Axonal Membranes and Their Domains: Assembly and Function of the Axon Initial Segment and Node of Ranvier

Neurons are highly specialized cells of the nervous system that receive, process, and transmit electrical signals critical for normal brain function. Here, we review the intricate organization of axonal membrane domains that facilitate rapid action potential conduction underlying communication between complex neuronal circuits. Two critical excitable domains of vertebrate axons are the axon initial segment and the nodes of Ranvier, which are characterized by the high concentrations of voltage-gated ion channels, cell adhesion molecules, and specialized cytoskeletal networks. The axon initial segment is located at the proximal region of the axon and serves as the site of action potential initiation, while nodes of Ranvier, gaps between adjacent myelin sheaths, allow rapid propagation of the action potential through saltatory conduction. The axon initial segment and nodes of Ranvier are assembled by ankyrins, spectrins, and their associated binding partners through the clustering of membrane proteins and connection to the underlying cytoskeleton network. Although the axon initial segment and nodes of Ranvier share similar protein composition, their mechanisms of assembly are strikingly different. Here we will cover the mechanisms of formation and maintenance of these axonal excitable membrane domains, specifically highlighting the similarities and differences between them. We will also discuss recent advances in super resolution fluorescence imaging which have elucidated the arrangement of the submembranous axonal cytoskeleton revealing a surprising structural organization necessary to maintain axonal organization and function. Finally, human mutations in axonal domain components have been associated with a growing number of neurological disorders including severe cognitive dysfunction, epilepsy, autism, neurodegenerative diseases, and psychiatric disorders. Overall, this review highlights the assembly, maintenance, and function of axonal excitable domains, particularly the axon initial segment and nodes of Ranvier, and how abnormalities in these processes may contribute to disease.