Main characteristics of the hindlimb locomotor cycle in the decorticate cat with special reference to bifunctional muscles.

In acute decorticate (thalamic) cats, efferent activities to various hindlimb muscles during the locomotor cycle were studied, using several complementary methods: muscle and motor nerve recordings, monosynaptic testing, intracellular recording from motoneurones, recording of fusimotor actions on single Ia afferents. The limb was fixed and peripheral influences could be either increased by exteroceptive stimulations or decreased by curarization and/or deafferentation. Our main results were the following: in all studied muscles, there is an alpha-gamma coactivation; there are pure flexor and pure extensor muscles with simple alternating activities; bifunctional pluriarticular muscles show complex activations which allow the division of the locomotor cycle into a flexion, an extension and two transition phases; alpha motoneurones of these muscles receive both flexor and extensor commands; the relative importance of the corresponding excitations depends on interactions between these central commands and peripheral inflow, especially through afferents acting according to the flexor reflex pattern. Based on these results, an attempt is made to explain how the complex and variable efferent activity can arise from an initially simple rhythmic command and produce the biomechanically adapted locomotor movement of the hindlimb.