Subdivisions

Axon structure can be analyzed in several ways. One way is to distinguish between cytoskeletal and membranous components. The cytoskeleton is considered to comprise three domains linked in a microtrabecular meshwork: the microtubules, neurofilaments, and, adjoining the peripheral membrane, the actin microfilaments. Neurofilaments are a type of intermediate filament, about 10 nm in diameter, which consist of a species-specific ratio of three proteins (68,150, and 200 kDa). Microtubules are somewhat larger (20-26 nm in diameter), and consist of two globular polypeptides, a and b tubulin, 50 kDa each. Microtubules have

Figure 2 Schematic illustration of (A) local circuit neurons, whose axons arborize in the immediate vicinity of the cell body, and (B) a cortical projection neuron, whose axon typically projects to distant targets, several millimeters or centimeters away. Open arrows point to axons. The caliber of the dendrites has been exaggerated to distinguish these from the axon.

Figure 2 Schematic illustration of (A) local circuit neurons, whose axons arborize in the immediate vicinity of the cell body, and (B) a cortical projection neuron, whose axon typically projects to distant targets, several millimeters or centimeters away. Open arrows point to axons. The caliber of the dendrites has been exaggerated to distinguish these from the axon.

Figure 3 (A) Overview of the six cortical layers in monkey (from visual association cortex). (B) Portion through the middle cortical layers of human temporal cortex from a surgical specimen. (C, D) Progressively higher magnification from the arrow in B. In D, clumps of rER are visible in the cell body. Numbers denote the conventional cortical layers 1-6. WM, white matter.

Figure 3 (A) Overview of the six cortical layers in monkey (from visual association cortex). (B) Portion through the middle cortical layers of human temporal cortex from a surgical specimen. (C, D) Progressively higher magnification from the arrow in B. In D, clumps of rER are visible in the cell body. Numbers denote the conventional cortical layers 1-6. WM, white matter.

polarity, by conventional designated as " + " or "—'' (respectively nearest the distal or proximal cell body region). Head-to-tail polymerization occurs, with subunits added to the plus end and released from the minus end ("treadmilling"). There are several microtubule-associated proteins (MAPS) that protrude as side arms and contribute to the microtrabe-cular meshwork.

Neurofilaments predominate in large axons, but microtubules predominate in small axons, and the total number of both structures is proportional to the caliber of the axon (Fig. 4). These structures play a role in skeletal support as well as in intracellular transport of ions, metabolites, and vesicles.

A major membranous component of the axon is the agranular ER, which is thought to consist of two subsystems: (i) clusters of tubules and flattened sacs at the outer wall of the axons and (ii) a complex of narrow tubules and sacs oriented parallel to the long axis of the axon, which is believed to extend the full length of the axon. The agranular ER has been thought to contribute to axonal transport, and it possibly contributes to the sequestration of Ca2+ ions and the provision of membrane for forming synaptic vesicles.

Table I

Characteristics of Axons and Dendrites"

Axon

Dendrite

1. Extends from either cell body or dendrite

2. Begins with a specialized initial segment (except dorsal root ganglion cell and autonomic ganglion cell)

3. May be absent as in amacrine cells of retina

4. Unique in most cells, but there are some examples of multiple origin

5. May be myelinated or unmyelinated

6. Almost never contains ribosomes (Except in initial segment)

7. Usually has smooth contours and cylindrical shape

8. Usually is the thinnest process of the cell at site of origin

9. Ramifies by branching at obtuse angles

10. Usually gives rise to branches of the same diameter as parent stem

11. Ramification can be close to cell body or at great distances; may extend long distances away from cell body, even into peripheral nervous system

12. Neurofilaments predominate in larger axons

13. Capable of generating action potentials, propagating them, and synaptic transmission

14. Primarily concerned with conduction and transmission

1. Extends from cell body

2. At least in proximal portions, continues cytoplas-mic characteristics of cell body

3. May be absent as in dorsal root ganglion cell

4. Usually multiple

5. Rarely myelinated (and if so, only thinly)

6. Contains granular endoplasmic reticulum or ribosomes, diminishing with distance from origin

7. Usually has irregular contours and specialized appendages

8. Usually originates as a thick tapering process

9. Ramifies by branching at acute angles

10. Usually subdivides into branches smaller than parent stem

11. Ramification usually confined to the vicinity of the cell body; if cell body lies in central nervous system, dendrites remain entirely within central nervous system

12. Microtubules predominate in larger stems and branches

13. Conducts in a decremental fashion, but may be capable of generating action potentials

14. Primarily concerned with receiving synapses.

"From The Fine Structure of the Nervous System: Neurons and Their Supporting Cells, Third Edition by Alan Peters, S. L. Palay, and H. Webster, Copyright © 1990 by Alan Peters, Used by permission of Oxford University Press, Inc.

Many long, type I axons have an associated specialization, the myelin sheath of the plasmalemma, which is elaborated by glial processes. The myelin wrapping is interrupted by nodes of Ranvier, which are involved in the process of saltatory impulse conduction.

Axons can also be subdivided into several specialized portions along their proximal to distal length. The proximal portion, at the juncture with the cell body, is termed the axon hillock (Fig. 1). This region is characterized by fascicles of microtubules and a high density of sodium-dependent channels. The high density of channels accords with the importance of this region as a "trigger zone'' for the initiation of axon impulses.

Adjoining the axon hillock distally is the initial segment (20-50 nm in length). This portion has clusters of microtubules and an undercoating of dense material below the plasmalemma. The initial segment leads into the body of the axon. This can range from <0.5 mm in the case of type II, local circuit axons to several centimeters or even meters in the case of axons projecting from motor cortex to the lower spinal cord of larger animals. Axons in some systems can send branches ("collaterals") to different target structures. Finally, at the distalmost portion, axons form terminal arbors (Fig. 5). Arbors consist of preterminal and terminal portions. The preterminal portions are where the axon begins to branch repeatedly, and, if myeli-nated, loses the myelin sheath. Terminal portions are thin-caliber branches, decorated with synaptic terminal specializations.

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