General Organization of the Basal Ganglia Sample Clauses

General Organization of the Basal Ganglia. The basal ganglia are a group of interconnected subcortical nuclei consisting of the striatum, the internal and external segments of the GP, STN, and the SN. The striatum, the largest of the basal ganglia nuclei (Yelnik, 2002), is the main entrance for excitatory transmission from the cerebral cortex and thalamus to the basal ganglia circuitry (Fig. 1.1) (Xxxxx et al., 1998). Interestingly, while both cortico-striatal and thalamo-striatal projections are glutamatergic, they express different types of vesicular glutamate transporters: vGluT1 and vGluT2, respectively. The striatum is often divided into dorsal and ventral regions because of differences in neuronal connectivity and function. The ventral striatum contains ventral parts of the caudate nucleus and putamen and the nucleus accumbens (Groenewegen et al., 1993; Xxxxx, 2003). In addition to receiving excitatory input from the cortex and the thalamus, the striatum also receives inhibitory projections from the globus pallidus, and a modulatory dopaminergic innervation from the substantia nigra pars compacta (SNc) (Xxxxxxx et al., 2002). Both cortex and thalamus contribute to glutamatergic innervation of striatum. Corticostriatal afferents from layers III and V form asymmetric synapses onto dendritic spines of medium spiny neurons. Unlike cortical inputs, thalamostriatal afferents, which arise predominantly from the intralaminar nuclei, display a more heterogeneous pattern of synaptic connectivity between dendrites and spines(Xxxxxxx et al., 2002; Xxxx et al., 1988). The centromedian (CM) and parafasicular (Pf) nuclei of the thalamus give rise to thalamostriatal terminals forming axo-dendritic synapses (Raju et al., 2006), while all other thalamic projections form axospinous synapses resembling those produced by the corticostriatal projection. The STN also receives input from the cortex (Nambu et al., 2002). The GPi and SNr are the output nuclei of the basal ganglia. Projections from the SNr and GPi innervate thalamic targets. However, in addition to the thalamus these output nuclei make other specific connections (Parent and Xxxxxxx, 1995). The SNr neurons project to the superior colliculus (SC), pedunculopontine nuclei (PPN), and the medullary reticular formation (RF). Nigrotectal fibers converge mainly on the tectospinal neurons in the intermediate layer of the SC and play a critical role in the control of visual saccades (Xxxxx and Hikosaka, 1986). The nigroreticular projection terminates in the parv...