- Dementia Slideshow Pictures
- Take the ADHD Quiz
- Brain Foods Slideshow
- Patient Comments: Spinal Cord Injury - Causes
- Patient Comments: Spinal Cord Injury - Symptoms
- Find a local Doctor in your town
- Spinal Cord Injury Facts
- What is the spinal cord injury?
- What are the causes of spinal cord injury?
- What are the symptoms of spinal cord injury?
- How is a spinal cord injury diagnosed?
- How is a spinal cord injury treated?
- What is the outlook for patients with spinal cord injury?
- Is there a cure for spinal cord injury?
- Where can I get more information on spinal cord injury?
- NIH spinal cord injury: treatments and rehabilitation
- What Is a Spinal Cord Injury?
- How Does the Spinal Cord Work?
- What Happens When the Spinal Cord Is Injured?
- What Are the Immediate Treatments for Spinal Cord Injury?
- How Does a Spinal Cord Injury Affect the Rest of the Body?
- How Does Rehabilitation Help People Recover From Spinal Cord Injuries?
- How Is Research Helping Spinal Cord Injury Patients?
- The Future of Spinal Cord Research
How Does the Spinal Cord Work?
To understand what can happen as the result of a spinal cord injury, it helps to know the anatomy of the spinal cord and its normal functions.
The soft, jelly-like spinal cord is protected by the spinal column. The spinal column is made up of 33 bones called vertebrae, each with a circular opening similar to the hole in a donut. The bones are stacked one on top of the other and the spinal cord runs through the hollow channel created by the holes in the stacked bones.
The vertebrae can be organized into sections, and are named and numbered from top to bottom according to their location along the backbone:
- Cervical vertebrae (1-7) located in the neck
- Thoracic vertebrae (1-12) in the upper back (attached to the ribcage)
- Lumbar vertebrae (1-5) in the lower back
- Sacral vertebrae (1-5) in the hip area
- Coccygeal vertebrae (1-4 fused) in the tailbone
Although the hard vertebrae protect the soft spinal cord from injury most of the time, the spinal column is not all hard bone. Between the vertebrae are discs of semi-rigid cartilage, and in the narrow spaces between them are passages through which the spinal nerves exit to the rest of the body. These are places where the spinal cord is vulnerable to direct injury.
The spinal cord is also organized into segments and named and numbered from top to bottom. Each segment marks where spinal nerves emerge from the cord to connect to specific regions of the body. Locations of spinal cord segments do not correspond exactly to vertebral locations, but they are roughly equivalent.
- Cervical spinal nerves (C1 to C8) control signals to the back of the head, the neck and shoulders, the arms and hands, and the diaphragm.
- Thoracic spinal nerves (T1 to T12) control signals to the chest muscles, some muscles of the back, and parts of the abdomen.
- Lumbar spinal nerves (L1 to L5) control signals to the lower parts of the abdomen and the back, the buttocks, some parts of the external genital organs, and parts of the leg.
- Sacral spinal nerves (S1 to S5) control signals to the thighs and lower parts of the legs, the feet, most of the external genital organs, and the area around the anus.
The single coccygeal nerve carries sensory information from the skin of the lower back.
Spinal Cord Anatomy
The spinal cord has a core of tissue containing nerve cells, surrounded by long tracts of nerve fibers consisting of axons. The tracts extend up and down the spinal cord, carrying signals to and from the brain. The average size of the spinal cord varies in circumference along its length from the width of a thumb to the width of one of the smaller fingers. The spinal cord extends down through the upper two thirds of the vertebral canal, from the base of the brain to the lower back, and is generally 15 to 17 inches long depending on an individual's height.
The interior of the spinal cord is made up of neurons, their support cells called glia, and blood vessels. The neurons and their dendrites (branching projections that help neurons communicate with each other) reside in an H-shaped region called "grey matter."
The H-shaped grey matter of the spinal cord contains motor neurons that control movement, smaller interneurons that handle communication within and between the segments of the spinal cord, and cells that receive sensory signals and then send information up to centers in the brain.
Surrounding the grey matter of neurons is white matter. Most axons are covered with an insulating substance called myelin, which allows electrical signals to flow freely and quickly. Myelin has a whitish appearance, which is why this outer section of the spinal cord is called "white matter."
Axons carry signals downward from the brain (along descending pathways) and upward toward the brain (along ascending pathways) within specific tracts. Axons branch at their ends and can make connections with many other nerve cells simultaneously. Some axons extend along the entire length of the spinal cord.
The descending motor tracts control the smooth muscles of internal organs and the striated (capable of voluntary contractions) muscles of the arms and legs. They also help adjust the autonomic nervous system's regulation of blood pressure, body temperature, and the response to stress. These pathways begin with neurons in the brain that send electrical signals downward to specific levels of the spinal cord. Neurons in these segments then send the impulses out to the rest of the body or coordinate neural activity within the cord itself.
The ascending sensory tracts transmit sensory signals from the skin, extremities, and internal organs that enter at specific segments of the spinal cord. Most of these signals are then relayed to the brain. The spinal cord also contains neuronal circuits that control reflexes and repetitive movements, such as walking, which can be activated by incoming sensory signals without input from the brain.
The circumference of the spinal cord varies depending on its location. It is larger in the cervical and lumbar areas because these areas supply the nerves to the arms and upper body and the legs and lower body, which require the most intense muscular control and receive the most sensory signals.
The ratio of white matter to grey matter also varies at each level of the spinal cord. In the cervical segment, which is located in the neck, there is a large amount of white matter because at this level there are many axons going to and from the brain and the rest of the spinal cord below. In lower segments, such as the sacral, there is less white matter because most ascending axons have not yet entered the cord, and most descending axons have contacted their targets along the way.
To pass between the vertebrae, the axons that link the spinal cord to the muscles and the rest of the body are bundled into 31 pairs of spinal nerves, each pair with a sensory root and a motor root that make connections within the grey matter. Two pairs of nerves - a sensory and motor pair on either side of the cord - emerge from each segment of the spinal cord.
The functions of these nerves are determined by their location in the spinal cord. They control everything from body functions such as breathing, sweating, digestion, and elimination, to gross and fine motor skills, as well as sensations in the arms and legs.
The Nervous Systems
Together, the spinal cord and the brain make up the central nervous system (CNS).
The CNS controls most functions of the body, but it is not the only nervous system in the body. The peripheral nervous system (PNS) includes the nerves that project to the limbs, heart, skin, and other organs outside the brain. The PNS controls the somatic nervous system, which regulates muscle movements and the response to sensations of touch and pain, and the autonomic nervous system, which provides nerve input to the internal organs and generates automatic reflex responses. The autonomic nervous system is divided into the sympathetic nervous system, which mobilizes organs and their functions during times of stress and arousal, and the parasympathetic nervous system, which conserves energy and resources during times of rest and relaxation.
The spinal cord acts as the primary information pathway between the brain and all the other nervous systems of the body. It receives sensory information from the skin, joints, and muscles of the trunk, arms, and legs, which it then relays upward to the brain. It carries messages downward from the brain to the PNS, and contains motor neurons, which direct voluntary movements and adjust reflex movements. Because of the central role it plays in coordinating muscle movements and interpreting sensory input, any kind of injury to the spinal cord can cause significant problems throughout the body.