Morphology of interneurones mediating Ia reciprocal inhibition of motoneurones in the spinal cord of the cat.

1. Interneurones identified by physiological criteria (Hultborn, Jankowska & Lindström, 1971b) to mediate Ia reciprocal inhibition of motoneurones in the spinal cord of the cat were stained by intracellular injection of a fluorescent dye (Procion Yellow).2. The somas of the stained cells were found in Rexed's lamina VII, just dorsal or dorsomedial to the motor nuclei. Their size was about 30 x 20 mum. The cells had four to five slender, weakly branching dendrites. The total extension of their dendritic trees was about 600 mum dorsoventrally, 400 mum mediolaterally and 300 mum rostrocaudally.3. The axons originated from a separate axon hillock or from the base of a dendrite. They were myelinated with external diameter of about 6-14 mum and projected to either the ipsilateral ventral or lateral funiculi. Early collaterals were found only exceptionally. Some axons bifurcated into an ascending and a descending branch within the funiculi.4. The possibility of identifying the Ia inhibitory interneurones on purely morphological grounds is discussed.