Correlated histological and physiological observations on a case of commonsensory output and motor input of the bag(1) fibre and a chain fibre in a cat tenuissimus spindle

In muscle spindles of the cat, independent control of dynamic and static co mponents of the response of the primary sensory ending to stretch is provid ed by separate motor inputs to the various kinds of intrafusal muscle fibre : dynamic axons (gamma or beta) to the bag(1) fibres and static axons to th e bag(2) (typically gamma only) and chain (gamma or beta) fibres. Nonlinear summation of separately evoked effects during combined stimulation of dyna mic and static motor axons appears to be due to mutual resetting by antidro mic invasion of separate encoding sites, leading to partial occlusion of th e momentarily lesser response by the greater. The encoding sites are though t to be located within the primary ending's preterminal branches which from first-order level are normally segregated to the bag, fibre and to the bag (2) and chain fibres. Here we describe the analysis of a special case that arose in a histophysiological study which had shown that the degree of occl usion was related to the minimum number of nodes between the putative encod ing sites. Three-dimensional reconstruction of the primary ending revealed that the terminals of one chain fibre were derived entirely from the first- order branch that supplied the bag(1) fibre, including one terminal that wa s shared directly with the bag, (sensory cross-terminal). The other first-o rder branch supplied the bag(2) and remaining chain fibres as normal. The d egree of occlusion seen during simultaneous stimulation of a dynamic beta a xon and a static gamma axon indicated that the encoding sites were separate d by both first-order branches. Schematic reconstruction of the motor inner vation revealed that the static gamma axon was most unlikely to have suppli ed the chain fibre which shared sensory terminals with the bag(1), but that these fibres also shared a motor input with histological characteristics o f beta type. Ramp-frequency stimulation of the dynamic beta axon at constan t length evoked a driving effect which persisted after fatiguing the extraf usal component and was therefore explicable on the basis of the observed pa ttern of motor innervation, though the identity of the axon could not be co nclusively proved. Individually, instances of shared sensory terminals and motor input of bag, and chain fibres are rare in the cat; their combination in a single spindle with correlated physiology is described here for the f irst time. The observation is considered in relation to the importance of d ynamic and static segregation in motor control, since it may imply that the re is a lower limit to the degree of segregation that the developmental pro gramme can provide.