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Spinal somatosensory evoked potentials (SEP) were recorded in 58 normal mice (C3H strain) divided into 4 groups according to age (3-, 6-, 9- and 12 weeks). Monopolar recordings of spinal SEP were made by subdermal needle electrodes from 3 vertebral levels, ‘low-lumbar’, ‘high-lumbar’ and ‘mid-thoracic’, by stimulating the tibial nerve bilaterally at the ankle. Three negative peaks, NI, NII and NIII, presumably due to conduction through muscle afferents, cutaneous afferents (in the dorsal root or dorsal white column) and spinocerebellar tract, respectively, were recorded at the high-lumbar level in the 12-week-old mouse. Besides the NI and NII peaks, a small ventral root potential was also occasionally recorded at the low-lumbar level. At the mid-thoracic level, only NI and NIII were recordable. At both the high-lumbar and mid-thoracic levels, the negative peaks were superimposed over long duration ‘summation potentials’ of opposite polarities. Well-defined standing potentials were also recorded at these two levels. The standing potentials could be the ‘entry point potential’ due to the entry of S1 root into the spinal cord at the T13 vertebral level. The summation potential presumably is due to a fixed generator located between the T7 and T12 vertebral levels resulting from intense synaptic activity at this level. In 3- and 6-week-old mice, the entry point potential was recorded in the low-lumbar SEP also, possibly due to less axial growth of the vertebral column at this stage of development. The summation potential and NIII peak were reduced in size in these young mice, possibly due to less developed collaterals and synaptic activity. The somatosensory conduction velocity, measured between the low-lumbar and mid-thoracic recording sites, showed a highly significant increase during the 3–6 and 6–9 week periods, suggesting that a significant amount of myelination occurs in proprioceptive fibres postnatally in mice.
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Accepted: October 20, 1993
Received: August 3, 1993
© 1994 Elsevier Science B.V. All rights reserved. Published by Elsevier Inc.