PERIPHERAL TARGET SPECIFICATION OF SYNAPTIC CONNECTIVITY OF MUSCLE-SPINDLE SENSORY NEURONS WITH SPINAL MOTONEURONS

The source of environmental cues determining the central connections o f muscle sensory neurons was investigated by manipulating chick embryo s so that sensory neurons supplied a duplicate set of dorsal thigh mus cles. These neurons projected out ventral nerve pathways and along mot or axons that normally project to ventral muscles but their ultimate t arget tissue was the duplicate set of dorsal muscles. The central conn ections of these sensory neurons to motoneurons supplying normal dorsa l muscles were then determined with intracellular recordings in isolat ed spinal cord preparations. Sensory neurons supplying individual dupl icate dorsal muscles made the same connections as those supplying the corresponding normal dorsal muscles; the pattern of these connections was different than that made by afferents supplying ventral muscles. S ensory neurons thus made synaptic connections appropriate for their ta rget muscle rather than for their more proximal ventral environment. T hese findings suggest that the target muscle is the source of cues tha t determine the central connections of the sensory neurons projecting to it. Motoneurons forced to innervate novel muscle received many of t he same sensory inputs they would normally receive, suggesting that mo toneurons are less influenced by their target tissue than sensory neur ons.