A mechanical model of the secondary endings of mammalian muscle spindles

SummaryA mechano-electric model of the secondary endings of the mammalian muscle spindle receptors has been developed. The model involves a mechanical system with second order transfer dynamics connected to a zero order mechano-electric transducer with constant gain.The mechanical system is supposed to describe the viscoelastic properties of the intrafusal muscle fibres, and this part of the model is identical to that used in connection with an earlier proposed model of the primary endings (Rudjord, 1970).The general transfer properties of the model are derived, together with a description of the particular response components obtained during a linear extension of the mechanical system.A simple direct electronic analogue was used for studying the effects of the individual parameters and for a preliminary estimate of an adequate set of parameter values.With appropriate parameter values, a close correspondence could be obtained between the behaviour of the model and the typical response properties of the secondary endings of the muscle spindles.The order of magnitude of the best parameter fit also appeared to correspond to the values previously found to yield an adequate description of the transfer properties of the primary endings.It is further shown that for the best fit of the mechanical parameters, the secondary ending model may be approximated by a simple first order system with lead-lag transfer behaviour.The transfer properties of this simplified model appear to agree with the reported response properties of typical secondary ending receptors except for extreme rates of stretch or very high vibrational frequencies of stretch during which the simplifying approximation evidently does not apply.