Figure 4.

Medium spiny neuron and spine morphology. (A) A medium spiny neuron contains small clusters of enkephalin immunoreactivity throughout the perikarya, dendrite, and nucleus. (B) A labeled spine (sp) with a large head and thin neck emerges from a labeled dendrite (den), and receives an asymmetric synapse (solid black arrow). An adjacent labeled spine head also receives an asymmetric synapse (solid black arrow). Note the symmetric axodendritic synapse (open arrow). All three axon terminals (at) are unlabeled. (C) A spine with a spinule (asterisk) emerges from a dendrite (den). The spine is lightly labeled and receives a perforated asymmetric synapse (solid black arrows) from an unlabeled terminal (at). The dendrite receives a symmetric synapse (open black arrow) from an unlabeled terminal (at). (D) A labeled spine (sp) with a large head and wide neck emerges from a dendrite (den) and receives an asymmetric synapse (solid black arrows) and a symmetric synapse (open black arrow) from unlabeled axon terminals (at). (E) A labeled spine (sp) with a thin neck and head emerges from a dendrite. An unlabeled terminal (at) makes contact with the spine head but does not meet all three synaptic criteria since a clear synaptic cleft is not present. A nearby labeled axon terminal (+at) is adjacent to the dendrite but does not form a synapse. (F) A mushroom shaped spine (sp) emerges from a labeled dendrite (den). An unlabeled axon terminal makes a perforated (asterisk) asymmetric synapse (solid black arrows). Reaction product is indicated by white arrows. Scale bars: A, 2 μm; B-F, 0.5 μm.