Changes in discharge rate of cat hamstring fusimotor neurones during fatiguing contractions of triceps surae muscles

Changes in the discharge rate of fusimotor neurones to hamstring muscles during long-lasting, fatiguing, contractions of the triceps surae muscles were studied in decerebrate cats. Discharges of fusimotor neurones were recorded from the nerve filaments. Muscle contractions were elicited by electrical stimulation of the muscle nerves applied until the muscle tension fell to about 30% of its initial value. Early and late changes could be recognized in fusimotor discharge rate. The early changes, at the onset of muscle contraction, occurred in 9 out of 22 neurones and varied in both sign and duration among the cells. The late change, encountered in 16 fusimotor neurones, was an increase in discharge rate developing towards the end of the muscle contraction and outlasting it. When the contracting triceps muscle was made ischaemic the late increase in discharge rate developed earlier, as did the muscle tension fall and started to subside after the arterial clamp was removed. After severing the muscle nerves their stimulation provoked either no changes or a slight sustained decrease in fusimotor discharge rate. It is supposed that the late increase in discharge rate of fusimotor neurones to hamstring muscles appears due to reflex excitation by discharges in group III and IV afferent fibres from the triceps muscle provoked and/or enhanced by metabolic products liberated during its fatiguing contraction. The possibility is raised that the excitation is elicited primarily by the discharges from chemosensitive afferent fibres. Its functional role in muscle fatigue is discussed.