Expert: Walter Hintz - 12/3/2011

Question
why doesn't the nerve impulse go backward?

why doesn't the invertible animals have myelin sheath?

Answer
I am not sure what you mean by backward. Nerves go to and from the central nervous system;sensory nerves carrying the impulse to the brain and motor nerves  from the brain. The impulse starts where the stimulus begins. So the word backwards has no meaning when we consider nerve impulses

The function of the myelin sheath is to control the speed of the impulse of the long nerves. The neuron is somewhat like an electrical wire and requires insulation. Of course a nerve impulse is not an electron flow. Electrons flow at the speed of light. Nerve impulses are slow and in long nerves must be sped up. The myelin sheath has gaps called the nodes of Ranvier and the impulse jumps from gap to gap making it faster. The simpler nervous system in invertebrates does not have a speed problem. The giant squid has long axons but the diameter of the axon is larger meaning less resistance. Also a fast response time the squid is less critical