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The actions of CO2 were studied on 48 lumbosacral motoneurones impaled with microelectrodes in spinal cats. CO2 produced a reversible depolarization in some cells tested and a reversible hyper-polarization in other cells tested. Both increases and decreases in membrane resistance were produced by CO2, and these were significantly correlated with hyperpolarizations and depolarizations of the membrane, respectively. The after-hyperpolarization following an antidromic response was always reduced by CO2, irrespective of the CO2-induced change in membrane potential. The firing threshold of the motoneurone in response to injected depolarizing currents was increased by CO2. Statistical analysis of excitatory postsynaptic potentials produced by activity in a single afferent fiber revealed that the principal depressant action of CO2 on this pathway is to block intraspinal branches of the primary afferent fibers. Neither the transmitter release mechanism nor the sensitivity of the postsynaptic membrane to the released transmitter was significantly affected.

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