Final answer:
The sympathetic division of the Autonomic Nervous System regulates pupil dilation in response to stimuli like fear or pleasure. The nerve pathway starts at the hypothalamus, goes to the spinal cord and through the sympathetic chain ganglion, where a chain of neuron signals leads to the release of norepinephrine, causing the dilation.
Step-by-step explanation:
When a person is startled or sees a 'pleasurable' object, causing pupil dilation, the division of the Autonomic Nervous System (ANS) involved is the sympathetic division.
The sympathetic division is responsible for inducing reactions to situations of stress or excitement, often termed as 'fight or flight' reactions. The nerve pathway involved begins with the hypothalamus, which sends a signal to the spinal cord. From the spinal cord, the signal travels through the sympathetic chain ganglion where preganglionic and postganglionic neurons lead to the release of norepinephrine, causing pupil dilation.
Learn more about Pupil Dilation