Neurons Are Highly Polarized Cells

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The structure of the nerve cell is well-suited for intracellular and intercellular information transfer. Neurons are highly polarized cells, with long processes that extend from the cell body to interact with other nerve cells, muscles, or glands (Figs. 1 and 2). Neurons are often classified according to the number of processes extending from their cell bodies as multipolar, bipolar, and monopolar. Neurons are the most morphologically diverse population of cells in an organism, varying widely in their size, form, and number of processes. The following will serve as a general description of the most common attributes of neurons, with a focus on vertebrate neurons. However, as with almost every aspect of the nervous system, exceptions to nearly every general statement can be found.

Two types of processes are distinguished in the typical nerve cell: dendrites and axons. These processes are sometimes referred to generically as neurites, particularly in developing neurons grown in culture and in invertebrates where many of the distinctions described in the following sections are less clear. Dendrites can be considered as extensions of the cell body specialized for receiving information from other nerve cells or the environment. The number ofprimary dendrites emanating from the cell body can vary from zero to many and is characteristic of a particular cell

Figure 2 Purkinje neuron from the rat cerebellum stained by intracellular injection of the fluorescent dye lucifer yellow. The magnificent dendritic tree is revealed in its entirety. The fuzzy appearance of the finer caliber dendritic branches is due to the high density of dendritic spines (spiny branchlets). A single axon is visible emerging from the cell soma, although it is stained very faintly compared to the dendritic tree due to its small caliber.

Figure 2 Purkinje neuron from the rat cerebellum stained by intracellular injection of the fluorescent dye lucifer yellow. The magnificent dendritic tree is revealed in its entirety. The fuzzy appearance of the finer caliber dendritic branches is due to the high density of dendritic spines (spiny branchlets). A single axon is visible emerging from the cell soma, although it is stained very faintly compared to the dendritic tree due to its small caliber.

type. The form of the dendritic tree can also be very distinctive, ranging from the simple morphologies of sensory cells to the magnificent tree of the Purkinje neuron (Fig. 2). The axon carries information away from the cell soma and transmits it to other neurons, muscles, and glands at a specialized cellular junction called the synapse. Information is conducted down the axon in the form of electrochemical signals known as action potentials. Transmission of signals to the next cell in the network usually involves the release of chemical neurotransmitters at the synaptic terminal, which interact with receptors in the target cell. The view that dendrites and axons are specialized for reception and transmission, respectively, was first proposed by Ramon y Cajal as the Law of Dynamic Polarization. However, examples where dendrites pass information between them (dendrodendritic transmission), signals are passed from the dendrite to the axon, and cells have no axons at all are numerous enough to suggest that this concept is only useful as a starting point for understanding neural circuits.

Since the time of Ramon y Cajal, neuroanatomists have spent considerable effort describing the morphology of nerve cells, from their overall form to their internal organization. The overall form has been studied by using techniques like the Golgi reaction (Fig. 1) and, more recently, by the injection of intracellular dyes, which reveal the pattern of the dendritic and axonal arbors of individual nerve cells (Fig. 2). The technique of intracellular dye injection is very powerful because it can be combined with intracellular recording in living tissue. By using this method, neuroscientists have produced elegant studies correlating the form, chemical identity, and physiology of individual neurons. The form of a neuron is important in several ways in determining its functional properties. For example, the distribution and extent of the dendritic tree and axon will determine how a neuron fits into neural circuits. A neuron like the pyramidal cell of the cortex has dendrites that extend hundreds of microns and will receive different types of information compared to a cell such as the stellate cell, which has a much more limited dendritic field. Simple properties such as the diameter of the axon will affect the speed of neurotransmission, with larger caliber axons conducting faster than smaller caliber axons. Similarly, the diameter and branching pattern of dendrites along with the distribution of ion channels and other signal transduction molecules can affect the spread of a signal in the dendritic tree.

To develop and maintain their characteristic forms, neurons have well-developed cytoskeletons. The cy-toskeleton is important not only for establishing and maintaining neuronal form but also for the transport of proteins and organelles throughout the neuron. The axons and dendrites of nerve cells can extend for hundreds or thousands of microns, necessitating an efficient mode of intracellular transport. The three major types of cytoskeletal elements found in all cell types, including neurons, are (1) microfilaments, e.g., actin, ~ 7 mm in diameter, (2) intermediate filaments, including neurofilaments, one of the neuron-specific classes of intermediate filaments, ~ 10 nm in diameter, and (3) microtubules, ~ 25 nm in diameter.

Although the morphology of a neuron is a good starting point for the discussion of neuronal diversity and function, the distinction between different types of neurons goes well beyond their unique morphology. Neurons are as heterogeneous in their biochemistry and physiology as they are in their morphology. Indeed, the nervous system by far displays the most molecular diversity of any tissue. The advent of immunocytochemical techniques to localize proteins and other small molecules and in situ hybridization to localize unique nucleic acid sequences has shown that different populations of neurons can be distinguished by their complement of neurochemical constituents. Such constituents include neurotransmitters, neuro-transmitter receptors, ion channels, calcium-binding proteins, and neuropeptides. Even morphologically identical populations ofcells can sometimes be divided into subclasses on the basis of their neurochemical signatures. Neurons are also distinguishable by their physiological properties, such as whether they excite or inhibit their targets, and by their pattern of firing. A major goal of modern neuroscience is to create an integrated view of nerve cells whereby the physiological properties of neurons can be understood in terms of their structural and biochemical specializations.

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