What is primarily responsible for relaying messages from the brain to the rest of the body?

Nerve cells, or neurons, are primarily responsible for relaying messages from the brain to the rest of the body due to their specialized structure and function. Neurons are designed to transmit electrical impulses through their axons and dendrites, enabling rapid communication between different parts of the body and the central nervous system. This transmission is crucial for coordinating all bodily functions, including movement, sensation, and reflexes.

The unique attributes of neurons, such as the myelin sheath which accelerates impulse conduction and synapses that allow communication between neurons, underscore their role in carrying signals. In contrast, structural cells, support cells, and red blood cells do not have the primary function of message relay. Structural cells typically provide support and form the framework of tissues, while support cells (glia) assist and protect neurons but do not transmit messages themselves. Red blood cells are involved in oxygen transport rather than nerve signal transmission. Thus, the correct answer reflects the fundamental role that nerve cells play in the communication network of the nervous system.