Two different types of thalamic fibers innervate the rat striatum.

Thalamostriatal projections were studied in rats by tracing the axons of small pools of thalamic neurons labeled anterogradely with biocytin. Single-cell mapping of these projections revealed two types of thalamostriatal fibers. The first type arises from the bushy relay cells of the central lateral and associative thalamic nuclei which arborize sparsely in the striatum by means of long varicose axon collaterals. The second type of fiber arises from large, reticular-like, relay cells located in the parafascicular and ethmoid nuclei. These latter fibers form dense clusters of terminations within the striatum and they also send branches to other components of the basal ganglia. These different morphological features suggest that the two types of fibers subserve different functions.