This is (the real) Spinal Tap...

I know I'm scheduled to start "From Brain to Mind" today, but I found one more entry in the mailbag that's worth answering:

"Dr. Roberts: What is a spinal tap?"

Oh, yeah, that's a really good question. To explain, let me start back in the brain. As with all tissues in the body, the brain has an extensive supply of blood vessels, however, there is a very tight wall made of specialized cells lining the blood vessels allowing only water and certain chemicals through the the spaces around the neurons. To cushion and bathe the neurons, the brain floats in a fluid called "Cerebrospinal Fluid" (CSF), which consists of the fluid which filters out of the blood and is generated inside 4 spaces within the brain called "ventricles". The figure at right shows these spaces (the two lateral ventricles are also labeled as the first and second ventricles). At the base of the cerebellum, the ventricles also connect to the space outside the brain that cushions between brain and skull. There is a tough tissue lining the inside of the skull (dura), and another tightly conforming to the surface of the brain (pia). The space between is filled with CSF and the combination of inner and outer lining plus CSF is quite commonly known as "meninges" - and is the root of the disease name "meningitis" (and inflammation of the tissue lining and increased fluid pressure due to bacterial or viral infection).

You will notice that below the fourth ventricle the space narrows. The cross section at left shows the continuation of this tube as the "central canal" down through the middle of the spinal cord. The central canal is continuous with the ventricles of the brain, forming a complete internal system draining fluid and chemicals from the brain. Likewise, the external space - the meninges - continue down along the outside of the spinal cord.

The fact that these spaces are continuous means that anything that appears in the CSF in the brain will eventually appear in the CSF of the spinal cord. Evidence of infection, inflammation, or neural damage will result in an increase in certain proteins and neurotransmitter chemicals in the CSF. Since flow of the CSF is from inside to outside, the best place to sample the CSF is from either the fourth ventricle or the central canal.of the spinal cord.
Justify Full
This is where the spinal tap comes in. Doctors can insert a long, hair-thin needle into the spinal cord and withdraw a small amount of CSF for analysis. Since the spinal cord is encased in bone (right), the needle has to be threaded between the vertebrae, into the spinal cord and right to the central canal. It's a bit tricky, but there are guiding features that allow the needle to be inserted in the proper place. In fact, the indentations that separate the left and right sides of the spinal cord (above) ensure that the needle is directed to the center, and do not enter the neuron- and axon-packed regions to either side.

One of the problems with spinal tap is the "spinal headache" that often follows. Injection sites don't necessarily completely seal up, and CSF can leak out into the meninges, the decrease in pressure throughout the central canal and ventricles, coupled to the increase in fluid in the "epidural" space between pia and dura, causes headaches until the injection site completely seals - either by clotting or scar formation. These days there are newer analysis techniques based on MRI that are replacing the need for spinal tap in many cases, but there are also very good reasons to directly sample the fluid in closest contact with the brain.

There is another reason to enter this space - injection of a very small amount of anesthetic (spinal block) allows surgeons to cut all muscle movement and tactile/pain sensation to a specif region of the body by targeting only that region where the appropriate nerves enter and exit the spinal cord. On a larger scale, infusion of anesthetic into the space inside the dura, surrounding the spinal cord (epidural), will block pain sensation to all body regions below the injection point.

And for those who have wondered, the spinal tap needle isn't *really* a foot long, it just seems that way. It's about 4 inches, and is so fine that you really don't feel it enter the skin or the spinal cord. What you feel is the pressure and the slight CSF loss that accompanies the needle entry. It's really not something to be worried or scared of, but it is becoming increasingly rare in this age of digital imaging and analyses.

Many thanks to my readers who send in such great questions!

Tune in next time for the next installment in The Lab Rats' Guide to the Brain: From Brain to Mind.