MRI of the Spinal Cord
The spinal cord is about 45 cm long in men and 43 cm long in women. However, this is much shorter than the length of the bony spinal column. In fact, the spinal cord extends down to only the last of the thoracic vertebrae. Therefore, nerves that branch from the spinal cord from the lumbar and sacral levels must run in the vertebral canal for a some distance before they exit the vertebral column. This collection of nerves in the vertebral canal is called the cauda equina (which means "horse tail").

Receptors in the skin send information to the spinal cord through the spinal nerve. The cell bodies for these nerve fibers are located in the dorsal root ganglion. The nerve fibers enter the spinal cord through the dorsal root. Some fibers make synapses with other neurons in the dorsal horn, while others continue up to the brain. Many cell bodies in the ventral horn of the spinal cord send axons through the ventral root to muscles to control movement.

In the figure below, note the differences in the shape and size of the spinal cord at different levels. The dark gray color in each segment represents "gray matter". If you use your imagination a bit, you can see that the gray matter looks a little bit like an H or even like a butterfly. The gray matter is where the nerve cell bodies are located. Surrounding the gray matter is white matter (lighter color shading) - this is where the axons of the spinal cord are located. Spinal Cord Segments Compare the relative amount of gray and white matter at each level of the spinal cord. In the cervical segment, there is a relatively large amount of white matter. This is because at this level of the spinal cord, there are many axons going up to the brain from all levels of the spinal cord AND there are many axons traveling from the brain down to different segments of the spinal cord. In lower segments of the spinal cord, there is less white matter because there are fewer axons traveling to and from the brain. There are also differences in the gray matter. In the cervical segment, the ventral horn (the lower half of the segment) is enlarged. Also in the lumbar segment that is illustrated, the ventral horn is large. These segments are those that contain motor neurons that control movement of the upper limbs (cervical segment) and lower limbs (lumbar segment).

The Spinal Cord The major "highway" connecting the brain and peripheral nervous system

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