What is the main component of the nervous system that runs the length of the body, dorsal to the notochord or vertebral column?

The main component of the nervous system that runs the length of the body, situated dorsal to the notochord or vertebral column, is the nerve cord. In many organisms, particularly invertebrates and vertebrates, the nerve cord serves as the critical structure through which signals are transmitted between the brain and the rest of the body, playing a vital role in coordination and reflex activities.

The nerve cord is typically composed of a central canal surrounded by nerve tissue, which is responsible for processing and transmitting information. In vertebrates, this structure becomes the spinal cord, which is encased within the vertebral column for protection.

In contrast, other options such as the endoskeleton and axial skeleton refer to structural components that provide support and protection for the body but do not play direct roles in the transmission of nerve impulses. The capillary option relates to the circulatory system and is responsible for the exchange of substances between blood and tissues but has no association with the nervous system's structural organization.