Sensory nerve: carries electrical message from receptor to spine
Motor nerve: carries electrical message from spine to effector.
Relay nerve: connects sensory and motor nerves. Also relays message to the brain.
Schwann cells: wrap around the axon of the long nerves, creating a thick layer of membrane, which insulates the nerve and allows for much faster conduction speed. The thick layer of membrane has gaps in it between adjacent Schwann cells, these are called Nodes of Ranvier.