Evidence for specialized rhythm-generating mechanisms in the adult mammalian spinal cord

Locomotion and scratch are characterized by alternation of flexion and extension phases within one hindlimb, which are mediated by rhythm-generating circuitry within the spinal cord. By definition, the rhythm generator controls cycle period, phase durations, and phase transitions. The aim was to determine whether rhythm-generating mechanisms for locomotion and scratch are similar in adult decerebrate cats. The regulation of cycle period during fictive scratching was evaluated, as were the effects of specific sensory inputs on phase durations and transitions during spontaneous fictive locomotion and pinna-evoked fictive scratching. Results show that cycle period during fictive scratching varied predominantly with flexion phase duration, contrary to spontaneous fictive locomotion, where cycle period varied with extension phase duration. Ankle dorsiflexion greatly increased extension phase duration and cycle period during fictive locomotion but did not alter cycle period during scratching. Moreover, stimulating the plantaris (ankle extensor muscle) nerve during flexion reset the locomotor rhythm to extension but not the scratch rhythm. Stimulating the plantaris nerve during extension prolonged the extension phase and cycle period during fictive locomotion but not during fictive scratching. Stimulating the sartorius nerve (hip flexor muscle) during early flexion reduced the flexion phase and cycle period during fictive locomotion, but considerably prolonged the flexion phase and cycle period during fictive scratching. These data indicate that cycle period, phase durations, and phase transitions are not regulated similarly during fictive locomotion and scratching, with or without sensory inputs, providing evidence for specialized rhythm-generating mechanisms within the adult mammalian spinal cord.