| neural tube | <embryology> The progenitor of the central nervous system. See: neural plate, neurulation. (18 Nov 1997) |
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| neural tube defect | <neurology, paediatrics> Abnormal development during embryonic life of the neural tube producing congenital malformations of the nervous system due to closure failure of the neural tube. The structure gives rise to the central nervous system (the brain and spinal cord), and failure to close results in anencephaly (absence of the cranial vault and absence of most or all of the cerebral hemispheres of the brain) and spina bifida or meningomyelocele (open spina with exposure and protusion of the spinal cord). The risk of neuroal tube defects can be decreased by the mother taking folic acid during pregnancy. (04 Jul 1999) |
| dorsal plate of neural tube | The thin layer of the embryonic neural tube connecting the alar plate's dorsally. Synonym: dorsal plate of neural tube. (05 Mar 2000) |
| dorsolateral plate of neural tube | The dorsal division of the lateral walls of the neural tube in the embryo; it gives rise to neurons relaying afferent impulses to higher centres; in the adult such neurons compose the sensory nuclei of the spinal cord and brainstem. Synonym: lamina alaris, alar plate of neural tube, dorsolateral plate of neural tube, lamina dorsalis, wing plate. (05 Mar 2000) |
| aberrant ventricular conduction | Abnormal intraventricular conduction of a supraventricular beat, especially where surrounding beats are normally conducted. Synonym: ventricular aberration. (05 Mar 2000) |
| accelerated conduction | Any pathologically increased speed of conduction; usually occurs between the atrium and ventricles as in the Wolff-Parkinson-White and Lown-Ganong-Levine syndromes; such accelerated pathways provide the bases for particular forms of reentry tachycardia. (05 Mar 2000) |
| A-H conduction time | Forward conduction of the cardiac impulse from atria to ventricles via the A-V node or any bypass tract, represented in the electrocardiogram by the P-R interval. P-H conduction time is from the onset of the P wave to the first high frequency component of the His bundle electrogram (normally 119 ± 38 msec); A-H conduction time is from the onset of the first high frequency component of the atrial electrogram to the first high frequency component of the His bundle electrogram (normally 92 ± 38 msec); P-A conduction time is from the onset of the P wave to the onset of the atrial electrogram (normally 27 ± 18 msec). (05 Mar 2000) |
| air conduction | In relation to hearing, the transmission of sound to the inner ear through the external auditory canal and the structures of the middle ear. (05 Mar 2000) |
| anomalous conduction | Conduction of cardiac electrical impulses through any abnormal pathway. (05 Mar 2000) |
| antegrade conduction | Conduction in the expected normal direction between any cardiac structures. Synonym: antegrade conduction, forward conduction, orthograde conduction. (05 Mar 2000) |
| anterograde conduction | Conduction in the expected normal direction between any cardiac structures. Synonym: antegrade conduction, forward conduction, orthograde conduction. (05 Mar 2000) |
| aphasia, conduction | A type of fluent aphasia in which there is normal comprehension of spoken language but words are repeated incorrectly. It results from a lesion of the arcuate fasciculus connecting broca's and wernicke's areas. Like patients with wernicke's aphasia (aphasia, wernicke), patients with conduction aphasia are fluent but have many paraphasic errors (incorrect words or sounds substituted for correct ones). The degree of fluency is less than that in wernicke's aphasia, but comprehension is good. (12 Dec 1998) |
| atrioventricular conduction | Forward conduction of the cardiac impulse from atria to ventricles via the A-V node or any bypass tract, represented in the electrocardiogram by the P-R interval. P-H conduction time is from the onset of the P wave to the first high frequency component of the His bundle electrogram (normally 119 ± 38 msec); A-H conduction time is from the onset of the first high frequency component of the atrial electrogram to the first high frequency component of the His bundle electrogram (normally 92 ± 38 msec); P-A conduction time is from the onset of the P wave to the onset of the atrial electrogram (normally 27 ± 18 msec). (05 Mar 2000) |
| atrioventricular conduction abnormalities | <cardiology, physiology> Any condition which involves abnormal (blocked, delayed or aberrant) electrical conduction through the atrioventricular node (for example atrioventricular block). Some medications may precipitate atrioventricular conduction abnormalities. (02 Jan 1998) |
| avalanche conduction | <physiology> The discharge of an impulse from a neuron into a large number of neurons of the same physiologic system. This produces an amplification effect, thus producing a very large amount of nervous energy for a given stimulus. (05 Mar 2000) |