Introduction

Nerve cells consist of three components: the cell soma, dendrites, and axon (Fig. 1). There are usually multiple dendrites and a single axon (there are instances of neurons with no axon and, rarely, of neurons with multiple axons). Dendrites are frequently viewed as extensions of the cell body and are the primary target for synaptic inputs to the neuron. Unlike the dendrites or cell body, the axon does not contain ribosomes, the apparatus of protein manufacture. Via the excitable plasmalemma and other specializations, it is mainly

Encyclopedia of the Human Brain Volume 1

Copyright 2002, Elsevier Science (USA).

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Figure 1 Photomicrographs at lower (A) and higher (B) magnifications of a cortical pyramidal neuron in layer 3, retrogradely labeled from an injection of tracer (biotinylated dextran amine). Curved arrow, cell body; arrowheads, three of the dendrites; open arrow, axon hillock; solid arrow, descending portion of axon (this continues toward the white matter but has been cut off in this section). Open arrows point to equivalent features in A and B.

Figure 1 Photomicrographs at lower (A) and higher (B) magnifications of a cortical pyramidal neuron in layer 3, retrogradely labeled from an injection of tracer (biotinylated dextran amine). Curved arrow, cell body; arrowheads, three of the dendrites; open arrow, axon hillock; solid arrow, descending portion of axon (this continues toward the white matter but has been cut off in this section). Open arrows point to equivalent features in A and B.

concerned with conduction and transmission of the nerve impulse and other signals.

Modern investigations of the axon are proceeding on several fronts, namely, the cell and molecular biology of the axon, especially concerning the mechanisms of axoplasmic transport; the specializations and mechanisms of the plasmalemma as an excitable membrane; and the network properties of the axon as a constituent of neural architectures. These issues are of major importance for further understanding of the basic science of neural function and bear directly on pathological conditions, such as peripheral neuropathies, demyelinating diseases, and diffuse axon injury subsequent to trauma. Recent progress has been rapid in all these areas owing to dramatic technical advances. The technical armament includes physiological techniques such as nodal voltage clamping and optical methods for studying electrical activity, sophisticated immunocytochemical and ultrastructural methods, and an array of fine molecular tools.

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