Our previous paper (Granit & Rutledge, 1960) showed that recurrent inhibition silences the discharge of a motoneurone which is feebly supported by excitatory drive, even in the face of constant depolarizing pressure as defined in that work. One answer was also provided thereby to the general question of why sense organs and interneurones fire at frequencies which are in excess of immediate apparent needs. In the present study every precaution was taken to maintain afferent excitation in excess of that barely needed to keep up a given rate of reflex firing to electrical stimulation of muscular afferents. This is because the main question here concerns the relationship between normal firing frequency Fn of a motoneurone and its rate of discharge Fi under recurrent inhibition. F, was now found to be a basically linear function of Fn. This finding proved to be of methodological interest in work on the physiological significance of recurrent inhibition. Holmgren & Merton (1954), making use of an analogy derived from electronics, suggested that recurrent inhibition has a stabilizing effect on the discharge from motoneurones. Defining stabilization for our purpose as the integrated net effect of one or several processes engaged in holding the rate of firing within relatively narrow limits, we have also tried below to scrutinize this concept as a biological proposition. (It is not our intention to elaborate an electronic analogy which may or may not be valid.) This means that special attention will be given to the factors which determine the upper and lower limits of the frequency range. Thus limitation of discharge frequency is possibly a more accurate term than 'stabilization'.We are not aware of any previous work concerned with the relation between Fn and Fi. The general problem of 'frequency limitation' may,
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