Changes in fusimotor discharge rate provoked by isotonic fatiguing muscle contractions in decerebrate cats

Changes in discharge rate of 26 fusimotor neurones to medial gastrocnemius muscle were studied during isotonic fatiguing contractions of lateral gastrocnemius and soleus muscles in decerebrate cats. Muscle contractions were elicited by either continuous or repetitive electrical stimulation of the muscle nerves. The muscles were considered to be fatigued when, against a load equal to one third of the tension developed at the onset of an isometric contraction, i.e. to the tension indicating isometrically-induced fatigue, they returned to the length at which the isometric contraction was elicited. At the onset of muscle contraction an increase in discharge rate, lasting for 5-220 s, occurred in all except one of the neurones. In 73% of the units a late increase developed in addition in parallel with muscle fatigue outlasting the contraction for 5-180 s. All but one of the remaining neurones exhibited a short lasting burst of spike discharges coincident with the end of contraction. Enhancement of the late increase by muscle ischaemia indicates contribution of chemosensitive small-diameter muscle afferents, while the short lasting burst is supposed to be elicited rather by the mechanosensitive units sensitized by metabolic products liberated during contraction and/or fatigue. Differences of the fusimotor reflex responses to isotonic vs. isometric contraction and/or fatigue and their possible functional role are discussed.