Search

Dystonia is a disorder of both motor and sensory function, clinically characterized by involuntary movements, abnormal postures, and abnormal sensory discrimination. Whereas most data on the physiology of motor cortices are consistent with a general frame of reduced inhibition and maladaptative plasticity, electrophysiological data in sensory cortices are controversial. Habituation refers to response decrement with repeated stimulation, and reflects mechanisms that alter synaptic effectiveness in the stimulated cortex through LTD or LTP-like phenomena. We investigated somatosensory evoked potentials (SEPs) habituation in patients with dystonia and hemifacial spasm (HSF). We recorded median-nerve SEPs in 28 patients (8 cranial, 4 cervical, 1 cranial-cervical dystonia, 15 HSF) and 18 age-matched controls. We measured N20-P25 amplitudes from 3 blocks of 200 sweeps, and assessed amplitude of block 1, and habituation from amplitude changes between the 3 sequential blocks. ANOVA showed a main effect of factor group ( F =4.4 p =0.015). In patients with dystonia, SEP amplitudes were normal in block 1, but thereafter failed to habituate ( p =0.03). Patients with hemifacial spasm had normal SEP amplitudes and habituation ( p =0.98). SEP habituation is altered in dystonia indicating that somatosensory cortex becomes hyperresponsive. This abnormality is not the result of involuntary movements because in patients with HSF SEPs habituate normally.

Habituation is defined as “a response decrement with repeated stimulation”. In normal cerebral cortex repetitive stimulation delivered with various sensory modalities elicits responses characterized by habituation, presumably owing to mechanisms that alter synaptic effectiveness. Habituation may represent an early, pre-clinical marker of synaptic dysfunction. In dementia, evidence of changes in synaptic functioning in sensory cortices is lacking. We investigated somatosensory evoked potentials (SEPs) habituation in 29 patients with mild dementia, grouped according to clinical, neuropsychological, and neuroimaging features. We recorded median-nerve SEPs in 15 AD-type and 14 VD-type, mild dementia, and 15 controls. We measured N20-P25 amplitudes from 3 blocks of 200 sweeps, and assessed amplitude of block 1, and habituation from amplitude changes between the 3 sequential blocks. ANOVA showed a main effect of factor group ( F = 3.43 p = 0.042). AD-type patients, had normal block1 SEP amplitudes, but abnormal habituation ( p = 0.036). VD-type patients had normal SEP amplitudes and habituation ( p = 0.58). SEP habituation is altered in AD-type dementia suggesting hyperresponsivity of the somatosensory cortex. This abnormality is not found in patients with VD-type mild dementia, which manifest normal synaptic function to repetitive stimulation. Further investigation is deserved to see whether abnormal SEP habituation may represent a neurophysiological marker of degenerative dementia.

To examine how anesthetized dogs compensate for the diaphragmatic shortening that occurs during head-up tilting, we measured the electroneurogram (ENG) of the C5 phrenic root and the electromyographic (EMG) activity of the parasternal intercostal and transversus abdominis muscles in eight spontaneously breathing animals during postural changes between supine (0 degree) and 80 degrees head-up. Both steady state ENG and EMG activities and first breath responses to tilting from 80 degrees head-up to supine were studied. These experiments have shown that: (1) anesthetized dogs respond to head-up tilting by increasing the neural drive to the costal diaphragm and parasternal intercostals; (2) this response, however, does not occur on the first breath and therefore cannot compensate for the immediate changes in diaphragmatic length; (3) the abdominal muscles, in contrast, show a first breath response to tilting and their activation is primarily responsible for the maintenance of tidal volume. Unlike in humans, increases in neural inspiratory drive in head-up anesthetized dogs are mediated by a chemoreceptive, rather than proprioceptive, feedback mechanism.

Partly phosphatized, oolitic-biogenic limestones were recovered 950–1300 m below sea level from two sites near the crest of a transverse ridge running parallel and adjacent to the Romanche fracture valley (equatorial Atlantic). Some of the biogenic contituents of the limestones (the benthonic foraminiferal genusAmphistegina; corals of the genusStylophora sp., scaphopods, etc.); their paleofacies assemblage (including echinoderms, gastropods, calcareous algae, etc.); and the presence of a well-developed oolitic facies, indicate that the limestones formed in very shallow water close to sea level. In addition, several features of the limestones (including the presence of stromatolite-like laminae, and dissolution features typical of subaerial diagenesis) suggest that the summit of the transverse ridge might have undergone episodes of emersion. The limestones were formed on a shallow carbonate bank at around the Miocene-Pliocene boundary, i.e., 5 ± 1 Myr ago, as determined by the age of fossil planktonic foraminifera and corals. The transverse ridge must have subsided since that time at an average rate of 0.2 mm/yr. It is unlikely that the vertical motions of the Romanche transverse crustal block were caused solely by accreting plate margin- or mantle plume-related volcanic and/or tectonic processes. It is suggested instead that such motions are related to vertical tectonism typical of large oceanic fracture zones.