CHANGES IN FUSIMOTOR ACTIVITY DURING REPETITIVE LENGTHENING MUSCLE CONTRACTIONS IN DECEREBRATE CATS

Responses of fusimotor neurons to lengthening vs isometric contraction s have been studied in decerebrate cats. Spike discharges of fusimotor neurons to the medial gastrocnemius muscle were recorded From this mu scle nerve filament during sequences of contractions and/or stretches of the lateral gastrocnemius and soleus muscles. The sequences lasted for 250-450 s (duty cycle 4:2 s). Isometric contractions were elicited by electrical stimulation (40 Hz, 1.3 times motor threshold) of the m uscle nerves. Lengthening contractions were elicited in the same way w hile the muscles were stretched by 4 mm at a velocity of 1 mm/s. Of 25 fusimotor neurons studied, 23 responded to muscle contractions with a n increase in firing rate, subsiding towards the end of the sequence. The increase was either modulated with each subsequent contraction or smooth throughout the sequence. Approximately 64% of fusimotor neurons , responding to muscle contractions, responded in a similar way to the sequences of muscle stretches, applied alone. Responses to sequences of the lengthening contractions were significantly larger, on average, than those to the isometric ones, but smaller than the sum of the res ponses to the contractions and stretches applied separately. On the ot her hand, they were also larger in fusimotor units, showing no overt r esponses to muscle stretches alone. (C) 1998 IBRO. Published by Elsevi er Science Ltd.