Biomedical subjects

Martin Schneider

Publications and source records attributed to Martin Schneider.

21 records · Page 2Linked to original sources

PubMed2003

The maximum-entropy method in superspace.

One of the applications of the maximum-entropy method (MEM) in crystallography is the reconstruction of the electron density from phased structure factors. Here the application of the MEM to incommensurately modulated crystals and incommensurate composite crystals is considered. The MEM is computed directly in superspace, where the electron density in the (3+d)-dimensional unit cell (d > 0) is determined from the scattering data of aperiodic crystals. Periodic crystals (d = 0) are treated as a special case of the general formalism. The use of symmetry in the MEM is discussed and an efficient algorithm is proposed for handling crystal symmetry. The method has been implemented into a computer program BayMEM and applications are presented to the electron density of the periodic crystal NaV(2)O(5) and the electron density of the incommensurate composite crystal (LaS)(1.14)NbS(2). The MEM in superspace is shown to provide a model-independent estimate of the shapes of the modulation functions of incommensurate crystals. The discrete character of the electron density is found to be the major source of error, limiting the accuracy of the reconstructed modulation functions to approximately 10% of the sizes of the pixels. MaxEnt optimization using the Cambridge and Sakata-Sato algorithms are compared. The Cambridge algorithm is found to perform better than the Sakata-Sato algorithm, being faster, always reaching convergence, and leading to more reliable density maps. Nevertheless, the Sakata-Sato algorithm leads to similar density maps, even in cases where it does not reach complete convergence.

Journal Article↗

PubMed2002

Rhabdomyomatous dysplasia of the newborn lung associated with multiple congenital malformations of the heart and great vessels.

Interstitial proliferation of striated muscle in the lung is extremely rare. Most cases are associated with other congenital malformations, such as lung sequestration, diaphragmatic hernia, or cardiac malformations. We describe a newborn with rhabdomyomatous dysplasia of the lung associated with multiple congenital malformations of the heart and great vessels. The female neonate was born at 37 weeks of gestation as the second child to a 31-year-old woman without relevant previous medical or family history. In week 26 of gestation, a complex heart malformation and polyhydramnion were diagnosed by ultrasound. Postnatally, right lung hypoplasia, a bilobar right and left lung, anomalous drainage of the pulmonary veins, atrial and ventricular septal defects, and double-outlet right-ventricle and multiple aortopulmonary collaterals were described. Histological examination of a biopsy of the right lung demonstrated the presence of numerous bundles of striated muscle fibers arranged randomly in the pulmonary interstitium. Unilateral resection of the right lung was not a therapeutic option, because the left lung had developed bronchopulmonary dysplasia with severe reduction in gas exchange as a consequence of long-term mechanical ventilation. Symptomatic relief and palliative cardiac surgery were offered. At age 5 months, the infant died of a pulmonary hemorrhage following cardiac surgery.

Abnormalities, Multiple↗

PubMed2002

Activity-dependent development of axonal and dendritic delays, or, why synaptic transmission should be unreliable.

Systematic temporal relations between single neuronal activities or population activities are ubiquitous in the brain. No experimental evidence, however, exists for a direct modification of neuronal delays during Hebbian-type stimulation protocols. We show that in fact an explicit delay adaptation is not needed if one assumes that the synaptic strengths are modified according to the recently observed temporally asymmetric learning rule with the downregulating branch dominating the upregulating branch. During development, slow, unbiased fluctuations in the transmission time, together with temporally correlated network activity, may control neural growth and implicitly induce drifts in the axonal delays and dendritic latencies. These delays and latencies become optimally tuned in the sense that the synaptic response tends to peak in the soma of the postsynaptic cell if this is most likely to fire. The nature of the selection process requires unreliable synapses in order to give successful synapses an evolutionary advantage over the others. The width of the learning function also determines the preferred dendritic delay and the preferred width of the postsynaptic response. Hence, it may implicitly determine whether a synaptic connection provides a precisely timed or a broadly tuned "contextual" signal.

Animals↗