RELATIVE EFFECTS OF PENTOBARBITAL AND CHLORALOSE ON THE RESPONSIVENESS OF NEURONS IN SENSORIMOTOR CEREBRAL-CORTEX OF THE DOMESTIC CAT

A sample of 583 individual neurons was obtained from the sensorimotor cerebral cortex in cats anesthetized with sodium pentobarbital. Four sites related to the contralateral forepaw were sampled: precoronal (field 3b), midsigmoid (field 3a), and postcruciate (field 4γ) and precruciate (field 4γ). The sample was compared with one previously obtained from the same four sites (4175 neurons) in cats anesthetized with α-chloralose. The average number of evokable neurons per electrode track was the same under both anesthetics at the precoronal site in somatosensory area I. Under pentobarbital anesthesia, the number of neurons/track decreased linearly through an order of magnitude from the precoronal to the precruciate sites, whereas under chloralose anesthesia it was nearly the same at three of the four sites, decreasing by about 25% at the midsigmoid site. The pentobarbital sample consisted almost exclusively of small-field neurons, whereas the chloralose sample contained a substantial number of bilateral-field and wide-field neurons. Two wide-field neurons found under pentobarbital anesthesia were found to become unresponsive to some, but not all, cutaneous inputs after a supplemental dose of pentobarbital. The small-field neurons found under pentobarbital anesthesia responded with lower probability but at significantly shorter latency than those found under chloralose. It is argued that although a few of the neurons classed as wide-field under chloralose anesthesia may be classed as small-field or bilateral-field under pentobarbital anesthesia, the majority simply become unresponsive to cutaneous input under pentobarbital anesthesia. They probably increase their tonic discharge under pentobarbital anesthesia, some showing 'burst' discharges. It is concluded that the sample of cutaneously excitable neurons obtainable under pentobarbital anesthesia is limited by comparison with that obtainable under chloralose anesthesia, and that a specific set of neurons is affected most strongly. This set-the wide-field neurons-includes the great majority of the pyramidal tract neurons. It is argued that not only do the views of the sensorimotor cerebral cortex differ between the two anesthetic conditions, but also that neither view relates in a simple or direct way to that obtained from awake, unanesthetized preparations. © 1979.