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.
| Identifer | oai:union.ndltd.org:ADTP/243111 |
| Date | January 2007 |
| Creators | Martin, Peter Glen, Medical Sciences, Faculty of Medicine, UNSW |
| Publisher | Awarded by:University of New South Wales. Medical Sciences |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright Peter Glen Martin, http://unsworks.unsw.edu.au/copyright |
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