The central nervous system ( CNS ) is the part of the nervous system that functions to coordinate the activity of all parts of the bodies of multicellula. In vertebrates , the central nervous system is enclosed in the meninges . It contains the majority of the nervous system and consists of the brain and the spinal cord . Together with the peripheral nervous system it has a fundamental role in the control of behavior . The CNS is contained within the dorsal cavity , with the brain in the cranial cavity and the spinal cord in the spinal cavity . The brain is protected by the skull, while the spinal cord is protected by the vertebrae. 
Nerve mechanics, or neurodynamics simply means the study of the mechanical properties of nerves, how they stretch, move, glide, and cause pain as the body moves. Science has made some exciting leaps forward in this area in the last twenty years,and these have been well summarized in some great books by Michael Shacklock and David Butler. Amazingly, this work is almost completely unknown to most doctors and physical therapists. This is unfortunate because nerve mechanics are easily understandable and help explain and treat a great many painful conditions. Here are some basic facts that will help you understand this issue. By the way, after reading this you will probably know more than your PT and doctor about musculoskeletal pain.
You can feel this easily by moving into a forward bend to touch your toes while moving your head and neck to stare at your navel. When you go low enough, you will feel a familiar intense stretch at the back of the knee. This is the sciatic nerve saying stretch no further (its not the hamstrings as most people think.) You can greatly reduce the intensity of the feeling in the hamstrings by lifting your neck to look up. This releases tension on the spinal cord from above and the resulting slack eases the tension on the sciatic nerve. Move your head back and forth to feel how the neck position affects the feeling at the back of your knee. It should be pretty obvious if you’re doing it right. The point here is that your whole body is connected . A lack of mobility in the neck can easily affect the low back or even the hamstring or vice versa. The converse is also true. You can reduce pain in one area by improving mobility in another.
Movement of the Nerves is Healthy . The movements of the nerves – bending, sliding, compressing and stretching – are completely normal and healthy under normal circumstances. In fact, proper nerve function depends on movement – alternating tension, compression and relaxation probably assists circulation and reduces the nerve’s inner viscosity. In other words, motion is lotion for the nerves just as it is for all the other parts of the body. However, as you might have guessed, once the nerves become sensitive, moving then can cause pain. In fact, a great many painful conditions that you may have thought were caused by a sore muscle or tendon such as carpal tunnel, plantar fasciaitis, tendonitis, a pulled hamstring etc, may actually be caused by sensitive, compressed or inflamed nerves.
In the next post we’ll discuss how nerves become sensitive to movement.
If you think of the brain as a central computer that controls all bodily functions, then the nervous system is like a network that relays messages back and forth from the brain to different parts of the body. It does this via the spinal cord, which runs from the brain down through the back and contains threadlike nerves that branch out to every organ and body part.
When a message comes into the brain from anywhere in the body, the brain tells the body how to react. For example, if you accidentally touch a hot stove, the nerves in your skin shoot a message of pain to your brain. The brain then sends a message back telling the muscles in your hand to pull away. Luckily, this neurological relay race takes a lot less time than it just took to read about it.
Three types of neurons occur. Sensory neurons typically have a long dendrite and short axon, and carry messages from sensory receptors to the central nervous system . Motor neurons have a long axon and short dendrites and transmit messages from the central nervous system to the muscles (or to glands). Interneurons are found only in the central nervous system where they connect neuron to neuron.