These all describe a group of conditions which all look the same, but are caused by different problems. The nervous system is effectively a two-way system. Messages from the brain travel via the upper motor neurones, down the spinal cord, through the lower motor neurones, such as the sciatic nerve in the leg, to the muscle. In the sensory system, the messages travel upward from the sensory input through the first sensory neurones, up the spinal cord, through to the brain. Peripheral nerves are in the limbs and are composed to two types of nerve fibre - motor nerves and sensory nerves.
Abnormal myelin means that the messages to and from the brain move very slowly.
In CMT Type 2 the fault lies in the core of the nerve, the axon. While the speed of signal transmission is nearly normal, there are other irregularities in the way the nerve functions. Type 1A is caused, usually, by a duplication in a gene on chromosome 17. These genes produce a protein called PMP22, but just what this protein does in the myelin is not known. There are also some cases where there is only one gene on each chromosome, but one is defective.