The epidural space is a very narrow space that is situated immediately outside the spinal fluid, and separated from it by a special membrane, called ‘dura mater’, or dural membrane (hence the name epi-dural, outside the dura).
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The basic anatomy is as follows: the extension from the brain into the back is the spinal cord, in which millions of nerve fibers are running upward connecting the body to the brain or downward connecting the brain to the body. Around the whole length of the spinal cord is spinal fluid, which is held together by the dural membrane. This membrane forms like a very narrow long sac in which the spinal cord and the spinal fluid houses, and runs from the skull all the way down to the sacrum.
This whole structure sits in the spinal canal, which is a long, narrow tunnel from the skull to the sacrum, inside the spinal column. The very small space between the dural membrane and the inside of the bony spinal canal, over the whole length of the back, is called the epidural space.
When the nerves leave the spinal cord, they have to go through the spinal fluid, through the dural ‘sac’ and through the epidural space, and then through the side holes in the spinal column, after which they run their different courses to the organs or limbs they innervate. The nerves transmitting signals from the body to the spinal cord and brain follow the same route of course, but in the opposite direction.
By injecting local anaesthetic into the epidural space, the nerves can be blocked when they run through this space, so that no more signals from the affected body area can reach the spinal cord or the brain, nor can signals from the brain and spinal cord reach the body.
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