Quantitative management of contraction in lowest level co-ordination. Hughlings Jackson Lecture. (Given January 29, 1931). By C.S. Sherrington. Brain 1931: 54; 1-28

Abstract

‘Compact in Hughlings Jackson was a fine vein of pithy thought and phrase.’ Thus, Sherrington introduces the printed version of his Hughlings Jackson lecture, delivered only 2 months before publication (in contrast to the Silliman lectures that took 2 years to see through the press). He reminds us of Jackson's hierarchies—the nervous centres rising in three tiers or levels: ‘no illustrations happier than the notable studies of asynergia, rigidity and tremor’ (in striatal disease) representing a ‘pulling to pieces from the top downwards … so likewise physiological experiment … starts lower … a rump of mechanism, a stump of spinal cord … these it interrogates through perhaps a single afferent for answer by a single efferent … its hope is, since bottom is basal, to reach bottom, though even there the elemental may not prove to be the simple’. At that lowest Jacksonian level is the ‘old-fashioned if time honoured entity, the motor centre’. Afferent stimulation shows that every reflex fractionates its responsive muscle, but not right down to the single muscle-fibre. As part of ‘nervous co-ordination’, to paraphrase Hughlings Jackson, the territory of each anterior horn cell selects around 150 muscle fibres from amongst 30–40 000 available within most muscles. These constitute what ‘may be called for short a motor unit’ dividing the (feline) limb muscles into a manageable 200–450 such units. Their ‘all or nothing’ responses yield contraction waves of around 2.5 g but, in reflex activities, those derived from separate motor units overlap to create a maximum tetanic response of up to 30 g in the cat gastrocnemius. Testing tetanic motor units parcels out the share of each reflex for a given muscle, revealing that every afferent nerve contributes reflex activity to practically every muscle within the limb. And, in turn, this distributed anatomy of the muscle fibre dependence on …