The G. L. Brown lecture. Adventures in anaesthesia

Abstract

The results presented in this lecture show that anaesthetic agents impede the transfer of information from the periphery to the cerebral cortex. This is shown both as a reduction in the amplitudes of the initial positive and negative waves of the cerebral cortical response evoked by simulation of the periphery and as an increase in the latency of this response. This effect is most probably a prime effect of anaesthesia since (a) it is common to all the anaesthetics used, (b) the potency of the anaesthetics is directly proportional to their lipid solubility, and (c) the effect is reversed by high ambient pressures. The major site at which information transfer is most susceptible to the action of anaesthetics is at the level of the ventrobasal thalamus, although the cells in cortical layer V also appear to have an enhanced susceptibility to anaesthetic action. This latter observation is seen both in whole animal and cortical slice preparations. None the less, the first site of synaptic transfer at which anaesthetics exert a profound effect is upon the monosynaptically generated responses of ventrobasal thalamic neurones to cuneothalamic input. A possible mechanism of action for anaesthetic agents acting at this site would be upon a hypothetical cortico-thalamic-reticular-thalamic loop with the theoretical ability to control the responsiveness of the ventrobasal thalamic cells. This action was proposed both from the activity of neurones in response to anaesthetic agents and the anatomical arrangement seen in the thalamus. The thalamic reticular nucleus is a curved sheath of cells situated between the internal capsule and the external medullary lamina, capping and bounding laterally the specific nuclei of the dorsal thalamus. There is both anatomical and physiological evidence that the thalamic reticular nucleus comprises part of the thalamic reticular formation: its cellular structure also resembles that of the brain stem regions of the reticular formation (Ramon-Moliner, 1975). Early degeneration and Golgi studies showed that ascending fibres from the medial parts of the pontine and mesencephalic components of the brain stem reticular formation innervated the thalamic reticular nucleus ventrally, by penetrating the zona incerta, and dorsally, via the intralaminar and dorsal thalamic nuclei (Scheibel & Scheibel, 1958). These observations have been confirmed and extended more recently and it appears that the major innervation of the thalamic reticular nucleus occurs via the ventral route which follows the entire course of the reticular nucleus. No fibres ascending from the dorsal column lemniscal system, the spino-cervico-lemniscal system or the spinothalamic tract have been observed to terminate within the thalamic reticular nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)