3. which type of neuron carries messages within the central nervous system?

Interneurons which are responsible for connecting one neuron to another. 

Schwann cells are a type of glial cell that are found in the peripheral system but function similarly to Oligodendrocytes which are found in the CNS. Motor neurons carry signals from the CNS to the target or effector organ i.e. muscle or skin, while Sensory neurons carry sensory messages from receptors to the CNS. The former typically have short dendrites to receive input and long axons to carry impulses while the latter tends to have longer dendrites and shorter axons.
Hope this helps you