PLASTICITY OF SPINAL REFLEXES
R.M. Herman (NeuroBiology Institute, Scottsdale, AZ, USA); S.C. D'Luzansky (Good Samaritan Regional Medical Center, Phoenix, AZ, USA)
Plasticity of spinal reflexes following chronic supraconal spinal cord lesions can be interpreted in terms of: 1) interruption of descending inhibitory pathways, thereby unmasking or releasing spinal reflex mechanisms inclusive of neurons 'switching' their fundamental function, 2) expansion and reorganization of afferent projections, 3) increased concentrations of neuropeptides pari passu with anatomical changes, and 4) denervation supersensitivity of various receptors in the dorsal horn. The latter suggests up-regulation of opioidergic, GABAergic, alpha-adrenergic, and other mechanisms which may be fundamental to functional plasticity of the spinal cord circuitry.
MOTOR REORGANIZATION IN THE MOTOR CORTEX
Yukio Mano M.D. (Hokkaido University, Sapporo, Japan)
Plasticity within the human central motor system has been studied with transcranical magnetic stimulation in patients with peripheral and central nervous diseases.
In 8 patients with chronic cerebral infarction with hemiplegia. Four of 8 patients did not show MEPs in paralytic hand muscles by contralateral cortex stimulation, but showed small MEPs by ipsilateral cortex stimulation. These cases had the huge cortical infarction unilaterally in CT. Another four of 8 patients showed small MEPs in paralytic hand muscles by contralateral cortex stimulation, but no MEPs by ipsilateral cortex stimulation. These cases had the subcortical infarction unilaterally in CT. These findings suggest that reorganization of motor cortex following unilateral cerebral infarction.
In 4 patients with a complete upper limb palsy due to traumatic cervical root avulsion, surgical anastomosis of intercostal to musculocutaneous nerves was performed to restore function in the biceps brachii muscle. The motor unit discharges became independent from respirations gradually over 1 to 2 years. Motor cortex mapping of the reinnervated biceps muscle showed a gradual change over 4 to 33 months from the area of the intercostal muscles to that of the arm area, which was more lateral on the motor cortex. These findings suggest that reorganization of the motor cortex to arm flexor muscles occurs following peripheral nerve anastomosis.
USE OF TRANSCRANIAL MAGNETIC STIMULATION IN REHABILITATION MEDICINE
J.C. ROTHWELL, MRC, Human Movement & Balance Unit, Institute of Neurology, Queen Square, London, WC1N 3BG, UK.
Transcranial magnetic stimulation over the motor cortex has been used traditionally to examine the integrity of the most rapidly conducting portion of the corticospinal tract. Studies in children and infants have documented an increase in conduction velocity and decrease in threshold of stimulation over the first 4 years of life because it is probably related to the continuing process of myelination of the corticospinal tract. The data can now be used to identify abnormalities of conduction at an early age in brain damaged children. At the other end of the age range, several studies have now confirmed that the response to transcranial magnetic stimulation in the first few days after stroke can give a reasonable prognostic indicator of final outcome. Patients in whom no response at all can be elicited tend to have poor or no recovery whereas those with normal or delayed responses go on to make reasonable recovery. There is little evidence that functional recovery after stroke is related to changes in the excitability of the large diameter component of the corticospinal system. However, it is now possible to study slow, probably poly-synaptic (perhaps cortico-reticular spinal) pathways from cortex to spinal cord using a double pulse method of stimulation. These pathways appear to be bilateral, at least to more proximal muscles, and therefore may allow the undamaged hemispheres to take control of muscles on the affected side.