We continually encounter different ground terrain such as slippery, compliant, uneven, rocky, and irregular terrain when walking, yet we know very little about how individuals safely negotiate this type of complex environment. Furthermore, we know little about how aging affects stability in these situations despite the increased risk of falls and fall-related injuries among older adults. Paramount to our comprehension of how individuals safely traverse challenging ground terrain is to understand how visual information is utilized as vision is the first line of defense for preparing for and/or avoiding potentially hazardous terrain or obstacles. Thus, the objective of this thesis was to provide a better understanding towards how individuals negotiate different ground terrain in the environment to maintain dynamic stability and prevent the occurrence of a fall. In particular, the role of vision and the effects of aging were investigated. Three studies focused on the role of vision while negotiating varying ground terrain while two studies examined stability across these surfaces. Two main conclusions can be drawn from the results of the three studies on the role of vision. First, regardless of age individuals fixate on highly task-relevant areas (i.e. surfaces eventually stepped on) in an on-line manner and by fixating approximately two steps ahead. Second, visual information from the lower visual field is important for negotiating varying ground terrain. This latter finding has implications for older adults who wear multi-focal glasses and suggests that these individuals should be cautious when wearing these glasses in complex environments. In terms of stability, the results suggest that young and older adults demonstrate greater instability when walking across varying unstable ground terrain compared to solid level ground. Older adults are particularly more unstable in the medial-lateral direction when negotiating the challenging terrain, which may explain the frequency of laterally directed falls and increased hip-fracture risk with advancing age. Interestingly, older adults appear more stable in the anterior-posterior direction; although, this can largely be explained by the cautious gait strategy (i.e. slower walking speed and shorter steps) adopted by these individuals. The results of the studies of my thesis provide valuable insight into how individuals safely negotiate different types of challenging ground terrain when walking. Importantly, this knowledge can serve as an initial step in attempting to reduce falling among those at risk.

Mice that are incapable of expressing the small G protein Rhes have been generated and have shown to have abnormalities in behaviors mediated by the striatum, a region in which Rhes is highly expressed. Moreover, conditions that result in dopamine supersensitivity and a breakdown in D1/D2 synergism in rodents, consistently decrease rhes mRNA in striatum. Thus, Rhes may play have relevance in dopamine signal modulation. For evaluating the role of Rhes in anxiety, stereotypy and basal motor activity, adult male and female wild-type (WT) mice, Rhes knockout (KO) mice, and mice heterozygous for the KO and WT alleles (Het) were tested. There was no genotype differences in the distance traveled in the open field. However, female KO mice showed lower anxiety than either WTs or Hets, based on the quantity of time spent in the periphery vs. the central area of the open field (p<0.05). With respect to striatally-mediated motor stereotypy, the mixed D1/D2 agonist apomorphine elicited a significant greater response in male KO and Het compared to WTs (p<0.05). In previous studies of D1/D2 synergism, it has been consistently found in rats and mice that when D2 receptors alone are stimulated, there is an early and brief, D1 independent peak in stereotypy that disappears by 20 minutes. In the present study, this effect was more intense in male KO mice compared to the other two genotypes during the interval between 5 and 10 minutes (p<0.05). The current findings favor the hypothesis that the GTP-binding protein Rhes interacts with as yet unidentified cellular proteins to buffer the transduction of synaptic dopamine signals into intracellular responses. Decreased or loss of Rhes therefore results in increased DA signal transduction.

Thesis (Ph. D.)--Ohio State University, 2006. / Title from first page of PDF file. Includes bibliographical references (p. 129-150).

This thesis examined two aspects of information processing by the feline spinal cord during centrally-evoked motor activity: 1) the modification of transmission from different sensory afferents and 2) the neuronal elements of reflex pathways from group II muscle afferents during fictive motor behaviours (i.e motoneuron activity under neuromuscular blockade). Fictive locomotion was evoked by electrical stimulation in the midbrain and fictive scratch was triggered by stimulation of the skin covering the ears following curare application to cervical dorsal roots in decerebrate in vivo feline preparations. Both monosynaptic and longer latency components of muscle and cutaneous afferent-evoked field potentials were reduced in amplitude during fictive locomotion and scratch, but field potentials evoked by muscle group II afferents were suppressed more than those evoked by cutaneous and group I muscle afferents recorded at the same spinal locations. The novel finding, that field potentials evoked at the same spinal locations by muscle and cutaneous afferents are suprressed differently, suggests that there is a preferential and non-uniform control of transmission from muscle and cutaneous fibres during motor activity. Extracellular recordings from neurons within the lumbar spinal segments showed that suppression of group II afferent input during fictive motor activity results in a powerful reduction of the activation of neurons with input from muscle group II afferents in 93% of the examined neurons after short trains of stimuli were delivered to peripheral nerves. However, more neurons remained recruitable by group II intensity stimulation if train duration was sufficiently long with only 33% showing a reduction in sensory-evoked firing. The majority of the neurons that remained responsive to muscle group II afferent input during fictive locomotion had axonal projections to supralumbar, or supraspinal areas and showed spontaneous, often rhythmic, firing activity. Overall, the studies presented in this thesis provide insights into the mechanisms by which the mammalian spinal cord processes sensory information and on how sensory input is able to control motor activity in spite of suppressive control provided by the nervous system. / October 2006

Thèse de doctorat--Sciences naturelles--Paris--Faculté des sciences, 1872. N°: 340.

Reproduction de : Thèse de doctorat : Sciences Naturelles : Faculté des sciences de Paris : 1874. / Titre provenant de la page de titre du document numérisé.

Thesis (M. Phty.)--University of Queensland, 2002. / Includes bibliographical references.

Thesis (M. Phil.)--University of Hong Kong, 2002. / Includes bibliographical references.

No description available.

No description available.