The Axon Hillock and the Initial Segment

Abstract

Axon hillocks and initial segments have been recognized and studied in electron micrographs of a wide variety of neurons. In all multipolar neurons the fine structure of the initial segment has the same pattern, whether or not the axon is ensheathed in myelin. The internal structure of the initial segment is characterized by three special features: (a) a dense layer of finely granular material undercoating the plasma membrane, (b) scattered clusters of ribosomes, and (c) fascicles of microtubules. A similar undercoating occurs beneath the plasma membrane of myelinated axons at nodes of Ranvier. The ribosomes are not organized into Nissl bodies and are too sparsely distributed to produce basophilia. They vanish at the end of the initial segment. Fascicles of microtubules occur only in the axon hillock and initial segment and nowhere else in the neuron. Therefore, they are the principal identifying mark. Some speculations are presented on the relation between these special structural features and the special function of the initial segment.

Squid Giant Axon Contains Neurofilament Protein mRNA but does not Synthesize Neurofilament Proteins

“…Nevertheless other energy-intensive processes such as axonal transport by kinesin motors are readily observed in the extruded axoplasm (Kang et al 2016;Prahlad et al 2000;Vale et al 1985). Another key observation against the idea of intra-axonal protein synthesis in the squid giant axon is the absence of morphological evidence for canonical ribosomes in the giant axon, and in vertebrate axons in general (Conradi 1966;Palay and Palade 1955;Palay et al 1968;Peters et al 1968;Peters et al 1970;Zelena 1972). This is in contrast to well-documented sites of "local protein synthesis" such as in dendrites where canonical ribosomes and polyribosomes have clearly been observed (Steward and Levy 1982;Steward and Schuman 2003).…”

Microdomains for Dopamine Volume Neurotransmission in Primate Prefrontal Cortex

“…In the CNS and peripheral nervous system, SSCs are neuron specific and localized exclusively in perikarya and the most proximal portions of their processes (Rosenbluth, 1962;Henkart et al, 1976;Peters et al, 1991). Likewise, SSCs occur in perikarya and large-caliber proximal dendrites of cortical neurons (Rosenbluth, 1962;Buschmann, 1979; for review, see Peters et al, 1991) and are unique to the axon initial segment of principal cells (Palay et al, 1968;Benedeczky et al, 1994). Double arrowheads in B point to an axo-spinous synapse (ax-sp) next to the SSC.…”

Structural complexes in the squid giant axon membrane sensitive to ionic concentrations and cardiac glycosides

“…The opaque axoplasmic material of the nodes of Ranvier has been thought to be related to specialized zones of membrane permeability (5), to areas of activity (17), or to diffusion barriers (I). A dense layer of fine granular material undercoating the plasma membrane of the axon initial segment, resembling that found at the node of Ranvier, has also been described in different neurons (13,22). It has been suggested that this undercoating represents a structural modification of the cell surface involved in the origin and propagation of the action potential (13).…”

“…A dense layer of fine granular material undercoating the plasma membrane of the axon initial segment, resembling that found at the node of Ranvier, has also been described in different neurons (13,22). It has been suggested that this undercoating represents a structural modification of the cell surface involved in the origin and propagation of the action potential (13). The distribution of the thickenings along the axolemma has been compared to that found, with the aid of histochemical techniques, in connection with sites of Na+-K+-Mg++-ATPase (19) and of acetylcholinesterase activities (26).…”