Magnetic stimulation coil and circuit design.

A detailed analysis of the membrane voltage rise commensurate with the electrical charging circuit of a typical magnetic stimulator is presented. The analysis shows how the membrane voltage is linked to the energy, reluctance, and resonant frequency of the electrical charging circuit. There is an optimum resonant frequency for any nerve membrane depending on its capacitive time constant. The analysis also shows why a larger membrane voltage will be registered on the second phase of a biphasic pulse excitation [1]. Typical constraints on three key quantities voltage, current, and silicone controlled rectifier (SCR) switching time dictate key components such as capacitance, inductance, and choice of turns.

Alumina gel injections into the temporal lobe of rhesus monkeys cause complex partial seizures and morphological changes found in human temporal lobe epilepsy.

The goal of the present study was to determine whether alumina gel injections into temporal lobe structures cause complex partial seizures (CPS) and pathological changes observed in human temporal lobe epilepsy. Rhesus monkeys with alumina gel injections in the amygdala, perirhinal and entorhinal cortices, or Ammon's horn and dentate gyrus all initially displayed focal pathological electroencephalographic (EEG) slowing limited to the site of injection. After clinical seizures developed, they also displayed widespread pathological EEG slowing over both hemispheres, interictal and ictal epileptiform EEG abnormalities limited to the mesial-inferior temporal lobe on the side of injection, and different degrees of spread to other ipsilateral and contralateral structures. Noninjected control and nonepileptic monkeys with injections into the middle and inferior temporal gyri displayed no hippocampal neuronal loss or mossy fiber sprouting. When alumina gel was injected into the amygdala, CPS began within 3-6 weeks and degeneration of neurons and gliosis occurred in the perirhinal cortex or the hippocampus, with consequent sprouting of mossy fibers in the dentate gyrus. Dispersion of the granule cell layer was also observed. Other monkeys with alumina gel in the perirhinal and entorhinal cortices developed CPS within 2-3 weeks after the injections and displayed mossy fiber sprouting only after 4 weeks after the injections. Alumina gel in Ammon's horn and the dentate gyrus also induced CPS, but mossy fiber sprouting was limited to sites immediately adjacent to the injection, probably because none survived more than 4 weeks after the injections. This nonhuman primate model of CPS displayed similar anatomical, behavioral, and EEG features as observed in human temporal lobe epilepsy and provides opportunities to analyze the chronological sequence of epileptogenesis and to test potential therapies.

Brain blood flow alterations induced by therapeutic vagus nerve stimulation in partial epilepsy: I. Acute effects at high and low levels of stimulation.

PURPOSE: Left cervical vagus nerve stimulation (VNS) decreases complex partial seizures (CPS) by unknown mechanisms of action. We hypothesized that therapeutic VNS alters synaptic activities at vagal afferent terminations and in sites that receive polysynaptic projections from these medullary nuclei. METHODS: Ten patients with partial epilepsy underwent positron emission tomographic (PET) measurements of cerebral blood flow (BF) three times before and three times during VNS. Parameters for VNS were at high levels for 5 patients and at low levels for 5. Resting BF measurements were subtracted from measurements during VNS in each subject. Subtraction data were averaged in each of 2 groups of 5 patients. t Tests were applied to BF changes in brain regions that receive vagal afferents and projections (significant at p < 0.05, corrected for repeated measures). RESULTS: In both the low- and high-stimulation groups during VNS, brain BF was (a) increased in the rostral, dorsal-central medulla; (b) increased in the right postcentral gyrus, (c) increased bilaterally in the hypothalami, thalami, and insular cortices, and in cerebellar hemispheres inferiorly; and (d) decreased bilaterally in hippocampus, amygdala, and posterior cingulate gyri. The high-stimulation group had greater volumes of activation and deactivation sites. CONCLUSIONS: Our findings suggest that left cervical VNS acutely increases synaptic activity in structures directly innervated by central vagal structures and areas that process left-sided somatosensory information, but VNS also acutely alters synaptic activity in multiple limbic system structures bilaterally. These findings may reflect sites of therapeutic actions of VNS.

EEG mean frequencies are sensitive indices of phenylalanine effects on normal brain.

We previously reported that changes in plasma phenylalanine (PHE) concentrations of 1000 microM or more adversely affected cognitive function and reduced mean frequency of the EEG power spectrum. In the present study, we characterized EEG effects of changes in plasma PHE from physiological to supraphysiological concentrations. Subjects were mentally normal children and adult volunteers with 3 different genotypes for phenylalanine hydroxylase (PHY): homozygous deficient, heterozygous, and homozygous normal. Double-blinded crossover studies were performed at equilibrium during PHE restriction and supplementation. The mean frequency of the power spectrum and the mean across a set of alpha-theta factors showed highly significant, reversible, generalized EEG slowing during PHE supplementation in subjects homozygous for PHY deficiency. Smaller but significant changes in EEG mean frequencies occurred in the heterozygous and normal subjects. Spectral profiles of EEG change were similar in both groups; the mean alpha-theta was more sensitive in the second group. Demonstration of EEG changes with PHE supplementation in normal individuals has important dietary implications.