Submitted on July 26, 2007
Accepted on January 31, 2008
Development of tonic firing behavior in rat soleus muscle
Torsten Eken1*, Geoffrey C. B. Elder2, and Terje Lomo2
1 Department of Physiology, Institute of Basic Medical Sciences, Faculty of Medicine, University of Oslo, Oslo, Norway; Department of Anaesthesiology, Aker University Hospital, Oslo, Norway
2 Department of Physiology, Institute of Basic Medical Sciences, Faculty of Medicine, University of Oslo, Oslo, Norway
* To whom correspondence should be addressed. E-mail: torsten.eken{at}medisin.uio.no.
Tonic firing behavior in soleus muscle of unrestrained rats aged 7 to at least 100 days was studied by chronic single-motor-unit and gross-EMG recordings. Median motor-unit firing frequency at 10 days was 19 – 26 Hz and did not change appreciably after this time, whereas interval-to-interval firing variability was reduced with age. Two units with median frequencies 40 and 59 Hz were encountered in one 7-day-old rat. Integrated rectified gross EMG developed from being phasic only to predominantly tonic during the second and third postnatal week. From the end of the third week, rather short tonic periods with irregular amplitude were replaced by longer lasting constant-amplitude periods. Quantitatively, median duration of gross-EMG activity episodes more than doubled, while 90-percentile values for episode duration increased 19-fold, from 7.4 s at 7 days to 140 s in adults. The main part of this increase took place after 22 days. Previous work in adult rats has indicated that descending monoaminergic innervation is essential for maintained tonic motoneuron activity, which probably is caused by depolarizing plateau potentials. Such innervation of the lumbar spinal cord matures gradually to an adult pattern and density about 3 – 4 weeks after birth. The present results, describing a concurrent considerable development of tonic firing behavior, support and extend these findings.
