No abstract

In unanaesthetized spinal cats, injected with L‐DOPA, volleys in the flexor reflex afferents (FRA) evoke a long‐latency, longlasting discharge in ipsilateral flexor and contralateral extensor motoneurones. It is postulated that this discharge is transmitted by a neuronal pathway which is inhibited in the normal acute spinal cat, presumably from the pathway, which in this state transmits the shortlatency effect from the FRA to motoneurones. The organization of the pathway released by an injection of DOPA has been analyzed by recording the discharges in efferents to flexors and extensors and with intracellular records from motoneurones. Combined stimulation of ipsilateral and contralateral FRA reveals a reciprocal organization in that either flexor or extensor motoneurones are activated. Transmission from the ipsilateral FRA to flexor motoneurones can be inhibited by volleys in the contralateral FRA, and transmissiqn from the contralateral FRA to extensor motoneurones by volleys in the ipsilateral FRA. These inhibitory effects are neither produced postsynaptically in the motoneurones nor presynaptically by depolarization of primary afferents and are hence exerted at an interneuronal level. The organization of reciprocal innervation at an interneuronal level is discussed in relation to the primary afferent depolarization evoked in Ia afferent terminals after DOPA and to rhythmic alternating movements.

In unanaesthetized spinal cats a systematic microelectrode exploration has been made in the lumbosacral spinal cord in order to find interneurones that may transmit the late longlasting reflex effects that volleys in the FRA (flexor reflex afferents) evoke after an intravenous injection of DOPA. Interneurones that may transmit these late effects are found in the lateral part of Rexed's layer VII and three main types are identified: A) cells activated from the ipsilateral FRA and inhibited from the contralateral FRA, B) cells activated from the contralateral FRA and inhibited from the ipsilateral FRA, C) cells activated from both the ipsilateral and contralateral FRA. The receptive fields and other criteria employed suggest that type A and B transmit excitatory action to flexor and extensor motoneurones respectively, and type C depolatization to Ia afferent terminals. The functional organization of these pathways is discussed mainly in relation to the mechanisms responsible for the long latency and duration of the discharge.