Development of the monosynaptic stretch reflex circuit.

Significant advances have been made during the past few years in our understanding of how the spinal monosynaptic reflex develops. Transcription factors in the Neurogenin, Runt, ETS, and LIM families control sequential steps of the specification of various subtypes of dorsal root ganglia sensory neurons. The initiation of muscle spindle differentiation requires neuregulin 1, derived from Ia afferent sensory neurons, and signaling through ErbB receptors in intrafusal muscle fibers. Several retrograde signals from the periphery are important for the establishment of late connectivity in the reflex circuit. Finally, neurotrophin 3 released from muscle spindles regulates the strength of sensory-motor connections within the spinal cord postnatally.