In the next post, The Mobile Nervous System, we will see just how physically active the structures lying within the dorsal cavity are. Let’s look first at a few of the design features of nerves, their blood vessels and their connective tissue that permit all this movement:
- Elastin, one type of connective tissue, is the most elastic tissue found in the body. It makes up 7% of the anterior aspect of the spinal dura, the fascial tube surrounding the spinal cord. But it accounts for a good 14% of the posterior surface of that dura. This difference stems from the fact that we mostly forward bend with the activities of daily life. With a bend forward, the posterior part of the dura is loaded more than the anterior portion and needs extra elastin to respond to the forces it experiences – first by accepting the stretch and then by recoiling.
- The filum terminale, a thin elastic band that attaches the bottom of the spinal dural sac to the coccyx, likely helps to protect the cord from overstretching.
- The nerves making up the spinal cord exit the safety of the bony spine via the intervertebral foramen, small apertures found between the vertebrae. They then form the dorsal and ventral nerve roots. See figure 1. These nerve roots in turn form the peripheral (spinal) nerves that bring in sensory information from the body and send out impulses to work the body’s muscles and glands. See figure 2. The nerve roots and peripheral nerves are exposed to much more traction, pinching and compression than the nervous tissue lying protected by the bony skull and vertebral column. To deal with these stresses, the otherwise fragile nerve tissue is supplied with a supporting skin of connective tissue. Now, collagen is the principle fiber making up that skin. And increasing the amount of collagen makes nerve roots and peripheral nerves stronger. As a result, nerve roots coming off the cord have 6x the collagen than does the spinal cord itself. And peripheral nerves have 5x more collagen than the nerve roots. Strikingly, connective tissue sheathing makes up a full half of the diameter of a peripheral nerve. See figure 3. This is also true, of course, for the median nerve we mentioned earlier. So the next time you send out the hand in brush knees or partitions, recall that the median nerve and its connective tissue wrappings lengthen 20% – only to return to their resting lengths as we move on to the next phase of the set. A constant cycle of expansion and contraction.
- The connective tissue surrounding nerve tissue is supplied with sensation that permits it to warn the nervous system of impending injury from excessive strain.
- The brain and spinal cord represent 2% of body weight but consume 20% of the available oxygen. Blood flow must be maintained even when nervous tissue is elongating. This is ensured, in part, by the tortuosity of the blood vessels that allows them to straighten or unfurl when the nerves are lengthening.
We see, then, that the nervous system is built to move, constantly turning, lengthening and shortening. This unceasing movement involves the nerves themselves, their accompanying blood supply and the pool of CSF that suspends the brain and cord.
To keep mobile over a lifetime, these tissues, and especially their connective tissue scaffolding, must remain supple, moist and flexible. On another occasion, we will look at the impact of the regular practice of the Taoist Tai Chi™ internal arts of health on collagen and how that speaks to the stiffness so often associated with aging and illness.
As for the CSF, I suspect our daily practice enhances its circulation in a number of ways. Likely mechanisms include:
- The rhythmic relaxation of weight that is so central to our art. This serves to lengthen and then shorten the vertebral column and the spinal cord within.
- The movements of the fourth ventricle and brainstem described in the next post, The Mobile Nervous System.
- The changes in CSF pressure associated with breathing. Exhalation lowers and inhalation raises the pressure of the CSF.
- The tenting outwards of the spinal dura produced by the tugging action of the nerve roots as we stretch out the arms and legs.
Atlas of Human Anatomy, 4th edition, Frank H. Netter, Saunders Elsevier, 2006, ISBN-13: 978-1-4160-3385-1
“The Sensitive Nervous System”, David Butler, 1st edition, 2000, ISBN 0 646 40251 X
© 2010 Taoist Tai Chi Society of Canada