Half-maximal Amplitude Research Articles

Temporal structure of in vivo growth hormone secretory events in humans

The time course of spontaneous growth hormone (GH) secretion was studied in 12 normal men by analyzing serum GH concentrations measured in blood collected at 5-min intervals over 24 h with a multiple-parameter deconvolution model to simultaneously resolve endogenous GH secretory and clearance rates. Twenty-four-hour profiles of serum GH concentrations were accounted for by an average of 12 +/- 1.2 (SE) discrete GH secretory bursts having a mean half-duration (duration at half-maximal amplitude) of 25 +/- 2.3 min, indicating that 95% of daily GH secretion occurred in 8.8 h. The majority (96%) of GH was secreted in volleys composed of multiple (4.0 +/- 0.4) discrete secretory bursts. Such volleys of GH secretion were separated by 171 +/- 19 min, whereas their constituent individual secretory events occurred every 36 +/- 1.7 min (P = 0.0001). Between secretory volleys, calculated GH secretory rates fell asymptotically to zero. Significant positive and negative autocorrelations were observed for the series consisting of successive GH secretory burst mass and interburst intervals, respectively. Linear regression analysis revealed significant negative correlations between the mass of GH secreted per burst and both the preceding and following interburst intervals. Estimates of the half-life of endogenous GH (17 +/- 1.7 min) and of the endogenous GH production rate (0.25 +/- 0.033 mg/m2 for an assumed distribution volume of 7.9% body wt) agreed well with earlier independent measurements. We conclude that the human pituitary gland secretes GH in volleys consisting of multiple secretory bursts, without measurable intervening tonic secretion. This pattern of in vivo GH release in normal humans is consistent with a model of high-frequency GH-releasing hormone secretory events superimposed on low-frequency episodes of somatostatin withdrawal.

Amplitude, but not Frequency, Modulation of Adrenocorticotropin Secretory Bursts Gives Rise to the Nyctohemeral Rhythm of the Corticotropic Axis in Man*

The ACTH-adrenal axis is a critical stress-responsive system with prominent circadian rhythmicity. To test the basis for the circadian ACTH physiology, we have used 1) a sensitive and specific two-site immunoradiometric assay to estimate plasma ACTH-(1-39) concentrations during intensive (every 10 min) and extended (24-h) blood sampling to capture complete diurnal ACTH profiles in eight normal men, and 2) a novel deconvolution model designed to resolve the number, amplitude, and duration of ACTH secretory bursts and simultaneously estimate subject-specific ACTH half-lives under physiological conditions in vivo. Deconvolution revealed 40 +/- 1.5 significant ACTH secretory bursts/24 h, with a mean interburst interval of 39 +/- 2.3 min. ACTH secretory bursts were discrete punctuated events arising without tonic interpulse secretion and had a half-duration (duration at half-maximal amplitude) of 19 +/- 2 min. The estimated half-life of endogenous ACTH was 15 +/- 1.2 min, and its daily production rate was 0.96 +/- 0.16 ng/mL (0.21 +/- 0.035 nmol/L) distribution volume. Cosinor analysis revealed a significant (3.8-fold) 24-h rhythm in the mass (or rate) of ACTH secreted per burst (maximal at 0818 h), but no nyctohemeral variation in ACTH secretory pulse frequency. The validity of ACTH pulse analysis was supported by the significantly nonrandom associations among ACTH, beta-endorphin, and cortisol peaks in the same subjects. Specifically, we found that ACTH and beta-endorphin bursts occurred simultaneously (P less than 10(-4)) and were both followed in 10 min by a cortisol pulse (P less than 10(-4)). We conclude that 1) selective amplitude control of a punctuated burst-like mode of ACTH secretion can give rise to the nyctohemeral corticotropic rhythm without the need to postulate any tonic (or interpulse basal) component of ACTH release; and 2) there is exquisite 3-fold temporal synchrony among bursts of ACTH, beta-endorphin, and cortisol release in normal men.

Amplitude modulation of a burstlike mode of cortisol secretion subserves the circadian glucocorticoid rhythm.

We have examined the mechanism subserving the in vivo circadian rhythm of cortisol in men. To this end, blood samples were withdrawn at 10-min intervals for 24 h in each of six men to yield well-defined profiles of episodic cortisol release. A novel multiple-parameter deconvolution model was applied to discriminate the number, amplitudes, and durations of all statistically significant underlying cortisol secretory bursts from the plasma hormone concentrations and simultaneously estimate the endogenous half-life of cortisol disappearance in each subject. These experiments disclosed randomly occurring cortisol secretory bursts at a mean frequency of 19 +/- 0.82 events per day (interpulse interval 77 +/- 4.0 min). Secretory bursts exhibited a mean half-duration (duration at half-maximal amplitude) of only 16 +/- 0.61 min indicating that 95% of daily cortisol secretion occurred in 8.2 h. Cortisol secretory burst frequency varied 2.2-fold over 24 h, whereas cortisol secretory burst amplitude varied 6.6-fold. We conclude that the nyctohemeral pattern of cortisol variation in vivo can be accounted for by an amplitude-modulated, random burstlike mode of cortisol secretion without the need to postulate a tonic mode of cortisol release.

An in vivo electrophysiological study of the ontogeny of excitatory and inhibitory processes in the rat hippocampus

Although several studies have compared hippocampal slices from young vs adult rats, a systematic in vivo characterization of the ontogeny of electrophysiologic responses in this structure has not been done. The current report describes the postnatal development of excitatory and inhibitory responses in the CA1 region of the hippocampus in rats 7–65 days of age. Under urethane anesthesia, a recording electrode was placed in one CA1 region to measure extracellular population spikes elicited by stimulation of the contralateral CA3 region. Age-related changes in the maximal population spike amplitude, the voltage required to elicit a half-maximal amplitude spike, the width at half-maximal spike amplitude, and latency to onset of the population spike (‘conduction velocity’) were monitored as parameters describing excitatory processes in the hippocampus. A paired-pulse paradigm was used to quantify the ontogeny of inhibitory processes. In younger animals, population spikes were broader, smaller in amplitude, and required higher stimulus intensities to be elicited. After postnatal (PN) day 14, excitability (voltage to elicit half-maximal population spike) and spike width were at fully mature levels. Maximal spike amplitudes were also smaller in rats younger than PN14, but not thereafter. The conduction velocity parameter steadily increased during development. In contrast, no evidence of inhibition was found prior to PN18, after which it steadily increased to reach adult levels by PN28. These results indicate that, in the rat hippocampus, excitatory processes are well established or fully mature within 2 weeks following birth, whereas the maturation of inhibitory processes to adult levels is not achieved until several weeks later.

Effects of temperature on a central synapse between identified motor neurons in the locust.

Changing the temperature from 10-40 degrees C modifies the transmission at an established monosynaptic connection between the fast extensor tibiae (FETi) and flexor tibiae motor neurons in the metathoracic ganglion of the locust Schistocerca gregaria (Forskål). Striking changes occur to the shape of the spikes, to membrane resistance, to the synaptic delay, and to the evoked synaptic potentials. In the presynaptic FETi motor neuron, raising the temperature reduces the amplitude of an antidromic spike recorded in the soma by a factor of 10 (40 mV to 4 mV), reduces the time taken to reach peak amplitude by 5 (3.5 to 0.7 ms) and decreases the duration at half maximum amplitude by 0.5. The conduction velocity of the spike in the axon is increased by 50% from 10 degrees C to 40 degrees C. Orthodromic spikes are affected by temperature in a similar way to the antidromic spikes. The membrane resistance of both pre- and postsynaptic motor neurons falls as the temperature is raised. The membrane resistance of FETi falls by a factor of 4 (about 4 M omega at 10 degrees C to 1 M omega at 40 degrees C). A contributory component to this fall could be the increase in the frequency of synaptic potentials generated as a result of inputs from other neurons. No temperature dependence could be demonstrated on the voltage threshold relative to resting potential for evoking orthodromic spikes, but because the resistance changes, the current needed to achieve this voltage must be increased at higher temperatures.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative electrophysiological studies of regenerating visuotopic maps in goldfish—I. Early recovery of dimming sensitivity in tectum and torus longitudinals

Refinement and connectivity in the regenerating retinotectal system of goldfish were studied quantitatively by electrophysiological methods. One optic nerve was crushed intraorbitally in fish kept at 25°C. At different postcrush times, visually-evoked multiunit activity was recorded from the superficial layers of tectum, and from the torus longitudinalis. Responses in torus longitudinalis were used as a test of retinotectal connectivity because torus longitudinalis derives a visuotopic map from a tectal projection. The stimulus, effective for both the early retinotectal projection and torus longitudinalis, was a 10° wide vertical black stripe rotated horizontally at 25°/s through both visual fields. Activity from repeated sweeps was averaged to yield receptive field profiles in the horizontal dimension. Normally, profiles from tectum were dual-peaked and 20° wide at half maximum amplitude; torus longitudinalis profiles were bell-shaped and 41° wide. Between 20 and 40 days postcrush, tectum gave broad low-amplitude (25% normal) profiles that were roughly visuotopic. Over the same period, torus longitudinalis gave profiles of relatively high amplitude (69% of normal) that were also broadened but normally visuotopic. The widths of both tectal and torus longitudinalis profiles declined with the same exponential timecourse, reaching normal values by 80–100 days. Torus longitudinalis profiles were on average 21.6° wider than tectal profiles at all stages of regeneration. The results agree with previous anatomical observations showing that optic fibers initially form much enlarged arbors that shrink over time, and suggest that arbors engage in widespread synaptic connections, at least with tecto-torus longitudinalis cells. Around 40 days, tectal profiles started to grow in amplitude and recover the dual-peaked shape, indicating a second phase in retinotectal regeneration.

Fast Optical Detector Deposited On Dielectric Channel Waveguides

We have fabricated a thin-film optical detector for detecting short optical pulses propagating in channel waveguides. The detectors show response times of 200 ps full width at half maximum amplitude when illuminated by guided, subpicosecond optical pulses. The detectors are formed by depositing hydrogenated amorphous silicon (a-Si:H) directly on the dielectric channel waveguides. Back-to-back Schottky photodiodes are then formed when interdigitated chrome-gold metal contacts are deposited on the a-Si:H.

The effects of ion implantation on optical spectra of SiO 2 glass

Optical absorption of ion implanted amorphous SiO2 was measured in the energy region 1.0 to 6.5 eV. Both sides of silica wafers (Spectrosil and Spectrosil WF) were implanted with either Cr+, Mn+ or Fe+ to doses of 1 × 1016, 3 × 1016, and 6 × 1016 ions/cm2 per side at 160 keV and 4 μA/cm2. Distributions of the implanted species were determined by ion backscattering techniques and, within experimental error, were the same for all doses and species. For each ion specie the spectra, when plotted as extinction coefficient per ion, increased with increasing dose throughout the entire range of measurement. The intensities of the optical absorption spectra were a function of ion specie. There was no evidence for effects of substrate type in the absorption spectra. Evidence for three previously reported bands associated with irradiation damage effects was observed in all optical spectra. These bands are: the B2 band (5.05 eV), the E1' band (5.8 eV), and the low energy tail of a band centered at 7.15 eV. Assuming a Gaussian shape for the B2 and E1' bands and a Lorentzian shape for the band at 7.15 eV and constraining the peak energies and full widths at half maximum amplitude (FWHM) to previously reported values, the observed data were fitted when two additional bands were included in the fitting. These are the DO and E2' bands at 4.8 and 5.31 eV, respectively, which were similarly constrained. Optical absorption due to the implanted species has not been detected. The chemical state of these ions is tentatively assumed to be a mixture of the chemical state of the ions preceding implantation, i.e. + 1, and higher states, e.g. +2 and +3, which are a consequence of the interactions between the implanted ion and the substrate. The effect of these states on the optical absorption will be discussed.

Voltage-clamp analysis of the ionic conductances in a leech neuron with a purely calcium-dependent action potential

1. The purely calcium-dependent action potential of the anterior lateral giant (ALG) cell in the leech Haementeria was examined under voltage clamp. 2. Analysis with ion substitutions showed that the ALG cell action potential is generated by only two time- and voltage-dependent conductance systems, an inward Ca-dependent current (ICa) and an outward Ca-dependent K current IK(Ca). 3. The kinetic properties of the inward current were examined both in Cs-loaded neurons with Ca as the current carrier as well as in Ba-containing Ringer solutions with Ba as the current carrier, since Ba effectively blocked all time- and voltage-dependent outward current. 4. During a maintained depolarization, Ba and Ca currents activated with a time constant tau m, they then inactivated with the decay following a single exponential time course with a time constant tau h. The time constants for decay of both Ba and Ca currents were comparable, suggesting that the mechanism of inactivation of ICa in the ALG cell is largely voltage dependent. In the range of potentials from 5 to 45 mV, tau m varied from 8 to 2 ms and tau h varied from 250 to 125 ms. 5. The activation of currents carried by Ba, after correction for inactivation, could be described reasonably well by the expression I'Ba = I'Ba(infinity) [1--exp(-t/tau m)]. 6. The steady-state activation of the Ba-conductance mBa(infinity) increased sigmoidally with voltage and was approximated by the equation mBa(infinity) = (1 + exp[(Vh-6)/3])-1. The steady-state inactivation hBa(infinity) varied with holding potential and could be described by the equation hBa(infinity) = [1 + exp(Vh + 10/7)]-1. Recovery from inactivation of IBa was best described by the sum of two exponential time courses with time constants of 300 ms and 1.75 s, respectively. 7. The outward current IK(Ca) developed very slowly (0.5-1 s to half-maximal amplitude) and did not inactivate during a 20-s depolarizing command pulse. Tail current decay of IK(Ca) followed a single exponential time course with voltage-dependent time constants of between 360 and 960 ms. The steady-state activation n infinity of IK(Ca) increased sigmoidally with depolarization as described by the equation n infinity = [1 + exp(Vh-13.5)/-8)]-1. 8. The reversal potentials of IK(Ca) tail currents were close to the expected equilibrium potential for potassium and they varied linearly with log [K]o with a slope of 51 mV. These results suggest a high selectivity of the conductance for K ions.(ABSTRACT TRUNCATED AT 400 WORDS)

Magnetic Properties of Ions Implanted in Glass; Fe in SiO2

ABSTRACTSilica glass platelets have been implanted with Fe(+) ions, 125 KeV and 10⋀-5 A to doses of 10⋀14 cm⋀-2, 10⋀15 cm⋀-2, 10⋀16 cm⋀-2 and 5 x 10⋀16 cm⋀-2. Ion scattering measurements show that the peak of the Fe ion distribution is ~95 nm below the sample surface, approximately Gaussian, with a width at half maximum amplitude of 100 nm. The intensity of a component in the EPR spectra of implanted samples with a width of 300 gauss and an approximately isotropic shape increases with increasing dose. The increase, proportional to dose for doses <⋀16 cm⋀-2, is larger for doses ≥10⋀16 cm⋀-2. In addition for doses ≥10⋀16 cm⋀-2, the component is orientation dependent. Subsequent heat treatments in air at 700° and 800° C alter the shape, intensity, and orientation dependence of the component. The spectral component in as-implanted samples is attributed to paramagnetic states of Fe ions. The threshold for magnetic exchange interactions and consequently long range magnetic ordering at ~300K occurs at a dose ≥10⋀16 cm⋀-2. Data on effects of thermal treatments on intensities and shapes are interpreted in terms of changing chemical composition of precipitating-particles.

Role of low and high spatial frequencies in the face-selective responses of neurons in the cortex in the superior temporal sulcus in the monkey

There are neurons in the cortex in the anterior part of the superior temporal sulcus of the macaque monkey with visual responses selective for faces. One aim of the present study was to analyze further the information which leads them to respond, by measuring their responses to parametrically filtered stimuli. The responses of 32 such single neurons were measured to faces which were digitized, lowpass filtered at spatial frequencies of 2, 4, 8, ... 128 cycles/face, highpass filtered at frequencies of 4, 8, ... 64 cycles/face, and presented in random sequence using a video framestore. It was found that many of the neurons could respond to blurred images of faces, with a mean frequency at half-maximum amplitude of the neuronal response to the series of lowpass filtered images of faces of 3.3 cycles/face. Almost all the neurons had lowpass cutoff frequencies defined in this way below 8 cycles/face. Many of the neurons could also respond to images of faces in which the only information remaining was a limited amount of high spatial frequency edge information. The mean frequency at half-maximum amplitude of the neuronal response to the series of highpass filtered images of faces was 29.7 cycles/face. Almost all the neurons had highpass cutoff frequencies above 8 cycles/face. Thus, many of the neurons could respond to a lowpass and a highpass filtered image of a face even when these had no spatial frequencies in common. The mean separation between the lowpass and highpass cutoff frequencies was 3.2 octaves. For comparison, face recognition in man can be performed with images which contain only information up to 8 cycles/face, or with highpass filtered images which contain only information down to 8 cycles/face. The response of the neurons was not always a smooth function of frequency, but could decrease as higher frequencies were included in the lowpass filtered images of faces, or as low frequencies were included in the highpass filtered images of faces. This indicates that information in certain frequency bands was able to inhibit these neurons. This was particularly likely to occur for the non-optimal face stimulus for a given neuron, indicating that the selectivity of these neurons to different faces was a combination of the excitation produced by some information in faces and inhibition produced by other.

AII amacrine cells quicken time course of rod signals in the cat retina.

1. In a perfused eyecup preparation, AII amacrine cells of the cat retina were penetrated with glass microelectrodes and their electrical responses to photic stimuli recorded. 2. Intracellular injections of the stains Procion, lucifer, or horseradish peroxidase revealed dendritic tree diameters of 30-80 micrometers (48 +/- 16 micrometers, mean +/0 SD) and cell body diameters from 7 to 12 micrometers (9 +/- 3 micrometers) for these cells. The dendrites were broadly stratified throughout the inner plexiform layer (IPL) but possessed large, terminal varicosities in the IPL and inner nuclear lyer (INL) proximal to the cell body. 3. The waveform of these cells in response to photic stimulation suggested division into four components: a) an initial rapid depolarization of the cell membrane followed by a slower decay toward the dark level; b) suppression of the dark noise of the cell; c) with dim or moderately intense stimuli, an off-hyperpolarization; d) in some cases a hyperpolarizing surround response. 4. The receptive fields of AII cells have been characterized using spatial stimuli consisting of long narrow slits. Curves have been fitted to spatial data using two space constants, one for the center mechanism and an opposing one for the surround. For the central mechanism, space constants ranged from 20 to 80 micrometers (46 +/- 22 micrometers), while for the surround they ranged from 60 to 130 micrometers (85 +/- 28 micrometers). The mean half-width of the center mechanism, calculated from the mean space constant, was about 0.25 degrees of visual angle (64 micrometers). The receptive-field properties of AII amacrine cells resemble those of center-depolarizing bipolar cells of other species. 5. Spectral studies of AII amacrine cells reveal that they are rod driven at all criterion voltage levels. Furthermore, adaptation of the rods by rod-saturating backgrounds eliminates 95% of the response amplitude of the AII amacrine cells. Under these conditions the tiny response component remaining is driven by the cat's long wavelength (556-nm peak) cones. 6. AII amacrine cells depolarize to rod stimulation more rapidly than other rod-dominated cells, such as rod bipolar cells, which hyperpolarize. For stimuli corresponding to about 10% of rod saturation, the latency to half-maximum amplitude is about 65 ms for AII cells, 40 ms faster than rod-dominated hyperpolarizing units. The leading edge of the response waveform for AII cells is also much more restricted in time. With the above stimulus it requires about 20 ms to increase from 25 to 75% of its peak, a period almost 4 times shorter than required by rod-dominated S-potential responses. With saturating stimuli the AII response requires only 5 ms to increase from 25 ot 75% of its peak. 7. Although prominent in the rod system, AII amacrine cells do not appear to be able to detect single quantum events. Threshold signals require the bleaching of about 200 rhodopsin molecules within a receptive field containing some 1,300 rods...

Changes in the intensity-response function of an insect's photoreceptors due to light adaptation

1. Intracellular recordings from the photoreceptors of the locustsLocusta migratoria andValanga irregularis and the flyCalliphora stygia, were used to examine the differences between theV/logI curve (intensity/response function) determined in the dark adapted and the light adapted state. 2. For response amplitudes less than half the maximum the fully light adaptedV/logI curve follows the simple self-shunting model, where the number of channels activated is proportional to the number of photons absorbed. 3. We fitted ourV/logI curves with the commonly employed hyperbolic function Eq. (1), and found a consistent deviation of the experimental data from the predicted curves when responses exceeded half maximal amplitude. 4. The light adaptedV/logI curves of locusts exhibit a ‘kink’ deformation (Fig. 3) and 4 of the 80 cells recorded from locusts had a small spike superimposed on the rising phase of the photo-response (Fig. 1). No kink or spike was observed in the flyCalliphora. 5. Transmembrane recording with a double electrode eliminated the ERG as a candidate for changing the slope and producing the kink or the spike. Stimulation of single ommatidia also ruled out interommatidial interactions. 6. Change in pulse duration did not affect the slope or the shape of theV/logI curves.

Comparison of the action of La3+ and Ca2+ on contraction threshold and other membrane parameters of frog skeletal muscle.

The influence of La3+ on contraction threshold, on membrane input resistance, and on action potential parameters was investigated in fibers of the sartorius muscle of the frog, and it was compared to that of Ca2+. The dependence of the contraction threshold on [La3+]0 in the presence of 0.5mM Ca3+ gave a sigmoid relationship between 0.1 and 5 mM La3+ with a shift of 23 to 34 mV to less negative potentials following a 10-fold increase of [La3+]0. The membrane input resistance was increased to various degrees in La-containing solutions, the increase being irreversible. The threshold of action potential generation was shifted to less negative potentials by 28 mV, and the duration at half-maximal amplitude was tripled by 0.5 mM La3+. In comparison a 10-fold increase of [Ca2+]0 in the range of 0.5 to 50 mM shifted the contraction threshold by 15 mV to less negative potentials. 17 mM Ca3+, a concentration having the same effect on contraction threshold as 0.5 mM La3+, increased membrane input resistance reversibly, shifted the action potential threshold by 16 mV to less negative potentials, and had only minor effects on action potential duration. Conduction was never blocked by Ca3+ as it was with 1 mM La3+. In the theoretical treatment, it is shown that the influence of Ca3+ on contraction threshold, but not that of La3+, may be accounted for by its screening and binding to negative surface charges according to the Gouy-Chapman theory of the diffuse double layer. To describe the action of La3+ on the contraction threshold an additional interaction of La3+ with neutral but amphoteric sites was considered.

Experimental density of states of liquid silicon

The K beta (3p to 1s) X-ray emission band has been recorded for liquid silicon. It provides a determination of the 3p partial density of states in the valence band. Comparisons are made with equivalent emissions from the crystalline and amorphous solid phases. It is found that the 3p distribution broadens by 1.7 eV in the liquid (full width at half maximum amplitude) compared to crystalline Si. Practically no evidence remains of the sp mixing observed in the bottom of the valence band of the crystalline and amorphous phases. The shape of the K beta emission band in the liquid closely resembles that expected of a simple metal, confirming the presence of four conduction electrons.

INTERMEDIATE PROCESSES IN PHOTOTRANSDUCTION: A STUDY IN DROSOPHILA MUTANTS

Abstract— A variety of procedures were used to modify the light response of Drosophila photoreceptors in order to find out if these manipulations produce effects that mimic some aspects of light adaptation. The ultimate goal of our approach is to use these experimental manipulations to dissect the different stages of the phototransduction process.The means which were used to modify the light response were as follows: (a) light adaptation of normal photoreceptors, (b) exposure of normal photoreceptors to various levels of CO2 (c) light adaptation of the trp Drosophila mutant in which the receptor potential decays to baseline during illumination, (d) exposure of the temperature sensitive norpAH52 mutant of Drosophila to elevated temperatures. Intensity‐response functions were obtained using intracellular and extracellular recordings. The maximal response amplitude (Vmax) was then plotted as a function of the light intensity (s̀) which evoked a response of half maximal amplitude. This plot was used to compare the effects of the procedures described above. Qualitatively all the manipulations had similar effects, they reduced Vmax and shifted s̀ to higher levels of light intensity, however, quantitatively the various effects were different.The finding that each experimental manipulation gave a different Vmax versus s̀ plot suggests that they affect different stages in phototransduction. We suggest that the slope of the Vmax versus a function can be interpreted in terms of an ordered cascade of events in phototransduction. This interpretation might give us a tool to distinguish between early and late stages in phototransduction.

The sensitivity of receptors in the posterior median eye of the nocturnal spider,Dinopis

1. Intracellular and extracellular (ERG) recordings were made of receptor responses in the posterior median eye of the nocturnal spiderDinopis subrufus. These are among the largest single lens eyes described among the arthropods and are used during prey capture at night. 2. The receptor potential is a depolarising wave, typical of the responses recorded from other spiders, and from rhabdomeric photoreceptors in general. By comparison with other spiders, the receptor responses are slow in time course. Large (4–12 mV) discrete potentials are seen at very low intensities. The evidence suggests that bumps are responses to single photons. If this is the case the quantum capture efficiency for receptors shown to be in their diurnal state of photoreceptive membrane depletion is 7%, measured relative to photons incident on the cornea. 3. Large responses to single photons and the known light gathering capacity of the lens (Blest and Land, 1977) make individual dark adapted photoreceptors very sensitive. A corneal flux of 5×105 photons·cm−2·s−1 produces an initial responses of half maximal amplitude. This corresponds to an intensity midway between starlight and moonlight under clear sky conditions. 2×105 more light is required to produce the same response in the diurnal jumping spiderPlexippus (Hardie and Duelli, 1978). 4. The spectral sensitivities of single photoreceptors are almost identical to that of the massed receptor response (ERG), and this indicates a retina dominated by photoreceptors containing a single 517 nm rhodopsin. UV sensitivity is depressed by selective attenuation in the lens matrix. 5. The mean receptor angular sensitivity function has a half-width of 2.3°, larger than expected from the optical data, but still sufficient for the resolution of spatial detail whose fineness matches the receptor mosaic. 6. ERG recordings fail to detect the sensitivity shifts that should be associated with the daily cycle of photoreceptive membrane (Blest, 1978). Perhaps such shifts may still exist because we find that our test lights block the synthetic phase of photoreceptive membrane turnover. 7. By comparison with vertebrate rods and cones,Dinopis photoreceptors exhibit the following properties of a scotopic system: large discrete responses to single photons, a slow time course of response, the possibility of electrical coupling between receptors and a spectral sensitivity in the blue/green. The resolving power ofDinopis retina is inferior to human rod vision at the same intensities, as expected of a smaller eye.

Excitation of Mytilus smooth muscle.

1. Membrane potentials and tension were recorded during nerve stimulation and direct stimulation of smooth muscle cells of the anterior byssus retractor muscle of Mytilus edulis L.2. The resting potential averaged 65 mV (range 55-72 mV).3. Junction potentials reached 25 mV and decayed to one half maximum amplitude in 500 msec. Spatial summation and facilitation of junction potentials were observed.4. Action potentials, 50 msec in duration and up to 50 mV in amplitude were fired at a membrane potential of 35-40 mV. No overshoot was observed.5. Contraction in response to neural stimulation was associated with spike discharge. Measurement of tension and depolarization in muscle bundles at high K(+) indicated that tension is only produced at membrane potentials similar to those achieved by spike discharge.6. Blocking of junction potentials, spike discharge and contraction by methantheline, an acetylcholine antagonist, supports the hypothesis that the muscle is excited by cholinergic nerves. However, evidence of a presynaptic action of methantheline complicates this argument.

Emission Spectra of ( Zn , Cd ) S Phosphors

Single emission bands of ‐type phosphors are represented as Gaussian curves and critically examined. The width (at half maximum amplitude) of the emission bands of almost all and phosphors activated by Cu or Ag and containing various coactivators is 0.36 ev. The colors of single emission bands, and of superpositions of the two main bands in case of copper activation, are represented in color diagrams. A red shift of the green copper emission band of phosphors with increasing high concentrations of activator and coactivator, as reported by Froelich for , Al, has also been observed in phosphors and with many different coactivators.