What is the primary structure in the peripheral nervous system?

The primary structure in the peripheral nervous system is nerves. The peripheral nervous system (PNS) consists of all nerve tissue outside of the brain and spinal cord, which together comprise the central nervous system (CNS). Nerves are responsible for transmitting sensory information to the CNS and carrying motor commands from the CNS to the muscles and glands throughout the body. Each nerve is composed of bundles of axons, which allow for the communication between the brain and the extremities.

The other options provided are components of the central nervous system. The brain and spinal cord, including specific regions like the cerebellum, are integral parts of the CNS, which processes information and coordinates responses. In contrast, the PNS is essential for connecting the CNS to the limbs and organs, making nerves the primary structures in this system.