Fig. 2.

Electron micrographs showing lack of M2R-ir in putative pyramidal cell perikarya, identified on the basis of synaptology (see Materials and Methods), but dense M2R-ir in processes arising from these perikarya (PUR as chromogen for M2R). (A) An unlabeled pyramidal cell perikaryon (Upk) gives rise to a densely-labeled proximal dendrite (M2d, center), with other M2R+ dendrites indicated. (B) An unlabeled pyramidal cell perikaryon (Upk) gives rise to an M2R+ process (M2p) that may be a small-caliber primary dendrite or an axon initial segment. Its identification as a putative axon initial segment is based on the fact that: (1) there appears to be some fasciculation of microtubules, (2) the conical shape of its origin from the perikaryon resembles that of an axon hillock, and (3) ribosomes and rough endoplasmic reticulum of the perikaryon do not extent into the putative axon hillock (Peters et al., 1991). M2R+ terminals (M2t) are also indicated, one forming a symmetrical synapse with the M2R+ process (arrowhead), and another forming an asymmetrical synapse with an M2R+ spine (asterisk). A DAB-labeled VAChT+ axon is shown to the right of the field (Vax). Scale bars = 0.5 μm.