Final answer:
The sympathetic nervous system is the division that triggers the startle response, preparing the body for 'fight or flight' during stressful situations. It is part of the autonomic nervous system and functions to rapidly enact physiological changes to respond to threats.
Step-by-step explanation:
The division of the nervous system that produces the startle response, such as when a person is frightened, is the sympathetic nervous system. This division is activated during stressful situations to prepare the body for a "fight or flight" response. The autonomic nervous system is comprised of the sympathetic and parasympathetic nervous systems, with the sympathetic division taking charge during times of stress to increase heart rate, breathing rate, redirect blood flow to essential muscle groups for rapid action, and induce sweating.
The sympathetic nervous system prepares the body for emergency responses by activating a broad array of changes throughout the body. This is unlike the parasympathetic nervous system, which is responsible for the body's rest and digest functions. The sympathetic system's widespread influence is due to its dense network of preganglionic neurons that diverge extensively, influencing many targets and preparing the body for a swift reaction to perceived threats.