Structural and topographical influences on functional connectivity in spinal monosynaptic reflex arcs in the cat

A greatly expanded version of spike-triggered averaging (Mendell & Henneman, 1971), performed off-line on tape-recorded signals, was utilized to determine the presence or absence of functional connexions between stretch-afferent fibres and homonymous motoneurones. As many as 264 possible connexions between eleven Ia or spindle group II fibres and twenty-four motoneurones were studied in each single, acute experiment. Morphological and topographical factors influencing functional connectivity were analysed with the aid of wiring diagrams and connectivity matrices. In all experiments the greater the conduction velocity (i.e. diameter) of a Ia or group II fibre, the higher was the probability of its having functional connexions with homonymous motoneurones. The greater the longitudinal distance between the spinal entry points of Ia fibres and the location of a motoneurone, the less was the same probability. The influence of axonal conduction velocity of motoneurones on functional connectivity was apparent in some experiments, but not in others. In pooled data large motoneurones received functional connexions from a higher percentage of group II fibres than did small cells. The projection percentage reached 100 only when both Ia fibres and motoneurones were large, suggesting that motoneurone size influences the probability of functional connexions from group Ia as well as group II fibres. On a cell-to-cell level, connectivity apparently does not follow strict, deterministic rules. The results raise the question of how probabilistic connexions between afferent fibres and motoneurones give rise to deterministic outputs from the whole pool.