The muscles in your body get their commands from the brain. For example, when you decide to walk, your brain tells your leg muscles to start moving. When you want to lift something, your brain sends signals to your arm muscles. Between your brain and your muscles are "wires" -- nerve cells -- that carry the commands. Every nerve pathway that controls your arms, hands, torso, legs and feet passes through your spinal cord to get to its destination. Your brain + your spinal cord = your central nervous system.
The bones -- vertebrae -- in your spine protect your spinal cord. Between the vertebrae, spinal nerves branch off and head toward different muscle groups. Nerves also collect signals for touch, pain, heat, cold, joint position and so on, and send this information back to the brain through the spinal cord.
But if the spinal cord gets injured, this communication system is interrupted, leaving parts of the body disconnected from the brain. Depending on the severity of the injury, a person may experience partial or complete paralysis, which can be temporary or permanent.
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The spinal cord (purple) runs down the spine and is protected by bone. Between the vertebrae, spinal nerves branch off and head toward different muscle groups.
If the damage to the cord is located below chest level, then the stomach and legs may be affected. This is called paraplegia. Injuries above the chest may result in tetraplegia (also known as quadriplegia), meaning all four limbs and the trunk have a loss of feeling or movement. Higher-level injuries can even make breathing difficult or stop altogether.
Injuries to the spinal cord can be caused by traumatic injuries like an auto or diving accident, but they can also occur because of an illness that causes nerve degeneration.
According to the spinal cord injury resource center:
Approximately 450,000 people live with spinal cord injuries in the US. There are about 10,000 new SCIs every year; the majority of them (82%) involve males between the ages of 16-30. These injuries result from motor vehicle accidents (36%), violence (28.9%), or falls (21.2%). Tetraplegia is slightly more common than paraplegia.