Motility in the chick embryo in the absence of sensory input

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Abstract
The following experiment was designed to test the role of sensory input in the motility of the chick embryo: Complete deafferentation of the leg level was accomplished in 2‐day embryos by extirpation of the dorsal half of the lumbar spinal cord including the precursor cells of spinal ganglia; and simultaneous extirpation of the entire spinal cord in the thoracic level to exclude sensory input from more rostral levels. In control embryos, only the thoracic gap was made. The embryos were reared to eight and one‐half, 11, 13, 15 and 17 days, respectively, and manual recordings were made of the leg motility before the embryos were sacrificed. It had been reported by us that the motility of normal chick embryos is periodic, activity phases alternating with inactivity phases. Both the overall activity, i.e., percent of activity during a standard 15‐minute observation period, and the durations of activity and inactivity phases were found to be the same in experimental embryos and in control embryos with thoracic gaps, at least up to and including 15‐day embryos. Histological checks showed the complete absence of lumbar spinal ganglia in the majority of embryos and, the presence of a few small thoracic ganglia in some cases. All embryos with lumbar ganglia were discarded. The motor roots formed a normal plexus, and the pure motor nerves emerging from it showed a normal peripheral distribution and muscle innervation. It is concluded that the leg motility results from autonomous discharges of the lateral motor or internuncial neurons, and that sensory input is not necessary for the triggering, the maintenance or the periodicity of the leg motility. A sharp decline of motility was observed in 17‐day embryos. A study of the histological condition of the residual spinal cord showed a numerical depletion and partial degeneration of the lateral motor column. This deterioration is considered to be responsible for the functional impairments.The implication of these results for the widespread notion that “self‐stimulation” and similar “experiences” during embryonic development play an important role in the molding of behavior is discussed.