Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale
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The purpose of this study was to examine the effect of fixation target distance on the human vestibuloocular reflex (VOR) during eccentric rotation in pitch. Such rotation induces both angular and linear acceleration. Eight normal subjects viewed earth-fixed targets that were either remote or near to the eyes during whole-body rotation about an earth-horizontal axis that was either oculocentric or 15 cm posterior (eccentric) to the eyes. ⋯ For the 3-m target distance, vergence during the VOR was stable over each entire trial but slightly exceeded the ideal value. For the 0.1-m near target, instantaneous vergence during the VOR typically declined gradually in a manner not corresponding to the time course of instantaneous VOR gain change; mean vergence over entire trials ranged from 60 to 90% of ideal, corresponding to target distances for which ideal gain would be much higher than actually observed. These findings suggest a dissociation between vergence and VOR gain during eccentric rotation with near targets in the frequency range from 0.8 to 2.0 Hz.
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In cats and monkeys, extrastriate visual areas that have been reported to be involved in the near triad of pupilloconstriction, convergence, and accommodation have well-defined projections to the pretectal olivary nucleus (PON), the retinorecipient pretectal nucleus mediating the pupillary light reflex in mammals. We have therefore used alert, behaving primates to investigate the possibility that PON neurons are involved in the pupillary near response in addition to the pupillary light reflex. ⋯ Thus the behavior of PON luminance neurons is appropriate for their participation in the pupillary light reflex, but is inappropriate for any proposed role in the pupillary near response. This result strongly suggests that neurons in the primate PON are solely related to the pupillary light reflex and that the cortical projections to this pretectal nucleus are related to this reflex and do not play a role in the pupillary near response.
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We have investigated the effects of prior activation of afferent inputs by a train of low-frequency stimulation (LFS) on the induction of long-term potentiation (LTP) induced by high-frequency stimulation (tetanus, 100 Hz, 100 pulses) in CA1 neurons of guinea pig hippocampal slices. The parameters of the LFS were altered systematically: the frequency (1 or 5 Hz); the number of pulses (80, 200 or 1000); and the time lag from the LFS to the tetanus (20, 60 or 100 min). Conditioning effects of the LFS on the induction of LTP were evaluated in terms of the slope of the field excitatory postsynaptic potential (S-EPSP) and the amplitude of the population spike (A-PS). ⋯ In contrast, the attempt to induce LTP 60 min after LFS of 200 or 1000 pulses at 1 Hz resulted only in short-term potentiation while the LFS itself produced no significant change in the responses. The suppressive effect on LTP was significantly reduced for 1 Hz LFS with a smaller number of pulses (80 pulses), or a shorter (20 min) or longer (100 min) time lag from the LFS to the tetanus, or with LFS at a higher frequency (5 Hz). When the LFS of 1000 pulses at 1 Hz was delivered in the presence of the N-methyl-D-aspartate (NMDA) receptor antagonist AP5 (D,L-4-amino-5-phosphonovalerate, 50 microM), which was washed out after the end of the LFS, the tetanus given 60 min after application of the LFS produced stable LTP, indicating the involvement of NMDA receptor/channels in the mechanisms of this particular form of synaptic plasticity-long-term suppression of LTP.
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Unmyelinated cutaneous mechano-heat fibers (CMH) in the peroneal nerve of healthy human volunteers were studied by means of a "marking" technique which allows stable recordings from identified single units over extended periods. Mechanoreceptive field sizes were 105 +/- 13 mm2 in 25 units. These large receptive fields indicate extensive terminal branching of C fibers in the skin of foot and lower leg. ⋯ In contrast, heat thresholds in the nontreated parts did not change significantly (42.1 +/- 3.4 degrees C vs 41.2 +/- 3.9 degrees C), i.e. "secondary sensitization" to heat was lacking. The absence of primary sensitization to probing with von Frey hairs indicates that sensitization of insensitive C fibers and recruitment of insensitive axon collaterals may be more important for mechanical hyperalgesia than sensitization of conventional CMH units-apart from the contribution of central mechanisms. The lack of spread of sensitization to untreated parts of the receptive fields o CMHs ("secondary sensitization") indicates that this fiber group is probably not involved in any form of secondary hyperalgesia to heating.
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Membrane potential changes and/or discharges from 36 inspiratory neurons were recorded intracellularly in the dorsal respiratory group (DRG; i.e., the ventrolateral subdivision of the nucleus tractus solitarii) in decerebrate, paralyzed, and ventilated cats. Electrical activities were recorded from both somata (n = 10) and axons (n = 26). Activities during quiet breathing were compared with those observed during fictive coughing and swallowing evoked by repetitive electrical stimulation of afferent fibers of the superior laryngeal nerve (SLN). ⋯ The membrane potentials of nine somata exhibited a brief bell-shaped depolarization during swallowing, the amplitude of which was similar to that observed during inspiration. These results suggest that some inspiratory premotoneurons and propriobulbar neurons of the DRG might be involved in nonrespiratory motor activities, even if clearly antagonistic to breathing (e.g., swallowing). We postulate the existence in the medulla oblongata of adult mammals of neurons exhibiting a "functional flexibility".