The use of state-dependent modulation of spinal reflexes as a tool to investigate the organization of spinal interneurons

This review examines the proposition that state-dependent modulation of tra nsmission through spinal reflex pathways can be used as an investigative to ol to reveal details about the organization of spinal interneurons into fun ctional circuits. The first set of examples includes the use of spinal and supraspinal lesions, as well as the administration of the drug l-dihydroxyp henylalanine (l-DOPA), to produce different, relatively stable "states" of the central nervous system (CNS), revealing previously unsuspected spinal p athways activated by the flexor reflex afferents (FRA). The second set of e xamples deals with the use of fictive locomotion and scratching to investig ate the organization of oligosynaptic excitatory and inhibitory reflex path ways from cutaneous and muscle afferents. As in the first set of examples, several hitherto unknown reflex pathways have been found only during the fl exion or extension phases of rhythmic locomotion, which are regarded as dif ferent CNS states. Differences in the patterns of control can be used to in fer the existence of distinct sets of reflex pathway interneurons that have remarkably precise input/output relations.