These data, together with previously published anatomic and radiologic studies, are consistent with activity-dependent corticospinal axonal withdrawal during development and maintenance of increased corticomotoneuronal projections from the intact hemisphere after unilateral perinatal lesions.

From studies of subhuman primates it has been assumed that functional corticospinal innervation occurs post-natally in man. We report a post-mortem morphological study of human spinal cord, and neurophysiological and behavioural studies in preterm and term neonates and infants. From morphological studies it was demonstrated that corticospinal axons reach the lower cervical spinal cord by 24 weeks post-conceptional age (PCA) at the latest. Following a waiting period of up to a few weeks, it appears they progressively innervate the grey matter such that there is extensive innervation of spinal neurons, including motor neurons, prior to birth. Functional monosynaptic corticomotoneuronal projections were demonstrated neurophysiologically from term, but are also likely to be present from as early as 26 weeks PCA. At term, direct corticospinal projections to Group Ia inhibitory interneurons were also confirmed. Independent finger movements developed much later, between 6 and 12 months post-natally. These data do not support the proposal that in man, establishment of functional corticomotoneuronal projections occurs immediately prior to and provides the capacity for the expression of fine finger movement control. We propose instead that such early corticospinal innervation occurs to permit cortical involvement in activity dependent maturation of spinal motor centres during a critical period of perinatal development. Spastic cerebral palsy from perinatal damage to the corticospinal pathway secondarily involves disrupted development of spinal motor centres. Corticospinal axons retain a high degree of plasticity during axon growth and synaptic development. The possibility therefore exists to promote regeneration of disrupted corticospinal projections during the perinatal period with the double benefit of restoring corticospinal connectivity and normal development of spinal motor centres.

SUMMARY1. Group Ia EPSPs were recorded from lumbosacral motoneurones in anaesthetized cats after almost complete section of the relevant dorsal roots. The EPSPs were usually of small amplitude (median value of 230 1V) and an averaging device was used to improve the definition of their time course.2. From a total of over 500 averaged EPSPs a smaller number (342) were subjected to analysis. The other EPSPs were rejected either because they showed signs of multiple origin in the rising phase of their time course (see Methods) or because the resting membrane potential of the cell was less than 50 mV. All the selected EPSPs had their rise time (from the 10 to the 90 % level) and half-width measured, and a semilogarithmic plot of their decay time course was made.3. 252 of the EPSPs showed an exponential decline in their later time course and the slope of this line was used to give an estimate of the membrane time constant. The range of the time constant for different motoneurones was 2 3-12 9 msec, with a mean value of 5-8 msec.4. In ten cells an EPSP was recorded which was judged to be generated exclusively by synaptic knobs located on the soma. J. J. B. JACK AND OTHERSThe estimated value of a ranged from 18 to 65. A positive correlation was found between ax and Tm, indicating that for these EPSPs the duration of current injection was independent of the membrane time constant. The peak time of the wave form of current injection was between 0.1 and 0*25 msec.The estimates ofpO, were not thought to be very accurate. A lower limit of 4 was assumed and the highest measured value was 12, but in three cells the time course of the EPSP could not be fitted even with a very high value of p,,. Some possible explanations for this discrepancy are mentioned in the Discussion.The electrotonic length of the dendrites (L) was usually greater than l O A and ranged between 0 75 and 1-5 A. Evidence for an open-circuit termination of the dendrites was found in some cells.5. The normalized values of the rise time and half-width were used to make an electrotonic distance allocation to the 246 EPSPs which were judged to be non-somatic. The method of allocation was not precise because individual values of p, and L were not available for these motoneurones. Instead, a maximum possible range was assumed: for po,, 4-25; for L, 0-75-1*5. The range of a was also assumed, from 12 to 100. With these values the motoneurone model (Jack & Redman, 1971 b) was used to set limits within which the normalized rise time and half-width of all EPSPs, generated by current at a single point, should lie. Twenty of the 246 EPSPs lay outside these boundary lines and hence they did not receive a distance allocation. The remaining 226 were assigned values between 0-2 and 1F6 A (in 0-2 A steps); the majority of the allocations (183) were to the proximal electrotonic part of the dendrites (0-2, 0-4 or 0-6 A).The relationship of these distance allocations to the histological results of Conradi (1969) is discussed.6. It is concluded that there is no good evidence a...

5. The conduction delays in the peripheral components of both motor and somatosensory pathways also decrease initially but then from the age of 5 years progressively increase in proportion to arm length.6. The threshold stimulus intensity for evoking muscle responses following electromagnetic stimulation of the cortex is high initially and falls progressively until the age of 16 years. A linear relationship exists between the threshold intensity and height for the height range 70-180 cm. 7. The threshold stimulus intensities for exciting peripheral motor and somatosensory nerves decrease up to the age of 5 years and then reach a plateau.8. The results support the conclusion, already reported in the literature that peripheral nerves attain maximum value for fibre diameter and conduction velocity at approximately 5 years of age.9. In contrast, it is concluded that the maximum fibre diameters in both motor and somatosensory central pathways increase in proportion to height, leading to constant central conduction delays with growth.

In adult paraplegic subjects one tibialis anterior muscle received daily electrical stimulation for 4 weeks at twice the motor threshold to determine changes of morphological and histochemical profiles (this paper) and of contractile properties (preceding paperV Bilateral biopsies, obtained 4 weeks before, and immediately after, electrical stimulation, were studied for fibre type proportions, fibre diameters, oxidative capacity, microvascu lature and histopathology, Before stimulation the biopsies showed disuse with increased type 2 fibre proportions and decreased oxidative capacity (succinate dehydrogenase (SOH) activity). The effects of two stimulus patternsl were compared. Following stimulation SOH activity increased significantly in all stimulated muscles. Inconsistent changes occurred in fibre type proportions, fibre diameters, capillary density and capillary/fibre ratios, Both stimulus patterns evoked similar results. In five/seven subjects sub sarcolemmal vacuolation was observed, Electrical stimulation for 4 weeks at only twice motor threshold improves oxidative capacity, but different stimulus parameters are probably needed for significant fibre type conversion.