Excitability Of Cerebral Cortex Research Articles

Repetitive transcranial magnetic stimulation and chronic tinnitus

Conclusion: There is a good theoretical basis and early research evidence suggesting that transcranial magnetic stimulation (TMS) may have treatment potential in tinnitus. Further studies with larger sample sizes and additional assessment of neurobiological effects are necessary. Objectives: Tinnitus is a common and often severely disabling disorder for which there is no satisfactory treatment. TMS is a new, non-invasive method of modifying the excitability of the cerebral cortex, which has proven effective in auditory hallucinations and other disorders. Some early studies have been published in which TMS has been used in the treatment of tinnitus. The objective of this paper is to examine the literature and consider the potential for TMS as a therapy in tinnitus. Methods: A thorough search of the tinnitus and TMS literature was conducted, and all available relevant material was examined. Results: Tinnitus is common, with a prevalence of 8.2% in subjects aged 50 years and over, and may be associated with great distress (tinnitus sufferers). There are no effective treatments. Tinnitus is frequently associated with deafness, and may be the result of a pathological plasticity process. Neuroimaging studies demonstrate increased activity within the central auditory system. TMS is a non-invasive method of modulating excitability in cerebral cortex. It uses electromagnetic principles and has been successfully employed in the treatment of other conditions associated with increased activity of the cerebral cortex. Meanwhile, a growing number of studies suggest that repetitive TMS may be effective in the treatment of chronic tinnitus.

Column-based model of electric field excitation of cerebral cortex.

A model to explain the orientation selectivity of the neurophysiologic effects of electric-field transients applied to cerebral cortex is proposed and supported with neuroimaging evidence. Although it is well known that transcranial magnetic stimulation (TMS) excites cerebral cortex in an orientation-selective manner, a neurophysiologically compelling explanation of this phenomenon has been lacking. It is generally presumed that TMS-induced excitation is mediated by horizontal fibers in the cortical surfaces nearest to the stimulating coil, i.e., at the gyral crowns. No evidence exists, however, that horizontal fibers are orientation selective either anatomically or physiologically. We used positron emission tomography to demonstrate that TMS-induced cortical activation is selectively sulcal. This observation allows the well-established columnar organization of cerebral cortex to be invoked to explain the observed orientation selectivity. In addition, Rushton's cosine principle can used to model stimulation efficacy for an electrical field applied at any cortical site at any intensity and in any orientation.

Amino acids as the source of considerable excitation in cerebral cortex

A number of different factors contributed to the rationale for providing a critical review of the field of excitatory amino acids in the cerebral cortex at this time. In addition to the recent growing realisation by clinicians that the excitatory amino acids are linked critically to a number of neurological conditions, including neurodegenerative syndromes such as Alzheimer's disease, cortical damage due to stroke and cerebral ischemia, epilepsy, amyotropic lateral sclerosis, and schizophrenia, the recent cloning and membrane reconstitution of specific receptors known as AMPA, NMDA, kainate, and metabotropic receptors and their known subunits have prompted a surge of renewed interest in this important family of synaptic transmitter molecules. Moreover, recent advances in our understanding of the molecular events involved in growth promotion in the early stages of the development of the cortex have shown that both NMDA and non-NMDA receptor subtypes perform important roles in certain aspects of target selection and neurite outgrowth, in cone stimulation and guidance, and in spine formation and morphological alterations. A recent explosive growth in interest in the possible roles of nitric oxide and related short-lived radicals in plasticity, disease, and synaptic transmission also is related closely with the actions of excitatory amino acids. All these relatively new advances have transpired in parallel with ongoing work that has extended our appreciation for the roles of excitatory amino acids in the areas of synaptic plasticity (long-term potentiation, long-term depression, receptive field reorganisation following drug-induced or peripheral sensory disturbances, such as denervation or amputation), in processes of normal transmission at functionally and electrophysiologically identified neurones of the cerebral cortex, and in distinct proposed roles for cortical glia. A greater appreciation of the diverse types and properties of the burgeoning family of receptors for the metabotropic receptor also contributed to our desire to feature that aspect of the field in the context of glia and neurones of the cerebral cortex. That part of the field of neuroscience concerned with the functions of excitatory amino acids has grown so large over the past 10 years or so, that a review paper focusing on the contributions to a specialized meeting devoted solely to cerebral cortex could easily be supported by material comprising a sufficient body of communications from top-quality research laboratories. The present account endeavours to summarize and discuss the biochemical characteristics, physiological roles, pharmacological properties, clinical relevance, developmental involvements, and anatomical-morphological aspects pertaining to the excitatory amino acid transmitters in cerebral cortex.

Modelling magnetic coil excitation of human cerebral cortex with a peripheral nerve immersed in a brain-shaped volume conductor: the significance of fiber bending in excitation

To help elucidate some basic principles of magnetic coil (MC) excitation of cerebral cortex, a model system was devised in which mammalian phrenic nerve, or amphibian sciatic nerve with its branches was suspended in appropriate Ringer's solution in a human brain-shaped volume conductor, an inverted plastic skull. The nerve was recorded monophasically out of the volume conductor. The site of nerve excitation by the MC was identified by finding where along the nerve a bipolar electrical stimulus yielded a similar action potential latency. MC excitation of hand-related corticospinal (CT) neurons was modelled by giving the distal end of nerve attached to the lateral skull an initial radial (perpendicular) trajectory, with subsequent bends towards the base and posterior part of the skull; this nerve was optimally excited by a laterally placed figure 8 or round MC when the induced electric field led to outward membrane current at the initial bend. By contrast, nerve given a trajectory modelling CT neurons related to the foot was optimally excited when the coil windings were across the midline, but again when membrane current flowed outward at the first bend. Corticocortical fibers were modelled by placing the nerve in the anteroposterior axis lateral to the midline; with the round MC vertex-tangentially oriented, optimal excitation occurred at the bend nearest the interaural line, i.e., near the peak electric field. The findings emphasize the importance of orientation and direction of current in the MC and fiber bends in determining nerve excitation. The findings in the peripheral nerve-skull model help explain (1) why lateral and vertex-tangentially oriented MCs preferentially excite arm-related CT neurons directly and indirectly (through corticocortical fibers), respectively, and (2) why the MC orientations for optimally exciting directly arm and leg-related CT neurons differ.

FACILITATION: ELECTRICAL RESPONSE ENHANCED BY CONDITIONAL EXCITATION OF CEREBRAL CORTEX.

Cats were conditioned to give a foot flexion in response to stimulation of the brain through implanted electrodes. The evoked electrical activity of an extracallosal interhemispheric pathway was much greater in the repeatedly stimulated direction than in the unstimulated direction in the brain. No difference in the two directions was observed in control animals that did not receive the unconditional foot shock.

Threshold movements produced by excitation of cerebral cortex and efferent fibers with some parametric regions of rectangular current pulses (cats and monkeys).

Threshold movements produced by excitation of cerebral cortex and efferent fibers with some parametric regions of rectangular current pulses (cats and monkeys).