Final answer:
An interneuron is the type of neuron that lies completely within the central nervous system, functioning to relay impulses between sensory and motor neurons.
Step-by-step explanation:
The type of neuron that lies totally within the central nervous system (CNS) is known as the interneuron. These neurons play a critical role in communication between sensory neurons and motor neurons.
Interneurons receive nerve impulses from sensory neurons, which carry information from sensory receptors to the CNS, and relay these signals to motor neurons, which take information away from the CNS to muscles and glands. Unlike sensory and motor neurons, interneurons do not extend outside the CNS, hence they are entirely contained within it. In fact, interneurons make up the vast majority of neurons in the brain and spinal cord, and their complex networks are crucial for interpreting, planning, and coordinating appropriate responses to sensory input.