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Back to basics: treating spinal-cord injury/disease involves treating all cell types.

When people think about spinal-cord injury and disease (SCI/D), most imagine damage to the entire spinal cord. But the spinal cord consists of many individual cells. Because SCI/D may cause different amounts of damage to nerve (neurons) as well as support (glia) cells, it is a complicated situation to treat. However, scientists have learned a great deal about how each cell type can he individually repaired to improve the overall response to in jury.

In this article, we wild look at cell types and parts that are important for nervous-system function. Part 2 will examine how SCI/D affects these cellular parts, and how different therapies can help repair nerve cells ante glia.


Neurons are cells that form a network of connections--the nervous system--throughout our bodies. The brain and spinal cord make Up the central nervous system (CNS), and our bodies' other nerves (for example, in the arms and legs) comprise the peripheral nervous system (PNS). Neurons communicate using special types of electrical signals, which control all of the actions we (and other organisms) perform on a daily basis. (See "The Human Nervous System: Master Organizer," September 1996.)

A neuron, or nerve cell, consists of three major regions (Figure 1):


* The cell body houses many, parts that keep the cell alive and healthy.

* Long, thin extensions of the cell bodies. axons send out information, coded as electrical signals, to other neurons.

* Dendrites are small fibers extending out of the cell bodies to receive incoming information, also coded as electrical signals, other neurons. In order to understand the function of these three main regions of neurons, you need to be familiar with the smaller parts of cells and the jobs they perform.


The cell membrane is a coating made up mostly of tats. It covers the outside of the entire cell and separates it from other cells and its environment (Figure 1). The membrane is important because many specialized proteins located within it create a "bridge" between the cell's inside and outside.

Two types of proteins within the cell membrane are channels and receptors. These are necessary for the proper passage of electrical signals in the nervous system.

Channels create holes in the cell membrane to allow specific chemical molecules (such as calcium or sodium) to pass into or out of the cell (Figure 2). Channels are located all over the neuron. One of the most important functions of channels is to help pass nerve signals, but these proteins also help maintain appropriate chemical balances inside and outside the nervous system's cells.


Receptors are frequently located in the membranes of the dendrites or cell bodies (Figure 2). One important feature of receptors is that they bind to specific types of nerve-signaling chemicals (neurotransmitters). In many cases, the binding of neurotransmitters can lead to the movement of specific molecules through the cell membrane (like a channel). This process helps to pass electrical impulses. In other instances, the binding can have more long-term effects on the cell.

Cytoplasm, the fluid material inside the cell membrane, contains all of the structures and chemicals that keep the cell functioning correctly (Figure 1).

The cytoskeleton is literally the cell's skeleton. Long chains of proteins form a network of fibers inside the cell to help it hold its shape and perform many functions--such as growth--that are essential for regeneration. Cytoskeletal proteins are found in all parts of the cell (body, dendrites, and axons) and perform different functions depending on their location.


The largest part of a cell, the cell body holds all of the general parts of a cell as well as the nucleus, which is the control center. The nucleus contains the cell's genetic material (DNA, located in the chromosomes). One of the nucleus's most important activities is the "expression" of genes. Gene expression is essential for normal cell function, but it can also play a key role in successful repair and regeneration after nervous-system injury (see "What Is Gene Expression?").

In addition to the nucleus, the cell body contains many other unique structures. Some of these are responsible for making energy for the cell, and others help get rid of cellular waste. Some structures help make proteins, while substances called enzymes help break things down. The cell body also contains many different types of chemicals, such as calcium, sodium, and potassium. The cell must maintain a very precise balance of these chemicals or it won't function properly.


Axons are long, thin fibers that extend from the nerve-cell body to contact target structures (for example, other neurons, muscles, etc.)--nearby and in other parts of the body. Some axons are extremely short, while others (by necessity) are very long.

Axons are important for the passage of electrical impulses in the nervous system. Normally, electrical signals move down an axon, along the cell membrane, away from the cell body. Membrane channels that allow sodium and potassium to pass through are essential for the movement of these signals. At the ends of the axons, nerve endings come into close contact with the cell membrane of the neuron's target

An axon contacts another neuron at a very specialized region of the cell membrane called a synapse. When a nerve impulse reaches a synapse, this releases signaling chemicals called neurotransmitters. These substances travel a short distance to the membrane of the next cell, where they attach to receptors. The binding of neurotransmitters to membrane receptors helps transfer electrical signals from cell to cell.

Many axons in the nervous system are coated with a fatty substance called myelin. The presence of myelin allows nerve impulses to travel quickly down an axon (Figure 3). Although myelin is important for axons, neurons do not produce it. Instead, it is made by special types of glial cells described below.



Dendrites are special fibers extending from the cell bodies of some--but not all--types of neurons. Through synapses, dendrites receive information from the axons of other nerve cells. Because most neurons have more than one dendrite, they may receive electrical signals from many other cells at once. For this reason, it can be complicated for the nerve-cell body to make sense of all the electrical information it receives from its dendrites and to decide what kind of signal it should send on to the next neuron.


In addition to neurons, glial cells are also present in the nervous system. These live in close contact with nerve cells and perform critical jobs under normal conditions as well as after injury. As a result of their many jobs, glial cells are mentioned frequently in reference to spinal-cord regeneration.

Glial cells are similar to neurons in numerous ways. For example, both have many of the same parts: cell body, nucleus, cell membrane, cytoplasm, and cytoskeleton.

They have fiber-like extensions that project from their cell bodies, but these extensions are neither dendrites nor axons. Instead, glial extensions have different jobs depending upon the cell's function and location.

Glial cells, like neurons, have membrane receptors and channels, but they do not conduct nerve impulses in the same way. Nevertheless they are "aware" when neighboring neurons are electrically activated and can respond to this activity.

The nervous system contains three types of glial cells:

* Astrocytes. These star-shaped cells help provide support for the neurons by maintaining a normal balance of chemicals in their environment and by secreting substances that help support neurons' hearth. In response to injury, astrocytes may become enlarged and form a type of scar tissue in the injury region. They may also produce chemical factors that affect repair and regeneration processes.

* Microglia. Activated following injury, these cells may help clean up the debris of damaged neurons. Like astrocytes, they may also produce chemicals that influence nervous-system repair.

* Glial cells that make myelin: In the CNS, cells called oligodendrocytes produce myelin (the fatty coating wrapped around axons), which enables them to send very rapid nerve impulses. In the PNS, Schwann cells make the myelin.


The nervous system contains many unique types of cells, each with different structures that are essential for the system's norm,al function. Unfortunately, when the spinal cord is injured or affected by disease, neurons and glia can be damaged in a variety of ways. Everything from receptors to the cytoskeleton can be disrupted. Thus, many research strategies focus on repairing very specific parts of these cells. For example, one therapy might try to repair the cell membranes of damaged neurons, while another might help restore the myelin coating around the axons.

Part 2 of this series examines how some of the most promising therapies for treating spinal-cord dysfunction help repair the different parts of the neurons and glia.

Dr. Melinda Kelley is associate director of research at the Paralyzed Veterans of America National Office, in Washington, D.C.


What do scientists mean when they talk about genes and gene expression?

Genes are sections of the genetic material (also called DNA or chromosomes) in all of our cells. Each gene provides a kind of blueprint or code the cell uses to build proteins--molecules of many different types that allow each of our cells to perform its own unique job. When a gene is "expressed," a group of specialized molecules in the nucleus "reads" the code and makes a kind of inverse copy of it.

This process is similar to taking a picture. When you take a photo through a camera, the negative is an inverse image from the one you see through the lens. The genetic negative has a name: a messenger RNA molecule. These messenger RNA negatives pass from the cell's nucleus, where they are made, into the cytoplasm. There, another group of specialized structures reads the information on the messenger RNA molecule and builds the appropriate protein. The protein produced "matches" the information provided on the original gene.

It's somewhat like printing a photographic negative: The final picture represents the image you photographed.
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Title Annotation:Cells & SCI, part 1
Author:Kelley, Melinda
Publication:PN - Paraplegia News
Date:May 1, 1998
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