Actions of general anaesthetics on synaptic transmission in the CNS.

This article reviews the actions of general anaesthetics on synapses in the mammalian central nervous system. It is shown that during general anaesthesia, anaesthetics act primarily on the chemical transmission process itself and do not affect the conduction of impulses in nerve axons or change the electrical excitability of neurones. Virtually all general anaesthetics depress excitatory synaptic transmission at concentrations required for surgical anaesthesia but some, notably the barbiturates, also increase the intensity of inhibitory synaptic transmission. Detailed analysis has found that small concentrations of barbiturates increase the amount of inhibitory transmitter released but decrease the amount of excitatory transmitter released. In addition to these effects on the neurosecretory process, anaesthetics directly affect the sensitivity of the postsynaptic receptors to transmitter substances, although the effects vary between anaesthetics and receptors. It is concluded that general anaesthesia results from a summation of a number of effects which together tend to depress the excitability of the CNS as a whole.