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published_or_final_version / Physiology / Master / Master of Philosophy

The mammalian visual cortex is believed to be parcellated into functional radial units called modules, which are composed of neurons with similar physiological properties. The first demonstration of modularity was provided in 1957 by Mountcastle in the somatosensory cortex, and has since been demonstrated in the visual cortex, where neurons within a vertical unit of the visual cortex process information about the same portion of the visual field. / A new approach has been proposed for identifying functionally similar neurons by examining their molecular characteristics. Indeed, the arrangement of neurons into functional arrays might be reflected by the presence of specific molecules (e.g Cat-301 patches, cytochrome oxidase blobs). / In this experiment, immunohistochemistry was used to examine the tangential and radial distribution and development of a subset of pyramidal neurons in the kitten and adult cat visual cortex using the monoclonal antibody SMI-32, that recognizes the non-phosphorylated form of neurofilament H. It was found that the neurons recognized by this antibody were grouped into clusters, forming regularly spaced patches in the infragranular and supragranular layers. These anatomical findings support the notion of an intrinsic columnar organization.

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published_or_final_version / Physiology / Master / Master of Philosophy

The detection of visual stimuli involves neurons which are selectively responsive to components of a visual scene. In the early stages of visual processing, it is commonly accepted that neurons respond to the changes in luminance associated with objects and object boundaries. However, recent experiments have demonstrated that some neurons can also respond to features which are not defined by luminance variations. These features are termed "second-order" because they require more complex processing, and neurons which respond to second-order features are necessarily nonlinear. / In this thesis, I undertook a three dimensional physiological characterization (i.e. tuning of orientation, spatial frequency and temporal frequency) of such nonlinear neurons in order to shed light on their processing capabilities. In particular we sought to address the following issues: (1) whether the temporal and spatial properties underlying second-order motion are similar to those underlying luminance based ("first-order") motion; (2) whether these properties remain constant using different types of second-order stimuli, suggesting that neurons' responses are invariant to the physical attributes comprising the stimulus; and (3) whether second-order processing is a cortical mechanism or can occur at an earlier stage of the visual system (e.g. in the lateral geniculate nucleus). Taken together these results have a dual function; they provide insight into the complex cellular processing of higher order features, and they provide a general framework for the generation of second-order models.

Normal levels of CO₂ are responsible for the maintenance of approximately 30% of sympathetic neurogenic vascular tone in intact cats. The central medullary chemoreceptors have been implicated as the major source of this CO₂ dependent neurogenic vascular tone. However, it is possible that spinal cord CO₂ sensitivity could also have mediated a portion of neurogenic vascular tone. Cats with acute and chronic spinal transections can maintain near normal levels of systemic arterial blood pressure, and show cardiovascular and sympathetic reflex changes in response to a variety of stimuli. Thus, it seemed likely that the acute spinal cat could exhibit the spinal component of CO₂ mediated sympathetic neurogenic vascular tone. Therefore the effects of systemic CO₂ increases on the perfusion pressures of vascularly isolated hindlimbs autoperfused at constant flow (as an indication of vascular resistance and sympathetic vascular tone) were studied in the acute cervical spinal cat. The contributions of the lumbar sympathetic system and the adrenal glands to perfusion pressure responses to CO₂ were evaluated. Experiments were carried out in mongrel cats with acute cervical (C2) transections. Each cat had one leg denervated by cutting and stripping the lumbar sympathetic chain from L₁-L₇. In all cats each hindlimb was vascularly isolated and perfused with blood taken from the abdominal aorta. Bilateral adrenalectomy was performed on 8 animals. CO₂ administration for 5 minutes resulted in biphasic increases in the perfusion pressures of both legs which were designated peak1 (P1) and peak2 (P2). Increasing PCO₂ from 16 to 38mm Hg, and from 16 to 62mm Hg resulted in significant P1 and P2 responses of the innervated leg. This also resulted in a significant P2 response, and an observable but insignificant P1 response, of the denervated leg. Adrenalectomy reduced P1 and P2 responses of the innervated leg, and abolished the P1 response and reduced the P2 response of the denervated leg. In non-adrenalectomized cats increasing PCO₂ also resulted in a significant increase in systemic arterial pressure (SAP) with no changes in heart rate (HR). In adrenalectomized cats increasing PCO₂ resulted in an observable but non-significant increase in SAP and a significant decrease in HR. These results suggested that: 1) The P1 response was primarily a sympathetic neurogenic response to increased CO₂. 2) The P2 response was primarily a hormonal response to CO₂ in the denervated leg, and a combination of hormonal and sympathetic neurogenic responses to CO₂ in the innervated leg. 3) The adrenal glands were mostly involved in the P2 response to CO₂, but possibly had a small role in the P1 response. 4) Other non-adrenal vasoconstrictor hormones may have played a role in the P2 response to C0₂. 5) Likely, CO₂ initially activated the sympathetic system to directly increase neurogenic tone, perhaps by stimulating sympathetic afferent or efferent neurons, or hypothetical spinal chemosensitive regions. Progressively the adrenal and possibly other unidentified vasoconstrictor hormone systems became activated, either directly by CO₂ or indirectly by CO₂ mediated sympathetic activation. These hormone systems may have also played a role in CO₂ mediated maintenance of vascular tone. / Medicine, Faculty of / Cellular and Physiological Sciences, Department of / Graduate