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.

Temporal Summation of C-Fiber Afferent Inputs: Competition Between Facilitatory and Inhibitory Effects on C-Fiber Reflex in the Rat

Abstract: Gozariu, Manuela, Dominique Bragard, Jean-Claude Willer, and Daniel Le Bars. Temporal summation of C-fiber afferent inputs: competition between facilitatory and inhibitory effects on C-fiber reflex...

Spatial summation for pain perception: interaction of inhibitory and excitatory mechanisms.

Abstract: To study the relation between size of the surface stimulated and perceived pain intensity (spatial summation effect), subjects sequentially immersed predetermined segments of the surface of their arm, between the fingertips and the shoulder, in circulating nociceptive hot water. Immersion sessions were of three types: (i) increasing session (immersion beginning at fingertips and increasing to shoulder); (ii) decreasing session (immersion beginning at shoulder and decreasing to fingertips); and (iii) whole arm+increasing session (preliminary immersion of the whole arm up to shoulder, followed by an increasing session from fingertips to shoulder). Results showed a positive spatial summation effect (pain perception positively correlated to the size of the surface stimulated) during both the decreasing session and the whole arm+increasing session. However, no spatial summation effect was found during the increasing session (fingertips to shoulder). In addition, pain perceived for a surface area was less intense during the decreasing session compared to the increasing session. One possible explanation for the lack of a spatial summation effect during the increasing session is that inhibitory mechanisms are gradually recruited at the same time as excitatory afferences, thus 'cancelling out' any measurable spatial summation effect. The results obtained during the decreasing session and the whole arm+increasing session may be explained by inhibitory mechanisms being fully recruited at the beginning of the session with the immersion of the largest surface area (whole arm). The results are a shift of the pain perception curve and a positive relation between the surface stimulated and pain perception.

Spatial summation in lateral geniculate nucleus and visual cortex.

Abstract: We have compared the spatial summation characteristics of cells in the primary visual cortex with those of cells in the dorsal lateral geniculate nucleus (LGN) that provide the input to the cortex. We explored the influence of varying the diameter of a patch of grating centred over the receptive field and quantitatively determined the optimal summation diameter and the degree of surround suppression for cells at both levels of the visual system using the same stimulus parameters. The mean optimal summation size for LGN cells (0.90°) was much smaller than that of cortical cells (3.58°). Virtually all LGN cells exhibited strong surround suppression with a mean value of 74%±1.61% SEM for the population as a whole. This potent surround suppression in the cells providing the input to the cortex suggests that cortical cells must integrate their much larger summation fields from the low firing rates associated with the suppression plateau of the LGN cell responses. Our data suggest that the strongest input to cortical cells will arise from geniculate cells representing areas of visual space located at the borders of a visual stimulus. We suggest that analysis of response properties by patterns centred over the receptive fields of cells may give a misleading impression of the process of the representation. Analysis of pattern terminations or salient borders over the receptive field may provide much more insight into the processing algorithms involved in stimulus representation.

Characteristics of postsynaptic potentials produced in single human motoneurons by homonymous group 1 volleys.

Abstract: Changes in the firing probability of voluntarily activated human tibialis anterior motor units occurred when the muscle nerve was stimulated below the threshold of motoneuron axons. A prominent period of increased firing probability was considered to result from the group 1 composite EPSP. The rise time of this composite EPSP, estimated from the duration of the period of increased firing probability, was between 2 and 5 ms. One estimate of the relative amplitude of the EPSP was derived from the cross-correlation and tested by inserting synthetic EPSPs into a computer simulation of a rhythmically discharging neuron. A second estimate of the amplitude was obtained by delivering stimuli with various delays within the interspike interval to determine the position (as a percentage of the mean interspike interval) at which the postsynaptic potential was capable of bringing the membrane potential to threshold. The two estimates were in reasonable agreement. The largest of these EPSPs could bring the motoneuron to threshold after 39% of the elapsed interspike interval. The falling phase of the postsynaptic potential was explored by delivering double stimuli within the interspike interval to produce temporal summation. The duration of the falling phase, so tested, was between 5 and 20 ms. An attempt was made to deduce the general form of the trajectory representing the effective distance of an EPSP from threshold during the interspike interval from the alterations in the cross-correlation profile that occurred when motoneuron firing rate was voluntarily altered.

Spatial summation of heat induced pain within and between dermatomes

Abstract: The aim was to study spatial summation within and between ipsi- and contralateral dermatomes at different painful temperatures. For heat stimulation we used a computer controlled thermofoil based thermode. The thermode area could be varied in five discrete steps from 3.14 to 15.70 cm2. When we applied the stimuli within a dermatome, the mean heat pain threshold decreased significantly from 45.6 to 43.5 C as the area was increased from minimum (3.14 cm2) to maximum (15.70 cm2). When the areas were increased involving different dermatomes (both ipsi- or contralateral), we found similar decreases in pain threshold. Spatial summation was also found within and between dermatomes at supra-threshold temperatures (46, 48, 50 C).The study shows that spatial summation of pain is most likely a mechanism acting across segments and is existing from pain threshold to tolerance.