal dominance in cardiac autonomic interaction, due to the central pooling of blood and/or the pressure of water on the trunk. Exercise caused a decrease in the total power in W and A. The LF% did not change up to about 50% V˙O_2max, thereafter decreasing towards nil in both conditions. The HF% decreased in similar ways in W and A to about half at 55%–60% V˙O_2max and then increased to reach 1.5 times the resting values at V˙O_2max. The central frequency of HF increased linearly with oxygen uptake, showing a tendency to be higher in W than in A at medium to high intensities. The VLF% remained unchanged. The lack of differences in the LF peak between W and A during exercise would suggest that blood distribution had no effect on the readjustments in control mechanisms of arterial pressure. On the other hand, the findings of similar HF powers and the very similar values for ventilation in W and A confirmed the direct effect of the respiratory activity in heart rate modulation during exercise. Accepted: 25 August 1997     

Visual pigments, oil droplets, ocular media and cone photoreceptor distribution in two species of passerine bird: the blue tit (Parus caeruleus L.) and the blackbird (Turdus merula L.)

The spectral absorption characteristics of the retinal photoreceptors of the blue tit (Parus caeruleus) and blackbird (Turdus merula) were investigated using microspectrophotometry. The retinae of both species contained rods, double cones and four spectrally distinct types of single cone. Whilst the visual pigments and cone oil droplets in the other receptor types are very similar in both species, the wavelength of maximum sensitivity (λ_max) of long-wavelength-sensitive single and double cone visual pigment occurs at a shorter wavelength (557 nm) in the blackbird than in the blue tit (563 nm). Oil droplets located in the long-wavelength-sensitivesingle cones of both species cut off wavelengths below 570–573 nm, theoretically shifting cone peak spectral sensitivity some 40 nm towards the long-wavelength end of the spectrum. This raises the possibility that the precise λ_max of the long-wavelength-sensitive visual pigment is optimised for the visual function of the double cones. The distribution of cone photoreceptors across the retina, determined using conventional light and fluorescence microscopy, also varies between the two species and may reflect differences in their visual ecology. Accepted: 8 January 2000     

Implementation of an elaborated neuromorphic model of a biological photoreceptor

We describe here an elaborated neuromorphic model based on the photoreceptors of flies and realised in both software simulation and hardware using discrete circuit components. The design of the model is based on optimisations and further elaborations to the mathematical model initially developed by van Hateren and Snippe that has been shown to accurately simulate biological responses in simulations under both steady-state and limited dynamic conditions. The model includes an adaptive time constant, nonlinear adaptive gain control, logarithmic saturation and a nonlinear adaptive frequency response mechanism. It consists of a linear phototransduction stage, a dynamic filter stage, two divisive feedback loops and a static nonlinearity. In order to test the biological accuracy of the model, impulses and step responses were used to test and evaluate the steady-state characteristics of both the biological (fly) and artificial (new neuromorphic model) photoreceptors. These tests showed that the model has faithfully captured most of the essential characteristics of the insect photoreceptor cells. The model showed a decreasing response to impulsive stimuli when the background intensity was increased, indicating that the circuit adapted to background luminance in order to improve the overall operating range and better encode the contrast of the stimulus rather than luminance. The model also showed the same change in its frequency response characteristics as the biological photoreceptors over a luminance range of 70,000 cd/m², with the corner frequency of the circuit ranging from 10 to 90 Hz depending on the current state of adaptation. Complex naturalistic experiments have also further proven the robustness of the model to perform in real-world scenario. The model showed great correlation to the biological photoreceptors with an r ² value exceeding 0.83. Our model could act as an excellent platform for future experiments that could be carried out in scenarios where in vivo intracellular recording from biological photoreceptors would be impractical or impossible, or as a front-end for an artificial imaging system.     

Cardiovascular control via angiotensin II and circulating catecholamines in the spiny dogfish, Squalus acanthias

The contributions of circulating angiotensin II (Ang II) and catecholamines to cardiovascular control in the spiny dogfish were investigated by monitoring the effects of exogenous and endogenous dogfish [Asn¹, Pro³, Ile⁵]-Ang II (dfAng II) on plasma catecholamine levels and blood pressure regulation. Bolus intravenous injections of dfAng II (30–1200 pmol kg⁻¹) elicited dose-dependent increases in plasma adrenaline and noradrenaline concentrations, caudal artery pressure (P _CA), and systemic vascular resistance (R _S), and a decrease in cardiac output (Q). Similar injections of Ang II in dogfish pre-treated with the α-adrenoceptor antagonist yohimbine (4 mg kg⁻¹) also elicited dose-dependent increases in plasma catecholamine levels yet the cardiovascular effects were abolished. Dogfish treated with yohimbine were hypotensive and had elevated levels of plasma Ang II and catecholamines. Intravenous injection of the smooth muscle relaxant papaverine (10 mg kg⁻¹) elicited a transient decrease in P _CA and R _S, and increases in plasma Ang II and catecholamine levels. In dogfish first treated with lisinopril (10⁻⁴ mol kg⁻¹), an angiotensin converting enzyme inhibitor, papaverine treatment caused a more prolonged and greater decrease in P _CA and R _S, an attenuated increase in plasma catecholamines, and no change in plasma Ang II. By itself, lisinopril treatment had little effect on P _CA, and no effect on R _S, plasma Ang II or catecholamines. In yohimbine-treated dogfish, papaverine treatment elicited marked decreases in P _CA, R _S, and Q, and increases in plasma Ang II and catecholamines. Among the three papaverine treatments, there was a positive linear relationship between plasma Ang II and catecholamine concentrations, and the cardiovascular and hormonal changes were most pronounced in the yohimbine + papaverine treatment. Therefore, under resting normotensive conditions, while Ang II does not appear to be involved in cardiovascular control, catecholamines play an important role. However, during a hypotensive stress elicited by vascular smooth muscle relaxation, Ang II indirectly contributes to cardiovascular control by dose-dependently stimulating catecholamine release. Accepted: 24 February 1999     

The photoreceptors and visual pigments of the garter snake (Thamnophis sirtalis): a microspectrophotometric, scanning electron microscopic and immunocytochemical study

Scanning electron microscopy, immunocytochemistry, and single cell microspectrophotometry were employed to characterize the photoreceptors and visual pigments in the retina of the garter snake, Thamnophis sirtalis. The photoreceptor population was found to be comprised entirely of cones, of which four distinct types were identified. About 45.5% of the photoreceptors are double cones consisting of a large principal member joined near the outer segment with a much smaller accessory member. About 40% of the photoreceptors are large single cones, and about 14.5% are small single cones forming two subtypes. The outer segments of the large single cones and both the principal and accessory members of the doubles contain the same visual pigment, one with peak absorbance near 554 nm. The small single cones contain either a visual pigment with peak absorbance near 482 nm or one with peak absorbance near 360 nm. Two classes of small single cones could be distinguished also by immunocytochemistry and scanning electron microscopy. The small single cones with the 360-nm pigment provide the garter snake with selective sensitivity to light in the near ultraviolet region of the spectrum. This ultraviolet sensitivity might be important in localization of pheromone trails. Accepted: 10 March 1997     

Temperature-dependent conduction properties in Arctic fish peripheral nerves

The conduction properties of peripheral nerves from the Arctic fish species Arctic eelpouts (Lycodes sp.), snake blenny (Lumpenus lampretaeformis) and polar cod (Boreogadus saida), permanently adapted to low temperatures, were studied. Nerves of these fishes have two types of fibres, characterised by extracellular compound action potentials with fast (7 m/s) and slow (4 m/s) conduction velocities, as measured at 12 °C. The temperature dependence of the conduction velocity was bimodal, changing its slope at about 16 °C. The Q ₁₀ above 16 °C was 1.12–1.49, while below 16 °C it was 1.82–2.16. Irreversible deterioration of the nerve was observed at temperatures around 19–27 °C. A comparison with data previously obtained from Mediterranean fishes indicates that Arctic fishes have similar temperature sensitivity of nerve conduction and a slight vertical displacement on the conduction velocity-temperature curves, which is insufficient to compensate the decrease of the conduction velocity at their physiological temperature, the conduction velocity of Arctic fishes being about one-half of that of temperate fishes. This suggests that this neurophysiological function is not fully cold-adapted in these Arctic fish species. Accepted: 3 June 2000     

Correlation between auditory sensitivity and vocalization in anabantoid fishes

Several anabantoid species produce broad-band sounds with high-pitched dominant frequencies (0.8–2.5 kHz), which contrast with generally low-frequency hearing abilities in (perciform) fishes. Utilizing a recently developed auditory brainstem response recording-technique, auditory sensitivities of the gouramis Trichopsis vittata, T. pumila, Colisa lalia, Macropodus opercularis and Trichogaster trichopterus were investigated and compared with the sound characteristics of the respective species. All five species exhibited enhanced sound-detecting abilities and perceived tone bursts up to 5 kHz, which qualifies this group as hearing specialists. All fishes possessed a high-frequency sensitivity maximum between 800 Hz and 1500 Hz. Lowest hearing thresholds were found in T. trichopterus (76 dB re 1 μPa at 800 Hz). Dominant frequencies of sounds correspond with the best hearing bandwidth in T. vittata (1–2 kHz) and C. lalia (0.8–1 kHz). In the smallest species, T. pumila, dominant frequencies of acoustic signals (1.5–2.5 kHz) do not match lowest thresholds, which were below 1.5 kHz. However, of all species studied, T. pumila had best hearing sensitivity at frequencies above 2 kHz. The association between high-pitched sounds and hearing may be caused by the suprabranchial air-breathing chamber, which, lying close to the hearing and sonic organs, enhances both sound perception and emission at its resonant frequency. Accepted: 26 November 1997     

Temporal resolution in olfaction III: flicker fusion and concentration-dependent synchronization with stimulus pulse trains of antennular chemoreceptor cells in the American lobster

To understand how chemoreceptor organs may extract temporal information from odor plumes, we investigated the frequency filter properties of lobster chemoreceptor cells. We used rapid stimulation and high-resolution stimulus measurement for accurate stimulus control and recorded extracellular responses from chemoreceptors in the lobster lateral antennule in situ. We tested 16 hydroxyproline-sensitive cells with a series of ten 100-ms pulses at 10, 100 and 1000 μmol l⁻¹ at stimulation frequencies from 0.5 Hz to 4 Hz. Receptor cell responses could accurately encode 10 μmol l⁻¹, but not 100 or 1000 μmol l⁻¹ pulses, delivered at rates of 4 Hz. Flicker-fusion frequency and synchronization with the stimulus pulse train were concentration dependent: performance rates above 1 Hz became poorer both with increasing pulse amplitude and frequency. Flicker fusion frequency was 3 Hz for 100 μmol l⁻¹ pulses and 2 Hz for 1000 μmol l⁻¹ pulses. Individual cells showed differences in their stimulus pulse following capabilities, as measured by the synchronization coefficient. These individual differences may form a basis for coding temporal features of an odor plume in an across-fiber pattern. Accepted: 7 July 1999     

Energetic cost of hovering flight in a nectar-feeding bat measured with fast-response respirometry

Hover-feeding glossophagine bats provide, in addition to the hummingbirds, a second vertebrate model for the analysis of hovering flight based on metabolic measurement and aerodynamic theory. In this study, the power input of hovering Glossophaga soricina bats (11.9 g) was measured by standard respirometry and fast-response (<0.2 s) oxygen analysis. Bats needed 5–7 s after a rest-to-flight transition to return to a respiratory steady state. Therefore, only hovering events preceeded by a 7-s flight interval were evaluated. V˙_O2 during hovering fluctuated with a frequency of 3–5 Hz, which corresponded in frequency to the licking movement of the tongue. During hovering, bats often may have hypoventilated as indicated by reduced V˙_O2 and a respiratory exchange ratio (RER) well below the steady-state value of 1. Steady-state oxygen consumption (and derived power input) during hovering was estimated to be 27 (25–29) ml O₂ g⁻¹ h⁻¹ (158 W kg⁻¹ or 1.88 W) in the 11.9-g bats as indicated by three independent findings: (1) V˙_O2 was 26 ml O₂ g⁻¹ h⁻¹ after 6.5 s of hovering, (2) the mean RER during single hovering events was at its steady-state level of 1 only at oxygen uptake rates of 25–29 ml g⁻¹ h⁻¹, and (3) when the oxygen potentially released from estimated oxygen stores was added to the measured oxygen uptake, the upper limit for oxygen consumption during hovering was found to be 29 ml O₂ g⁻¹ h⁻¹. Hovering power input was about 1.2 times the value of minimum flight power input (Winter and von Helversen 1998) and thus well below the 1.7–2.6 difference in power output postulated by aerodynamic theory (Norberg et al. 1993). Mass specific power input was 40% less than in hummingbirds. Thus, within the possible modes of hovering flight, Glossophaga bats seem to operate at the high-efficiency end of the spectrum. Accepted: 28 April 1998     

Frequency-intensity characteristics of cricket cercal interneurons: units with high-pass functions

Interneurons in the cercal sensory system of crickets respond in a cell-specific manner if the cercal hair sensilla are stimulated by air-particle oscillations at frequencies below about 2000 Hz. We investigated the filter properties of several of these interneurons, and tested the effect of stimulus intensity (typically 0.3–50 mm s⁻¹ peak-to-peak air-particle velocity) on the frequency response in the range 5–600 Hz. We focus on three interneurons (the lateral and medial giant interneurons and interneuron 9-3a) of Acheta domesticus which are characterized by a relatively high sensitivity above ca. 50–200 Hz. The responses of the medial giant interneuron usually increase monotonically with frequency and intensity. Interneuron 9-3a and the lateral giant interneuron exhibit saturation or response decrement at high frequencies and intensities. The lateral giant interneuron has an additional peak of sensitivity below about 40 Hz. Small individual variations in the relative locations of the two response areas of this interneuron within the frequency-intensity field are responsible for a large variability obtained if frequency-response curves are determined for particular intensities. Stimulus frequency does not affect the principal directional preferences of the three interneurons. Nevertheless, if tested individually, the lateral giant interneuron and interneuron 9-3a exhibit small changes of directional tuning. Accepted: 12 November 1997     

Optimization of Hydrogels for Transdermal Delivery of Lisinopril by Box–Behnken Statistical Design

The aim of this study was to investigate the combined influence of three independent variables on the permeation kinetics of lisinopril from hydrogels for transdermal delivery. A three-factor, three-level Box–Behnken design was used to optimize the independent variables, Carbopol 971 P (X ₁), menthol (X ₂), and propylene glycol (X ₃). Fifteen batches were prepared and evaluated for responses as dependent variables. The dependent variables selected were cumulative amount permeated across rat abdominal skin in 24 h (Q ₂₄; Y ₁), flux (Y ₂), and lag time (Y ₃). Aloe juice has been first time investigated as vehicle for hydrogel preparation. The ex vivo permeation study was conducted using Franz diffusion cells. Mathematical equations and response surface plots were used to relate the dependent and independent variables. The regression equation generated for the cumulative permeation of LSP in 24 h (Q ₂₄) was Y ₁ = 1,443.3–602.59X ₁ + 93.24X ₂ + 91.75X ₃ − 18.95X ₁ X ₂ – 140.93X ₁ X ₃ – 4.43X ₂ X ₃ – 152.63X ₁ ² – 150.03X₂ ² − 213.9X ₃ ². The statistical validity of the polynomials was established, and optimized formulation factors were selected by feasibility and grid search. Validation of the optimization study with 15 confirmatory runs indicated high degree of prognostic ability of response surface methodology. The use of Box–Behnken design approach helped in identifying the critical formulation parameters in the transdermal delivery of lisinopril from hydrogels.     

Sensitivity and response dynamics of elasmobranch electrosensory primary afferent neurons to near threshold fields

Elasmobranch fishes localize weak electric sources at field intensities of <5 ηV cm⁻¹, but the response dynamics of electrosensory primary afferent neurons to near threshold stimuli in situ are not well characterized. Electrosensory primary afferents in the round stingray, Urolophus halleri, have a relatively high discharge rate, a regular discharge pattern and entrain to 1-Hz sinusoidal peak electric field gradients of ≤20 ηV cm⁻¹. Peak neural discharge for units increases as a non-linear function of stimulus intensity, and unit sensitivity (gain) decreases as stimulus intensity increases. Average peak rate-intensity encoding is commonly lost when peak spike rate approximately doubles that of resting, and for many units occurs at intensities <1 μV cm⁻¹. Best neural sensitivity for nearly all units is at 1–2 Hz with a low-frequency slope of 8 dB/decade and a high-frequency slope of −23 dB/decade. The response characteristics of stingray electrosensory primary afferents indicate sensory adaptations for detection of extremely weak phasic fields near 1–2 Hz. We argue that these properties reflect evolutionary adaptations in elasmobranch fishes to enhance detection of prey, communication and social interactions, and possibly electric-mediated geomagnetic orientation. Accepted: 20 June 1997     

Responses of the infrared sensilla of Melanophila acuminata (Coleoptera: Buprestidae) to monochromatic infrared stimulation

The pit organs of the beetle Melanophilaacuminata were stimulated with monochromatic infrared radiation using a continuous wave CO overtone infrared laser. Best sensitivity was in the wavelength range 2.8–3.5 μm. In this range a stimulus intensity of 14.7 mW cm⁻² was sufficient to generate single action potentials. At a wavelength of 5 μm receptor performance significantly decreased. An increase in stimulus intensity caused a decrease in response latency and an increase in the number of action potentials elicited. At a given wavelength (3.4 μm) the dynamic amplitude range of action potential responses covered 12 dB. At high stimulus intensities (94.2 mW cm⁻²) a stimulus duration of 4 ms was sufficient to generate one to two action potentials and a stimulus duration of 60 ms already caused response saturation (with up to nine action potentials). In a repetitive stimulus regime distinct receptor potentials were visible up to a frequency of 600 Hz. Accepted: 18 March 2000     

Incorporating diffuse photosynthetically active radiation in a single-leaf model of canopy photosynthesis for a 56-year-old Douglas-fir forest

A simple top-down model of canopy photosynthesis (P) was developed and tested in this study. The model (referred to as the Q_e-MM model) is P = αQ _e P _max/(αQ _e + P _max), α and P _max are quantum-use efficiency and potential P, respectively. Q _e is given by Q _{d 0} + kQ _{b 0}, where Q _{d 0} and Q _{b 0} are the diffuse and direct photosynthetically active radiation (PAR) incident on the canopy, respectively. Q _e can be considered to be the effective incident PAR contributing to P and k is a measure of the contribution of Q _{b 0} to Q _e. When k = 1, the Q_e-MM model becomes the regular Michaelis-Menten type model of P (referred to as the MM model). A major objective of this study was to determine how well the Q_e-MM model could estimate P of a 56-year-old coastal Douglas-fir stand. To this end, we parameterized the Q_e-MM model using five and half years of eddy-covariance measurements of CO₂ flux above the Douglas-fir stand. The Q_e-MM model, with the incorporation of a function of air temperature, accounted for 74% of the variance in over 34,000 half-hourly P measurements. P estimated using the Q_e-MM model had no systematic errors with respect to Q _{d 0}. Although the Q_e-MM model has only one more parameter than the MM model, it accounted for 30% more variance in P than the latter when total incident PAR exceeded 900 μmol m⁻² s⁻¹. On average, k was found to be 0.22. We show that this small value of k reflects the significant effect of the scattering of the solar beam and the fraction of light-limited sunlit leaves. We also show that the success of the Q_e-MM model was due to the fact that a large fraction of the sunlit leaves were light-limited as a result of their orientation to the solar beam.     

Cold induced vasodilatation and cardiovascular responses in humans during cold water immersion of various upper limb areas

To study the physiological responses induced by immersing in cold water various areas of the upper limb, 20 subjects immersed either the index finger (T1), hand (T2) or forearm and hand (T3) for 30 min in 5°C water followed by a 15-min recovery period. Skin temperature of the index finger, skin blood flow (Q_sk) measured by laser Doppler flowmetry, as well as heart rate (HR) and mean arterial blood pressure (ˉBP_a) were all monitored during the test. Cutaneous vascular conductance (CVC) was calculated as Q_sk / ˉBP_a. Cold induced vasodilatation (CIVD) indices were calculated from index finger skin temperature and CVC time courses. The results showed that no differences in temperature, CVC or cardiovascular changes were observed between T2 and T3. During T1, CIVD appeared earlier compared to T2 and T3 [5.90 (SEM 0.32) min in T1 vs 7.95 (SEM 0.86) min in T2 and 9.26 (SEM 0.78) min in T3, P < 0.01]. The HR was unchanged in T1 whereas it increased significantly at the beginning of T2 and T3 [+13 (SEM 2) beats · min⁻¹ in T2 and +15 (SEM 3) beats · min⁻¹ in T3, P < 0.01] and then decreased at the end of the immersion [−12 (SEM 3) beats · min⁻¹ in T2, and −15 (SEM 3) beats · min⁻¹ in T3, P < 0.01]. Moreover, ˉBP_aincreased at the beginning of T1 but was lower than in T2 and T3 [+9.3 (SEM 2.5) mmHg in T1, P < 0.05;  +20.6 (SEM 2.6) mmHg and 26.5 (SEM 2.8) mmHg in T2 and T3, respectively, P < 0.01]. The rewarming during recovery was faster and higher in T1 compared to T2 and T3. These results showed that general and local physiological responses observed during an upper limb cold water test differed according to the area immersed. Index finger cooling led to earlier and faster CIVD without significant cardiovascular changes, whereas hand or forearm immersion led to a delayed and slower CIVD with a bradycardia at the end of the test. Accepted: 26 November 1996     

The photoreceptors and visual pigments of two species of Acipenseriformes, the shovelnose sturgeon (Scaphirhynchus platorynchus ) and the paddlefish (Polyodon spathula )

Scanning electron microscopy, microspectrophotometry, and spectrophotometry of digitonin extracts were employed to characterize the photoreceptors and visual pigments of two freshwater Acipenseriformes. The retinas of the shovelnose sturgeon, Scaphirhynchus platorynchus (Acipenseridae), and the paddlefish, Polyodon spathula (Polyodontidae) are dominated by large rods with long, broad outer segments. A second rod, rare and much narrower than the dominant rod, is present in Scaphirhynchus but not seen in Polyodon. The absorbance maximum of the visual pigment in the rods of Polyodon is near 540 nm; that of Scaphirhynchus near 534 nm. The retinas of both species contain substantial numbers of large, single cones, about 33% of the photoreceptors in Scaphirhynchus; 37% in Polyodon. Scaphirhynchus cone pigments have absorbance maxima near 610 nm, 521 nm and 470 nm, respectively. Polyodon cone pigments absorb maximally near 607 nm and 535 nm, respectively. All visual pigments are based on vitamin A₂. The data are compared to those from other Acipenseriformes and are discussed in terms of lifestyle and behavior. Accepted: 7 October 1998