The Pause During Contraction in the Discharge of the Spindle Afferents from Primary End Organs in Gat Extensor Muscles

Abstract

Abstract Recently much detailed information has become available about the structural design of the muscle spindles. It seems highly pertinent that the large spindle‐afferents from primary end organs possess two kinds of terminals, one set deriving from the nuclear bag of special ‘nuclear bag’‐fibres, another from purely muscular, short, so‐called ‘nuclear chain’‐fibres. Also spindles may vary in length over more than a tenfold range (cat). The pause in the discharge of the muscle spindles in isometric contraction has been studied and the findings evaluated from the point of view of spindle anatomy. The primary end organs were found to fall into two groups, long‐pause spindles and short‐pause spindles. This grouping is assumed to express their anatomical length. This may vary between 2 and 22 mm. The pause either ended with a phasic burst, often succeeded by a brief, secondary pause, or else the discharge was resumed in a tonic fashion. Thus spindles were ‘phasic’ or ‘tonic’ with respect to how they resumed firing after the ‘pause’. Nearly all the ‘tonic’ spindles were found among the long‐pause spindles. The hypothesis adopted to explain the phasic‐tonic differentiation is that phasic spindles are dominated by ‘nuclear bag’‐fibres, tonic ones by ‘nuclear chain’‐fibres. Spindles are also known to vary considerably with regard to the relative number of these two different fibre types.