THRESHOLD-SPACING IN MOTONEURON POOLS OF RAT AND CAT - POSSIBLE RELEVANCE FOR MANNER OF FORCE GRADATION

In the context of an analysis concerning factors of importance for the relative contributions of recruitment and rate gradation of muscle fo rce, the distribution of electrical excitability was analyzed for medi al gastrocnemius (MG) motoneurones of rat and cat. The experimental da ta came from previously collected intracellular measurements in animal s anaesthetized with pentobarbitone. Electrical excitability was measu red as the threshold (nanoamperes) for single spike generation (rheoba se) in rat and for maintained repetitive firing (rhythmic threshold) i n cat. Furthermore, the data included measurements of axonal conductio n velocity and of contractile properties of the muscle units innervate d by the studied motoneurones. The units were categorized into types S (slow-twitch, fatigue-resistant), FR (fast-twitch, fatigue-resistant) and FF (fast-twitch, fatiguable) on the basis of the combined criteri a of twitch-speed and sensitivity to fatigue. We confirmed that, in sp ite of the presence of normal-looking symmetrical distributions of axo nal conduction velocity, there was a positive skew in the distribution of electrical excitability (relatively high numbers of cells with low thresholds, few with high ones). Within each unit category (S, FR, FF ), we ranked the motoneurones according to their relative electrical e xcitability and calculated the threshold difference between consecutiv e cells (''threshold spacing''). In accordance with the skewed distrib ution of electrical excitability, we found that the mean threshold spa cing was ranked in the same way as the mean thresholds, i.e. S<FR<FF; the statistical analysis showed that, for cats as well as rats, small threshold-spacing steps were significantly more common for S than for FF motoneurones. In the discussion it is pointed out that the narrow t hreshold-spacing for S units, as compared to FF units, would tend to d ecrease the relative amount of recruitment-parallel rate modulation in these cells. Thus, the spacing of recruitment thresholds tends to all ow the easily recruited S motoneurones to remain firing at relatively low rates during ongoing recruitment gradation, which would be of pote ntial value in promoting a high degree of endurance for long-lasting p ostural contractions.