Thesis (Ph. D.)--University of Wisconsin--Madison, 1970. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 70-73).

No

The leading cause of death in patients who survive subarachnoid hemorrhage (SAH) is stroke as a result of cerebral arterial vasospasm1. Such vasospasms involve a vasoactive response, but they remain enigmatic and no clinical treatment has proven effective in prevention or reduction2. Arteries remodel in response to diverse mechanical loads and chemical factors. Following SAH, the surrounding vasculature is exposed to a radically altered chemo-mechanical environment. It is our hypothesis that chemical stimuli associated with the formation of an extravascular blood clot dominates the maladaptive growth and remodeling response early on, thus leading to important structural changes. However, it is not clear which of the many chemical factors are key players in the production of vasospasm. Before an accurate picture of the etiology of vasospasm can be produced, it is imperative to gain a better understanding of the non-pathogenic cerebral vasculature. In particular, the rabbit basilar artery is a well established model for vasospasm. However, surprisingly little is known about the mechanical properties of the rabbit basilar artery. Using an in vitro custom organ culture and mechanical testing device, acute and cultured basilar arteries from male White New Zealand specific pathogen free rabbits underwent cyclic pressurization tests at in vivo conditions and controlled levels of myogenic tone. Sections of basilar arteries were imaged for collagen fiber orientation at 0, 40 and 80 mmHg at in vivo stretch conditions using nonlinear optical microscopy. The nonlinear stress-strain curves provide baseline characteristics for acute and short-term culture basilar arteries. The active and passive testing creates a framework for interpreting the basal tone of arteries in our culture system. Nonlinear optical microscopy second harmonic generation provides unique microstructural information and allows imaging of live, intact arteries while maintaining in vivo geometries and conditions. Collagen fibers were found to be widely distributed about the axial direction in the adventitial layer and narrowly distributed about the circumferential direction in the adventitial layer. The quantified collagen fiber angles within the artery wall further support the development of accurate mathematical models.

Cerebrovascular disease continues to be responsible for significant morbidity and mortality. There is a pressing need to better understand the biomechanics of both intracranial arteries and the extracranial arteries that feed these vessels. Histology and immunohistochemistry were performed on basilar and carotid arteries and elucidated their structural differences. Nonlinear optical microscopy (NLOM) provided collagen fiber orientation and distribution, which offered motivation for a more microstructrually-based model. Biaxial testing was performed on acute basilar and carotid arteries and cultured basilar arteries. We used a validated 2D, four fiber family constitutive relation to model passive biaxial stress-stretch behaviors of basilar and common carotid arteries, and we developed a new relation to model their active biaxial responses. Residual stress information from opening angles and collagen fiber orientation were used to create a 3D fiber distribution constitutive relation. Passive biaxial stress-stretch behavior of basilar arteries was modeled with the 3D fiber distribution constitutive relation and was combined with our new active relation to model the active response of basilar arteries. These data and 2D, four fiber family and active constitutive relation allow the first full comparison of circumferential and axial biomechanical behaviors between a muscular (basilar) and an elastic (carotid) artery from the same species. Our active model describes the responses by both types of vessels to four doses of the vasoconstrictor endothelin-1 (10^-10, 10^-9, 10^-8, 10^-7 M) and predicts levels of smooth muscle activation associated with basal tone under specific in vitro testing conditions. Cultured arteries revealed smooth muscle tone is necessary for vascular remodeling. Our 3D model allowed for the calculation of stress through the wall in passive basilar arteries. These results advance our understanding of the biomechanics of intracranial and extracranial arteries, which is needed to understand better their differential responses to similar perturbations in hemodynamic loading.

The leading cause of death in patients who survive subarachnoid hemorrhage (SAH) is stroke as a result of cerebral arterial vasospasm1. Such vasospasms involve a vasoactive response, but they remain enigmatic and no clinical treatment has proven effective in prevention or reduction2. Arteries remodel in response to diverse mechanical loads and chemical factors. Following SAH, the surrounding vasculature is exposed to a radically altered chemo-mechanical environment. It is our hypothesis that chemical stimuli associated with the formation of an extravascular blood clot dominates the maladaptive growth and remodeling response early on, thus leading to important structural changes. However, it is not clear which of the many chemical factors are key players in the production of vasospasm. Before an accurate picture of the etiology of vasospasm can be produced, it is imperative to gain a better understanding of the non-pathogenic cerebral vasculature. In particular, the rabbit basilar artery is a well established model for vasospasm. However, surprisingly little is known about the mechanical properties of the rabbit basilar artery. Using an in vitro custom organ culture and mechanical testing device, acute and cultured basilar arteries from male White New Zealand specific pathogen free rabbits underwent cyclic pressurization tests at in vivo conditions and controlled levels of myogenic tone. Sections of basilar arteries were imaged for collagen fiber orientation at 0, 40 and 80 mmHg at in vivo stretch conditions using nonlinear optical microscopy. The nonlinear stress-strain curves provide baseline characteristics for acute and short-term culture basilar arteries. The active and passive testing creates a framework for interpreting the basal tone of arteries in our culture system. Nonlinear optical microscopy second harmonic generation provides unique microstructural information and allows imaging of live, intact arteries while maintaining in vivo geometries and conditions. Collagen fibers were found to be widely distributed about the axial direction in the adventitial layer and narrowly distributed about the circumferential direction in the adventitial layer. The quantified collagen fiber angles within the artery wall further support the development of accurate mathematical models.

Thesis (M.Eng.)(Bioengineering)--University of Pretoria, 2005. / Title from opening screen (Viewed March 20, 2006). Summaries in English and Afrikaans. Includes bibliographical references.

Esta pesquisa analisa experimentalmente as diferenças de comportamento entre a versão clássica e a modificada da craniotomia subtemporal quanto à retração do lobo temporal. A retração é medida pelo ângulo de visão, através do microscópio, obtido ao mirar-se estruturas previamente estabelecidas. Estudou-se a correlação estatística entre a retração cerebral e os diâmetros transversos do crânio, a profundidade da fossa temporal e a altura da bifurcação medida em relação à tenda do cerebelo e à clinóide posterior. É considerada a relação entre estes achados e os aspectos pertinentes à opção entre as craniotomias pterional e subtemporal para o tratamento cirúrgico dos aneurismas da porção superior da artéria basilar. / This paper is an experimental analysis between the classic and the modified subtemporal procedures, regarding the cerebral retraction required to approach the interpeduncular cistern and the basilar artery bifurcation. The retraction is assumed to be proportional to the angle of sight, through the microscope, required to observe the basilar bifurcation and other structures. The statistical correlations between the degree of brain retraction and the transverse diameters of the skull, the vertical length of the temporal fossa and the position of the basilar bifurcation in relation to the posterior clinoid and the tentorium are analysed. Considerations are made regarding these findings and the many aspects involved in the options between the pterional and the subtemporal approaches in the surgical treatment of the basilar bifurcation aneurysms

Esta pesquisa analisa experimentalmente as diferenças de comportamento entre a versão clássica e a modificada da craniotomia subtemporal quanto à retração do lobo temporal. A retração é medida pelo ângulo de visão, através do microscópio, obtido ao mirar-se estruturas previamente estabelecidas. Estudou-se a correlação estatística entre a retração cerebral e os diâmetros transversos do crânio, a profundidade da fossa temporal e a altura da bifurcação medida em relação à tenda do cerebelo e à clinóide posterior. É considerada a relação entre estes achados e os aspectos pertinentes à opção entre as craniotomias pterional e subtemporal para o tratamento cirúrgico dos aneurismas da porção superior da artéria basilar. / This paper is an experimental analysis between the classic and the modified subtemporal procedures, regarding the cerebral retraction required to approach the interpeduncular cistern and the basilar artery bifurcation. The retraction is assumed to be proportional to the angle of sight, through the microscope, required to observe the basilar bifurcation and other structures. The statistical correlations between the degree of brain retraction and the transverse diameters of the skull, the vertical length of the temporal fossa and the position of the basilar bifurcation in relation to the posterior clinoid and the tentorium are analysed. Considerations are made regarding these findings and the many aspects involved in the options between the pterional and the subtemporal approaches in the surgical treatment of the basilar bifurcation aneurysms

A cochlear implant is a prosthetic device that can provide severe-to-profoundly deaf individuals with partially restored hearing. It emulates the function of a normal cochlea through combined functioning of externally situated electronics and an electrode array surgically implanted into the cochlea. Speech coding strategies implemented in speech processors aim to stimulate the auditory nerve in a way similar to that of a normal working cochlea by modelling the way the cochlea processes sound. Current speech processing strategies rely on the tonotopicity of the cochlea, i.e. the relation between distance from the base of the cochlea and the specific frequency that causes the highest amplitude of deflection at the specific point. The phenomenon of the travelling wave on the basilar membrane is thus reduced to its point or points of maximal deflection. In this study, the behaviour along the full length of the basilar membrane will be investigated in the time domain, i.e. the deflection along the whole membrane for any point in time, in order to evaluate the relevance of the travelling wave in coding sound in a cochlear implant system. The additional information acquired by emulating the motion of the fluid and the basilar membrane in the cochlea, will be transmitted to the recipient in electrical stimulus patterns, to assess whether it provides recipients of cochlear implants with better pitch perception. It will be shown that for the individuals that partook in the experiments, improvement of discrimination around 100 Hz were obtained when compared to current speech coding strategies like the advanced combination encoder (ACE) speech coding strategy in the same recipient. / Dissertation (MEng (Bio-Engineering))--University of Pretoria, 2007. / Electrical, Electronic and Computer Engineering / unrestricted

Intracellular membrane potential (Em) and force development were measured in rabbit basilar artery to help elucidate the mechanism of action of dopamine in this preparation. There was a strong correlation between membrane depolarization and contraction (r=0.95) between 3×10-7 M to 10-4 M dopamine. When the vascular muscle cells were depolarized by elevating [K]o there was a Em dependent decrease in force development in response to dopamine. Significant reduction of dopamine stimulated force development was observed when the vessel was depolarized by 5-6 mV by excess extracellular K+ and 90% inhibition was seen when the artery was depolarized to -20mv. When Ca++ influx was blocked, dopamine no longer induced force development. Such findings suggest that dopamine cotracts rabbit basilar artery by a mechanism involving membrane depolarization. This process may involve an influx of extracellular Ca++ through voltage sensitive channels.