Effects of hypoxia on polysynaptic hind-limb reflexes in new-born lambs before and after carotid denervation

Polysynaptic reflex responses in the hind limb were obtained by stimulating the peroneal nerve in unanaesthetized lambs between 4 and 12 days from birth, before and after carotid denervation. In intact lambs isocapnic reduction of the Pa,O2 from 88 to 31 or to 23 mmHg caused an insignificant decrease in reflex amplitude. Denervation of the carotid bifurcations induced hypoventilation. When the Pa,O2 was then reduced to 22 mmHg the reflex amplitude fell by 77%. In carotid denervated lambs after complete transection of the spinal cord at T12-L1, reduction of the Pa,O2 to 23 mmHg did not alter the size of the reflex. The same result was observed after section of the dorsal part of the lateral funiculi. It is concluded that, in the young lamb a few days after birth, the afferent activity of the carotid chemoreceptors during hypoxia plays an important role in maintaining the excitability of motor reflexes. The possible mechanisms are discussed.