Morphological features of cat cervicothalamic tract terminations in different target regions

Using biotinylated dextran amine to label cervicothalamic tract termination s of cats, three types of terminal arrangements were recognized. The ventra l posterior lateral nucleus contains the largest proportion of the cervicot halamic tract terminals (79%) and most (72%) of these are type I terminals (form compact clusters of 5-30 boutons). In contrast, type II (form less co mpact clusters of 3-10 boutons) and type III (widely spaced boutons along t hin axons) terminals dominate in the medial nucleus of the posterior comple x (78%) and in the ventral periphery of the ventrobasal complex (86%). In t he magnocellular medial geniculate nucleus, type I terminals (38%) are foun d close to medially located clusters of Cat-301 immunoreactive neurons, whe reas type II and type III terminals locate in the surrounding Cat-301-negat ive regions. These findings indicate a high degree of synaptic security in the transmission between cervicothalamic tract fibers and neurons in the ve ntral posterior lateral nucleus and highlight the role of this nucleus in f aithful transmission of cervicothalamic tract input to the cerebral cortex. Also, the Cat-301-positive neurons in the magnocellular medial geniculate nucleus may faithfully transmit cervicothalamic tract signals. The dominati on of type II and type III terminals in the medial nucleus of the posterior complex and in the ventral periphery of the ventrobasal complex indicates a more divergent cervicothalamic input to these regions, in line with the l arge receptive fields and multimodal responses of neurons in the posterior complex. (C) 2001 Elsevier Science B.V. All rights reserved.