Summation

About: Summation is a research topic. Over the lifetime, 954 publications have been published within this topic receiving 45593 citations. The topic is also known as: summation & sum of a sequence.

Single photon signals in fly photoreceptors and first order interneurones at behavioral threshold.

Abstract: 1. The contrast sensitivity of the optomotor response of the fly Musca domestica was measured using a moving sinusoidal grating as the stimulus. In parallel experiments intracellular recordings were made from photoreceptors and first order visual interneurones to to determine their responses to the same threshold stimuli. Measurements of the spatial modulation transfer function for photoreceptors confirm that the optics of the eye were intact during recordings. 2. At the lowest intensity at which one can obtain an optomotor response, the photoreceptor signal is a train of discrete depolarizations, or bumps. With constant intensity stimuli, the temporal distribution of bumps followed the Poisson distribution with a mean rate of proportional to luminance. The mean bump rate at the threshold intensity for a behavioural response is 1.7 +/- 0.7 s-1 (mean +/- S.D., n = 25). 3. Calibrations and the statistical properties of the bump train indicate that a bump represents one effective photon, implying that the bump : photon ratios are quantum capture efficiencies. 4. At low intensities the first order interneurones (the large monopolar cells or LMCs) show hyperpolarizing bumps each triggered by a receptor bump. Using a point source stimulus, centred in the field of view, the LMC bump rate is six times that in a single receptor viewing the same stimulus, as expected from the known projection of six receptor axons to each LMC. When using an extended stimulus (the grating), the bump rate is 18-20 times that in receptors. Comparison with earlier work suggests that this increased lateral summation of receptor inputs to LMCs only occurs at very low intensities. 5. In both receptor and LMCs the amplitudes and wave forms of bumps depend upon the position of a point source stimulus within the field of view. With the light in the periphery of the field the bumps are smaller and slower than when the light is in the centre. This difference in response suggests that spatial stimulation is brought about by lateral interactions, possibly between receptors. 6. At higher mean intensities the signal-to-noise ratios in receptors responding to the appropriate threshold stimuli increase with intensity. This is suggestive of a decrease in the extent of spatial and/or temporal summation in the optomotor pathway.

Dorsal horn convergent neurones: negative feedback triggered by spatial summation of nociceptive afferents

Abstract: In order to investigate the effects of spatial summation on the spinal transmission of nociceptive information, we compared in intact and spinal anaesthetized rats, responses of lumbar convergent neurones elicited by noxious heat stimuli applied to areas of the body much greater in size than their individual excitatory receptive fields, located distally on the hindpaw. Twenty-four neurones were recorded in each group of animals. For each neurone, 4 successive immersions of increasing areas (1.9–18 cm2) of the ipsilateral hindpaw in a 48°C water bath (15-sec duration) were performed with 10-min intervals in a randomized and balanced order. In intact animals, the responses of convergent neurones progressively decreased when the area of noxious thermal stimulation reached and then exceeded approximately twice the area of their individual excitatory receptive fields. This decrease was highly significant for 18 cm2 which represents approximately 10-fold the mean of the receptive field areas. Such a phenomenon was not observed for neurones recorded in spinal animals although their excitatory receptive field areas were not significantly different. These results suggest that the activation of a large population of nociceptive afferents triggers supraspinally mediated negative feed-back loop modulating the responses of convergent neurones.

Temporal Summation Phenomena at Threshold: Their Relation to Visual Mechanisms

Abstract: Threshold energies were determined for brief flashes as a function of their duration in order to determine the maximum duration for which the flash intensity and duration could be varied reciprocally without affecting detectability (the Bunsen-Roscoe effect). A pair of threshold-level flashes for which reciprocity obtained in the determination of threshold were shown to be discriminable from each other at several imperfectly detectable energy levels. Thus equal detectability of flashes of equal energy does not imply identical neural responses to such stimuli. It is suggested that the summation reflects primarily the operation of the detection mechanism rather than of the peripheral visual mechanism. Some general implications for the interpretation of threshold measures are also discussed.

Integration of Asynchronously Released Quanta Prolongs the Postsynaptic Spike Window

Abstract: Classically, the release of glutamate in response to a presynaptic action potential causes a brief increase in postsynaptic excitability. Previous reports indicate that at some central synapses, a single action potential can elicit multiple, asynchronous release events. This raises the possibility that the temporal dynamics of neurotransmitter release may determine the duration of altered postsynaptic excitability. In response to physiological challenges, the magnocellular neurosecretory cells (MNCs) in the paraventricular nucleus of the hypothalamus (PVN) exhibit robust and prolonged increases in neuronal activity. Although the postsynaptic conductances that may facilitate this form of activity have been investigated thoroughly, the role of presynaptic release has been largely overlooked. Because the specific patterns of activity generated by MNCs require the activation of excitatory synaptic inputs, we sought to characterize the release dynamics at these synapses and determine whether they contribute to prolonged excitability in these cells. We obtained whole-cell recordings from MNCs in brain slices of postnatal day 21-44 rats. Stimulation of glutamatergic inputs elicited large and prolonged postsynaptic events that resulted from the summation of multiple, asynchronously released quanta. Asynchronous release was selectively inhibited by the slow calcium buffer EGTA-AM and potentiated by brief high-frequency stimulus trains. These trains caused a prolonged increase in postsynaptic spike activity that could also be eliminated by EGTA-AM. Our results demonstrate that glutamatergic terminals in PVN exhibit asynchronous release, which is important in generating large postsynaptic depolarizations and prolonged spiking in response to brief, high-frequency bursts of presynaptic activity.

Vibrotactile threshold in young and old observers: The effects of spatial summation and the presence of a rigid surround

Abstract: Human psychophysical detection thresholds for ten frequencies of sinusoidal vibration ranging from 10 to 400 Hz were obtained on the left index fingertip and thenar eminence of young and older observers using a three‐alternative forced‐choice tracking procedure. The first experiment utilized a 7‐mm (0.38 cm2) contactor and rigid surround with 1‐mm gap. In the second experiment, three contactor sizes (1.6‐, 7.0‐, and 25.4‐mm diameter) and two surround configurations (1‐mm gap between contactor and surround, and no surround) were used. The results indicate that, although the shapes of the threshold versus frequency functions were similar in the two age groups, there was a reduction in sensitivity for the older group at all frequencies. Furthermore, taking into account the difference in sensitivity between the two age groups, spatial summation and the effects of a surround did not seem to differ between the two groups. These results are discussed in the context of physiological models of cutaneous sensitivity and changes in receptor function with age.