Properties of single-fiber spindle group II EPSPs in triceps surae motoneurons

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Abstract
Excitatory postsynaptic potentials (EPSP) produced by the action of single medial gastrocnemius muscle spindle group II fibers in the cat were examined using intracellular recording and spike-triggered averaging techniques. Spindle group II afferents were divided into 2 functional connectivity groups based on the proportion of homonymous motoneurons receiving a monosynaptic projection. Connectivity was closely related to afferent conduction velocity: fast afferents (> 52 m/s) had high connectivity (69%); slow afferents (< 52 m/s) had low connectivity (27%). Maps of the location of homonymous motoneurons connected with single spindle group II fibers suggest that major collateral branches of fast and slow afferents descend into the motoneuron pool at 1- to 1.5 mm intervals. Terminal ramification is apparently much greater for fast than for slow spindle group II fibers. Mean amplitude of single-fiber spindle group II EPSP was 24.3 .mu.V in homonymous motoneurons and 17.0 .mu.V in heteronymous motoneurons. There was no relation between EPSP amplitude and the conduction velocity of either the afferent or the target motoneuron. The rise time vs. half-width relationship in spindle group II EPSP showed considerable scatter. EPSPs generated by slow group II fibers had shorter EPSP latencies (measured from the terminal potential) and shorter rise times than did EPSP generated by fast group II fibers. Analysis of single traces of spindle group II motoneuron single-fiber EPSP showed a limited and continuous range of EPSP amplitudes. These data are interpreted as further evidence for the monosynaptic nature of the transmission. Conduction velocity of spindle group II afferent fibers in the dorsal columns through segment L2 is proportional to their conduction velocity in the peripheral nerve. The merits of considering spindle group II fibers as comprising a continuum or as 2 separable types are considered. Possible functional roles of spindle group II fibers are discussed.

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