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Cenciarini, Massimo; Peterka, Robert J.
doi: 10.1152/jn.00856.2004pmid: 16467429
Humans maintain stable stance in a wide variety of environments. This robust behavior is thought to involve sensory reweighting whereby the nervous system adjusts the relative contribution of sensory sources used to control stance depending on environmental conditions. Based on prior experimental and modeling results, we developed a specific quantitative representation of a sensory reweighting hypothesis that predicts that a given reduction in the contribution from one sensory system will be accompanied by a corresponding increase in the contribution from different sensory systems. The goal of this study was to test this sensory-reweighting hypothesis using measures that quantitatively assess the relative contributions of the proprioceptive and graviceptive (vestibular) systems to postural control during eyes-closed stance in different test conditions. Medial/lateral body sway was evoked by side-to-side rotation of the support surface (SS) while simultaneously delivering a pulsed galvanic vestibular stimulus (GVS) through electrodes behind the ears. A model-based interpretation of sway evoked by SS rotations provided estimates of the proprioceptive weighting factor, W p , and showed that W p declined with increasing SS amplitude. If the sensory-reweighting hypothesis is true, then the decline in W p should be accompanied by a corresponding increase in W g , the graviceptive weighting factor, and responses to the GVS should increase in proportion to the value of W g derived from responses to SS rotations. Results were consistent with the predictions of the proposed sensory-reweighting hypothesis. GVS-evoked sway increased with increasing SS amplitude, and W g measures derived from responses to GVS and from responses to SS rotations were highly correlated. Address for reprint requests and other correspondence: R. J. Peterka, Neurological Sciences Institute, OHSU West Campus, 505 NW 185th Ave., Beaverton, OR 97006, USA (E-mail: [email protected] )
Heiser, Laura M.; Colby, Carol L.
doi: 10.1152/jn.00054.2005pmid: 16291805
We explore the world around us by making rapid eye movements to objects of interest. Remarkably, these eye movements go unnoticed, and we perceive the world as stable. Spatial updating is one of the neural mechanisms that contributes to this perception of spatial constancy. Previous studies in macaque lateral intraparietal cortex (area LIP) have shown that individual neurons update, or "remap," the locations of salient visual stimuli at the time of an eye movement. The existence of remapping implies that neurons have access to visual information from regions far beyond the classically defined receptive field. We hypothesized that neurons have access to information located anywhere in the visual field. We tested this by recording the activity of LIP neurons while systematically varying the direction in which a stimulus location must be updated. Our primary finding is that individual neurons remap stimulus traces in multiple directions, indicating that LIP neurons have access to information throughout the visual field. At the population level, stimulus traces are updated in conjunction with all saccade directions, even when we consider direction as a function of receptive field location. These results show that spatial updating in LIP is effectively independent of saccade direction. Our findings support the hypothesis that the activity of LIP neurons contributes to the maintenance of spatial constancy throughout the visual field. Address for reprint requests and other correspondence: C. L. Colby, 115 Mellon Inst., 4400 5th Ave., Pittsburgh, PA 15213 (E-mail: [email protected] )
Sanada, Takahisa M.; Ohzawa, Izumi
doi: 10.1152/jn.00955.2005pmid: 16394073
How are surface orientations of three-dimensional objects and scenes represented in the visual system? We have examined an idea that these surface orientations are encoded by neurons with a variety of tilts in their binocular receptive field (RF) structure. To examine whether neurons in the early visual areas are capable of encoding surface orientations, we have recorded from single neurons extracellularly in areas 17 and 18 of the cat using standard electrophysiological methods. Binocular RF structures are obtained using a binocular version of the reverse correlation technique. About 30% of binocularly responsive neurons have RFs with statistically significant tilts from the frontoparallel plane. The degree of tilts is sufficient for representing the range of surface slants found in typical visual environments. For a subset of neurons having significant RF tilts, the degrees of tilt are correlated with the preferred spatial frequency difference between the two eyes, indicating that a modified disparity energy model can account for the selectivity, at least partially. However, not all cases could be explained by this model, suggesting that multiple mechanisms may be responsible. Therefore an alternative hypothesis is also examined, where the tilt is generated by pooling of multiple disparity detectors whose preferred disparities progressively shift over space. Although there is evidence for extensive spatial pooling, this hypothesis was not satisfactory either, in that the neurons with extensive pooling tended to prefer an untilted surface. Our results suggest that encoding of surface orientations may begin with the binocular neurons in the early visual cortex. Address for reprint requests and other correspondence: I. Ohzawa, Graduate School of Frontier Biosciences and School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531 Japan (E-mail: [email protected] )
doi: 10.1152/jn.00685.2005pmid: 16452261
The purpose of this study was to investigate the occurrence of motor unit doublet discharges in young and older individuals at different rates of increasing force. Participants included eight young (21.9 ± 3.56 yr) and eight older (74.1 ± 8.79 yr) individuals, with equal numbers of males and females in each group. Motor unit activity was recorded from the tibialis anterior during isometric dorsiflexion using a four-wire needle electrode. Subjects performed three ramp contractions from zero to 50% maximal voluntary contraction (MVC) force at each of three rates: 10, 30, and 50% MVC/s. Overall, the occurrence of doublets was significantly higher in the young than in the older individuals. However, neither group showed differences in the occurrence of doublets across the three rates of force production. Doublet firings were observed in 45.6 (young) and 35.1% (old) of motor units at 10% MVC/s; 48.6 (young) and 22.5% (old) of motor units at 30% MVC/s; and 48.4 (young) and 31.4% (old) at 50% MVC/s. The maximal firing rate was significantly higher and the force at which the motor units were recruited was significantly lower for those units that fired doublets than those that did not. The force at which doublets occurred ranged from 3.42 to 50% MVC in the young subjects and from 0 (force onset) to 50% MVC in the older subjects. The results of this study suggest that the occurrence of doublets is dependent on both motor unit firing rate and force level. The lower incidence of doublets in older individuals may be attributable to changes in the intrinsic properties of the motoneurons with aging, which appear to play a role in doublet discharges. Address for reprint requests and other correspondence: G. Kamen, Dept. of Exercise Science, Univ. of Massachusetts at Amherst, Totman Bldg., Amherst, MA 01003-9258 (E-mail: [email protected] )
doi: 10.1152/jn.01122.2005pmid: 16452257
Functionally, γ-aminobutyric acid receptor (GABAR)–mediated inhibition can be classified as phasic (synaptic) and tonic (extrasynaptic). The GABARs underlying tonic inhibition assemble from subunits different from those responsible for phasic inhibition. We wanted to assess the excitability of hippocampal pyramidal cell (PC) networks following a selective impairment of tonic inhibition. This is difficult to accomplish by pharmacological means. Because the GABAR α5 subunits mostly mediate the tonic inhibition in CA1 and CA3 PCs, we quantified changes in tonic inhibition and examined network excitability in slices of adult gabra5 –/– mice. In gabra5 –/– CA1 and CA3 PCs tonic inhibitory currents were 60 and 53%, respectively, of those recorded in wild type (WT), with no alterations in phasic inhibition. The amount of tonic inhibition recorded in slices was significantly affected by the method of slice storage (interface or submerged chamber). Field recordings in gabra5 –/– CA3 pyramidal layer showed an increased network excitability that was decreased by the GABAR agonist muscimol at a concentration that restored the tonic inhibition of gabra5 –/– PCs to the WT level without altering phasic inhibition. Through a battery of pharmacological experiments, we have identified δ subunit–containing GABARs as the mediators of the residual tonic inhibition in gabra5 –/– PCs. Our study is consistent with an important role of tonic inhibition in the control of hippocampal network excitability and highlights selective enhancers of tonic inhibition as promising therapeutic approaches for diseases involving network hyperexcitability. Address for reprint requests and other correspondence: I. Mody, UCLA School of Medicine, Department of Neurology, Neuroscience Research Building Room 571, 635 Charles E. Young Dr. South, Los Angeles, CA 90095 (E-mail: [email protected] )
Fitch, Thomas E.; Sahr, Robert N.; Eastwood, Brian J.; Zhou, Feng C.; Yang, Charles R.
doi: 10.1152/jn.01210.2005pmid: 16452256
The medial septum/vertical limb of diagonal band complex (MS/vDB) consists of cholinergic, GABAergic, and glutamatergic neurons that project to the hippocampus and functionally regulate attention, memory, and cognitive processes. Using tyrosine hydroxlase (TH) immunocytochemistry and dark-field light microscopy, we found that the MS/vDB is innervated by a sparse network of TH-immunoreactive (putative catecholaminergic) terminals. MS/vDB neurons are known to fire in rhythmic theta burst frequency of 3–7 Hz to pace hippocampal theta rhythm. Extracellular single-unit recording in theta and non-theta firing MS/vDB neurons and antidromically identified MS/vDB-hippocampal neurons were made in urethan-anesthetized rats. Tail-pinch noxious stimuli and ventral tegmental area (VTA) stimulation (20 Hz) evoked spontaneous theta burst firing in MS/vDB neurons. Systemic D1/5 antagonists SCH23390 or SCH39166 (0.1 mg/kg iv) alone suppressed the spontaneous theta bursts, suggesting a tonic facilitatory endogenous dopamine D1 "tone" that modulates theta bursts in vivo. Activation of D1/5 receptor by dihydrexidine (10 mg/kg iv) led to an increase in mean firing rate in 60% of all theta and non-theta MS/vDB neurons with an increase in the number of theta bursts and spikes/burst in theta cells. In strong theta firing MS/vDB neurons, D1/5 receptor stimulation suppressed the occurrence of theta burst firing, whereas the overall increase in spontaneous mean firing rate remained. In low baseline theta MS/vDB neurons D1/5 receptor stimulation increases the occurrence of theta bursts along with a net increase in mean firing rate. Atropine injection consistently disrupts theta burst pattern and reduced the time spent in theta firing. Collectively, these data suggest that dopamine D1/5 stimulation enhances the mean firing rate of most MS/vDB neurons and also provides a state-dependent bidirectional modulation of theta burst occurrence. Some of these MS/vDB neurons may be cholinergic or GABAergic that may indirectly regulate theta rhythm in the hippocampus. Address for reprint requests and other correspondence: C. R. Yang, Neuroscience Discovery, Eli Lilly & Co., Lilly Corporate Ctr., Indianapolis, IN 46285-0510 (E-mail: [email protected] )
Berkowitz, Ari; Yosten, Gina L. C.; Ballard, R. Mark
doi: 10.1152/jn.01246.2005pmid: 16452255
It has been difficult to predict the behavioral roles of vertebrate CNS neurons based solely on their morphologies, especially for the neurons that control limb movements in adults. We examined the morphologies of spinal interneurons involved in limb movement control, using intracellular recording followed by Neurobiotin injection in the in vivo adult turtle spinal cord preparation. We report here the first description of a class of spinal interneurons whose somato-dendritic morphologies predict their robust activity during multiple forms of ipsilateral and contralateral fictive hindlimb scratching and fictive hindlimb withdrawal. These "transverse interneurons" or T cells have a mediolaterally elongated soma and a simple dendritic tree that is extensive in the transverse plane but restricted rostrocaudally. During fictive scratching, these cells display strong rhythmic modulation with higher peak firing rates than other scratch-activated interneurons. These higher peak firing rates are at least partly caused by T cells having larger phase-locked membrane potential oscillations and narrower action potentials with briefer afterhyperpolarizations than other scratch-activated interneurons. Many T cells have axon terminal arborizations in the ventral horn of the spinal cord hindlimb enlargement. Identification of this morphological and physiological class of spinal interneurons should facilitate further exploration of the mechanisms of hindlimb motor pattern selection and generation. Address for reprint requests and other correspondence: A. Berkowitz, Dept. of Zoology, Univ. of Oklahoma, 730 Van Vleet Oval, Norman, OK 73019 (E-mail: [email protected] )
Sacchi, Oscar; Rossi, Maria Lisa; Canella, Rita; Fesce, Riccardo
doi: 10.1152/jn.01032.2005pmid: 16452258
A biophysical description of the axotomized rat sympathetic neuron is reported, obtained by the two-electrode voltage-clamp technique in mature, intact superior cervical ganglia in vitro. Multiple aspects of neuron functioning were tested. Synaptic conductance activated by the whole presynaptic input decreased to 29% of the control value (0.92 µS per neuron) 1 day after axotomy and to 18% after 3 days. Despite the decrease in amplitude of the macroscopic current, miniature excitatory postsynaptic current (mEPSC) mean conductance, acetylcholine (ACh) equilibrium potential, and EPSC decay time constant were unaffected. Synaptic efficacy was tested during paired-pulse or maintained stimulation (5, 10, and 15 Hz, 10-s duration). Quantal release in axotomized neurons was preserved during the tetanus despite the reduction of the initial EPSC amplitude, suggesting that ACh secretion depended on the number of surviving synapses; each of them exhibited dynamic behavior during trains similar to that of normal synapses. Facilitation of EPSC amplitude was noted in 2-day axotomized neurons during the first few impulses in the train. Voltage-dependent potassium currents (the delayed I KD and the transient I A ) exhibited an early drastic decrease in peak amplitude; these effects persisted 7 days after axotomy. Marked changes in I A kinetics occurred after injury: the steady-state inactivation curve shifted by up to +17 mV toward positive potentials and the voltage sensitivity of inactivation removal became steeper. I A impairment was reflected in a reduced inward threshold charge for discharge and reduced spike repolarization rate. Synaptic and somatic data were applied in a mathematical model to describe the progressive decrease in the safety factor, and the eventual failure of ganglionic transmission after axotomy. Address for reprint requests and other correspondence: O. Sacchi, Department of Biology—Section of Physiology and Biophysics, Via Borsari, 46, I-44100 Ferrara, Italy (E-mail: [email protected] )
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