Thalamocortical system

Steriade, M., Datta, S., Pare, D., Oakson, G. 8r Curro Dossi, R. C. (1990a). Neuronal activities in brain-stem cholinergic nuclei related to tonic activation processes in thalamocortical systems. J. Neurosci. 10, 2541-59. 

An epidemiological study found significant increases in neurasthenia (i.e., fatigue, depressed mood, lack of initiative, dizziness, and sleep disturbances) in workers who were chronically exposed to jet fuels by inhalation, oral, and/or dermal exposure (Knave et al. 1978). Also, attention and sensorimotor speed were impaired, but no effects were found on memory function or manual dexterity. EEG results suggest that the exposed workers may have had instability in the thalamocortical system. The limitations of the study are discussed in detail in Section 2.2.1.2 under Respiratory Effects. 

Ethosuximide is most commonly used antiepileptic agent in the treatment of petitmal epilepsy. It acts on thalamocortical system by selectively suppressing T current without affecting other types of Ca " or Na" currents. It is completely absorbed from gastrointestinal tract and present in plasma in free form and approximately 20% is excreted unchanged in urine and remaining portion is metabolized in liver. 

Under what circumstances does conscious experience arise in the forebrain Few neuroscientists now doubt that the distributed and interconnected cortical circuits that are the physical substrate of conscious experience need to be synchronously activated, probably via the widely distributed thalamocortical system. 

What is the source of the activation of the thalamocortical system and the distributed forebrain circuits underlying consciousness Few neuroscientists now doubt that the brainstem reticular formation, and especially its pontine-mesencephalic and diencephalic components, regulate the cortex via its interaction with the thalamocortical system. 

The contention that the thalamocortical system is essential to the synchronous activation of the forebrain and hence to consciousness is supported by the loss of consciousness in subjects with disease destruction of the thalamus and by the capacity to restore consciousness by activating the thalamocortical system if that system (and of course, the cortex) is intact. The case of Karen Ann Quinlan is well known her profound coma was caused by a very small, restricted thalamic lesion and was irreversible because the thalamocortical system could not be activated by any known means. 

Human hearing arises from airborne waves alternating 50 to 20,000 times a second about the mean atmospheric pressure. These pressure variations induce vibrations of the tympanic membrane, movement of the middle-ear ossicles connected to it, and subsequent displacements of the fluids and tissues of the cochlea in the inner ear. Biomechanical processes in the cochlea analyze sounds to frequency-mapped vibrations along the basilar membrane, and approximately 3,500 inner hair cells modulate transmitter release and spike generation in 30,000 spiral ganghon cells whose proximal processes make up the auditory nerve. This neural activity enters the central auditory system and reflects sound patterns as temporal and spatial spike patterns. The nerve branches and synapses extensively in the cochlear nuclei, the first of the central auditory nuclei. Subsequent brainstem nuclei pass auditory information to the medial geniculate and auditory cortex (AC) of the thalamocortical system. 

The central auditory system consists of the cochlear nuclei groups of brainstem nuclei including the superior olivary complex (SOC), nuclei of the lateral lemniscus (LL), and inferior colliculus (1C) and the auditory thalamocortical system consisting of the medial geniculate in the thalamus and multiple areas of the cerebral cortex. Figure 5.4 schematically indicates the nuclear levels and pathways. Efferent pathways are not shown. Page constraints prevent us from providing uniform detail for all levels of the auditory system. 

The MGB and AC form the auditory thalamocortical system. As with other sensory systems, extensive projections to and from the cortical region exist in this system. The MGB has three divisions, the ventral, dorsal, and medial. The ventral division is the largest and has the most precise tonotopic organization. Almost aU its input is from the ipsilateral ICC through the brachium of the 1C. Its large bushy cells have dendrites oriented so as to He in isofrequency layers, and the axons of these neurons project to the AC. 

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