What type of fibers make up the dorsal root of the spinal nerve?

The dorsal root of the spinal nerve is composed exclusively of sensory fibers, which are responsible for transmitting sensory information from the peripheral nervous system to the central nervous system. These sensory fibers carry signals related to touch, pain, temperature, and proprioception from sensory receptors throughout the body. The structure of the spinal nerve allows this information to enter the spinal cord through the dorsal root, where it can then be processed and relayed to different parts of the brain for interpretation.

In contrast, motor fibers originate from the ventral root of the spinal nerve, which is responsible for carrying motor commands from the central nervous system to skeletal muscles. Mixed fibers contain both sensory and motor components, but since the question specifically asks about the dorsal root, which is solely involved in sensory transmission, they are not applicable here. Lastly, autonomic fibers are involved in involuntary functions and are primarily associated with the autonomic nervous system, which also does not pertain to the dorsal root structure. Thus, the primary role of the dorsal root as a pathway for sensory information underlines why sensory fibers are the correct answer.