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All motor behaviours are expressed via the activation of alpha motoneurones, the final common path of the central nervous system. The corticospinal tract conveys neural information from the motor cortex to motoneurones. This thesis focuses on the corticospinal control of human motoneurones during voluntary contraction and fatigue. First, output of motoneurones to corticospinal inputs is described for a wide range of contraction strengths. Results show that motoneurones become less responsive during strong contractions whereas motor cortical output cells are not limited in the same way. Comparison of motoneurone output to different strength corticospinal inputs and of different motoneurone pools demonstrates an important role for motor unit firing rates in determining the excitability of motoneurones during strong contractions. Next, the reflex actions of group III and IV muscle afferents on motoneurones are investigated. These studies address a long and ongoing debate about the role of these afferents to the slowing of motor unit firing rates during sustained contractions. It was believed that these afferents inhibit motoneurones and contribute to fatigue. However, findings demonstrate that human motoneurones innervating extensor and flexor muscles are not uniformly affected by fatigue-sensitive afferents. Thus afferent inputs from homonymous and antagonist muscles depress extensor motoneurones but facilitate flexor motoneurones. When group III and IV muscle afferents are activated by hypertonic saline, motoneurones of both extensors and flexors are facilitated. This demonstrates parallel excitatory and inhibitory pathways from group III and IV muscle afferents to extensor motoneurones, which are activated under different conditions. Furthermore, the excitation is more pronounced for high-threshold motoneurones. In addition to the effects mediated at motoneurones, activity in group III and IV afferents inhibits motor cortical cells. The final studies investigate changes in the cervical propriospinal pathway with fatigue. This pathway transmits part of the voluntary drive to motoneurones, in parallel with the direct corticospinal pathway. The studies demonstrate that during fatigue, there are coordinated changes in the excitation mediated via this pathway to motoneurones of both fatigued and non-fatigued muscles of the upper limb. In summary, this thesis demonstrates novel aspects of the corticospinal control of motoneurones during voluntary contraction and fatigue.

Thesis (M.Sc.)--York University, 2004. Graduate Programme in Kinesiology and Health Science. / Typescript. Includes bibliographical references. Also available on the Internet. MODE OF ACCESS via web browser by entering the following URL:http://gateway.proquest.com/openurl?url%5Fver=Z39.88-2004&res%5Fdat=xri:pqdiss&rft%5Fval%5Ffmt=info:ofi/fmt:kev:mtx:dissertation&rft%5Fdat=xri:pqdiss:MQ99349

The focus of this research is on the control of random cracking in concrete paving by using sawcut notch locations in the early stages of construction. This is a major concern in concrete pavement construction. This research also addresses a probabilistic approach to determine the optimum time and depth of sawcutting for newly constructed portland cement concrete pavements. Variability in climate conditions and material characteristics during the hardening process affects the potential of cracking at any sawcut depth. Several factors affecting the probability of crack initiation are material strength parameters, method and quality of curing, slab/subbase stiffness, the amount and depth of steel reinforcement, friction between the slab and the subbase, and concrete shrinkage. Other factors relevant to concrete mixture characteristics such as cement content and type of coarse aggregate affect development of early aged stresses caused by shrinkage and thermally induced contraction. A probabilistic analysis of the factors that affect crack control using sawcut notches is presented in relation to different weather conditions (concrete placement temperature) at the time of construction, and concrete mixture characteristics such as fly ash replacement (FA) and cement factor (CF). Both of these significantly affect sawcut timing and depth requirement. The determination of crack initiation is based on fracture mechanics. Estimation of the time of cracking is based on predicted tensile strength and stress in the concrete at the bottom of the sawcut notch to assess the feasibility of crack control in the early stages of construction.

Although lymphatic vessel behavior is analogous to hearts (e.g. systole and diastole) and blood vessels (e.g. basal tone), hearts and blood vessels have fundamentally different contractile properties. While summation during contraction is minimized in the heart, summation is necessary for tonic contraction in blood vessels. Because lymphatic vessel behavior mimics cardiac and vascular behavior, we hypothesized that above a critical contraction frequency there is significant summation, evidenced by significantly increased diastolic active tension (i.e. basal tone). We used an isovolumic, controlled-flow preparation to examine the interaction of contraction cycle-time with contraction frequency. Using segments of isolated lymphatic vessels (~1 cm in length and 3-4 mm in diameter) from bovine mesentery, we measured transmural pressure and diameter for end-diastole and end-systole during spontaneous contractions for 10 volume steps. We found time between contractions (beat-to-beat period) decreases with increasing diameter, and total contraction time (vessel twitch length, 11.08 ± 1.54 s) slightly increases with increasing diameter. At the intersection of these relationships, there is a critical period, below which the vessel does not have time to fully relax. Above the diameter at the critical period, diastolic active tension (end-diastolic minus passive vessel tension) significantly increases with increases in diameter (309 to 562% change in slope, p<0.0001), and, below the critical period, diastolic active tension increases with decreases in beat-to-beat period (712 to 2208% change in slope, p<0.0014). Because this transition occurs within a physiological range, it suggests summation may be crucial for lymphatic vessel function as a pump and a conduit.

The first chapter comprises a survey of valuations on totally ordered structures, developing notation and properties. A contraction map is induced by the exponential map on the value group $G$ of an ordered exponential field $K$ with respect to the natural valuation $v_{G}$. By studying the algebraic properties of Abelian groups with contractions, the theory of these groups is shown to be model complete, complete, decidable and to admit elimination of quantifiers. Hardy fields provide an example of non-archimedean exponential fields and of differential fields and therefore, they play a very important role in our research.<p>In accordance with Rosenlicht we define asymptotic couples and then give a short exposition of some basic facts about asymptotic couples. The theory $T_{P}$ of closed asymptotic triples, as defined in Section 2.4, is shown to be complete, decidable and to have elimination of quantifiers. This theory, as well as the theory $T$ of closed $H$-asymptotic couples do not have the independence property. The main result of the second chapter is that there is a formal connection between asymptotic couples of $H$-type and contraction groups.<p>A given valuation of a differential field of characteristic zero is a differential valuation if an analogue of l'Hospital's rule holds. We present in the third chapter, a survey of the most important properties of a differential valuation. The theorem of M. Rosenlicht regarding the construction of a differential field with given value group is given with a detailed proof. There exists a Hardy field, whose value group is a given asymptotic couple of Hardy type, of finite rank. We also investigate the problem of asymptotic integration.

The first chapter comprises a survey of valuations on totally ordered structures, developing notation and properties. A contraction map is induced by the exponential map on the value group $G$ of an ordered exponential field $K$ with respect to the natural valuation $v_{G}$. By studying the algebraic properties of Abelian groups with contractions, the theory of these groups is shown to be model complete, complete, decidable and to admit elimination of quantifiers. Hardy fields provide an example of non-archimedean exponential fields and of differential fields and therefore, they play a very important role in our research.<p>In accordance with Rosenlicht we define asymptotic couples and then give a short exposition of some basic facts about asymptotic couples. The theory $T_{P}$ of closed asymptotic triples, as defined in Section 2.4, is shown to be complete, decidable and to have elimination of quantifiers. This theory, as well as the theory $T$ of closed $H$-asymptotic couples do not have the independence property. The main result of the second chapter is that there is a formal connection between asymptotic couples of $H$-type and contraction groups.<p>A given valuation of a differential field of characteristic zero is a differential valuation if an analogue of l'Hospital's rule holds. We present in the third chapter, a survey of the most important properties of a differential valuation. The theorem of M. Rosenlicht regarding the construction of a differential field with given value group is given with a detailed proof. There exists a Hardy field, whose value group is a given asymptotic couple of Hardy type, of finite rank. We also investigate the problem of asymptotic integration.

Although lymphatic vessel behavior is analogous to hearts (e.g. systole and diastole) and blood vessels (e.g. basal tone), hearts and blood vessels have fundamentally different contractile properties. While summation during contraction is minimized in the heart, summation is necessary for tonic contraction in blood vessels. Because lymphatic vessel behavior mimics cardiac and vascular behavior, we hypothesized that above a critical contraction frequency there is significant summation, evidenced by significantly increased diastolic active tension (i.e. basal tone). We used an isovolumic, controlled-flow preparation to examine the interaction of contraction cycle-time with contraction frequency. Using segments of isolated lymphatic vessels (~1 cm in length and 3-4 mm in diameter) from bovine mesentery, we measured transmural pressure and diameter for end-diastole and end-systole during spontaneous contractions for 10 volume steps. We found time between contractions (beat-to-beat period) decreases with increasing diameter, and total contraction time (vessel twitch length, 11.08 ± 1.54 s) slightly increases with increasing diameter. At the intersection of these relationships, there is a critical period, below which the vessel does not have time to fully relax. Above the diameter at the critical period, diastolic active tension (end-diastolic minus passive vessel tension) significantly increases with increases in diameter (309 to 562% change in slope, p<0.0001), and, below the critical period, diastolic active tension increases with decreases in beat-to-beat period (712 to 2208% change in slope, p<0.0014). Because this transition occurs within a physiological range, it suggests summation may be crucial for lymphatic vessel function as a pump and a conduit.

The focus of this research is on the control of random cracking in concrete paving by using sawcut notch locations in the early stages of construction. This is a major concern in concrete pavement construction. This research also addresses a probabilistic approach to determine the optimum time and depth of sawcutting for newly constructed portland cement concrete pavements. Variability in climate conditions and material characteristics during the hardening process affects the potential of cracking at any sawcut depth. Several factors affecting the probability of crack initiation are material strength parameters, method and quality of curing, slab/subbase stiffness, the amount and depth of steel reinforcement, friction between the slab and the subbase, and concrete shrinkage. Other factors relevant to concrete mixture characteristics such as cement content and type of coarse aggregate affect development of early aged stresses caused by shrinkage and thermally induced contraction. A probabilistic analysis of the factors that affect crack control using sawcut notches is presented in relation to different weather conditions (concrete placement temperature) at the time of construction, and concrete mixture characteristics such as fly ash replacement (FA) and cement factor (CF). Both of these significantly affect sawcut timing and depth requirement. The determination of crack initiation is based on fracture mechanics. Estimation of the time of cracking is based on predicted tensile strength and stress in the concrete at the bottom of the sawcut notch to assess the feasibility of crack control in the early stages of construction.