Cell Biology Of The Neuron

To appreciate the molecular biology presented in this chapter, it is necessary to describe the components of the neuron that process signals that directly or indirectly modify the aspects of the genome described below. Neurons —specialized cells that function to transmit signals to other neurons, muscles, and secretory cells—contain four basic domains (Figure 2-1), and these domains serve to receive signals, process and integrate signals, conduct impulses, and release transmitter.

FIGURE 2-1. Diagram of a typical neuron.

Input Regulation Conduction Transmission

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As described in the text, this neuron is divided into zones for the reception of signals (input = dendrites), integration of signals (regulation = nucleus and soma), conduction of signals (axon), and transmission of signals (axon terminal).

The nucleus resides in the cell body (the signal processing domain) and contains the DNA that codes for the genes expressed by neurons. Activation of a given gene results in the generation of a messenger RNA (mRNA), which is then translated into a protein (see below). Although such events are common to all cells, neural cells are unique in some aspects of molecular signaling. Of importance, the variety of gene expression is far greater in the brain than in any other organ or tissue. Some estimates are that in aggregate, the brain expresses as much as 10 times the number of genes expressed in any other tissue. This does not mean that individual neurons undergo a much greater gene expression. Rather, it suggests an extraordinary heterogeneity among neurons and glia, which allows for a rich regulation when those neurons and glia assemble into the elaborate network of the human brain.

mRNA molecules exported from the nucleus are translated into proteins by ribosomes in the endoplasmic reticulum. Note that most of the protein production occurs in the cell body, although there is also some mRNA in the dendrites (Steward and Wallace 1995). This means that newly made proteins must be transported from that cell body to the axon terminal, a distance as great as 1 m. These proteins are often packaged in vesicles, and specialized "motor" molecules transport packaged proteins down microtubule "tracks" at the cost of adenosine triphosphate (ATP) hydrolysis (Setou et al. 2000).

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