Pacemaker activity in a sensory ending with multiple encoding sites:The cat muscle spindle primary ending

1. A combined physiological, histological and computer modelling study was carried out on muscle spindles of the cat tenuissimus muscle to examine whether there was any correlation between the functional interaction of putative encoding sites, operated separately by static and dynamic fusimotor neurones, and the topological structure of the preterminal branches of the primary sensory ending.2. Spindles, whose Ia responses to stretch and separate and combined static and dynamic fusimotor stimulation were recorded in physiological experiments, were located in situ. Subsequently the ramifications of the sensory ending were reconstructed histologically, and the topology of the branch tree was used in computer simulations of I a responses to examine the effect of the electrotonic separation of encoding sites on the static-dynamic interaction pattern.3. Interactions between separate static and dynamic inputs, manifest in responses to combined stimulation, were quantified by a coefficient of interaction (C-1) which, by definition, was 1 for strictly linear summation of separate inputs and zero for maximum occlusion between inputs.4. For the majority of spindles static-dynamic interactions were characterized by pronounced occlusion (C-1 <0.35). In these spindles putative encoding sites (the peripheral heminodes of the branches supplying the intrafusal fibres activated by individual fusimotor efferents) were separated by a minimum conduction path of between three and ten myelinated segments (2-9 nodes of Ranvier). In contrast, significant summation (C-1, similar to 0.7) was found in only one spindle. In this case putative encoding sites were separated by a single node.5. Occlusion was not due to encoder saturation and it could not be accounted for by any other known physiological mechanisms (intrafusal fatigue or unloading). It is therefore attributed to competitive pacemaker interaction between encoding sites which are largely selectively operated by static and dynamic fusimotor efferents.6. Model simulations of real preterminal-branch tree structures confirmed that short conduction paths between encoding sites were associated with manifest summation, whereas longer minimum conduction paths favoured pronounced occlusion.7. In the extreme, occlusion could be so pronounced as to give rise to negative values of C-1 during critical segments of response cycles. This was associated with lower discharge rates during combined static and dynamic stimulation than the higher of the individual stimulation effects. This phenomenon is referred to as hyperocclusion. Computer simulations demonstrated that hyperocclusion could be accounted for by a slow ionic adaptation process, e.g. by a very slowly activating K+ conductance.