DUAL RESPONSE-MODES IN LATERAL GENICULATE NEURONS - MECHANISMS AND FUNCTIONS

Relay cells of the lateral geniculate nucleus, like those of other tha lamic nuclei, manifest two distinct response modes, and these represen t two very different forms of relay of information to cortex. When rel atively hyperpolarized, these relay cells respond with a low threshold Ca2+ spike that triggers a brief burst of conventional action potenti als. These cells switch to tonic mode when depolarized, since the low threshold Ca2+ spike, being voltage dependent, is inactivated at depol arized levels. In this mode they relay information with much more fide lity. This switch can occur under the influence of afferents from the visual cortex or parabrachial region of the brain stem. It has been pr eviously suggested that the tonic mode is characteristic of the waking state while the burst mode signals an interruption of the geniculate relay during sleep. This review surveys the key properties of these tw o response modes and discusses the implications of new evidence that t he burst mode may also occur in the waking animal.