The action of carbon dioxide on afferent transmission in the dorsal column-lemniscal system.

1. Transmission in the lemniscal afferent pathway was studied in thirteen decerebrate, unanaesthetized cats while changing the concentration of inspired P(CO2).2. 2-20% CO(2), when inhaled for </= 10 min, raised the mean tissue P(CO2), recorded from the surface of the medulla or cerebellum, from 22 torr to between 26 and 93 torr.3. Medial lemniscal potentials were evoked by stimulation of a forelimb nerve and recorded from the transected surface of the contralateral mid-brain. The transmission of supramaximal responses was progressively and reversibly depressed as tissue levels of P(CO2) were raised and lowered. The time course of the changes in transmission corresponded closely to changes in the tissue P(CO2).4. The amplitude of the main early lemniscal peak was decreased to 80% by the inhalation of 20% CO(2). A late component of the lemniscal response, presumably due to repetitive firing and conduction in smaller fibres and polysynaptic pathways, was more affected than the early main response.5. The failure of transmission was unrelated to threshold changes in the peripheral nerve, since potentials recorded close to the site of stimulation showed no changes.6. Increases in the transmission time of the main lemniscal potentials were uniform and < 10% during the administration of CO(2) and did not appear to contribute to amplitude changes.7. The inhibition of afferent transmission from one nerve by a preceding conditioning volley in a second nerve was not altered by hypercarbia.8. It is concluded that CO(2) has a blocking action on afferent transmission in the pre-thalamic lemniscal system. The site of this block may be at the synapses and/or other regions of low safety factor in the afferent fibres.