Showing papers on "Summation published in 1988"

Long-term facilitation in Aplysia involves increase in transmitter release

Abstract: In a variety of vertebrates and invertebrates, long-lasting enhancement of synaptic transmission contributes to the storage of memory lasting one or more days. However, it has not been demonstrated directly whether this increase in synaptic transmission is caused by an enhancement of transmitter release or an increase in the sensitivity of the postsynaptic receptors. These possibilities can be distinguished by a quantal analysis in which the size of the miniature excitatory postsynaptic potential released spontaneously from the presynaptic terminal is used as a reference. By means of microcultures, in which single sensory and motor neurons of Aplysia were plated together, miniature excitatory postsynaptic potentials attributable to the spontaneous release of single transmitter quanta from individual presynaptic neurons were recorded and used to analyze long-term facilitation induced by repeated applications of 5-hydroxytryptamine. The results indicate that the facilitation is caused by an increase in the number of transmitter quanta released by the presynaptic neuron.

Cutaneous pain and detection thresholds to short CO2 laser pulses in humans: evidence on afferent mechanisms and the influence of varying stimulus conditions.

Abstract: Summary Pam and detection thresholds to short COa laser pulses were studied in healthy human subjects. Pam thresholds were significantly higher than detection thresholds in both hairy and glabrous skin; in the glabrous skin both thresholds were higher in the hairy skin. The range from detection threshold to pain threshold was larger in the glabrous skin. The minimal energy per surface area needed to produce any sensation (detection) or pain sensation decreased with increasing stimulus surface, and this spatial summation effect was to equaf magnitude in the hairy and the glabrous skin. With decreasing stimulus pulse duration (from 45 to 15 msec) the detection and pain thresholds were elevated: this effect was stronger on pain thresholds. With increasing adapting skin temperature, less energy was needed to produce any sensation (detection) or pain sensation. The effect of adapting skin temperature was equai on pain and detection thresholds. The conduction veiocity of fibers mediating laser evoked first sensations was in the thin fiber range { < 30 msec), according to a reaction time study. The rest&s suggest that short CO, laser pulses produce both non-pain and pain sensations, but that both these sensations are based on the activation of the same primary afferent fiber population of slowly conducting nociceptive fibers. Central summation of primary afferent impulses is needed to elicit a liminal non-painful sensation, and an increased number of impulses in the same fibers produces pain.

Nonlinear measurement and classification of receptive fields in cat retinal ganglion cells.

Abstract: Originally, modeling of ganglion-cell responses in cat was based mainly on linear analysis. This is satisfactory for those cells in which spatial summation of excitation is approximately linear (X-cells) but it fails for Y-cells, where summation has strong nonlinear components. Others have shown the utility of using sinusoidal analysis to study harmonic and intermodulation nonlinearities in the temporal frequency domain. We have used Wiener-kernel analysis to obtain directly both temporal and spatial impulse responses and their nonlinear interactions. From these, we were able to predict accurately the responses that a counterphase modulated grating elicited in both X-cells and Y-cells. In addition, we show that the first-order responses can measure the two-dimensional spatial features of the receptive field with high resolution. Thus, nonlinear analysis of responses to white-noise stimuli may be sufficient to both classify and measure the receptive fields of many different types of ganglion cells.

Facilitation and depression in the responses of spinal Renshaw cells to random stimulation of motor axons

Abstract: 1. We investigated the responses of cat lumbosacral Renshaw cells to pseudo-Poison stimulus sequences (of three different mean rates) delivered to motor axons in ventral roots or various muscle nerves. The Renshaw cell responses were evaluated by computation of peristimulus time histograms (PSTHs). 2. PSTHs computed with respect to all the stimuli showed, before the reference time, near-constant bin contents corresponding to the mean firing probability (rate), and an initial excitatory component (increase in discharge probability) after the reference time, followed by a small but longer-lasting reduction of firing rate. These two response components were strongly correlated linearly. It is suggested that the postexcitatory rate reduction is predominantly due to afterhyperpolarization. 3. In general, Renshaw cell responses to any stimulus in a stimulus train depended upon the stimulation history. In the averaged record, the response to the second of a pair of stimuli was affected by the first stimulus independently of intervening (random) stimuli. Very often, the second response showed a long-lasting depression (from 25 to greater than 250 ms). In a number of cases a briefer facilitating effect preceded the depression. 4. These conditioning effects were largely homosynaptic, i.e., confined to the particular input channel that was stimulated. This was shown by stimulating two different nerves (or nerve branches) with independent random patterns of similar mean rates and determining the cross-conditioning exerted by one input channel on the excitatory effects of the other. At small intervals between conditioning and test stimuli of some tens of milliseconds, a facilitatory effect could often be seen, which almost certainly reflected spatial summation. However, the subsequent depressant effect was largely accounted for by the postexcitatory rate reduction consequent to the conditioning stimulus in the parallel channel. Autoconditioning was still present. 5. The amount of facilitation and depression as well as their balance depended on the average Renshaw cell response. This in turn depended, at each mean stimulus rate, on the strength of synaptic coupling between an input channel and the cell, and on the mean stimulus rate, declining with an increase in mean rate. That is, the facilitation increased and the depression decreased with decreasing synaptic coupling and increasing mean stimulus rate. 6. Several factors may contribute to facilitation and depression; these are discussed with respect to their relative quantitative significance.(ABSTRACT TRUNCATED AT 400 WORDS)

Membrane properties of nociceptive neurones in lamina II of lumbar spinal cord in the cat.

Abstract: 1. Intracellular recordings were made from neurones in lamina II of the dorsal horn of the lumbar spinal cord of the cat, and the electrotonic responses to brief rectangular current pulses of up to 0.5 nA passed through the recording microelectrode measured. 2. The majority of penetrations were associated with input resistances lower than 70 M omega, low resting potentials (-25 to -45 mV) and frequent firing of action potentials. Stable resting potentials of -50 to -75 mV were recorded in twenty neurones which exhibited continuous ongoing synaptic activity without action potentials. The threshold for action potential initiation was around -42 mV. The current-voltage relationships were linear over most of the range of currents used; with depolarizing currents rectification became apparent close to the firing threshold. Input resistances ranged from 80 to 150 M omega. 3. The time course of the decay of the electrotonic response was exponential with a time constant of 0.8-2.0 ms. The morphology of the cells--small soma with a small number of fine processes--and these short time constants suggest that axial current flow is limited and that the charge is dissipated locally within the soma through the membrane capacitance. 4. Effective membrane capacities were calculated from the estimated soma surface area of typical neurones in lamina II stained with HRP, and assuming a specific membrane conductance of 1.0 microF cm-2 they ranged from 3.1 to 15.7 pF. Membrane capacities were calculated for the twenty neurones in this study from measurement of input resistance and time constants (6.4-15.0 pF) and lay within this range. 5. Three neurones which had their electrical properties measured were also stained with horseradish peroxidase (HRP). Their specific membrane capacitances (1.1-1.2 microF cm-2) and specific resistances (0.9-1.1 k omega cm2) were within the range of values measured for other neurones in the CNS. 6. The short time constants found for these neurones suggests that temporal summation of postsynaptic potentials evoked by short-acting neurotransmitters will be limited. This may help to explain why action potentials arise singly from discrete, short-lived EPSPs. There is anatomical evidence for multiple connections from terminal branches of A delta and C afferent fibres within the superficial dorsal horn; this suggests that spatial summation of EPSPs is a major factor in synaptic integration of some of the primary afferent inputs to these neurones.

Temporal summation in hearing-impaired listeners.

Abstract: A study was conducted to explore variations in auditory temporal summation in listeners with normal hearing, and impairment due to otosclerosis, sensori-neural hearing loss and acoustic neuroma. Using a two-interval forced-choice procedure the detection threshold was measured for one-third octave noise bands centered at either 1000 or 4000 Hz, in combination with eight signal durations (2.5, 5, 10, 20, 40, 80, 160, and 640 ms). The results indicated that for normal listeners: (1) the slope of the function relating the detection threshold and the signal duration varied inversely with the frequency tested, and (2) the variability in the detection threshold was greater for 4000 Hz than for 1000 Hz. A comparison of performance across groups showed that the magnitude of the slope of the temporal integration function decreased as the site of lesion moved from middle ear to eighth nerve. For listeners with normal hearing and those with otosclerosis, temporal integration appeared to be incomplete at 640 ms.