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.

Mechanisms of intracortical I-wave facilitation elicited with paired-pulse magnetic stimulation in humans.

Abstract: In order to elucidate the mechanisms underlying intracortical I-wave facilitation elicited by paired-pulse magnetic stimulation, we compared intracortical facilitation of I1-waves with that of I3-waves using single motor unit and surface electromyographic (EMG) recordings from the first dorsal interosseous muscle (FDI) We used a suprathreshold first stimulus (S1) and a subthreshold second stimulus (S2) In most experiments, both stimuli induced currents in the same direction In others, S1 induced posteriorly directed currents and S2 induced anteriorly directed currents When both stimuli induced anteriorly directed currents (I1-wave effects), an interstimulus interval (ISI) of 15 ms resulted in extra facilitation of the responses to S1 alone The latency of this effect was equivalent to that of the I2-wave from S1 When S1 evoked posteriorly directed currents (I3-wave recruitment), facilitation occurred at a latency corresponding to the I3-wave from S1 This facilitation occurred at an ISI of 15 ms when both S1 and S2 flowed posteriorly, and at an ISI of approximately 35 ms when S1 was posteriorly and S2 was anteriorly directed Based on these findings, we propose the following mechanisms for intracortical I-wave facilitation When S1 and S2 induce currents in the same direction, facilitation is produced by summation between excitatory postsynaptic potentials (EPSPs) elicited by S1 and subliminal depolarization of interneurones elicited by S2 directly When S1 and S2 induce currents in the opposite direction, facilitation is produced by the same mechanism as above or by temporal and spatial summation of EPSPs elicited by two successive stimuli at interneurones or corticospinal neurones of the motor cortex

Membrane Properties Underlying the Firing of Neurons in the Avian

Abstract: Neurons of the avian nucleus magnocellularis (NM) relay auditory information from the Vlllth nerve to other parts of the auditory system To examine the cellular properties that permit NM neurons to transmit reliably the temporal characteristics of the acoustic stimulus, we performed wholecell recordings in neurons of the chick NM using an in vitro thin slice preparation NM neurons exhibited strong outward rectification near resting potential; the voltage responses to depolarizing current steps were substantially smaller than to equivalent hyperpolarizing steps Suprathreshold current steps evoked only a single action potential at the start of the step In contrast, stimulation with trains of brief current pulses evoked repetitive firing that was phase-locked to the stimulus cycle The number of action potentials evoked by the pulses during the train decreased with increasing stimulus rate Voltage-clamp experiments revealed a rapidly activating, slowly inactivating, outward current with a threshold near -65 mV During depolarizing voltage steps, the outward current rose sigmoidally to a peak and then decayed slowly, reaching steady state within 5 sec Application of 200 PM 4-aminopyridine (4-AP) reduced the peak of the outward current by 64%, leaving a small, persistent component Under current clamp, application of 200 @A 4-AP reduced the outward rectification and increased the amplitude and duration of the action potentials Moreover, NM neurons could no longer sustain firing during high rates of stimulation with the current pulses: increased temporal summation of the potentials caused sufficient depolarization to inactivate the sodium conductance underlying the action potential These results suggest that the outward current is necessary for NM neurons to transmit well-timed events reliably for the dura

Distinct FGFs promote differentiation of excitatory and inhibitory synapses

Abstract: The differential formation of excitatory (glutamate-mediated) and inhibitory (GABA-mediated) synapses is a critical step for the proper functioning of the brain. An imbalance in these synapses may lead to various neurological disorders such as autism, schizophrenia, Tourette's syndrome and epilepsy. Synapses are formed through communication between the appropriate synaptic partners. However, the molecular mechanisms that mediate the formation of specific synaptic types are not known. Here we show that two members of the fibroblast growth factor (FGF) family, FGF22 and FGF7, promote the organization of excitatory and inhibitory presynaptic terminals, respectively, as target-derived presynaptic organizers. FGF22 and FGF7 are expressed by CA3 pyramidal neurons in the hippocampus. The differentiation of excitatory or inhibitory nerve terminals on dendrites of CA3 pyramidal neurons is specifically impaired in mutants lacking FGF22 or FGF7. These presynaptic defects are rescued by postsynaptic expression of the appropriate FGF. FGF22-deficient mice are resistant to epileptic seizures, and FGF7-deficient mice are prone to them, as expected from the alterations in excitatory/inhibitory balance. Differential effects of FGF22 and FGF7 involve both their distinct synaptic localizations and their use of different signalling pathways. These results demonstrate that specific FGFs act as target-derived presynaptic organizers and help to organize specific presynaptic terminals in the mammalian brain.

Responses of human cutaneous afferents to CO2 laser stimuli causing pain.

Abstract: Microelectroneurographic studies in man allow the comparison of stimulus induced activity in the single peripheral nerve unit with the subject's ratings of sensation. Relationships between stimulus intensity, single unit discharges, and pain ratings were investigated using a CO2 laser stimulator which delivers radiant heat pulses of 50 ms duration. Recordings were performed percutaneously from the radial nerve at the wrist. Receptor types were identified by their response to different stimulus modalities and by their reaction delay to electrical test stimuli within the receptive field. Receptive fields of identified units were stimulated with randomised series of different radiant heat intensities between half and double the individual pain threshold (5 to 20 W; stimulation area 64 mm2). The largest receptor class observed to be activated by CO2 laser stimuli were polymodal C-nociceptors. None of them was spontaneously active. High discharge rates up to 75/s were not necessarily associated with pain but, if pain was felt, the impulse trains usually lasted for more than 60 ms. Inter-spike intervals were distributed over a wide range between 8 and 145 ms with a peak at about 25 ms. This peak was only slightly shifted by increasing the stimulus intensity. Higher correlations were found between the number of spikes and stimulus intensity. Measures of Signal Detection Theory indicated that the single unit discharges discriminated stimulus intensities better than the subjects' ratings. These findings underline the importance of temporal summation in the processing of C-fibre input with a considerable loss of information in the nociceptive system.