Neurophysiology Of Spared Motor Tracts In Spinal Cord Injury

Recent experimental and therapeutic initiatives have been directed towards enhancing the survival and function of preserved central axons following spinal cord injury (SCI). The continued development of these initiatives depends largely on the sensitivity of techniques to detect the presence of residual innervation in descending motor tracts. Detection of preserved innervation in SCI patients provided the focus of the present thesis.;Preserved motor innervation was investigated in patients with established SCI using transcranial magnetic stimulation of the motor cortex to elicit motor evoked potentials (MEPs) in muscles innervated below the level of the lesion. In particular, a set of experiments was designed to enhance the probability of eliciting MEPs or detecting subliminal innervation in patients with SCI.;Experiment 1 tested the hypothesis that cutaneous afferent stimulation facilitates MEPs in lower limb muscles. This was demonstrated and therefore may be used to reveal latent but preserved innervation in SCI patients.;Experiment 2 tested the hypothesis that induced whole body hypothermia would enhance the detection of MEPs in control subjects and patients with SCI. While MEP amplitudes were significantly (p {dollar}\u3c{dollar}.05) enhanced in control subjects and some high functioning SCI patients, hypothermia was not helpful in revealing latent innervation in patients with severe SCI.;Experiments 3 and 4 used subthreshold and suprathreshold cortical conditioning of lower limb H-reflexes to reveal preserved short and long latency facilitation of lumbosacral motor neurons in control subjects and SCI patients. The principal finding was that residual subthreshold descending influences in patients with SCI, which were previously undetected by clinical assessment or cortical stimulation, were detected by cortical conditioning of H-reflexes in some patients with severe SCI.;A second important finding was the detection of late facilitation (60-150 ms) following subthreshold cortical stimulation. This result establishes descending supraspinal innervation as a potential source of the late excitatory synaptic inputs. Cortical conditioning of H-reflexes provides a viable new means to detect preserved innervation in descending motor tracts.;Collectively, these results provide support for the emerging concept that patients with SCI may possess intact but latent innervation despite the absence of useful sensory or motor function