Specification of synaptic connections mediating the simple stretch reflex

A variety of mechanisms underlie the specification of synaptic connections during development. In the monosynaptic stretch reflex in vertebrates, sensory neurones innervating muscle spindles are not determined until they make contact with a particular muscle. Instead, the muscle they supply appears to specify the pattern of central connections they establish with motoneurones. Developing thoracic sensory neurones made to project to novel peripheral targets in the forelimb of tadpoles project into the brachial spinal cord, something they never do in normal frogs. Moreover, these foreign sensory neurones make monosynaptic connections with the now functionally appropriate brachial motoneurones. Normal patterns of neuronal activity are not necessary for the formation of specific central connections. Neuromuscular blockade of developing chick embryos with curare during the period of synaptogenesis does not prevent the formation of correct sensory-motor connections. Competitive interactions among the afferent fibres also do not appear to be important in this process. When the number of sensory neurones projecting to the forelimb is drastically reduced during development, each afferent fibre still makes central connections of the same strength and specificity as normal. Together, these results suggest that peripheral targets induce some molecular change in developing sensory neurones such that they can recognize their appropriate synaptic partners in the spinal cord.