Summary diagram. (A) Localization and molecular composition of the axon initial segment (AIS), para-AIS and juxtapara (JXP)-AIS in α motor neurons (MNs). α MNs have either a soma-derived AIS (85% of α MNs) or a dendrite-derived AIS (15%). From the MNs with a soma-derived AIS, 79% of them express voltage-gated sodium channel (Nav)1.1 and Nav1.6, while 20.2% express Nav1.6 only. The remaining AIS, Para-AIS and JXP-AIS proteins were found in 100% of α MNs: KCNQ2 throughout the AIS only, while voltage-gated potassium channel (Kv)1.1/Kv1.2/Kvβ2, PSD-93 and contactin-associated protein-like 2 (Caspr2) in the distal AIS and JXP-AIS, Caspr in the para-AIS, and protein 4.1B in the para-AIS, JXP-AIS and the internode. (B) Kv1 channels are expressed in the AIS and JXP-AIS by mechanisms independent of transient axonal glycoprotein-1 (TAG-1)/Caspr2 and PSD-93, while protein 4.1B was shown to control Caspr expression in the para-AIS and thus the proper exclusion of Kv1 channels from the para-AIS and the proper segregation of AnkG and Caspr from one another.
Duflocq et al. BMC Biology 2011 9:66 doi:10.1186/1741-7007-9-66