Figure 8.

Myenteric neuron identity and morphology. (A–C) Neurons expressing CGRPA in the (Figure 2B1) proximal and (Figure 2B2) middle colon, (Figure 2B3) with few to none in the distal colon. CGRPA fibers in the distal colon likely arise from extrinsic dorsal root ganglion afferents. (D1) CGRPA neurons express (D3) calbindin as do (D1–D4) IPANs and (D1 and D4) CGRPA and CGRPB neurons are labeled with CGRP-specific antibody. (E–H) CGRPA axons are (E and F) mostly smooth, but those that ramify within ganglia and circumferentially (F) are varicose. (G) Clusters of neurons surrounded by networks of CGRPA fibers associated with synaptophysin-labeled (localizes synapses) terminals. (H) Some CGRPA terminals surrounding myenteric neurons appear as modified calyxes. (I1 and I2) Calbindin forms basket-like structures around neurons expressing ChAT (likely IPANS) and (J1–J3) some of which do not express ChAT but that do express substance P; the identity of this neuron is unknown. Substance P also forms dense basket-like structures around ChAT + calbindin-expressing neurons, which could be IPANS or excitatory motor neurons, and around neurons that do not express ChAT. The identity of this neuron is unknown but the basket structure likely arises from an IPAN. (K–O) CGRPA neuron morphologies; (L) Dogiel Type II, (M) adendritic with a single process, and (K) neurons with lamellar dendrites but with more than 1 process. (N and O) Filamentous tyrosine receptor kinase B (TrkB) reporter neuron fibers run (N) longitudinally and (O) circumferentially. (D–O) Samples from colon areas B1–B4 (Figure 2). Scale bars: 100 μm (A–C), 50 μm (D1–K), and 25 μm (L–O).