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medium spiny neuron
Medium spiny neurons (MSNs), also known as spiny projection neurons, are a special type of GABA-ergic inhibitory cell representing 95% of neurons within the human striatum, a structure located in the basal ganglia.〔 Medium spiny neurons have two primary phenotypes (i.e., characteristic types): D1-type MSNs of the "direct pathway" and D2-type MSNs of the "indirect pathway".〔〔〔
Direct pathway MSNs excite their ultimate basal ganglia output structure (e.g., the thalamus) and promote associated behaviors;〔 these neurons express D1-type dopamine receptors, adenosine A1 receptors, dynorphin peptides, and substance P peptides.〔〔 Indirect pathway MSNs inhibit their output structure and in turn inhibit associated behaviors;〔 these neurons express D2-type dopamine receptors, adenosine A2A receptors (A2A), heterotetramers, and enkephalin.〔 Both types express glutamate receptors (NMDAR and AMPAR) and CB1 receptors.〔 A subpopulation of MSNs contain both D1-type and D2-type receptors, with approximately 40% of striatal MSNs expressing both DRD1 and DRD2 mRNA.〔〔 In the nucleus accumbens (NAcc), these mixed-type MSNs that contain both D1-type and D2-type receptors are mostly contained in the NAcc shell.〔
The dorsal striatal MSNs play a key role in initiating and controlling movements of the body, limbs, and eyes. The ventral striatal MSNs play a key role in motivation, reward, reinforcement, and aversion. Dorsal and ventral medium spiny neuron subtypes (i.e., direct D1-type and indirect D2-type) are identical phenotypes, but their output connections differ.
== Appearance and location ==
The medium spiny neurons are medium-sized neurons (~15 microns in diameter, ~12-13 microns in the mouse) with large and extensive dendritic trees (~500 microns in diameter). Striatal direct pathway MSNs (dMSNs) project directly to the globus pallidus internal (GPi) and substantia nigra pars reticulata (SNpr) whereas striatal indirect pathway MSNs (iMSNs) ultimately project to these two structures via an intermediate connection to the globus pallidus external (GPe) and ventral pallidum (VP).〔 The GPe and VP send a GABAergic projection to the subthalamic nucleus, which then sends glutamatergic projections to the GPi and SNpr.〔 Both the GPi and SNpr send inhibitory projections to nuclei within the thalamus.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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