4: The Cells of the Nervous System

Introduction

• Neurons and Glia Share Many Structural and Molecular Characteristics

• The Cytoskeleton Determines Cell Shape

• Protein Particles and Organelles Are Actively Transported Along the Axon and Dendrites

  • Fast Axonal Transport Carries Membranous Organelles

  • Slow Axonal Transport Carries Cytosolic Proteins and Elements of the Cytoskeleton

• Proteins Are Made in Neurons as in Other Secretory Cells

  • Secretory and Membrane Proteins Are Synthesized and Modified in the Endoplasmic Reticulum

  • Secretory Proteins Are Modified in the Golgi Complex

• Surface Membrane and Extracellular Substances Are Recycled in the Cell

• Glial Cells Play Diverse Roles in Neural Function

  • Glia Form the Insulating Sheaths for Axons

  • Astrocytes Support Synaptic Signaling

  • Choroid Plexus and Ependymal Cells Produce Cerebrospinal Fluid

  • Microglia in the Brain Are Derived from Bone Marrow

• An Overall View

Neurons and glia—the cells from which the nervous system is assembled—share many characteristics with cells in general. However, neurons are specially endowed with the ability to communicate precisely and rapidly with other cells at distant sites in the body. Two features give neurons this ability.

First, they have a high degree of morphological and functional asymmetry: Neurons have receptive dendrites at one end and a transmitting axon at the other. This arrangement is the structural basis for unidirectional neuronal signaling.

Second, neurons are both electrically and chemically excitable. The cell membrane of neurons contains specialized proteins—ion channels and receptors—that facilitate the flow of specific inorganic ions, thereby redistributing charge and creating electrical currents that alter the voltage across the membrane. These changes in charge can produce a wave of depolarization in the form of action potentials along the axon, the usual way a signal travels within the neuron. Glia are less excitable, but their membranes contain transporter proteins that facilitate the uptake of ions as well as proteins that remove neurotransmitter molecules from the extracellular space, thus regulating neuronal function.

There are about 100 distinct types of neurons. This cytological diversity is also apparent at the molecular level. Although neurons all inherit the same complement of genes, each expresses a restricted set and thus produces only certain molecules—enzymes, structural proteins, membrane constituents, and secretory products—and not others. In large part this expression depends on the cell's developmental history. In essence each cell is the set of molecules that it makes.

Neurons and Glia Share Many Structural and Molecular Characteristics

Neurons and glia develop from common neuroepithelial cells of the embryonic nervous system and thus share many structural and molecular characteristics (Figure 4–1). The boundaries of these cells are defined by the cell membrane or plasmalemma, which has the asymmetric bilayer structure of all biological membranes and provides a hydrophobic barrier impermeable to most water-soluble substances. Cytoplasm has two main components: cytosol and membranous organelles.