Postnatal development of the discharge pattern of phrenic motor units in the kitten.

The postnatal change of the mean frequency (F), the maximal frequency (FM) and the onset frequency (FO) of discharge of kitten phrenic motor units was studied and compared to adult values. The latency (recruitment time) and duration of discharge of phrenic units were also analyzed. In kittens less than 3 weeks old, there were relatively few early units (latency less than or equal to 10% of phrenic discharge duration), TI). The duration of discharge of early and late units, expressed in percentage of TI, was the same in kittens and adult cats, and the duration of discharge of the early units was greater than that of the late units. In kittens, F, FM and FO of the early and late units were always greater than in adult cats. In adult cat, as in Kittens, F, FM and FO of early units were not significantly greater than those of late units. In conclusion, the relatively small number of early units in kittens may reflect either a small number of active early bulbo-spinal neurons or may be linked to the electrical and morphological properties of phrenic motoneurons in the kitten.