Effects and site of action of a single-breath of 100% CO2 on the monosynaptic reflexes in cats.

A single breath of 100% CO2 produces depression of the monosynaptic reflexes, recorded from L7 or S1 ventral root, after stimulation of the Posterior Biceps and Semitendinosus nerve (PBST) in anaesthetised cats. As the depression could not be attributed to the J-reflex(1), the possible site of action of the CO2 induced depression of monosynaptic reflexes was worked out. PBST nerve threshold did not change after CO2 introduction and the depression persisted in paralysed cats under controlled ventilation, thus eliminating the possibility of movement effect of the spinal cord due to tachypnoea. Spinal cord sections at the level of L1 and C1 abolished the depression, whereas the depression persisted in the decerebrate preparation. Thus it is concluded that a single-breath of 100% CO2 depresses the monosynaptic reflexes at the supraspinal level. Blood gas tensions (PO2 and pCO2) measured before and after CO2 introduction showed a shortlasting increase in pCO2 and not much significant change in pO2 compared to the long lasting depression of monosynaptic reflexes.