Anatomy 101: Spinal Cord and Central Nervous System
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The Human Spinal Cord
The spinal cord is a long column of nervous tissue that acts as the pathway for transmitting information between the brain and the body. It is encased by a series of bones, the vertebrae, which allow for flexibility of the back and protect the very delicate spinal tissue. Different levels of the spinal cord control different parts of the body. There are 31 segments in the spinal cord and each controls a different movement. For example, cervical level 1-2 (C 1-2), the highest and closest to the skull, controls breathing, C 5-6 (the most common injury site) is at the bump where your neck meets your back and controls fingers and hands. At each segment, nerves leave the cord, nerves that are responsible for specific parts of the body. Nerves from the body also enter the cord at each segment, providing sensory information about specific body parts to the brain.
In additional to nerves coming into and out of the spinal cord at different segments, the spinal cord also conveys information between the brain and spinal cord and vice versa. Large bundles of nerves, called tracts or columns, run, for example, from the motor centers in the brain down the spinal cord and connect to pools of motor neurons that are responsible for movement.
The organization of the spinal cord is elegant. In cross-section, it looks like a butterfly on an oval background. The butterfly shape in the middle of the cord, or the gray matter, contains nerve cell bodies, and the surround, or white matter, has the axons, extensions from nerve cells that carry messages. In the section shown, axons are stained dark purple and the gray matter appears red. In unstained tissue, the white matter appears white because axons are wrapped with an fatty insulating substance called myelin.
Cells of the Nervous System
Neurons
The brain sends messages to the spinal cord and body through neurons. Neurons have a cell body and branching arms, called dendrites, that receive input from other nerve cells. They also have a long arm, called an axon, that reaches out to make contact with other neurons. The messages are sent via electrical signals that are converted into chemical signals at the synapse, the place where the axon of one neuron contacts another neuron.
Glia
Only 10% of brain and spinal cord cells are neurons. The remaining 90% are support cells called glia. The term glia comes from Latin for glue, as these cells bind the nervous system together. Many neuroscientists thought that glia were passive cells that did little except provide support for neurons. Because of this and a lack of good imaging techniques, glia have been all but ignored until recently. In the last few years, new technology for staining and imaging glia have been developed that allow scientists to better understand what these cells do. Glial cells include astrocytes, oligodendrocytes and Schwann cells.
Oligodendrocytes
Nerve cells send messages via electrical and chemical signals. Electrical signals are sent from the cell down a long snaky projection, the axon, to the next cell. Axons, like power cords, need insulation to keep the electricity flowing. The insulation in the spinal cord and brain is a fatty substance called myelin. Myelin is made by oligodendrocytes, which are a type of glial cell or neural support cell. Each oligodendrocyte sends out many arms that wrap around axons forming insulating myelin. Normal myelin is formed by approximately 30 wraps round an axon, although remyelination studies have shown that messages can be sent with as few as 3-10 wraps. When oligodendrocytes are damaged and myelin in disrupted, the neurons cannot communicate and the result is paralysis or motor dysfunction.
Myelin
If neurons are the electric wires allowing the brain to sending messages to the body and receive information from the body, then myelin is the insulation that allows the signal to move along the wires. Myelin is a fatty substance and is produced by Oligodendrocytes. Neurons have a cell body, which contains the machinery for keeping the cell alive, dendrites, which receive messages from other neurons, and axons, which send messages to other cells. To move the electrical signal down the axon, the axon must be encased, or sheathed, in insulating myelin. So myelin is critical for central nervous system communication. However, after a spinal cord injury, myelin releases chemicals that stop neurons from regenerating. Thus myelin, while essential for normal communication in the nervous system, also inhibits regeneration after injury.
Schwann Cells
Myelin has been big news in spinal cord injury research since it was found to release chemical signals that inhibit axon regeneration. But, there are two myelin making cells in the body, oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. Both provide the necessary insulation that allows nerve cells to send electrical signals and both support nerve cells and their axons by releasing beneficial chemicals and molecules. One minor difference between the two cell types is that one Schwann cell myelinates only one axon, while oligodendrocytes myelinate many axons.
The major difference between the two myelin makers is that Schwann cells do not inhibit regeneration. In fact, Schwann cells support axon regrowth. In the peripheral nervous system, Schwann cells act as a guide for regenerating axons by creating a tunnel system. Schwann cells form a tube that the nerve sprout can grow through. Indeed, regenerating axons that contact a Schwann cell tube will grow 3-4 mm a day, while sprouts that don't contact tubes die off.
Research has shown that Schwann cells may be beneficial in the central nervous system, even though this is not their natural home.
Astrocytes
Astrocytes make chemicals, or neurotrophic factors, that are like vitamins for neurons. These factors nourish and give energy to nerve cells. In addition, astrocytes are thought to play an important role in nerve cell communication. Astrocytes not only influence how neurons talk to each other, but may actually control the messages sent. Neurons communicate by releasing chemical molecules called neurotransmitters into the tiny gap between nerve cells. Astrocytes mop up and break down extra neurotransmitter, as the extra can be toxic to surrounding cells. An excess of the neurotransmitter glutamate is particularly dangerous and is thought to be a major culprit in the secondary cell death after spinal trauma. Certain types of astrocytes are also responsible for building the scar around an injury site. The scar is a physical barrier to regeneration.
Macrophages
Macrophages are one of the body's most talented cell types. These cells, which are born in bone marrow and circulate in the blood until needed, clean up cellular garbage, marshal the body's response to injury or disease, attack invading bacteria and pathogens, promote the formation of scar tissue and are able to produce over 100 different substances. Macrophages modulate the inflammatory response and stimulate the immune system. They are a critical part of the body's response to injury. However, there is controversy about the role of macrophages in central nervous system injury.
The brain and spinal cord are immunologically privileged sites, that is in the healthy human there is very little immune function in the central nervous system. When the brain or spinal cord are injured, the blood-brain barrier is broken and macrophages run into the injury site. In the periphery, this is a good thing. In the central nervous system, however, macrophages may cause more damage than good. A primary function for macrophages is to clean up cellular debris. After central nervous system injury, they may consume and destroy cells that have been damaged but could recover, leading to an increase in secondary damage.
