Effect of hypoxia on polysynaptic hind-limb reflexes of unanaesthetized fetal and new-born lambs.

Polysynaptic reflex excitation of the hind-limb flexor muscles was produced by stimulation of the distal peroneal or common sciatic nerve in ten fetal and six new-born unanaesthetized lambs. In the fetal lambs, stimulation at 1.1-3.25 times threshold for direct activation of the nerve produced a reflex response which was greatest during high-voltage electrocortical activity, being reduced 69 +/- 5.0% (mean +/- S.E. of the mean) during low-voltage electrocortical activity. Reduction of the fetal arterial PO2 from 23.3 +/- 0.58 to 12.2 +/- 0.4 mmHg caused a rapid and reversible reduction of the reflex in all fetal lambs with intact spinal cord, including two whose brain stems were transected at the level of the colliculi. Hypoxia did not alter the amplitude of the reflex in two fetuses in which the spinal cord at L1-2 had been cut several days previously. In new-born lambs reduction of the arterial PO2 to 43 mmHg did not change the reflex amplitude during episodes of quiet wakefulness or slow-wave sleep. Brief reduction of the Pa,O2 to 20-30 mmHg caused a statistically insignificant reduction of reflex amplitude during quiet sleep before behavioural arousal. It is concluded that hypoxaemia activates reticular mechanisms in the pons and medulla of fetal lambs which decrease the excitability of spinal reflex pathways. In the new-born lamb such effects are counteracted or prevented by mechanisms associated with arousal.