Spiny projection neuron input

Fig. 9. Summary of the major synaptic inputs to spiny projection neurons. Inputs to the cell bod> arise mainly from striatal intemeurons. Inputs to proximal dendrites are mainly from striatal interneurons and other spiny projection neurons. Inputs to distal dendrites arise from extrastriatal sources, from the cortex (asymmetric/ glutamatergic) to the spine heads, from dopamine neurons in the midbrain (symmetric/dopamine) to the necks of spines and to interspine shafts. Other spiny projection neurons also provide symmetric inputs to the necks of spines and to interspine shafts.

Spiny projection neurons receive inputs from the cortex, thalamus and amygdala, which make asymmetric synapses on dendritic spines, and to a lesser degree, dendritic shafts. These inputs provide the major excitatory input to these neurons. In addition, a number of inputs from outside the striatum, and from within the striatum provide inputs that function to modify the responsiveness of spiny neurons to the excitatory input. These include inputs from dopamine afferents from the substantia nigra, inhibitory GABA inputs from the axon collaterals of other spiny neurons, inhibitory inputs from GABA (and peptide containing) striatal interneurons, and inputs from cholinergic striatal interneurons. 

Fig. 13. A) Diagram showing an example of inputs to the globus pallidus (GP) from striatal spiny projection neurons. Typically there are two major sites of axonal arborization, one in the region immediately adjacent to the striatum and a second in the central region of the GP. B) Stylized drawing of two pallidal neurons showing how the dendrites of neurons are confined within the two regions of the GP that conform to the pattern of striatal inputs. C) The axonal projection of a globus pallidus neuron of the type with discoid dendrites, which provides collaterals to the striatum (CP), to the entopeduncular nucleus (EP), subthalamic nucleus (stn) and substantia nigra (SN). Adapted from Kita and Kitai 1994.

Fig. 24. Patch and matrix striatal compartments are labeled with neurochemical markers. A) The patch compartment is labeled with 3H-naloxone binding to mu opiate receptors (white in the darkfield photomicrograph). B) The matrix compartment is labeled with calbindin-immunoreactivity, which labels spiny projection neurons that provide inputs to the substantia nigra pars reticulata. The correspondence between calbindin-poor zones (black arrows) and mu opiate binding sites (white arrows) is seen to occur in all regions of the striatum. Calbindin-immunoreactivity is relatively weak in the dorso-lateral striatum, which nonetheless contains opiate receptor patches.

The ultrastructural organization of dopaminergic boutons in the striatum has been extensively investigated (see, inter alia, the reviews of Smith and Bolam, 1990 Sesack, 2003). Extrastriatal inputs, including the dopaminergic one, terminate mainly on the more distal part of the dendritic tree of medium spiny neurons, while intrinsic inputs terminate mainly on the proximal parts of the dendritic shaft and on the cell body. A small proportion of dopaminergic axons also contact the cell body of striatal projection neurons (Fig. 18B). 

Consistent with the asymmetric character of corticostriatal synapses onto spiny neurons electrophysiologic studies have demonstrated that corticostriatal input evokes a monosynaptic excitatory post-synaptic potential (EPSP) (Kitai et al. 1976 Wilson 1986). At least two types of corticostriatal afferents have been identified, on the basis of the electrophysiologic effects of these inputs (Jinnai and Matsuda 1979 Wilson 1986). One is a fast conducting collateral of neurons projecting to the brainstem and evokes an EPSP with a latency of 3 msec. A second type, which appears to be the major corticostriatal afferent, is a slower conducting afferent that evokes an EPSP with a latency of 10 msec. 

Inputs from midbrain dopamine neurons that project to the striatum make synaptic contact with spiny neurons. These aflferents have been identified at the ultrastructural... 

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