Structure and Maintenance of Neurons

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Neurons occur in a variety of sizes and shapes; nevertheless, as shown in Figure 8-2, most of them contain four parts: (1) a cell body, (2) dendrites, (3) an axon, and (4) axon terminals.

As in other types of cells, a neuron's cell body contains the nucleus and ribosomes and thus has the genetic information and machinery necessary for protein synthesis. The dendrites form a series of highly branched outgrowths from the cell body. They and the cell body receive most of the inputs from other neurons, the dendrites being vastly more important in this role than the cell body. The branching dendrites (some neurons may have as many as 400,000!) increase the cell's receptive surface area and thereby increase its capacity to receive signals from a myriad of other neurons.

The axon, sometimes also called a nerve fiber, is a single long process that extends from the cell body to its target cells. In length, axons can be a few micrometers or a meter or more. The portion of the axon closest to the cell body plus the part of the cell body where the axon is joined are known as the initial segment, or axon hillock. The initial segment is the "trigger zone" where, in most neurons, the electric signals are generated that then propagate away from the cell body along the axon or, sometimes, back along the den-drites. The main axon may have branches, called collaterals, along its course; near the ends both the main axon and its collaterals undergo further branching (Figure 8-2). The greater the degree of branching of the axon and axon collaterals, the greater the cell's sphere of influence.

Each branch ends in an axon terminal, which is responsible for releasing neurotransmitters from the axon. These chemical messengers diffuse across an extracellular gap to the cell opposite the terminal. Alternatively, some neurons release their chemical messengers from a series of bulging areas along the axon known as varicosities. Different parts of nerve cells serve different functions because of the segregated distribution of various membrane-bound channels and pumps as well as other molecules and organelles.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

II. Biological Control Systems

8. Neural Control Mechanisms

© The McGraw-Hill Companies, 2001

Neural Control Mechanisms CHAPTER EIGHT

Neural Control Mechanisms CHAPTER EIGHT

FIGURE 8-1

The central nervous system (green) and the peripheral nervous system (blue). Some of the peripheral nerves connect with the brain (these nerves are not shown) and others with the spinal cord.

FIGURE 8-1

The central nervous system (green) and the peripheral nervous system (blue). Some of the peripheral nerves connect with the brain (these nerves are not shown) and others with the spinal cord.

Neurons And Neuroglia

Axon terminal

FIGURE 8-2

(a) Diagrammatic representation of a neuron. The proportions shown here are misleading because the axon may be 5000 to 10,000 times longer than the cell body is wide. This neuron is a common type, but there are several other types, one of which has no axon. (b) A neuron as observed through a microscope. The axon terminals cannot be seen at this magnification. %

Axon terminal

FIGURE 8-2

(a) Diagrammatic representation of a neuron. The proportions shown here are misleading because the axon may be 5000 to 10,000 times longer than the cell body is wide. This neuron is a common type, but there are several other types, one of which has no axon. (b) A neuron as observed through a microscope. The axon terminals cannot be seen at this magnification. %

The axons of some neurons are covered by myelin (Figure 8-3), which consists of 20 to 200 layers of highly modified plasma membrane wrapped around the axon by a nearby supporting cell. In the central nervous system these myelin-forming cells are the oligodendroglia (a type of neuroglia, or simply glial cell to be described later in the chapter), and in the peripheral nervous system they are the Schwann cells. The spaces between adjacent sections of myelin where the axon's plasma membrane is exposed to extracellular fluid are the nodes of Ranvier. The myelin sheath speeds up conduction of the electric signals along the axon and conserves energy, as will be discussed later.

Various organelles and materials must be moved as much as one meter from the cell body, where they are made, to the axon and its terminals in order to maintain the structure and function of the cell axon. This movement is termed axon transport. The substances and organelles being moved are linked by proteins to microtubules in the cell body and axon. The microtubules serve as the "rails" along which the transport occurs. The linking proteins act both as the "motors" of axon transport and as ATPase enzymes, providing energy from split ATP to the "motors."

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

-Myelin-forming cell

Myelin-forming cell

Node of Ranvier

Nucleus

Cell body

Axon

Nucleus

Node of Ranvier

Myelin-forming cell

Axon

Myelin

FIGURE 8-3

(a) Cross section of an axon in successive stages of myelinization. The myelin-forming cell may migrate around the axon, trailing successive layers of its plasma membrane or, as shown here, it may add to its tip, which lies against the axon, so that the tip is pushed around the axon, burrowing under the layers of myelin that are already formed. The latter process must be used in the central nervous system where each myelin-forming cell may send branches to as many as 40 axons. (b) Adjacent myelin-forming cells are each separated by a small space, the node of Ranvier. (c) A myelinated neuron.

Part a redrawn from Meyer-Franke and Barres.

Axon transport of certain materials also occurs in the opposite direction, from the axon terminals to the cell body. By this route, growth factors and other chemical signals picked up at the terminals can affect the neuron's morphology, biochemistry, and connectivity.

This is also the route by which certain harmful substances, such as tetanus toxin and herpes and polio viruses, taken up by the peripheral axon terminals can enter the central nervous system.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Neural Control Mechanisms CHAPTER EIGHT

Neural Control Mechanisms CHAPTER EIGHT

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  • GARY
    How is the branching shape of a neuron related to a neuron's function?
    8 years ago

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