Thalamus

The thalamus, located in the diencephalon, is functionally interposed in the sensory and motor pathways from the brain stem to the basal ganglia and cortex. The thalamus is subdivided according to functional criterion, with the principal component being sensory and motor relay nuclei and the intralaminar nuclei.

1. Thalamic Relay Nuclei

Each of the relay nuclei may be distinguished and defined by their cortical connectivity. For example, the medial and lateral geniculate nuclei are auditory and visual, respectively. The anterior group is connected to cingulate cortex and has "limbic" functions, the medial group is connected to prefrontal cortex, presumably mediating cognitive, or so-called executive, functions, whereas the lateral group (which includes the pulvinar) is associated with visual and somatosensory association cortices and premotor cortex and mediates higher order sensorimotor inte-grative functions. The relay nuclei are not connected to each other but, rather, might be regarded as independent channels of information.

The oscillation of thalamic cells is the origin of the synchronous activity measured in the cortical EEG. Release of acetylcholine in the thalamus, which precedes EEG desynchrony and wakefulness, returns the thalamic cells to the mode in which information can be relayed to cortex. Release of acetylcholine in the thalamic relay nuclei depolarizes the relay cells, increasing their excitability. There is a simultaneous hyperpolarization of the local inhibitory interneurons: Because this releases the relay cells from local circuit inhibition, the likelihood of the relay cell firing is further increased. Thus, acetylcholine release in tha-lamic relay nuclei influences the speed, likelihood, and veracity of information transmission to cortex.

2. Intralaminar Nuclei

The intralaminar nuclei comprise cell groups located in the paramedian thalamus, within the white matter called the internal medullary lamina. The mesence-phalic reticular formation provides noradrenergic and cholinergic input to the intralaminar nuclei. There are also inputs to the intralaminar nuclei from cortex, globus pallidus, cerebellum, and spinal cord. Output is to the dorsal (caudate nucleus and putamen) and ventral (nucleus accumbens) striatum of the basal ganglia and to widespread areas of cortex. The anterior intralaminar nuclei project to prefrontal cortex, parietal cortex, and primary sensory areas. The posterior nuclei project to the parietal cortex and premotor (including the frontal eye fields) cortex. These intrala-minar-cortical projections are likely to provide the route by which thalamus directly influences cortical arousal. Whereas the cortical projections of the relay nuclei transmit specific sensory and motor information to the relevant areas of cortex, the cortical projections of the intralaminar nuclei do not follow the boundaries of cortical areas. Electrical stimulation of the intralaminar nuclei results in eye movements. When neurons in the intralaminar nuclei are driven by input from the mesencephalic reticular formation, EEG desynchrony results. Thus, the intralaminar nuclei can mediate the effects of mesencephalic influence on the activity of frontal cortical-basal ganglia-thalamic circuits carrying complex sensory and motor information as well as directly influencing cortical tone.

3. The Thalamic Reticular Nucleus

Reflecting the organization of the relay nuclei, the thalamus is often referred to as the "gateway" to cortex. Surrounding the thalamus on its anterior, ventral, and lateral surfaces is the thalamic reticular nucleus, which is a sheet of GABAergic cells. Corti-cothalamic and thalamocortical fibers pass through the thalamic reticular nucleus and fibers passing in both directions have axon collaterals that synapse in the reticular nucleus. Within the thalamic reticular nucleus, signals are segregated by sensory modality, maintaining the topography accorded by their thala-mic and cortical origins. The cells of the thalamic reticular nucleus do not project to cortex but rather to the thalamus (including to nuclei other than those from which they received signals) and to other sectors of the thalamic reticular nucleus. These connections imply that the reticular formation is privy to information passing between thalamus and cortex and can moderate that flow of information. Activity in the thalamic reticular nucleus may gate the flow of information from thalamus to cortex by sharpening receptive fields and response times of thalamic neurons and by modulating cortical arousal.

The thalamic reticular nucleus also receives choli-nergic input from the reticular formation, specifically from the cholinergic cells of the laterodorsal and pedunculopontine tegmental nuclei, as well as from the basal forebrain nucleus basalis of Meynert. The reticular nucleus also plays a role in the origin of EEG desynchrony by hyperpolarizing the thalamic relay cells, increasing the probability of burst firing and oscillatory activity. Because of the topography of cortical connections, the thalamic reticular nucleus is able to control the functional state (synchronous oscillatory activity as opposed to desynchronous activity compatible with information transmission) of individual specific channels to cortex.

In a series of studies in the 1970s, Skinner and Yingling examined the role of the system comprising the reticular formation, the thalamic reticular nucleus, and the corticothalamic circuits. Yingling and Skinner proposed that the circuits linking cortex and thalamus control the implementation of intention. The input from the mesencephalic reticular formation to the thalamic reticular nucleus is able to interrupt this pathway by controlling the inhibition by thalamic reticular nucleus of specific thalamic relay nuclei. Position emission tomography and functional magnetic resonance imaging and functional magnetic resonance imaging scans in human subjects have confirmed the role of thalamus and the mesencephalic reticular formation when subjects are mentally engaged and performing a task compared to resting scans.

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