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Threshold-spacing in motoneurone pools of rat and cat: possible relevance for manner of force gradation

Experimental Brain Research volume 102, pages 69–74 (1994)Cite this article

Abstract

In the context of an analysis concerning factors of importance for the relative contributions of recruitment and rate gradation of muscle force, the distribution of electrical excitability was analyzed for medial gastrocnemius (MG) motoneurones of rat and cat. The experimental data came from previously collected intracellular measurements in animals anaesthetized with pentobarbitone. Electrical excitability was measured as the threshold (nanoamperes) for single spike generation (rheobase) in rat and for maintained repetitive firing (rhythmic threshold) in cat. Furthermore, the data included measurements of axonal conduction velocity and of contractile properties of the muscle units innervated 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 criteria of twitch-speed and sensitivity to fatigue. We confirmed that, in spite of the presence of normal-looking symmetrical distributions of axonal 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 excitability and calculated the threshold difference between consecutive cells (“threshold spacing”). In accordance with the skewed distribution of electrical excitability, we found that the mean threshold spacing 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 threshold-spacing for S units, as compared to FF units, would tend to decrease the relative amount of recruitment-parallel rate modulation in these cells. Thus, the spacing of recruitment thresholds tends to allow the easily recruited S motoneurones to remain firing at relatively low rates during ongoing recruitment gradation, which would be of potential value in promoting a high degree of endurance for long-lasting postural contractions.

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Author notes

  1. Rob Bakels & Daniel Kernell

    Present address: Department of Medical Physiology, University of Groningen, Bloemsingel 10, NL-9712, Groningen, KZ, The Netherlands

Affiliations

  1. Graduate School Neurosciences Amsterdam, Department of Neurophysiology, University of Amsterdam, Academisch Medisch Centrum, Meibergdreef 15, NL-1105, Amsterdam, AZ, The Netherlands

    Rob Bakels & Daniel Kernell

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  1. Rob Bakels
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  2. Daniel Kernell
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Bakels, R., Kernell, D. Threshold-spacing in motoneurone pools of rat and cat: possible relevance for manner of force gradation. Exp Brain Res 102, 69–74 (1994). https://doi.org/10.1007/BF00232439

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  • DOI: https://doi.org/10.1007/BF00232439

Key words

  • Motoneurone
  • Recruitment
  • Force modulation
  • Rat
  • Cat