Figure 14.

Characteristic anatomy of a typical SPN and a spiny EGFP–TH⁺ Type I neuron filled with biocytin after whole-cell recording from adult mouse slices. A, Low-magnification photomicrograph of an SPN shows a soma ∼12 μm in diameter that gives rise to six primary dendrites that are aspiny for the initial 20 μm that then become densely invested with spines and give rise to secondary and higher-order spiny dendrites extending for a radius of ∼300 μm. 1–5, Higher-magnification images of spiny dendrites (A2–A5) and axons (A1, A2) from regions marked with the same numbers in A above. Note the high density of dendritic spines. The mushroom-like appearance of many of the spines is particularly apparent in A4 and A5. The local axon collaterals are varicose and exhibit intermittent varicosities. A6, Whole-cell current-clamp recording of responses to intracellularly injected current pulses from this neuron is shown in 6 and is very characteristic of SPNs. B, A spiny Type I neuron shows a slightly larger soma and four thick primary dendrites that branch less frequently and follow a straighter course than the dendrites of an SPN. The dendrites are invested with spine-like appendages. These spine-like processes are of much lower density than the dendritic spines on the SPN, many of them are quite long (arrows in B), and most appear to lack spine heads (B2). Like the SPN, the axonal arborization of the Type I neuron is quite dense. The intervaricose segments appear thinner than those of the SPN, and the entire axonal arborization is studded with larger and more prominent varicosities than that of the SPN (B1). B3, The whole-cell current-clamp recording from this neuron is typical for Type I neurons and completely distinct from that of a typical SPN.