Cortico-thalamic interplay and the security of operation of neural assemblies and temporal chains in the cerebral cortex.
Much evidence suggests that the mammalian thalamus is not merely a set of nuclei relaying signals to the cerebral cortex, but is engaged in two-way interplay with it. Three important features constrain ideas about the nature of this interplay: (i) thalamic projection neurones lack local axon collaterals; (ii) most corticothalamic projections have very long axonal conduction time; (iii) in the waking state the membrane potential of thalamic projections cells appears to be poised just beneath threshold for firing. It is proposed that corticothalamo-cortical pathways represent connections between different cortical loci which have higher security than the direct cortico-cortical route. Thus each thalamo-cortical projection neurone can have a singular and pivotal role in the activation of one or more cortical neural assemblies. The long delays of cortico-thalamic conduction suggest that the cortico-thalamo-cortical loop also plays a crucial role in the operation of time-structured neural assemblies ('synfire chains'; Abeles), by providing a high-security link from one active node of the chain to nodes activated at a later time in the sequence. It is predicted that, in the waking animal, thalamic projection cells should have a response selectivity to complex percepts and concepts, conferred on them by the cortical assemblies in whose activation they participate.
Miller
R
R
Department of Anatomy and Structural Biology, University of Otago Medical School, Dunedin, New Zealand.
D000344:Afferent Pathways / Q000502:physiology; D000818:Animals; D001369:Axons / Q000502:physiology / Q000648:ultrastructure; D002540:Cerebral Cortex / Q000502:physiology*; D004525:Efferent Pathways / Q000502:physiology; D006801:Humans; D008322:Mammals / Q000502:physiology; D008959:Models, Neurological; D009415:Nerve Net / Q000502:physiology*; D009431:Neural Conduction; D009474:Neurons / Q000502:physiology; D010465:Perception / Q000502:physiology; D017966:Pyramidal Cells / Q000502:physiology; D009435:Synaptic Transmission; D013788:Thalamus / Q000502:physiology*; D013997:Time Factors