Cortico thalamic

Figure 15.2(a) A schematic presentation of possible normal basal ganglia circuitry. Activity in the cortico-thalamic pathway is modulated by striatal control of the globus pallidus (pallidum) through two pathways, the indirect pathway (Ind Path) to the external pallidum/globus pallidus (GPext) and the subthalamic nucleus (SThN) and the direct pathway (Dir Path) to GPint. Scheme... 

Figure 15.2(b) A schematic presentation of possible basal ganglia circuitry in Parkinson s disease. In PD there is little or no inhibitory nigrostriatal input to the striatum so the Ind Path is active and GPext is inhibited. This will then have less depressant effect on the SThN which will be free to drive the GPint (and SNr) and so reduce cortico-thalamic traffic and produce akinesia. See text for detail. Pathway activity — low — normal — high... 

Figure 15.9 Peptide modulation of striatal input to the globus pollidus. Enkephalin released from axon terminals of neurons of the indirect pathway (see Fig. 15.2 for details) is thought to inhibit GABA release from the same terminals so that feedback (auto) inhibition is reduced. This will free the neurons to inhibit the subthalamic nucleus (SThN) and its drive to GPint and SNr which in turn will have less inhibitory effect on cortico-thalamic traffic and possibly reduce akinesia. Dynorphin released from terminals of neurons of the direct pathway may also reduce glutamate release and excitation in the internal globus pallidus and further depress its inhibition of the cortico-thalamic pathway. High concentrations of these peptides may, however, result in dyskinesias. (See Henry and Brotchie 1996 and Maneuf et al. 1995)...

Figure 22.5 Pathways involved in cortico-thalamic synchrony and EEG arousal. The ascending reticular activating system (ARAS) extends from the cephalic medulla through the pons and midbrain to the thalamus (see Moruzzi and Mayoun 1949). It is activated by impulses in collaterals of the spinothalamic sensory pathway running to specific thalamic nuclei (SpThNc) and in turn activates much of the cortex, partly through the non-specific thalamic nuclei (NspThNc), which also receive inputs from SpThNc and also via the nucleus basalis (NcB). Its stimulation is followed by EEG arousal. It is probable that reciprocal links between cortical areas and the thalamus, particularly NspThN, lead to slow-wave (8 Hz) cortical EEG synchrony and, in the absence of appropriate sensory input and ARAS activity, a sleep state...

Setting aside the general anaesthetics, which do not directly modify the function of any particular neurotransmitter, all the drugs that are used to induce sleep, i.e. the hypnotics , augment the function of GABA and so directly depress neuronal function and probably facilitate cortico-thalamic synchrony. Most of them are benzodiazepines... 

Fonnum F., Storm-Mathisen J, and Divac I. (1981) Biochemical evidence for glutamate as neurotransmitter in the cortico-striatal and cortico-thalamic fibers in rat brain. Neuroscience. 6, 863-875... 

Augment, or more probably, break up thalamic-cortico synchrony and its tendency to promote slow-wave EEG activity and non-REM sleep. Whether this results in full arousal, or merely a temporary disruption of sleep to give REM periods without full awaking, will depend on the balance of inputs and the overall state of cortical activity. 

Hnhancement of neurotransmission through prefrontal and paralimbic cortico-basal ganglia-thalamic circuits may account for the most commonly reported... 

A global view of consciousness is that it is generated throughout the entire brain, as a result of synchronisation of relevant neural networks. Specific systems or regions—for example the cerebral cortex, brainstem reticular formation and thalamic nuclei—may be key anatomical integrators. Areas with the most widespread interconnections are pivotal, and on this basis the cortex and thalamus are more relevant than cerebellum and striatum for example. Frontal cortex for example connects with every other brain region, both in terms of input and output, with 80% of such connections accounted for by cortico-cortical connections. Thalamic intralaminar nuclei are, in conjunction with the reticular nucleus, reciprocally connected to all cortical areas. By contrast the cerebellum has very few output pathways and striatal-cortical input is (via the thalamus) confined to frontal lobe. 

Liddle, P., Lane, C., 8c Ngan, E. (2000). Immediate effects of risperidone on cortico-striato-thalamic loops and the hippocampus. British Journal of Psychiatry, 177, 402-407. 

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