Thalamocortical Activating System

The midline-intralaminar nuclei of the thalamus are part of the ascending arousal system. Functional imaging studies have shown that activation of these nuclei leads to higher levels of arousal and attention.1,2 They have long been considered "nonspecific" since they project broadly within frontal cortex and can recruit other areas of cortex to become active.3 However, the different groups of nuclei appear to have fairly specific domains within which they promote attention and arousal: viscero-limbic functions (dorsal group), cognitive functions (lateral group), multimodal sensory processing

Evelyn K. Lambe, Department of Psychiatry, Yale University School of Medicine, New Haven CT 06508. George K. Aghajanian, Departments of Psychiatry and Pharmacology, Yale University School of Medicine, New Haven CT 06508.

(ventral group), and limbic motor functions (posterior group).4 These nuclei are shown in Figure 1.

Lesions of midline-intralaminar thalamic nuclei give rise to disturbances of attention, initiative, and executive functions similar to those seen with prefrontal cortex lesions. These deficits fit with the heavy projection from these thalamus nuclei to prefrontal cortex. Midline-intralaminar neurons have monosynaptic projections which terminate on the apical dendrites of cortical pyramidal neurons.5 Anterograde labeling studies have shown that these axons terminate predominantly in layers I and V of

Figure 1. Midline and intralaminar nuclei of the thalamaus shown in coronal section from most anterior to most posterior from left to right (adapted from Van der Werf et al., 2002).4 These nuclei include paraventricular (PV), paratenial (Pt), central lateral (CL), central medial (CeM), interomediodorsal (IMD), paracentral (PC), rhomboid (Rh), reuniens (Re) and the caudal intralaminar nucleus (the most ventral region shown with dotted lines on the far right image).

Figure 2. The cortical axonal arbor of a single central lateral thalamic neuron, shown in sagittal section of dorsal prefrontal cortex (adapted from Deschenes et al., 1996).6 The most anterior part of cortex is on the left. Such extensive arborization means that midline-intralaminar neurons innervate overlapping regions of cortex.

prefrontal cortex.3 Single axons have been shown to arborize extensively,6 as illustrated by the arbor from a single central lateral neuron in a sagittal section of prefrontal cortex shown in Figure 2.

Thalamocortical axons are unusual in that they are susceptible to the generation of terminal spikes.7,8 It has been suggested that the induction of ectopic spikes in the overlapping terminal fields would co-ordinate bursting of groups of thalamic neurons.9,10 Midline-intralaminar thalamic neurons are unusual in that they displaying bursting behavior during waking,11 the opposite pattern to that observed in sensory neurons which burst during slow wave sleep. It is not understood what regulates the bursting of midline-intralaminar thalamic neurons during waking.

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