What type of neuron carries messages from one part of the CNS to another?

The relay neuron is responsible for transmitting messages within the central nervous system (CNS). It acts as a connector between sensory neurons, which carry information from sensory organs, and motor neurons, which convey signals to muscles and glands. Relay neurons play a crucial role in processing information and coordinating responses by integrating incoming messages and forwarding them to appropriate areas of the CNS.

In the context of the options provided, the motor neuron specializes in sending signals from the CNS to effectors like muscles and glands, while the sensory neuron transmits information from sensory receptors to the CNS. The term "interneuron" can sometimes be used interchangeably with relay neuron, but in the context of your question, relay neuron is the more specific term for those that primarily link different parts of the CNS. Thus, the correct choice emphasizes the function of transmitting messages within the central processing areas of the nervous system.