The Neuron

Neurons are highly polarized cells, meaning that they develop, in the course of maturation, distinct subcellular domains that subserve different functions. Morphologically, in a typical neuron, three major regions can be defined: (1) the cell body, or perikaryon, which contains the nucleus and the major cytoplasmic organelles; (2) a variable number of den-drites, which emanate from the perikaryon and ramify over a certain volume of gray matter and which differ in size and shape, depending on the neuronal type; and (3) a single axon, which extends in most cases much farther from the cell body than does the dendritic arbor (Fig. 1.1). The dendrites may be spiny (as in pyramidal cells) or nonspiny (as in most interneurons), whereas the axon is generally smooth and emits a variable number of branches (collaterals). In vertebrates, many axons are surrounded by an insulating myelin sheath, which facilitates rapid impulse conduction. The axon terminal region, where

Dendritic branches with spines

Dendritic branches with spines

Pyramidal Cells And Purkinje Cells

Purkinje cell of cerebellar cortex

Pyramidal cell of cerebral cortex

FIGURE 1.1 Typical morphology of projection neurons. On the left is a Purkinje cell of the cerebellar cortex, and on the right, a pyramidal neuron of the neocortex. These neurons are highly polarized. Each has an extensively branched, spiny apical dendrite, shorter basal dendrites, and a single axon emerging from the basal pole of the cell.

Purkinje cell of cerebellar cortex

Pyramidal cell of cerebral cortex

FIGURE 1.1 Typical morphology of projection neurons. On the left is a Purkinje cell of the cerebellar cortex, and on the right, a pyramidal neuron of the neocortex. These neurons are highly polarized. Each has an extensively branched, spiny apical dendrite, shorter basal dendrites, and a single axon emerging from the basal pole of the cell.

From Molecules to Networks

Copyright 2004, Elsevier Science (USA).

All rights reserved.

dritic arbor and the distribution of axonal terminal ramifications confer a high level of subcellular specificity in the localization of particular synaptic contacts on a given neuron. The three-dimensional distribution of the dendritic arborization is also important with respect to the type of information transferred to the neuron. A neuron with a dendritic tree restricted to a particular cortical layer may receive a very limited pool of afferents, whereas the widely expanded dendritic arborizations of a large pyramidal neuron will receive highly diversified inputs within the different cortical layers in which segments of the dendritic tree are present (Fig. 1.2) (Mountcastle, 1978; Peters and Jones, 1984; Schmitt et al., 1981; Szentagothai and Arbib, 1974; Lund et al., 1995; Bjorklund et al., 1990). The structure of the dendritic tree is maintained by surface interactions between adhesion molecules and, intracellularly, by an array of cytoskeletal elements (microtubules, neurofilaments, and associated proteins), which also take part in the movement of organelles within the dendritic cytoplasm.

An important specialization of the dendritic arbor of certain neurons is the presence of large numbers of dendritic spines, which are membrane-limited organelles that project from the surface of the den-

Inhibitory Synapses Dendritic Tree

in layer V

FIGURE 1.2 Schematic representation of four major excitatory inputs to pyramidal neurons. A pyramidal neuron in layer III is shown as an example. Note the preferential distribution of synaptic contacts on spines. Spines are labeled in red. Arrow shows a contact directly on the dendritic shaft.

in layer V

FIGURE 1.2 Schematic representation of four major excitatory inputs to pyramidal neurons. A pyramidal neuron in layer III is shown as an example. Note the preferential distribution of synaptic contacts on spines. Spines are labeled in red. Arrow shows a contact directly on the dendritic shaft.

drites. They are abundant in large pyramidal neurons and are much sparser on the dendrites of interneu-rons. Spines are more numerous on the apical shafts of the pyramidal neurons than on the basal dendrites. As many as 30,000 to 40,000 spines are present on the largest pyramidal neurons. Spines constitute the region of the dendritic arborization that receives most of the excitatory input. Each spine generally contains one asymmetric synapse; thus, the approximate density of excitatory input on a neuron can be inferred from an estimate of its number of spines. The cytoplasm within the spines is characterized by the presence of polyribosomes and a variety of filaments, including actin and a- and p-tubulin, as well as a spine apparatus comprising cisternae, membrane vesicles, and stacks of dense lamellar material (see Box 1.1) (see (Berkley, 1896; Gray, 1959; Ramón y Cajal, 1955; Coss and Perkel, 1985; Scheibel and Scheibel, 1968; Steward and Falk, 1986; Zhang and Benson, 2000; Nimchinsky et al, 2002).

The perikaryon contains the nucleus and a variety of cytoplasmic organelles. Stacks of rough endoplas-mic reticulum are conspicuous in large neurons and, when interposed with arrays of free polyribosomes, are referred to as Nissl substance. Another feature of the perikaryal cytoplasm is the presence of a rich cytoskeleton composed primarily of neurofilaments and microtubules, discussed in detail in Chapter 2. These cytoskeletal elements are dispersed in "bundles" that extend into the axon and dendrites (Peters and Jones, 1984). Whereas the dendrites and the cell body can be characterized as the domains of the neuron that receive afferents, the axon, at the other pole of the neuron, is responsible for transmitting neural information. This information may be primary, in the case of a sensory receptor, or processed information that has already been modified through a series of integrative steps. The morphology of the axon and its course through the nervous system are correlated with the type of information processed by the particular neuron and by its connectivity patterns with other neurons. The axon leaves the cell body from a small swelling called the axon hillock. This structure is particularly apparent in large pyramidal neurons; in other cell types, the axon sometimes emerges from one of the main dendrites. At the axon hillock, microtubules are packed into bundles that enter the axon as parallel fascicles. The axon hillock is the part of the neuron from which the action potential is generated. The axon is generally unmyeli-nated in local-circuit neurons (such as inhibitory interneurons), but it is myelinated in neurons that furnish connections between different parts of the nervous system. Axons usually have larger numbers

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