Neostriatal spiny neuron
Neostriatal spiny neurons are neurons found in Vertebrates.
Neostriatal spiny neurons are found in the neostriatum in the basal ganglia. The neostriatum consists of two types of neurons - Aspiny and Spiny Neurons. Spiny Neurons account for the majority (almost 90%) of the neurons present in the neostriatum. Neostriatal spiny neurons are called so because of the large number of dendritic spines. The axons of neostriatal spiny neurons are gathered into small fiber fascicles that perforate the gray matter of neostriatum, giving the neostriatum the striated appearance for which it is named. These fibers were identified using Golgi staining and labeling the neurons with Horse Radish Peroxidase. These efferent neurons transmit information generally from the cerebral cortex and relay it to the globus pallidus and the substantia nigra regions in the basal ganglia. Neotriatal medium spiny neurons are GABAergic providing inhibitory inputs to adjacent spiny neurons via local axon collaterals, to the globus pallidus (external), and to both basal ganglia output nuclei.
Neuronal Type: Neostriatal spiny neurons are Golgi Type 1 and projection or principal neurons.
Neostriatal spiny neurons have ovoid or polygonal cell bodies with dendrites radiating in all directions. The somata of the neurons is smooth and the dendrites, except for their most proximal portions, are rich in spines (hence called spiny neurons). The smooth trunks divide to give rise to spiny secondary dendrites, which may further branch one or two times. The axon of the spiny neuron arises from a well defined initial segment on the soma or from a large proximal dendritic trunk. This main axon tapers rapidly and forms several smaller initial collaterals before leaving the vicinity of the cell body. These arborize extensively throughout an area of about the same size as, and highly overlapping with, the dendritic field of the cell, while the main axon extends up to 1 mm in the direction of the globus pallidus. In the globus pallidus and substantia nigra, the axons of neostriatal spiny neurons arborize in a very characteristic longitudinal axodendritic pattern. This pattern is characterized by individual neostriatal efferent axons running parallel to dendrites of the pallidal and nigral target neurons, making multiple synaptic contacts that almost completely ensheath the dendrites of the postsynaptic cells.
Neostriatal Spiny neurons are separated into two subpopulations based on their axonal targets. One subpopulation of neurons has highly collateralized axons, direct projections to the internal and external segments of the GP (Globus Pallidus)and substania nigra, and immediate access to the output of basal ganglia. These neurons form the direct pathway from the neostriatum. The other subpopulation of efferent neurons projects no farther than the external segment of the GP. These constitute the neurons of the indirect pathway. These neurons can affect the basal ganglia output only by way of the subthalamic nucleus and its projections to the GPi (internal segment of GP) and the substantia nigra, or by projections of the GPe to the output neurons. Both subpopulations of spiny neurons are GABAergic, forming GABA-containing synapses in the neostriatum, GP and substantia nigra. In addition to GABA, these axons contain peptide neurotransmitters, with indirect pathway axons containing enkephalin, whereas the axons of the direct pathway contain substance P and dynorphin. Direct neurons have D-1 type dopamine receptors while indirect neurons have D-2 type dopamine receptors.
Three major synaptic types have been observed to be in contact with spiny neurons. Boutons containing small round synaptic vesicles form synapses exclusively with spiny regions of the dendrites, and most being axo-spinous. The second type of bouton comprises small, very pleomorphic synaptic vesicles which make contacts with the somata, initial segments, and dendrites, but not dendritic spines. The third type consists of boutons, containing large pleomorphic synaptic vesicles, have the most widespread distribution, contacting all regions including dendritic spines. Spines receiving these contacts are postsynaptic to boutons containing small round vesicles. Axon collaterals of spiny cells form synapses with large pleomorphic vesicles and make synapses with somata, initial segments of axons, dendrites, and dendritic spines of striatal neurons, including other spiny cells. The neostriatal spiny neurons receive inputs from the cerebral cortex,thalamus and the whole of neostriatum. These neurons are a major target of modulatory afferents. The output axons of the Neostriatal spiny neurons form synapses in the substantia nigra and globus pallidus.
Neostriatal spiny neurons are mostly silent. They fire very rarely and in episodes that last for about 0.1-3 seconds and become silent once again
Parkinson's and Huntington's Disease
The major symptoms of Parkinson's disease arise as a consequence of a deficiency in striatal dopamine due to the progressive degeneration of neostriatal spiny neurons. In the case of Parkinson's disease, the subunit composition and/or phosphorylation state of the N-methyl-D-aspartate (NMDA) subtype glutamatergic receptors expressed on the dendritic spines of the neostriatal spiny neurons changes in ways that compromise motor performance. Alterations in signaling systems linking dopaminergic and glutamatergic receptors within the neostriatal spiny neurons may be the possible cause of induction of NMDA receptor modification. Huntington's disease is also characterized by the depletion of the GABAergic and cholinergic systems in the neostriatal spiny neurons especially in the caudate-putamen region in the neostriatum. As in Parkinson's disease, changes are also observed in the NMDA receptors.
- HT Chang, CJ Wilson, and ST Kitai,"Single neostriatal efferent axons in the globus pallidus: a light and electron microscopic study'", Science 21 August 1981 213: 915-918 [DOI: 10.1126/science.7256286]
- Gordon M.Shepherd, "The synaptic organization of the Brain".5th ed:Ch 9
- Wilson CJ, Groves PM ,"Fine structure and synaptic connections of the common spiny neuron of the rat neostriatum: a study employing intracellular inject of horseradish peroxidase"," J Comp Neurol" 1980 Dec 1;194(3):599-615.
- Thomas N. Chase, Justin D. Oh, and Pierre J. Blanchet,"Neostriatal mechanisms in Parkinson's disease","Neurology" 1998;51:S30-S35