I see in ms & gbs the immune system only seems to attack the mylein sheath. what stops the immune system going further & directly attacking the nerves?

Read on : Nerves are affected too early axonal degeneration is well known in ms the antibody is specific even in examples given, in ms the antibody is against oligodendrocytes forming the myelin in the central nervous system and in guillain barre is against the schwan cells forming the myelin in the peripheral nervous system!

Specific antibodies: ...Only against myelin are the hallmark of ms and guillain-barre (also transverse myelitis.) other types of autoimmune neurologic disease target other parts of the nervous system: myasthenia gravis, sydenham's chorea, and post-varicella cerebellitis are all examples.

Neuropathy: Although much is known about the mechanisms involved in multiple sclerosis, the cause remains unknown. Theories include genetics or infections. Guillain-barre sydrome involves autoimmune responses but has multiple inciting factors, and the reasons why autoimmune responses are to schwann cells is not understood. Good questions. If you could answer - nobel prize?

Variable: Yes, in many cases, the myelin sheath alone is involved, and the underlying axon is spared, BUT immune systems are variable, and the more aggressive forms of MS and GBS can result in axonal damage. It is the body's ability to turn off the immune attack which quells the ultimate damage.