The arrangement of forearm motoneurons in young and adult rats and the possibility of naturally occurring motoneuron death.

The normal number and arrangement of motoneurons contributing fibres, via the median and radial nerves, to the forearm muscles of the rat were determined at different stages of postnatal development. Horseradish peroxidase was applied directly to the proximal cut ends of the nerves in 5, 10, 21 days old and adult rats. At each stage of development, the motoneuronal pool for each nerve was confined to the ipsilateral, dorsolateral area of Lamina IX. In general, median nerve motoneurons were found dorsal and medial to those of the radial nerve and were located between the rostral parts of the C6-T1 segments of the spinal cord. The radial nerve motoneuronal pool extended between the caudal end of the C4 and rostral tip of the T1 segment. There was considerable overlap between the two populations, especially along the dorsoventral axis. Counts of labelled motoneurons revealed that significantly fewer were present in adult compared to 5 days old rats (P less than 0.05 for each nerve). Approximately 50% of the motoneurons were lost from each nerve over this period. Although results from other studies suggest that part of the apparent loss may have been due to deficiencies in the HRP tracing technique, the possibility of motoneuronal death cannot be entirely excluded in this study.