Views about the functional role of Ib pathways have evolved more over the years than for any other spinal circuit. The initial opinion that Ib inhibition subserved an autogenetic protective reflex has been replaced by the view that tendon organs continuously provide information about the extent of muscle contraction. Projections of ‘Ib’ interneurones were then shown to be more widely distributed than implied by the term ‘autogenetic inhibition’, and, because of the extensive convergence from peripheral afferents onto the relevant interneurones, the term of ‘non-reciprocal group I inhibition’ has been introduced to refer to inhibition conveyed by this pathway. Finally, the recent finding that, during locomotion, Ib (or non-reciprocal group I) inhibition is replaced by di- and poly-synaptic excitation has completely altered views on the functional significance of Ib pathways. The multitude of controls (pre- and post-synaptic, peripheral and descending) on Ib pathways and the numerous possible alternative patterns suggest that they might play multiple roles. Studies during various motor tasks in human subjects could be particularly important in helping to understand these roles but, because of the difficulty in investigating Ib pathways selectively in human subjects, they have not yet been explored to any great extent during human movement.

Background from animal experiments

Initial findings

In the chronic spinal dog, forced flexion of the knee produces, after the initial stretch reflex, the clasp-knife phenomenon, in which the reflex resistance suddenly ‘melts away’ (Sherrington, 1909).