Brain and spinal cord = Central nervous system (CNS)
Nerves = peripheral nervous system
There are 3 main types of cells (neurones), each with specific functions:
Nerves are a mixture of fibres of sensory and motor neurones, i.e. 2 types - with impulses moving in each direction.
However in each type of neurone the impulse is unidirectional. No impulse passes back to the sensory neurone, nor from the muscle to the motor neurone.