(PPTg) Also called the pedunculopontine nucleus (PPN) or tegmental pedunculopontine nucleus (TPP). A nucleus situated in the MESOPONTINETEGMENTUM, Caudal to the SUBSTANTIA NIGRA and rostral to the PARABRACHIAL NUCLEUS. It is best known as the site of the Ch5 CHOLINERGIC neurons (see CH1–CH8), which constitute the largest cholinergic input to the THALAMUS. The nucleus is also the target of the major GABA output from the substantia nigra zona reticulata, and receives innervation from every other site in the BASAL GANGLIA, as well as the NUCLEUS ACCUMBENS and VENTRAL PALLIDUM. Neurons in the PPTg atrophy during the course of PARKINSON’S DISEASE and PROGRESSIVE SUPRANUCLEAR PALSY. What role this loss has in generating the symptoms of these disorders is not clear, though the anatomical reciprocity between the PPTg and STRIATUM provides evidence for a fundamental role in the behavioural deficits in basal ganglia diseases.
The boundaries of the PPTg have been drawn with respect to the location of the cholinergic neurons. These are most densely packed in the posterior part of the nucleus—the pars compacta—but are more scattered further anteriorly—the pars dissipata. The cholinergic neurons are also contain other transmitters, including SUBSTANCE P and GLUTAMATE, and NITRIC OXIDE, a novel neuromodulator. Together with the Ch6 neurons of the LATERODORSAL TEGMENTAL NUCLEUS, they form the cholinergic component of the ASCENDING RETICULAR ACTIVATING SYSTEM. They have long projection neurons and influence activity in many structures in the FOREBRAIN, most notably the thalamus, through which they can also influence the CEREBRAL CORTEX.
Functionally, mesopontine cholinergic neurons interact with those in the dorsal RAPHE NUCLEI and LOCUS COERULEUS, locally and at the level of the thalamus, for the regulation of BEHAVIOURAL STATE. Within the PPTg, the terminal fields of basal ganglia and ventral striatalventral pallidal output neurons are found in the MIDBRAIN EXTRAPYRAMIDAL AREA. This area is located in the medial aspect of the PPTg, adjacent to the decussation of the SUPERIOR CEREBELLAR PEDUNCLE. In human and primate brains, the cholinergic neurons form a crescent around the striatal output neurons at this level, characteristic of the relationship between the substantia nigra pars compacta and zona reticulata. In the rat, the location of the cell bodies and terminal fields partially overlap (although output neurons do not project to the cholinergic cell bodies). Behavioural data suggest the importance of interactions between ascending (cholinergic/glutamatergic) and descending (striatal outflow) neurons. It is possible that the small GABA neurons in the PPTg are the link between these systems.
Reference
Inglis W.L. & Winn P. (1995) The pedunculopontine tegmental nucleus: where the striatum meets the reticualr formation. Progress in Neurobiology 47:1–29.
WENDY L.INGLIS
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