Browsing by Subject "Neural Conduction"

Now showing 1 - 3 of 3
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    Composition and topographic organization of signals sent from the frontal eye field to the superior colliculus.
    (J Neurophysiol, 2000-04) Sommer, MA; Wurtz, RH
    The frontal eye field (FEF) and superior colliculus (SC) contribute to saccadic eye movement generation, and much of the FEF's oculomotor influence may be mediated through the SC. The present study examined the composition and topographic organization of signals flowing from FEF to SC by recording from FEF neurons that were antidromically activated from rostral or caudal SC. The first and most general result was that, in a sample of 88 corticotectal neurons, the types of signals relayed from FEF to SC were highly diverse, reflecting the general population of signals within FEF rather than any specific subset of signals. Second, many neurons projecting from FEF to SC carried signals thought to reflect cognitive operations, namely tonic discharges during the delay period of a delayed-saccade task (delay signals), elevated discharges during the gap period of a gap task (gap increase signals), or both. Third, FEF neurons discharging during fixation were found to project to the SC, although they did not project preferentially to rostral SC, where similar fixation neurons are found. Neurons that did project preferentially to the rostral SC were those with foveal visual responses and those pausing during the gap period of the gap task. Many of the latter neurons also had foveal visual responses, presaccadic pauses in activity, and postsaccadic increases in activity. These two types of rostral-projecting neurons therefore may contribute to the activity of rostral SC fixation neurons. Fourth, conduction velocity was used as an indicator of cell size to correct for sampling bias. The outcome of this correction procedure suggested that among the most prevalent neurons in the FEF corticotectal population are those carrying putative cognitive-related signals, i.e., delay and gap increase signals, and among the least prevalent are those carrying presaccadic burst discharges but lacking peripheral visual responses. Fifth, corticotectal neurons carrying various signals were biased topographically across the FEF. Neurons with peripheral visual responses but lacking presaccadic burst discharges were biased laterally, neurons with presaccadic burst discharges but lacking peripheral visual responses were biased medially, and neurons carrying delay or gap increase signals were biased dorsally. Finally, corticotectal neurons were distributed within the FEF as a function of their visual or movement field eccentricity and projected to the SC such that eccentricity maps in both structures were closely aligned. We conclude that the FEF most likely influences the activity of SC neurons continuously from the start of fixation, through visual analysis and cognitive manipulations, until a saccade is generated and fixation begins anew. Furthermore, the projection from FEF to SC is highly topographically organized in terms of function at both its source and its termination.
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    Effects of frequency-dependent membrane capacitance on neural excitability.
    (Journal of neural engineering, 2015-10) Howell, Bryan; Medina, Leonel E; Grill, Warren M

    Objective

    Models of excitable cells consider the membrane specific capacitance as a ubiquitous and constant parameter. However, experimental measurements show that the membrane capacitance declines with increasing frequency, i.e., exhibits dispersion. We quantified the effects of frequency-dependent membrane capacitance, c(f), on the excitability of cells and nerve fibers across the frequency range from dc to hundreds of kilohertz.

    Approach

    We implemented a model of c(f) using linear circuit elements, and incorporated it into several models of neurons with different channel kinetics: the Hodgkin-Huxley model of an unmyelinated axon, the McIntyre-Richardson-Grill (MRG) of a mammalian myelinated axon, and a model of a cortical neuron from prefrontal cortex (PFC). We calculated thresholds for excitation and kHz frequency conduction block, the conduction velocity, recovery cycle, strength-distance relationship and firing rate.

    Main results

    The impact of c(f) on activation thresholds depended on the stimulation waveform and channel kinetics. We observed no effect using rectangular pulse stimulation, and a reduction for frequencies of 10 kHz and above using sinusoidal signals only for the MRG model. c(f) had minimal impact on the recovery cycle and the strength-distance relationship, whereas the conduction velocity increased by up to 7.9% and 1.7% for myelinated and unmyelinated fibers, respectively. Block thresholds declined moderately when incorporating c(f), the effect was greater at higher frequencies, and the maximum reduction was 11.5%. Finally, c(f) marginally altered the firing pattern of a model of a PFC cell, reducing the median interspike interval by less than 2%.

    Significance

    This is the first comprehensive analysis of the effects of dispersive capacitance on neural excitability, and as the interest on stimulation with kHz signals gains more attention, it defines the regions over which frequency-dependent membrane capacitance, c(f), should be considered.
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    Neurodevelopmental outcomes of umbilical cord blood transplantation in metachromatic leukodystrophy.
    (Biology of blood and marrow transplantation : journal of the American Society for Blood and Marrow Transplantation, 2013-04) Martin, Holly R; Poe, Michele D; Provenzale, James M; Kurtzberg, Joanne; Mendizabal, Adam; Escolar, Maria L
    Metachromatic leukodystrophy (MLD) is an inherited demyelinating disease that causes progressive neurologic deterioration, leading to severe motor disability, developmental regression, seizures, blindness, deafness, and death. The disease presents as a late-infantile, juvenile, or adult form. Hematopoietic stem cell transplantation has been shown to slow disease progression. The purpose of this longitudinal study was to evaluate long-term treatment outcomes after unrelated donor umbilical cord blood (UCB) transplantation in pediatric patients according to disease burden and age at onset (ie, late-infantile versus juvenile). Engraftment, survival, treatment-related toxicity, graft-versus-host disease, neurophysiologic measures, and neurodevelopmental function were assessed. To evaluate whether signal intensity abnormalities on magnetic resonance imaging (ie, modified Loes scores) predict post-transplant cognitive and gross motor development, a general linear mixed model was fit to the data. Twenty-seven patients underwent transplantation after myeloablative chemotherapy; 24 patients engrafted after the initial transplantation. Seven patients died of infection, regimen-related toxicity, or disease progression. Twenty patients (6 with late-infantile onset and 14 with juvenile onset) were followed for a median of 5.1 years (range, 2.4 to 14.7). We found that patients with motor function symptoms at the time of transplant did not improve after transplantation. Brainstem auditory evoked responses, visual evoked potentials, electroencephalogram, and/or peripheral nerve conduction velocities stabilized or improved in juvenile patients but continued to worsen in most patients with the late-infantile presentation. Pretransplant modified Loes scores were highly correlated with developmental outcomes and predictive of cognitive and motor function. Children who were asymptomatic at the time of transplantation benefited most from the procedure. Children with juvenile onset and minimal symptoms showed stabilization or deterioration of motor skills but maintained cognitive skills. Overall, children with juvenile onset had better outcomes than those with late-infantile onset. As in other leukodystrophies, early intervention correlated with optimal outcomes. We conclude that UCB transplantation benefits children with presymptomatic late-infantile MLD or minimally symptomatic juvenile MLD.