Which part of the nervous system is typically involved in processing external stimuli?

The brain is the central processing unit of the nervous system and plays a crucial role in interpreting external stimuli. It receives sensory information from various sensory organs and systems, processes this information, and integrates it to form perceptions, responses, and actions. This processing allows the organism to react appropriately to their environment.

While the peripheral nervous system does convey sensory information to the brain and the cranial nerves are part of this system responsible for transmitting sensory data from the head and neck to the brain, the actual processing and interpretation of that data occur in the brain itself. The sympathetic nervous system primarily regulates involuntary bodily functions, particularly during stress responses, rather than handling the initial processing of external stimuli. Thus, the brain is the correct answer, as it is fundamental to understanding and reacting to sensory input from the environment.