Development of spinal reflexes in the rat fetus studied in vitro.

1. The onset and development of spinal reflex activity was investigated using the isolated spinal cord of the rat fetus. The potential changes generated in motoneurones were recorded extracellularly from L3 ventral roots. 2. A spike potential was recorded from the ventral root at embryonic day 13.5 in response to stimulation of the cord surface close to the ventral root. The discharge persisted in Ca2+-free solution but was blocked by tetrodotoxin. 3. At embryonic day 14.5, trans-synaptically evoked discharges were detected in motoneurones. 4. Stimulation of the dorsal root was first effective in eliciting reflex discharges at embryonic day 15.5. The reflex response then consisted of a prolonged depolarization upon which were superimposed small spikes, and was probably polysynaptic. 5. A spike potential, presumably a monosynaptic reflex, was generated at the end of fetal life. This discharge appeared first at embryonic day 17.5 in a primitive form. 6. Between embryonic day 16.5 and 17.5, stimulation of the dorsal root of diffferent segments (L1-L6) elicited responses similar to those induced by the corresponding (i.e. L3) dorsal root stimulation. These inter-segmentally induced responses were then reduced in size toward the birth. However, in the presence of strychnine, a train of spike discharges of similar shape to the segmentally induced response was also evoked by stimulation of the dorsal root at L4 or L5. These spikes disappeared during further post-natal development. 7. It is concluded that synapses in the segmental polysynaptic pathway become functional in a retrograde sequence with respect to the direction of normal reflex impulse flow. The reflex responses, elicited by stimulation of the dorsal roots of different segments, are suggested to be suppressed first by the development of inhibitory mechanisms and then by neuronal cell death or by elimination of the synapses responsible for generating the inter-segmental reflexes.