Synaptic connexions between primary afferents and thoracic motoneurones in the frog.

Synaptic actions were evoked in thoracic motoneurones of the isolated frog spinal cord by 6th or 7th dorsal root volleys or impulses in single primary afferents. In a few cases the impaled motoneurones or the whole dorsal root were filled with horseradish peroxidase. Although HRP labelling has revealed that motoneurones have extensive dendritic arborizations extending into the dorsal horn, the zone in which thoracic dorsal root afferents terminate, there is no positive evidence for a direct link between them. Synaptic actions produced in thoracic motoneurones by dorsal root stimulation are mediated via interneurones and do not involve direct sensorimotor synapses. This conclusion is in agreement with results of simultaneous intracellular recording from motoneurones and individual dorsal root fibres afferent to them.