J. CELICHOWSKI, P. KRUTKI, D. £OCHYÑSKI, K. GROTTEL, W. MRÓWCZYÑSKI TETANIC DEPRESSION IN FAST MOTOR UNITS OF THE CAT GASTROCNEMIUS MUSCLE Department of Neurobiology, University School of Physical Education, Poznañ, Poland Ability of muscle fibers to generate force is decreased when higher frequency of stimulation of motor units immediately follows lower frequency. This phenomenon called tetanic depression was found in rat medial gastrocnemius. However, it was not clear whether tetanic depression occurred only in rat muscle or it concerns all mammals. This study was conducted on motor units of cat medial gastrocnemius. Analyses were made at three successive trains of stimulation: 30 Hz, 20 and 30 Hz and again 30 Hz (the first pattern) or 40 Hz, 25 and 40 Hz and 40 Hz (the second pattern). In all fast units force generated within the middle tetanus was lower than force generated at the same, but constant frequency of stimulation applied earlier or later. The mean tetanic depression in 30 Hz tetani amounted to 10.9% for fast fatigable (FF) and 15.9% for fast resistant (FR) motor units, whereas in 40 Hz tetani mean values were 5.6% and 7.3% for FF and FR motor units, respectively. In slow motor units tetanic depression was not observed. These results proved the existence of tetanic depression in the feline muscle and indicated that its intensity depends on the fusion of tetanus. It has been concluded, that the tetanic depression is a general property of fast motor units in mammals. Key words: contractile force; skeletal muscle; stimulation frequency; fusion index; cat. INTRODUCTION Fast motor units are very sensitive to even very small changes in the pattern of interpulse intervals of a motoneuronal firing (1-3) and therefore, force of unfused tetani of fast motor units is highly susceptible to modifications during their activity. The motor unit force may be influenced by several phenomena: potentation of the unfused tetanus (4, 5), the presence of "sag" profile (6, 7), as well as "catch effect" (1). Tetanic depression is the next factor modifying force JOURNAL OF PHYSIOLOGY AND PHARMACOLOGY 2004, 55, 2, 291303 www.jpp.krakow.pl