As we discussed earlier, the cranial fascia is one of 4 fascial layers found in the body. The space this particular layer creates, the dorsal cavity, serves to house and protect the brain, spinal cord and cerebrospinal fluid (CSF).
Now, the specialized function of all nerves, including those found in the brain and cord, is communication. They achieve this through conduction, an electrochemical process that proceeds along the surface of each nerve and within its internal, fluid environment. This work of our nervous system goes on even as our everyday movements subject the nerves to continual lengthening and shortening, winding and unwinding, bending, straightening, compression and release.
In the previous post, How the Nervous System is Designed for Movement, we learned that mobility of the nerve tissue is built into its design.
Today, we will draw on David Butler’s description¹of what has been learned about neurodynamics – the interplay between the physical abilities of the nervous system and its capacity to pass on messages. We will see just how physically active nervous tissue is.
This examination of the physical life of nerves gives us a better understanding of how Taoist Tai Chi™ internal arts of health help to maintain the vitality of nerve tissue lying deep within the body.
Nervous tissue is constantly moving as it communicates:
- The brainstem consists of the midbrain, pons and medulla. Sitting just below the main bulk of the brain and in front of the cerebellum, it connects the 2 cerebral hemispheres above with the spinal cord below. As might be guessed from its central location, it plays a crucial role in maintaining life – ensuring, among other things, continued respiratory effort, regular contractions of the heart and the maintenance of blood pressure, all without any conscious effort on our part. See figure 1 below. So what happens to this region of the brain when we move? As the neck (cervical spine) goes from full extension (backward bend) to full flexion (forward bend), a number of events occur. The brainstem lengthens 1.5 cm. The cranial nerves 5 to 12 are drawn tight to their exit zones in the base of the skull as they leave the protection of the skull to connect with the outside structures of the head. The fourth ventricle becomes longer and narrower. The medulla moves forward, its circumference decreasing by 4mm, and the angle between the spine and the medulla changes.
- The columns of nerve cells running down the posterior aspect of the spinal cord lengthen with flexion and shorten with extension. With side bending, the nerve tracts on the convex side stretch more than those on the concave aspect.
- As it moves from extension to flexion, the cervical spinal cord drifts forward in the spinal canal, lengthens by 20% and narrows by 30%. Because the nerves of the cord arrange themselves in spirals and folds, they are able to straighten as the cord elongates and refold as it shortens.
- In the upper limb, the median nerve provides so much sensory information that it is known as the eye of the hand. Formed just above the armpit, it runs the full length of the upper limb down into the fingers. See figure 2. Each time we send the hand out from the body, the median nerve lengthens about 20%. Undue stress exerted anywhere along its length will interfere with its function. The carpal tunnel syndrome, for example, develops when there is chronic crowding of the median nerve at the wrist. This common condition illustrates how altering the physical circumstance of a nerve has a major impact on its function.
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